(51) Earth Science

The most beautiful gems & minerals you'll ever see!

Minerals and Gems (National Geographic)

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Minerals and Gems

When you hear the word minerals, what comes to mind? Do you picture a cereal box advertising extra vitamins and minerals? Do you think of miners spending years searching for a glimpse of a shiny nugget or a brilliant stripe across a rock face? Or the many-faceted beauty of a friend’s diamond ring? Rock found on the Earth’s crust is a solid material created by three main geological processes: magma solidification, sedimentation of rock layers, and metamorphism. As a result, three basic rock types are formed.

* Igneous rock (volcanic or plutonic) is formed by the solidification of molten magma from the mantle.

* Sedimentary rock is formed from the burial, compaction, and lithification of deposited rock debris or surface sediments.

* Metamorphic rock is created when existing rock is chemically or physically modified by intense heat or pressure.

Geologists usually consider rocks to be a jumble of naturally occurring materials, mainly minerals. They can contain a mix of minerals and other organic substances ranging from microscopic mineral grains or organic matter to rough mineral agglomerates. Rocks can range in size from pebbles to mountains.

When people talk about their ‘‘rock collection,’’ they usually mean their ‘‘mineral collection.’’ Although some people collect rocks, mineral collectors are more common. They are the people looking for the ‘‘perfect’’ example of a specific mineral or the ‘‘rarest’’ specimen within a mineral group.

Amateur mineralogists and collectors are a lot like people who show dog breeds, like German Shepherds or English Pointers, to name a few. They get more points for having a specimen that meets the standard characteristics for the rock type and is of a high priority.

People value things that are rare and perfect. Flawless diamonds are much more valuable than those with flecks and flaws.

In fact, people have decorated themselves with shells, pieces of bone, teeth, and pebbles for the past 25,000 years (Paleolithic Period). But at that time most of the stones they chose were soft and brightly colored. Red carnelian and crystals were common choices.

From the time between 3000 and 2500 BC, lapis lazuli from Dadakshan reached Egypt and Sumer (Iraq). China, Greece, and Rome got their gemstones from many of the same regional mines.

Then, as people traveled and traded more, stones were made into family or governmental seals. They had different textures and some were carved.

When rolled on damp clay, an imprint was made that identified a product. Seals were part of a leap in commercial trade. Some stone seals were worn around the neck and considered a status symbol. Kings and rulers had ring seals that were recognized as symbols of identity and power.

Ancient people thought gems and crystals had special powers. In an uncertain world, people wore them for protection. Color was important in the imagery. Gold was related to the Sun, blue to the sea, sky, or heavens, red to blood or the life force, and black for death. Wearing powerful gems was thought to protect the wearer’s health, and bring wealth, luck, and love.

When the mummy of King Tutankhamun (1341BC–1323BC) a Pharaoh of ancient Egypt, was discovered, it was decorated with gold, red carnelian, turquoise, crystals, jasper, obsidian, alabaster, amazonite, jade, and lapis lazuli. These were the amulets of wealth and strength at that time. These stones were worn during life and some placed on the mummy after death as a protection against harm in the afterlife.

Some minerals and gems were thought to be powerful by themselves, while others were thought to wield power through the figures and words written on them.

Minerals and gems were also thought to contain medicinal powers. The early Greeks recorded these claims in medical papers known as lapidaries. The Greek philosopher, Theophrastus (372–287 BC) wrote the oldest surviving book on minerals and gems, called On Stones. He grouped 16 minerals into metals, earths, and stones (gemstones). A natural geologist, he accurately described physical characteristics of color, luster, transparency, hardness, fracture, weight, and medicinal benefits.

Pliny the Elder (AD 23–79) pulled together everything that earlier scholars had written into his 37-volume series, Historia Naturalis. Pliny’s work provided a lot of useful information on sources, mining methods, uses, trade, and gem value.

Since the 1600s, scientists have become even more questioning. The study of minerals and gems has become a part of the study of chemistry, optics, and crystallography.

Minerals are often described by their chemical formulas in order to note the chemical substitutions of one or more atoms. For example, topaz, a prismatic crystal with the formula, Al2SiO4 (F5OH)2, has been found to be as large as 100 kg. It can be colorless, white, gray, yellow, orange, brown, bluish, greenish, purple, or pink.

Gems and minerals are at the heart of the study of geology. Whether in the Earth or found on other planets, minerals tell the story of a planet’s chemical and physical developments. They have specific characteristics with unique physical and chemical properties. This adds to their great variety and makes the study of minerals interesting.

The study of minerals, minerology, is usually focused on the external microscopic study of minerals in polished sections. People who hunt for and collect rough mineral specimens as a hobby are often called ‘‘rock hounds.’’

Mineral Groups and Properties

All minerals belong to a specific chemical group, which represents their affiliation with certain elements or compounds. The chemical structure of minerals is exact, or can vary slightly within limits. They have specific crystalline structures and belong to different groups according to the way the mineral’s atoms are arranged. Elements like gold, silver, and copper are found naturally and considered minerals.

A mineral is a naturally found, inorganic substance with a specific crystalline structure.

Minerals are classified into the following chemical groups: elements, sulfides, oxides, halides, carbonates, nitrates, borates, sulfates, chromates, phosphates, arsenates, vanadates, tungstates, molybdates, and silicates. Some of these chemical groups have subcategories, which may be categorized in some mineral references as separate groups.

Nine Classes of Minerals

Geologists have identified over 3000 minerals. In order to study them more closely, they have divided minerals into nine different groups.. Minerals occur naturally as inorganic solids with a crystalline structure and distinct chemical make up.

Major mineral groups are determined by chemical composition.

Type Chemical structure

1. Elements

2. Sulfides

3. Halides

4. Oxides and hydroxides

5. Nitrates, carbonates, borates

6. Sulfates

7. Chromates, molybdates, tungstates

8. Phosphates, arsenates, vanadates

9. Silicates

Minerals are divided by different groupings

Mineral – Group (element-e, halide-h, oxide-o, silicate-si, sulfide-su, phosphate-p, molybdate-m, borate-b, carbonate-c) – Hardness (Mohs’ scale) – Chemical (Composition)

Antimony – e - 3 – 3.5 – Sb

Arsenic e 3.5 As

Bismuth – e - 2–2.5 Bi

Carbon (diamond and graphite) –e - Graphite 1–2 Diamond 10 - C

Copper – e - 2.5 –3 - Cu

Gold - e - 2.5 –3 - Au

Nickel – iron - e - 4 –5 - Ni,Fe

Platinum - e - 4 –4.5 - Pt

Silver - e 2.5 – 3 Ag

Sulfur e -1.5 – 2.5 - S

Fluorite - h 4 - CaF2

Halite – h - 2.5 - NaCl

Corundum - o - 9 Al2O3 - (ruby, sapphire)

Cuprite - o - 3.5 – 4 - Cu2O

Hematite - o - 5 – 6 - Fe2O3

Albite – si - 6 – 6.5 - NaAlSi3O8

Anorthite – si - 6 – 6.5- CaAl2Si2O8

Beryl - si 7 –8 - Be3Al2(SiO3)6

Dioptase – si – 5 - CuSiO2 (OH) 2

Jadeite - si - 6 – 7 - Na (Al, Feþ3) Si2O6

Labradorite – si - 6 – 6.5 - (Na, Ca) Al1 – 2Si3 – 2O8

Microcline – si - 6 – 6.5 - KAlSi3O8

Olivine – si - 6.5 – 7 - (Mg, Fe) 2SiO4

Orthoclase – si - 6 – 6.5 - KAlSi3O8

Quartz – si - 2.65 - SiO2

Topaz – si – 8 - Al2SiO4 (F, OH) 2

Zircon - si - 7.5 - ZrSiO4

Cinnabar – su - 2 – 2.5 - HgS

Galena – su - 2.5 - PBS

Pyrite - su - 6 – 6.5 - FeS2

Molybdenite – su - 1 – 1.5 - MoS2

Gypsum - su - 2 CaSO4-2 - (H2O)

Lazulite - p - 5.5 – 6 - (Mg, Fe) Al2 (PO4) 2 (OH)

Turquoise – p - 5 – 6 CuAl6 (PO4) 4 (OH)8  4H2O

Wulfenite m - 2.5 – 3 - PbMoO4

Borax b 2 – 2.5 - Na2B4O5 (OH) 4 8H2O

Calcite - c - 3 - CaCO3

Malachite - c - 3.5–4 - Cu2 (CO3) (OH) 2

Rhodochrosite - c - 3.5–4 - MnCO3

Earth minerals are composed of different elements. Oxygen 47%, Silicon 28%, Alumnium 8%, Iron 5%, Calcium 4%, Sodium 3%, Potassium 3%, Magnesium 2%

ELEMENTS

The elements include more than 100 known minerals. Many of the minerals in this class are made up of only a single element. Geologists sometimes subdivide this group into metal and nonmetal categories. Of all of the elements, 80% are metals. Gold, silver, and copper are examples of metals.

Carbon produces the minerals diamond and graphite, which are nonmetals. Elements like phosphorus and selenium are also nonmetals. For a complete listing of the known chemical elements, scientists use the Periodic Table of Elements. This is a chart that lists all the elements known today, along with a lot of other useful information. Besides the computer, the Periodic Table is probably the most important tool that scientists use.

Geologists use the Periodic Table to figure out the chemical composition of new minerals and to learn possible ways that different elements might bond.

The Periodic Table of Elements lists an element’s symbol (shorthand name, like C for carbon, Al for aluminum), atomic number (equal to the number of protons), atomic weight, and sometimes the atomic energy levels of the element. When a certain element is described, it is written with the atomic number in superscript and the atomic weight in subscript. On a Periodic Table, magnesium, with atomic number 12 and an atomic weight 24.31, is written as:

12

Mg

24:31

While the simplest of Periodic Tables show just an element’s atomic number and weight, complete charts give a broader amount of information. To give you an idea of the usefulness of the Periodic Table, the information listed for titanium in most Periodic Tables is shown below.

22

Ti

47:90

Ti

Titanium

Atomic Number – 22

Atomic Weight – 47.90

Group – 4

Period – 4

Transition Metal

Electrons per orbital layer – 2, 8, 10, 2

Valence electrons – 1s2 2s2p6 3s2p6d2 4s2

Knowing specifics about elements, like their electron arrangement, allows chemists and other scientists to figure out the bonding possibilities and types of compounds that can be formed with other elements. From this information, the mineral content of new and unknown samples is worked out. This information is also helpful when creating new compounds in the laboratory.

HALIDES

The halides are a group of nonmetals whose main chemical components include chlorine, fluorine, bromine, and iodine. Most halides are very soluble in water. They also form highly ordered molecular structures with a high degree of symmetry. Halite is the most common mineral of this group. It is known to most people as rock salt. Other halites include the minerals, cryolite, atacamite, fluorite, and diabolite.

OXIDES

A group of minerals, made up of one or more metals combined with oxygen, water, or hydroxyl (OH), is known as the oxides (and hydroxides) group. The minerals in this group show a great variety of physical characteristics compared to other more nonchanging groups. Some oxides are hard and others soft. Some have a metallic luster, while some are clear and transparent. Some of the oxide minerals include anatase, corundum, chromite, and magnetite, while hydroxides include manganite, goethite, tungstite, and diaspore.

SILICATES

The silicates encompass the largest mineral group. As the name implies, these minerals have varying amounts of silicon and oxygen. Silicates are often opaque and light weight. Silicate minerals are different from other groups in that they are all formed as tetrahedrons. However, it can be tough to identify individual minerals within the silicates group. A tetrahedron is a chemical structure where a silicon atom is bonded to four oxygen atoms (SiO4). Some representative silicates include albite, andesine, hornblende, microcline, labradorite, sodalite, leucite, and quartz.

SULFIDES

The minerals of the sulfide group are often made up of a metal combined with sulfur. They are recognized by their metallic luster. The sulfides are an economically important group of minerals. The extraction of sulfide ores from composite metals is a standard process in industry. Specific ores are known for certain metal extractions, like cinnabar (a major source of mercury), molybdenite (molybdenum, an alloy in steel), pyrite (iron source), and galena (lead, used in piping and pewter).

SULFATES

The sulfate mineral group usually combines one or more metals with the sulfate compound, SO4. Most sulfates are transparent to translucent, light in color, and soft. They usually have low densities. Gypsum, the most plentiful sulfate, is found in evaporite deposits. Common sulfates include anhydrite (CaSO4) and celestine (SrSO4).

Sometimes, sulfates contain substituted groups like chromate, molybdate, or tungstate in place of the sulfate group. Chromates are compounds in which metals combine with chromate (CrO4). The minerals crocoite (PbCrO4), wulfenite (PbMoO4), and scheelite (CaWO4) are all examples of different group replacements that form different minerals. These compounds are usually dense, brittle, and brightly colored.

PHOSPHATES

The mineral group, known as the phosphates, is made up of one or more metals chemically combined with the phosphate compound (PO4). The phosphates are sometimes grouped together with the arsenate, vanadate, tungstate, and molybdate minerals. These minerals have substituted arsenic, tungsten, and molybdenum elements, respectively.

Although geologists list several hundred different types of these minerals, they are not common. Apatite is the most common phosphate mineral. Most minerals in these groups are soft, but their hardnesses can range from 11/2 to 5 or 6 (turquoise). Although brittle, they have well-formed crystals in beautiful colors like lazulite (blue) and vanadinite (red or orange).

CARBONATES

This is an easy one. Carbonates are minerals which contain one or more metals bonded with carbon in the compound (CO4). Most pure carbonates are light colored and transparent. All carbonates are soft and brittle. They are usually found as well-formed rhombohedral crystals. Carbonates react with, bubble up, and dissolve easily in hydrochloric acid. Calcite is the most common carbonate. Other colorful carbonate minerals include rhodochrosite (pink to red), smithsonite (blue green), and azurite (deep blue), and malachite (medium to dark green).

Nitrates and borates are often thought of as a subgroup of carbonates. They are formed when metal compounds combine with nitrogen and boron. When metals bond with nitrate, minerals like nitratine, a rare rhombohedral, transparent, often twinned mineral is formed.

When metals bond with borate, minerals like borax, kernite, and ulexite are formed. Most people have seen white borax, but it can also be colorless, gray, greenish, or bluish. Borax forms near hot springs, in ancient inland lakes, and places from which water has evaporated.

ORGANIC MINERALS

Minerals originally from organic sources (plants) are not usually classified as true (pure) minerals. However, some crystalline organic substances look and act like true minerals. These substances, formed primarily from carbon, are called organic minerals. Amber (petrified tree sap) is an example of an organic mineral.

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(50) Earth Science

Metamorphic Rocks Video

Metamorphic Rocks

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Chemical Changes

Characteristics that cause chemical changes in rocks also add to the formation of metamorphic rocks. Very hot liquids and gases can, under extreme pressures, fill the pores of existing rocks. These liquids and gases cause chemical reactions to occur, and over time, change the chemical composition of the existing rock. Metamorphism can take place instantly as in rock shearing at plate boundaries or can take millions of years as in the slow cooling of deeply buried magma.

It is important to remember that the changes that go on in metamorphism are mostly in rock texture. The chemical composition of metamorphic rock is altered very little. The basic changes that do occur include the addition or loss of water and carbon dioxide. The biggest changes of metamorphic transformation, then, have to do with the way minerals are rearranged.

A chemical shift in the composition of metamorphic rock can also be changed by the addition or removal of different elements. This can happen as a result of the intrusion of magma bringing new minerals into contact with existing rock. Sometimes this can be seen through color changes in minerals of the same basic chemical composition.

When hot, mineral-rich waters rise through magma, they carry a variety of elements. Some of these elements include sulfur, copper, sodium, potassium, silica, and zinc ions to name a few. These minerals come from magma and intruded rock, during the time that water is filtering upward through the crust. On this journey, they interact with other minerals and chemicals replacing some of their own minerals with others. This type of chemical interaction and substitution is called metasomatism. Metal deposits like copper and lead are formed in this way.

Index Minerals

A Scottish geologist, George Barrow, noticed that rocks having the same overall mineral make up (like shale) could be seen to go through a series of transformations throughout specific zones in a metamorphic region. He found that minerals in individual zones had specific mineral configurations. As he studied minerals across a zone, he found that when new metamorphic mineral configurations were created, it was predictable.

The first appearance of index minerals marks the boundary of lowto high-grade metamorphic rock changes in a specific regional zone.

Barrow found that mineral (shale) configuration changes happened with regard to index minerals. These index minerals acted like milestones in the low- to high-grade metamorphic rock transformation process. Barrow found that the domino effect of metamorphism happened in the following series:

chlorite -> biotite -> garnet -> staruolite -> kyanite -> sillimanite ->

Low grade->High grade

When Barrow and his team studied the geological maps of the Scottish Highlands, they were able to plot where certain minerals started and stopped. They marked the locations of certain minerals and called these connected places isograds.

An isograd is a marker line on a map connecting different areas of certain minerals found in metamorphic rock.

Metamorphic Rock Textures

Metamorphic rocks are divided into two categories, foliated and nonfoliated. Foliate comes from the Latin work folium (meaning leaf ) and describes thin mineral sheets, like pages in a book. Metamorphic minerals that align and form bands, like granite gneiss and biotite schist, are strongly banded or foliated.

When metamorphic mineral grains align parallel in the same plane and give rock a striped appearance, it is called foliation or foliated rock.

Initially, the weight of sedimentary rock strata keeps the sheet-like formation of minerals parallel to the bedding planes. As the mineral layers are buried deeper or compressed by tectonic stresses, however, folding and deformation take place. The sedimentary strata are shoved sideways and are no longer parallel to the original bedding. In fact, metamorphism changes the texture enough that when broken, the metamorphic rock breaks in the direction of the foliation not the original mineral’s composition.

Foliates are made up of large concentrations of mica and chlorite. These minerals have very clear-cut cleavage. Foliated metamorphic rocks split along cleavage lines that are parallel to the alignment of the rock’s minerals. For example, mica can be separated into thin, flat nearly transparent sheets.

Mica is said to have good schistosity, from the Latin word schistos meaning easily cleaved.

Schistosity is the parallel arrangement of coarse grains of sheetstructure minerals formed during metamorphism and increasing pressure.

For fine-grained rocks with microscopic mineral grains, the breakage property is known as rock cleavage or slaty cleavage.

Slaty cleavage is found in an environment of low temperature and pressure. In these less-intense conditions, grain sizes increase and single grains are easily seen. Foliation is present with slaty cleavage, but not in a flat plane. Intermediate and high-grade metamorphic rock commonly breaks along rolling, or somewhat distorted surfaces similar to the orientation of the grain of quartz, feldspar, and other minerals.

Rock cleavage or slaty cleavage describes the way rock breaks into plate-like pieces along flat planes.

Large crystal textures can also be formed in a fine-grained, support rock during metamorphism. When this happens, crystals found in both contact and regional metamorphic rock are called porphyroblasts. They grow as the elements are rearranged by heat and temperature.

We learned that structural deformation goes on during metamorphism. When two rock surfaces deep in the Earth’s crust grind against each other, crushing and stretching into bands, myolites are formed. These rocks are deformed under very high pressure. This deformation can take place before, during, or after metamorphic changes have happened and is part of the ongoing recycling of rock.

For example, shale may be changed into schist during deep burial without any deformation. Then, much later, when tectonic action hauls the schist layer upward in mountain building, higher-grade metamorphism may cause foliation and deformation. Then, if the rock is living a really interesting life, it may be heated during contact metamorphism and change yet again.

Naming

When naming metamorphic rock, the rules are more flexible than that of igneous or sedimentary rock naming. Since metamorphic rock tends tochange in composition and texture as temperatures and pressures change, the naming changes.

For example, shale is a fine-grained, clastic sedimentary rock containing quartz, clays, calcite, and some feldspar. With the start of low-grade metamorphism, muscovite and chlorite begins to form. Transformed shale is called slate. If the slate meets with further metamorphism, the mineral grains grow and intermediate-grade metamorphism happens with foliation and mica forms. Continued metamorphism causes the formation of even larger, coarsegrained rock with high schistosity and is known as schist. Then at high-grade metamorphism, the minerals group into separate bands with layers of mica-like minerals such as quartz and feldspar. This type of high-grade metamorphic rock is called gneiss from an old German word, gneisto, meaning to sparkle. So naming then depends on what can be seen. Slate and phyllite describe textures, while gneiss is described by the large mineral grains (that are easily seen) being named first. So specific gneiss might be named, quartz–plagioclase–biotite–garnet gneiss. In this way, another geologist would have a pretty good idea of all that the rock contained. Nongeologists would probably just call it garnet!

The Internet has several sites that provide photos of metamorphic rock types. There are even photos that illustrate the complete metamorphic rock series. 

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(49) Earth Science

Metamorphism of rocks

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Hydrothermal Metamorphism

 This type of metamorphism is common with mid-ocean ridges where the crust is spreading and growing as a result of the outpouring of hot lava. The ocean water that bubbles through the hot, fractured basalts of the ridge margins becomes heated, causing chemical reactions between the surrounding ridge rock and seawater. These chemical changes produce metamorphosed basalt.

Hydrothermal metamorphism can also take place on land, when fluids from igneous rock intrusions percolate through surrounding country rock, causing a regional metamorphism.

Migmatites

Higher temperature and pressure metamorphic boundaries mark the lower limits of magma production. With a good amount of water, magma formation starts at a lower temperature. When there is little water, magma doesn’t form until higher temperatures are reached. This allows different types of metamorphic rock (schists, gneisses, and amphibolites) to form in different areas depending on the amount of fluid present.

Different types of layering are also possible depending on fluid intrusion, as well as temperature and pressure factors. When there is a variety of metamorphic rock types in an area, geologists find that a combination (mixed) rock has formed. Alternating layers of granite and schist form a mixed rock called migmatite.

A combination metamorphic rock type that contains both igneous and metamorphic rock is known as migmatite.

Burial Metamorphism

When layers of sedimentary rock become heavier and heavier, they get pushed further down into the crust, where they heat up and take on the temperature of the surrounding rock. We learned that when this happens, digenesis causes the transformation of sedimentary rock minerals and their textures. It happens at temperatures below 2008C.

As a result of increasing temperature and pressure in sedimentary rock layers, by ever heavier upper layers, diagenesis slowly continues and changes sedimentary rock layers over time through the process of low-grade burialmetamorphism.

This type of metamorphism often causes partial mineral changes in sedimentary rock with some bedding layers left unaffected. Burial metamorphism usually causes wide folding of sedimentary rock layers within the greater changes of regional metamorphism.

Cataclastic Metamorphism

Cataclastic metamorphism takes place in the same areas as igneous activity along plate margins, oceanic, and continental hot spots, and deformed mountain ranges.

Tectonic plate movement causes high-pressure metamorphism by crushing and shearing rock away as a result of plate movement. When metamorphism happens along a fault, the transforming heat comes from intense friction and pressure going on between massive plates as they grind past each other.

Broken and metamorphic rock fragments found along a metamorphic rock fault are called fault breccia. This rock type has minerals that crystallize at either extreme temperature or the high pressure and low temperature associated with extreme frictional stress. This type of metamorphism is often part of regional metamorphism.

Regional Metamorphism

Regional metamorphism is the most widespread kind of metamorphism. This takes place over a much greater crustal area where both temperatures and pressures are high. Geologists use the term regional metamorphism when talking about large-scale metamorphism rather than that found locally near specific igneous rock intrusions or faults. Most regional metamorphism takes place in the deeper levels of the crust, along the margins of clashing and subducting tectonic plates, where rock is deformed and forced into a new direction. Regional metamorphism is fueled by the Earth’s internal heat.

Regional metamorphism happens when a chunk of strata originally at the surface becomes deeply buried and subjected to squeezing horizontal stresses. When this happens, the sedimentary rock cracks, buckles, and is folded gently or severely depending on the amount of ongoing pressure. As the folds are shoved further down, heating increases and crystals begin to form as the sedimentary rock is changed into metamorphic rock. The speed and length of sedimentary burial affects the temperature and pressure it sees. For example, if the sediment is pushed down quickly in a subduction zone, it doesn’t have time to heat up because of the high-pressure environment. However, if the downward movement is slow, the temperatures usually keep pace with the surrounding rock and mineral formation is slower, more complete, and gradual.

Regional metamorphism affects large structures across a broad stroke of the landscape. It involves the uplifting and down warping of stressed and deformed landmasses in the middle of mountain building. When both pressure and temperature increases are involved in regional metamorphism, it is called dynamothermal metamorphism.

Since regional metamorphism covers a large geographical area, the minerals and textures throughout the area are found in zones. Some areas may be near magma intrusion sources and contain zones of metamorphic and igneous rock. Some fairly undisturbed areas will look very different than those found nearer active tectonic areas. The main thing to remember is that in a broad region of metamorphism, the areas of changed rock can be found in horizontal and vertical positions.

Regional metamorphism produces rocks such as gneiss and schist.Regional metamorphism is caused by large geologic processes such as mountain building. These rocks, when exposed to the surface, show the unbelievable pressure that causes rocks to be bent and broken during the mountain uplifting process.

Schist rocks are metamorphic in origin. In other words, they started out as something else and were changed by external factors. Schists can be formed from basalt, an igneous rock; shale, a sedimentary rock; or slate, a metamorphic rock. Through tremendous heat and pressure, these rocks were transformed into this new kind of rock.

Schist is a medium-grade metamorphic rock. Medium-grade rock has been subjected to more heat and pressure than another rock such as slate. Slate, a low-grade metamorphic rock, needs lower temperatures for metamorphic changes to take place.

Schist is a coarse-grained rock with easily seen individual mineral grains. Since it has been compressed tighter than slate, schist is often found folded and crumpled. A lot of its original minerals have been transformed into larger flakes. Schists are usually named with reference to their original minerals. Biotite mica schist, hornblende schist, garnet mica schist, and talc schist are all different types of schist that come from different original minerals.

Gneiss rocks are also metamorphic in origin. Some gneiss rocks started out as granite, an igneous rock, but are changed by heat and pressure. Many gneiss rock samples have flattened mineral grains that have been smoothed flat by extreme heat and pressure and are aligned in alternating horizontal patterns.

Gneiss is a high-grade metamorphic rock. It has been the focus of much more heat and pressure than schist. Gneiss, a coarser rock form than schist, has distinct and easily seen banding. This banding is made up of alternating layers of different minerals. Gneiss can be formed from sedimentary rock such as sandstone or shale, or it can be created from the metamorphism of the igneous rock, granite. Since gneiss can come from granite, the same minerals found in gneiss are also found in granite. Along with mica and quartz, feldspar is the most important mineral found in gneiss. Gneiss is often used as a paving and building stone due to its attractive banding.

Dynamic Metamorphism

Dynamic metamorphism also results from mountain building. Huge extremes of heat and pressure cause rocks to be bent, crinkled, smashed, compacted, and sheared. Metamorphic rocks are generally harder than sedimentary rocks because of their tough formation environment and are hard or harder than igneous rocks. They form the bases of many mountain chains and are exposed as outcrops only after short-lived outer rock layers have beenworn away. Metamorphic rocks discovered in mountainous regions today provide geologists with clues as to the location of ancient mountains on modern-day plains.

Geologists use these clues to figure out the temperatures that change different rock types into metamorphic rock. The crystal arrangement of different rock samples gives them a good idea as to the temperatures that the specific sample has been exposed to during its lifetime.

Retrograde Metamorphic Rock

Sometimes a rock type is changed into a high-grade rock at one point, then later exposed to low temperatures and changed to another type of rock. When this happens, it is known as retrograde metamorphism. Retro means to go backwards in development.

An easy way to think of it is to picture butter. When butter is heated, it melts and turns into a liquid. When the temperature cools, the butter, which has separated into slightly different forms, goes back into a solid state. Later, if the butter is left out and melts at room temperature, it will eventually sour and return to its basic components.

Sometimes, geologists find rock that has been through more than one change. This is usually seen during microscopic crystal examination or through chemical analyses.

Pressure

There are three main factors that cause pressure increases and the formation of metamorphic rocks. These are:

* The huge weight of overlying sedimentary layers,

* Stresses caused by plates clashing during mountain building, and

* Stresses caused by plates sliding past each other, like the shearing forces along the San Andreas Fault (western United States).

Pressure or stress from tectonic processes or the weight of overlying rock causes changes in mineral texture. The two types of pressure that are applied to existing rock are confining pressure and directed pressure. Confining pressure is an all around pressure. Like atmospheric pressure at the surface of the Earth, confining pressure is present within the mantle’sdepths. Extreme confining pressure changes a mineral’s structure by squeezing its atoms tighter and tighter until new minerals with denser crystalline structures are formed.

Directed pressure happens in a specific direction. When extreme squeezing pressure is applied in one direction, it’s like toothpaste in a tube; it is forced in one direction. When clashing plates are compressed, the force is applied in one direction. Since heat decreases a rock’s strength, when pressure is applied in one direction, a lot of folding and deforming goes on when temperatures are high.

Depending on the type of stress applied to a rock, the minerals in metamorphic rock are squeezed, stretched, and rotated to line up in a specific direction. This is how directed pressure affects the size and shape of metamorphic rock minerals undergoing change by heat and stress. For example, during recrystallization of micas, crystals grow within the planes of their sheet-silicate structures and align perpendicular to the directed pressure. Geologists use this type of metamorphic mineral to figure out the pressures that specific samples have been exposed to during their history.

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(48) Earth Science

Intro to Metamorphic Rocks

ES46 

Metamorphic Rock

 Igneous rock is formed as a result of the Earth’s internal ‘‘engine,’’ while sedimentary rock formation depends on external climate and conditions. Metamorphic rock, however, takes place after these rock types have already formed. It is created by transforming igneous or sedimentary rock into something new.

Of the three major rock types, igneous, metamorphic, and sedimentary, metamorphic rock is the chameleon rock. It transforms into different types of rocks depending on the factors that it is exposed to within the Earth. This rock type is both a wonder and a headache to geologists. Since metamorphic rock begins originally as something else, it can be confusing as to whether it is the original rock or a transformed version. To solve this problem, geologists gather clues from the surrounding area or an outcrop from which the sample rock is found.

Besides being intruded upon by magma regularly, the Earth’s crust is subjected to stresses within the crust and mantle that cause it to break and bend forming fault folds. These forces often center along thin, winding belts when folding. They also combine with magma intrusion and extrusion while pushing up mountain ranges. The rocks within a mountain range are not onlyunder extreme pressure, but heated by magma intrusion as well. These stresses deform and recrystallize rock to different degrees. Pressure and temperature can also change previously metamorphosed rocks into new types.

Rock-forming and destroying processes have been active since the Earth was first formed. When sedimentary and igneous rocks are exposed to extreme pressure or medium heat, they are changed. They become metamorphic rocks, which form while deeply buried within the Earth’s crust. It is important to remember that metamorphism does not just melt existing igneous or sedimentary rock, but transforms it into a denser, compacted rock.

Metamorphic rocks are formed from rocks that were originally another type and were changed into a different form.

 

The name metamorphic comes from the Greek words, meta and morph, which mean ‘‘to change form.’’ Geologists have found that nearly any rock can become a metamorphic rock. When existing rock is shoved and pressurized, its minerals become unstable and out of equilibrium with the new conditions, causing them to change.

Remember the chameleon? When a chameleon moves from a gray rock to a bright green leaf, he changes his skin color to the same as his environment. By adjusting to his new conditions, the chameleon protects himself and comes into equilibrium with his surroundings. The process of metamorphism is similar. When a rock is slowly moved through tectonic processes to a new temperature or pressure environment, its original chemical and physical conditions are changed. In order to regain stability in the new conditions, chemical and physical changes take place. With metamorphism, mineral changes always move toward reestablishing equilibrium. Common metamorphic rocks include slate, schist, gneiss, and marble with many grades in between.

Most of the time, metamorphic rock is buried many kilometers below the crust which allows increasing temperatures and pressures to affect it.

However, metamorphism can also happen at the surface. When geologists study soils under hot lava flows, they find metamorphic changes. The three main forces responsible for the transformation of different rock types to metamorphic rock are:

* Internal heat from the Earth,

* Weight of overlying rock, and

* Horizontal pressures from rock that changed earlier.

Temperature

Temperature increases in sedimentary layers that are found deeper and deeper within the Earth. The deeper the layers are buried, the more the temperature rises. The great weight of these layers also causes an increase in pressure, which raises the temperature even more.

This cycle of heat and pressure that describes the transformation of existing rock is called the rock cycle. It is a constantly changing feedback system of rock formation and melting that links sedimentary, igneous, and metamorphic rock.

The pushing down of rock layers at subduction zones causes metamorphism in two ways: the shearing effect of tectonic plates sliding past each other causes the rocks to be deformed that are in contact with the descending rocks. Some of the descending rocks melt from this friction. These melted rocks are considered igneous rock not metamorphic. Then secondly, nearby solid rock that lies alongside melted igneous rock can be changed by high heat to also form metamorphic rock.

The temperature of the Earth increases the deeper you go. On average, the temperature increases 308C/km, but can vary from 20 to 608C/km in depth. For example, the temperature at a depth of 15km is equal to 4508C. At the same depth, the pressure of the overlying rock is equal to 4000 times the pressure at the surface.

This heat and pressure gradient, changing with depth, allows metamorphism to happen in a graded way. The deeper you go, the hotter the temperature and pressure, the greater the metamorphic changes. Depending on the conditions under which rock is changed, the rock gradient forms new metamorphic rock into high-grade or low-grade metamorphic rock.

As rock adjusts to new temperature or pressure conditions, the crystal structure of its minerals are affected. Ions and atoms are energized. They begin breaking their chemical bonds and creating new mineral linkages and forms. Sometimes, crystals grow larger than they were in the original rock.

New minerals are created either by rearrangement of ion bonding or by reactions with fluids that enter the rocks.

There are five main ways that metamorphic rocks are created. These different metamorphic rock processes include contact, regional, dynamic metamorphism, cataclastic, hydrothermal, and burial metamorphism. A closer look at each one of these will show how they are different.

Contact Metamorphism

Contact metamorphism takes place when igneous intrusion of magma heats up surrounding rock by its extreme temperatures. This surrounding rock is called country rock. When igneous intrusion happens, the country rock’s temperature heats up, and becomes filled with fluid brought along by the traveling magma. The area affected by hot magma contact is usually between 1 and 10km in size.

When contact metamorphism happens on the surface because of an outpouring of lava, it is restricted to a fairly thin rock layer. Since lava cools quickly and gives heat little time to penetrate the underlying country rock, the metamorphism that takes place is limited.

An aureole or rock halo is formed by metamorphosed rock around a hightemperature source. The metamorphic rock close to the magma pocket contains high-temperature minerals, while rock found further away has lower-temperature minerals. These heat sources are commonly closer to the surface crust in contact metamorphism than other types.

When a plutonic magma pocket is rimmed by a contact ring of metamorphic rock, it is known as an aureole.

A special type of contact metamorphism, impact metamorphism, is caused by the high-speed impact of a meteorite. As the meteorite hits the Earth’s surface, it causes shock waves. These are sent out from the impact site as a way to scatter the energy from impact. Depending on the speed and angle of impact, the surface at impact is immediately compacted, fractured, melted, and may be vaporized. Following the initial slam and shock wave, the rock decompresses sending rock flying in all directions and forming an impact crater.

Have you ever seen high-speed photography of a droplet of water hitting the surface of a still pool? The impact compresses the water’s surface downward for an instant, followed immediately by a rebounding ring of droplets shooting upward. The shock-wave impact is absorbed throughout the liquid as ripples.

Unlike deep mantle metamorphism, shock metamorphism happens in the instant of a high-velocity impact.

A meteorite impact has much greater velocity and energy than a freefalling droplet, but impacts in much the same way. For example, an iron meteorite measuring 10m across and hitting the surface at a velocity of 10 km/sec would create a crater over 300km in diameter.

The shock wave from a meteorite impact causes high-pressure shock metamorphism effects such as specific fracture patterns and crystal structure destruction. In fact, the formation of polymorphs, or in-between shock-related minerals like coesite or stishovite, not commonly found on the surface, helps geologists to find ancient impact craters.

Contact metamorphism produces nonfoliated rocks (without any lines of cleavage) such as marble, quartzite, and hornfels.

Nonfoliated rock is made up of crystals in the shape of cubes and spheres that grow equally in all directions

Marble is formed from metamorphosed limestone or dolomite that has recrystallized into a different texture after contact with high heat. It is made up of calcite, but if it contains a large amount of dolomite, then it is called dolomitic marble. Both limestone and dolomite have large amounts of calcium carbonate (CaCO3) and many different crystal sizes. The different minerals present during the formation of marble give it many different colors. Some of marble’s colors include white, red, pink, green, gray, black, speckled, and banded.

Since marble is much harder than its parent rock it can be polished. Marble is used as a building material, for kitchen and bathroom countertops, bathtubs, and as carving material for sculptors. Grave stones are made from marble and granite because they weather very slowly and carve well with sharp edges.

Quartzite is the product of metamorphosed sandstone containing mostly quartz. Since quartzite is formed from sandstone that contacted hot, deeply buried magma, it is much harder than its parent rock. As it is transformed, the quartz grains recrystallize into a denser, tightly packed texture. Unlike matte-finished sandstone, quartzite has more of a shiny, glittery look. While sandstone shatters into many individual grains of sand, quartzite fractures across the grains.

Hornfels is a fine-grained, nonfoliated, large crystal metamorphic rock formed at intermediate temperatures by contact metamorphism. These can be further defined as pyroxene-hornfels and hornblende-hornfels formed at still lower temperatures.

The high heat coming from the deep magma chamber changes these sedimentary rocks into the metamorphic rocks, such as marble, quartzite, and hornfels. These changed rocks are listed to the right of the figure in relation to their original rock types.

 

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(47) Earth Science

Sedimentary Environments

ES45

BIOTUBATION

Sometimes you will see sedimentary rock with tubes crossing vertically or at an angle through several layers. This is known as biotubation. These fossilized sedimentary structures are the remains of burrows and tunnels made by worms, clams, and other marine organisms. These primitive ocean bottom-dwelling residents burrow through sedimentary layers in search of organic material. Geologists study their vacant homes and waste for clues to the ancient environment during the time they lived.

Sedimentary Environments

Sedimentary environments are places where sediments collect and sedimentary rocks form. They can be grouped into three main areas: terrestrial (land), marine, and transitional (border) environments.

1. Terrestrial sedimentary environments (land)

(a) Rivers, streams, and ponds

(b) Lakes

(c) Swamps

(d) Deserts

(e) Glacial environment

2. Transitional environments (border areas between the land and marine environments)

(a) Beach and barrier islands

(b) Delta

(c) Lagoons

(d) Estuaries

3. Marine environments

(a) Continental shelf

(b) Continental slope and rise (deep sea fans)

(c) Abyssal plain

(d) Reefs

TERRESTRIAL SEDIMENTARY ENVIRONMENTS

The first of the sedimentary environments is the best known since most people have visited streams, rivers, or lakes at one time or another. A good amount of clastic fragments are deposited into sedimentary layers within terrestrial sedimentary environments. This happens through the action of water current or blowing wind. Depending on the way the sediments are laid down, different layering patterns are seen.

TRANSITIONAL ENVIRONMENTS

An in-between or transitional sedimentary environment is found where major sources of water currents meet the ocean. In delta areas, there is a rich mixture of sediments that arrive from all along the route of the current. The mouth of the Mississippi River delta near New Orleans, Louisiana (United States) deposits many tons of silt into the Gulf of Mexico in a wide fan of sand and mud that can be seen from space. Beach environments have a lot of wave and tidal energy that moves particles constantly. This back-and-forth grinding movement polishes and sorts them according to size. Fine sediments are washed away to settle further out in the tidal flats, where the wave action is less.

MARINE ENVIRONMENTS

Marine environments include fine sediments that settle to the ocean bottom as the remains of marine organisms and plants. The finest marine sediments are found far from the continental margins. Pelagic (from the Greek word pelagos, meaning sea) sediments are so tiny as to be found suspended in salt water most of the time. Think of the super fine dust that is always settling out of the air onto furniture at home. You can’t even see it unless a ray of sunlight makes it visible. Pelagic sediments are made up of the calcium-containing shells of microorganisms such as foraminiferans, radiolarians, and diatoms. These microorganisms live near the ocean’s surface and when they die, their shells sink, decay, and become part of the fine-grained mucky ooze on the ocean’s bottom. Pelagic sediments, dispersed all through the oceans, settle out and form layers of fine sediment onto deep ocean plains.

Unique marine sediment is created by chemical precipitation in seawater. Precipitates of manganese oxides and hydroxides form golf ball to basketball size lumpy nodules strewn around the ocean floor.

When we talk about ocean environments, we’ll go into much greater detail on the currents, inhabitants, and characteristics of the planet’s ceans. We will see what makes them different and the same in several areas around the globe.

Weathering

Water, wind, and ice all work together to breakdown solid rocks into small rocky particles and fragments. These bits of rock are swept away by rain into streams. Gradually these particles get deposited at the bottom of stream beds or in the ocean. As more and more sediment builds up, it gets crushed together and compacted into solid rock.

Weathering wears away existing rocks and produces lots of small rock bits.

With every tick of the clock, day after day, rock surfaces are worn away by wind and rain. Small bits of dirt, sand, mud, and clay are slowly ground away and washed into streams, rivers, lakes, and oceans. After these tiny bits of sand and rock settle at the bottom, they become sediment.

Water minerals and microscopic or tiny organisms also get mixed with the dirt and sand to form sediment. Over time, more and more sediment piles up on top of what was there before. After millions of years the sediment builds up into deep layers. The heavy weight and extreme pressure from the constantly added sediment turns ocean sediment at the bottom into sedimentary rock. The oldest ocean sedimentary rocks are thought to be around 600 million years old.

These oldest sedimentary rocks were formed long ago, but since then, they have been crushed, heated, and transformed into what is known as metamorphic rock. 

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(46) Earth Science

Sedimentary Rocks

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Carbonate Sedimentary Rocks

Carbonate rocks all have carbon-related compounds in their composition. The two most important minerals found in carbonate rocks are:

* Calcite (CaCO3)

* Dolomite (CaMg(CO3)2)

Carbonate sedimentary rocks are formed through chemical and biochemical processes. They include the limestones, which contain over 80% of the carbonates of calcium and magnesium, and dolostones. Limestone is made up of calcium carbonate (CaCO3) from carbonate sands and mud, while dolostone is made up of calcium–magnesium carbonate (CaMg (CO3)2).

Dolomite formation is little different from some of the other evaporite and chemical sediments. Dolomite is formed by the reaction between sedimentary calcite or aragonite with magnesium ions in any seawater that trickles down through any sedimentary spaces. As the ions are exchanged, some of the calcium ions are switched with magnesium ions and calcium carbonate is then changed into dolomite.

Carbonate rocks are separated by their texture and contents. They include everything from fine mud to a mix-mash of fossils and debris.

Unlike igneous rock, carbonate sedimentary rocks have a fine-grained texture. There are a variety of different forms found. Some of these include the following:

* Micrite (microcrystalline limestone) – very fine-grained; may be light gray or tan to nearly black in color; made of lime mud (calcilutite),

* Oolitic limestone (look for the sand-sized oolites),

* Fossiliferous limestone (fossils in a limestone matrix),

* Coquina (fossil hash cemented together; may resemble granola),

* Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid),

* Crystalline limestone,

* Travertine (evaporates of calcium carbonate, CaCO3) stalactites and stalagmites, and

  • Other – intraclastic limestone, pelleted limestone

Siliceous Rocks

This type of sedimentary rock is commonly formed from silica-secreting organisms such as diatoms, radiolarians, or some types of sponges. It is most commonly called diatomaceous earth. Many expert gardeners use high silica containing diatomaceous earth to aerate and balance the acidity in soil.

Siliceous (silica-containing) rocks are sedimentary rocks with high silica (SiO2) content.

Biologic sedimentary rocks form when large numbers of living organisms die, pile up, and are compressed and cemented to form rock. Accumulated and pressurized carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert. Diatomite looks like chalk and fizzes easily in acid. It is made up of microscopic plankton (tiny plants) called diatoms. When the silica from diatom remains is dried and powdered, it is used as one of the main ingredients in dynamite.

Chert (also known as flint) is very different in appearance from diatomite. It is made of hard, extremely fine, microcrystalline quartz and can be dark or light in color. Chert is formed when silica in solution goes through chemical changes within limestone. It often replaces limestone and does not fizz in acid.

Flint was used by early hunters for spear and arrowheads. It was easily formed into points and sharp, cutting edges. Opal is a white or multicolored, less-developed crystalline form of chert used in jewelry. Opal has high water content.

COALS (ORGANIC SEDIMENTARY ROCKS)

Organic sedimentary rock is made up of rocks that were originally organic material (like plants). Because of this, they don’t contain inorganic elements and minerals. These organic sedimentary rocks are known as coals. Coals are usually described in the order of their depth, temperature, and pressure. They are made almost completely of organic carbon from the diagenesis of swamp vegetation. Coals contain the following types of materials:

* Peat (spongy mass of brown plant bits a lot like peat moss),

* Lignite (easily broken and black),

* Bituminous coal (dull to shiny and black; sooty; sometimes with layers), and

* Anthracite coal (very shiny and black, a bit of a golden gleam; low density; not sooty; could be a metamorphic rock from exposure to high temperatures and pressures).

Coal is formed from peat, which is a collection of rotting plants found in and around swamps. The conversion from peat to coal is called coalification.

In the various stages of coalification, peat is changed to lignite, lignite is changed to subbituminous coal, subbituminous coal is changed to bituminous coal, and bituminous coal is changed to anthracite coal.

In the United States, coal is found in areas of eastern and western Kentucky, where it is layered between shales, sandstones, conglomerates, and thin limestones. The time span from approximately 320 million years ago and until about 30 million years is commonly called the Coal Age.

Sedimentary Stratification

We saw how sedimentary layers can gather in one location, as a result of natural processes such as waves, currents, drying, wind, and other factors, when we looked at different stratas.

Geologists often use the words sedimentary bed and sedimentary layer to mean much the same thing. I have followed this pattern and will use both words to define sedimentary rock layers. The sedimentary rock strata are laid down in certain well-known structures such as:

* Lamination bedding,

* Uniform layers,

* Cross-bedding,

* Graded beds,

* Turbidity layers, and

* Mud cracks.

We will look at the differences between these types and how they give a different look to a variety of sedimentary rock layers.

BEDDING

Nearly all sedimentary rocks are laid down in layers or beds. Layers can be very thin, like a few millimeters or as thick as 10–20m or more. This sedimentary layering or bedding gives it the characteristic striped look seen in the Grand Canyon and deserts of the United States. The exposed mesas and arches are made of layered sedimentary rock.

A bedding plane is a specific surface where sediments have been deposited. Bedding most often happens in a flat plane as wind or water has layered it over and over onto the same area. When a bedding plane has a different color from surrounding rock, it makes it easier to spot one layer from another. Although we think of bedding as horizontal, this is not always the case.

Bedding is the formation of parallel sediment layers by the settling of particles in water or on land.

LAMINATION BEDDING

When an area has fine, thin (less than 1 cm in thickness) bedding layers, it is known as lamination or lamination bedding. Over millions of years, a single bed made up of very thin individual layers can be several meters thick. Different sedimentary lamination layers can be set apart by grain size and composition. These differences are caused by the different environments in place over long stretches of geologic time.

UNIFORM LAYER

A sedimentary rock layer, made up of particles, all about the same size, is known as a uniform layer. A uniform layer of clastic rock has particles of a single size that have been tumbled by a current of a constant speed. If a uniform bed is made up of layers of single particle sizes, it is thought that currents of different speeds caused the uniform layering of like particles at different times. When nonclastic minerals precipitate out of a solution, the crystals that form uniform layers are all the same size.

CROSS, GRADED, AND TURBIDITY BEDDING

Cross bedding happens when wind or water causes sedimentary layers to be laid down at inclined angles to each other. These can be up to 358 from the horizontal and are found when sediments are laid down on the downhill slopes of sand dunes on land or sandbars in rivers or shallow seas. Crossbedding of wind-deposited sediments can be beautifully complex with many changes in direction. Figure 7-6 gives you an idea of cross-bedding found in sandstone. Graded bedding comes about through the sorting of particles by a current. A graded or gradient bed is layered with heavy, coarse particlesat the bottom, medium particles in the middle layers, and fine particles on top.

A graded layer is made up of particles that are layered from coarse to fine with the heaviest particles on the bottom.

It’s like a jar full of beach sand, small shells, sea glass, and seawater. If you shake it up, everything swirls around for a few minutes before settling. When settling according to weight, the heavier glass pieces settle first, followed by the shells, before everything is coated finally by sand. Over geologic time, these graded beds are piled on top of each other, many meters thick, by deep ocean currents along the sea floor.

Turbidity bedding is found as ripples in the sedimentary rock record. Just as parallel lines of beach sand near the water line are caused by the constant pounding of the surf, sedimentary rock layers are hardened in these same patterns.

When bedding of sediments happens in water, it is almost always horizontal. But currents can affect the look of sedimentary rock as well, with constant wave action giving sedimentary rock layers a symmetrical look. Water currents making swirls and eddies cause permanent overlaying of sedimentary rock. Waves at the beach, constantly depositing sediment with a back-and-forth movement, produce bedding with evenly shaped (symmetrical) peaks. Sediments deposited by a current in only one direction cause sedimentary peaks to be tilted away (asymmetrical) from the direction of the current..

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(45) Earth Science

Formation of an Evaporite

ES44

SAND AND SANDSTONE

Sandstone is made up of mineral grains (mostly quartz) cemented together by silica, iron oxide, or calcium carbonate. Sandstones are commonly white, gray, brown, or red. Iron oxide impurities give the red and brown color to the darker colored sandstones. Most sandstones are gritty, while some are easily crushed ( friable) and break apart to form sand. The pores or spaces between the separate grains of sand in sandstones controls how porous the sandstone is. The amount and size of this spacing is called porosity. The porosity of sandstone allows sandstone to serve as good reservoirs for oil and natural gas. Petroleum engineers and geologists oftenlook for these natural resources in sandstone areas.

Sandstone is made up of one or more of the following:

* Silt (grain size 1/256 to 1/16mm (gritty)),

* Siltstone,

* Clay (grain size less than 1/256mm (smooth)),

* Shale (most abundant of sedimentary rock types),

* Claystone,

* Mudstone (a mixture of silt and clay or mudshale if it fractures along sedimentary lines), and

* Ironstone (clay ironstone).

Sandstones are very resistant to erosion and form bluffs, cliffs, ridges, rapids, arches, and waterfalls. Loose sands have many colors, but are commonly seen as white to light brown. Silica (quartz-rich) sands and sandstones of high purity (white color) are used widely in the glass industry for making window glass, light bulbs, vases, and utility containers. Tightly cemented sandstone is often used as a building stone. Sand sediments are moved along by medium-speed currents like those of rivers, shoreline waves, and the wind. These can be rounded, which tells a geologist that they have traveled far (probably by water), or rough edged which usually points to shorter treks.

The amount of sand grains sorting in one area is another clue as to sediment origins. The average size of grains tells a lot about the strength of the current that carried them to a new spot and the type of parent crystal the grain was originally part of. If the grain sizes in sandstone are all the same, they are well sorted. If many grains are larger, with a lot of smaller and inbetween grains, then they are poorly sorted. Sorting takes place during the sand’s travels. Well-sorted sand grains commonly come from beaches, while poorly sorted grains are often the result of glacial travels.

ROCK ASPHALT

Rock asphalt is a medium- to coarse-grained sandstone with asphalt (bitumen) filling the pore spaces. It is squishy to solid, brown to black, has a pitchy to resinous luster, and is very sticky when completely saturated.

Bitumen is made up of various mixtures of liquid, viscous, flammable, or solid naturally occurring hydrocarbons, excluding coal, that are soluble in carbon disulfide

Rock asphalt deposits were formed when erosion of the surface rocks exposed oil-bearing rocks and allowed the more volatile hydrocarbons to escape. The asphalt-based crude gradually thickened until only a heavy tar or asphalt remained. The bitumen content of commercial rock asphalt varies from 3 to 15%. When asphalt is produced as part of some petroleum refining processes, it is called artificial asphalt.

Rock asphalt was once mined widely in Kentucky in the eastern United States. Large deposits were found in several areas. During the early 1980s, attempts were made to recover the petroleum in the rock asphalt by heat treatment, distillation, and other processes. Rock asphalt is used most commonly for surfacing streets and roads. It is also used for roofing, waterproofing, and mixing with rubber.

IRONSTONE

Iron oxide sediments are sedimentary rocks containing more than 15% iron in the form of iron oxides, iron silicates, and iron carbonates. Geologists think that most sedimentary iron was formed early in the Earth’s history when there was less oxygen in the air and iron was more soluble. When soluble iron was carried to the ocean, it formed iron oxides and other compounds that then settled in layers to the bottom.

Ironstone, heavy, compact fine-grained stone is found mostly in nodules and in uneven beds with carboniferous and other rocks. It has 20–30% iron content and a clay-like texture, with large amounts of iron oxide, mostly limonite, in nodular form. Much of the iron produced in the United Kingdom is made from ironstone.

EVAPORITES

This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous rocks. Evaporites form from chemical elements dissolved in seawater. These compounds can be removed from saltwater and crystallized into rock by chemical processes or through biological processes (such as shell growth). Sometimes it’s tough to sort between the two (carbonates and siliceous rocks), so evaporites are commonly grouped as chemical/biochemical.

Evaporites that form as elements become more and more concentrated in an evaporating solution (usually seawater).

Marine evaporites are the sediments and sedimentary crystalline rocks formed as seawater becomes more and more salty through evaporation. Some marine evaporites are hundreds of meters thick. Huge amounts of seawater would have to evaporate for this amount of crystal formation to have been possible.

The most common types of evaporites include the following:

* Carbonates – mostly calcite and dolomite by diagenesis,

* Gypsum – made up of calcium sulfate (CaSO4 and water),

* Halite (rock salt) – made up of sodium chloride (NaCl), and

* Travertine – made up of calcium carbonate (CaCO3) (forms in caves and around hot springs).

Geologists have found that three things must happen in a bay for large amounts of evaporites to form. They are: (1) freshwater that flows into the bay from rivers and streams is limited, (2) connections to the open sea are constricted, and (3) the climate is parched and dry. In these bays, seawater evaporates constantly, but is replenished at a steady rate remaining supersaturated all the time. Evaporite minerals then settle steadily to the floor of the bay in sedimentary layers.

Phosphorite is another marine evaporite formed from chemical and biochemical sediments. It is made up mostly of calcium phosphate from places along the continental margins where ocean water is cold and deep. The phosphorite forms from an interaction between phosphate-rich seawater and muddy or carbonate-containing sediments. Land (nonmarine) evaporites form in areas usually far from the sea. These are found in desert-region lakes with little or no river outlet. In these places, minerals come into the lake from chemical weathering and erosion, but without water current can’t move on. One of the best known examples of this is the Great Salt Lake in Utah in the western United States. Rivers bring ions into the lake, but there they stay when the water evaporates. The concentrated dissolved ions in the Great Salt Lake make it one of the saltiest places on Earth, with levels eight times saltier than seawater. 

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(44) Earth Science

 The Rock Cycle

 ES43

CEMENTATION

Cementation of sediments happens when compacted grains stick together.

Since most sediment is deposited in water, they have water molecules in the spaces between particles. The surrounding water contains different dissolved minerals that eventually fall out of solution and stick to the sediment grains. Minerals like calcite, silica, iron oxide, and magnesium cement the grains together into a solid mass that dries, is compressed further, and becomes rock.

Compaction and cementation can happen at the same time. The squashed sediments can be so tightly packed that they shut out the flow of mineralcontaining water.

Additionally, minerals within the sediments can be dissolved away when water flows through. This creates pockets and places for other minerals or oils to gather. Petroleum geologists look for oil in these types of pockets.

When sedimentary minerals dissolve and react with minerals in water to form other compounds, it is called dolomitization.

 Dolomitization happens when limestone turns into dolomite by a mineral substitution of magnesium carbonate for calcium carbonate.

 CRYSTALLIZATION AND CHEMICAL CHANGES

Chemical and biochemical sediments and sedimentary rocks can be classified by their chemical makeup and properties. The ions of the most common elements dissolved into seawater are shown in Fig. 7-3. Although silica (SiO2) and phosphorus play a big part in the makeup of sedimentary rock, they are only found in small amounts in seawater. When the water evaporates, the ions crystallize to form rock.

Carbonate sediments come from the biochemical precipitation of the decayed shells of microorganisms. Other chemical sediments that are high in calcium (Ca2+) and bicarbonate (HCO3-) are precipitated out of seawater as calcium carbonate (CaCO3) and carbonic acid (H2CO3) by inorganic processes and are much less common.

 Types of Sedimentary Rocks

Unlike igneous rock, most sedimentary rocks have a fine-grained texture.

Since a lot of the reason they have layered or settled in one place is due to water or wind, the particles of sediment are usually small and fine.

The way that sedimentary rock is deposited can also be related to size. Since wind can’t blow or carry away boulders (well, maybe tornadoes can), generally it is the lighter, finer grains of silt that are transported by the wind.

In contrast to that, water tumbles rocks of different sizes. With the water deposit of sedimentary rock, current plays a big part. The stronger the current, the larger the rock and the farther it is carried. The relationship between current and particle size is the reason why many beds have the same types of particles. They sort and group according to size when flowing in the same current stream. So you see sand together with sand, river pebbles with other river pebbles.

 CLASTIC

Clastic or detrital sedimentary rocks are formed from the weathering of existing rocks, which have been carried to a different spot from where they were originally and then turned into rock. They have a clastic (broken) texture made up of clasts (bigger pieces, like sand or gravel) and are grouped according to their grain size.

 Detritus is igneous, sedimentary, or metamorphic rock that has been moved away from its original location

Clastic sedimentary rocks are made up of pieces of other rocks. These pieces of rock are loosened by weathering, and then carried to some low area or crack where they are trapped as sediment. If the sediment gets buried deeply enough, it becomes compacted and cemented, forming sedimentary rock.

Clastic sedimentary rocks have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. Beginning with the smallest grains, there are clay, then silt, then sand. Grains that are larger than 2mm are called pebbles.

Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and a conglomerate is made of pebbles surrounded by a covering of sand or mud.

 * Coarse-grained clastics

Gravel (grain size greater than 2 mm; rounded clasts¼conglomerate; angular clasts¼breccia)

* Medium-grained clastics

Sand (grain size from 1/16 to 2mm)

Sandstone (mostly quartz grains¼quartz sandstone (also called quartz arenite); mostly feldspar grains¼arkose; mostly sand-sized rock fragment grains¼lithic sandstone (also called litharenite or greywacke)

* Cement (the glue that holds it all together) like calcite, iron oxide, silica

* Fine-grained clastics

Silt and siltstone (grain size from 1/16 to 1/256 mm)

Mud (clay), mudstone (claystone), and shale mud (grain size <1/256mm)

 Ironstone

The deposit of these sedimentary rock types by different currents is as you might guess. The larger gravel, rocks, and pebbles are only carried along by strong currents. These are rushing mountain streams, rocky beaches with high waves, and glaciers’ melt water. Strong glacial currents also carry sand. That is why you usually see sand between the gravel and pebbles. Pebbles and small stones are tumbled along and become smooth very quickly while bouncing along the land or in the water. Beach gravels and broken bits of glass, constantly rolled back and forth in the surf, also get smooth and rounded.

The coarse-grained clastic rock that doesn’t easily smooth or erode is not a conglomerate, but instead a breccia. These sharp-edged rock fragments are found close to their source where sedimentary rock has been layered on top of them before they travel very far. Although some breccias are sedimentary in origin, others come from igneous rock and volcanic beginnings. They were deposited onto a sedimentary rock layer after first being shot out during an eruption or broken away from igneous rock along a fault during an earthquake.

 

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