(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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