SEDIMENTARY ROCK - GEO STREAM

SEDIMENTARY ROCK

Rocks that are formed when sediments are carried away by flow of water and sediments are joined together by water acting as ions are known as sedimentary rocks.

Definition: Sedimentary rocks are formed by COMPACTION and CEMENTATION of surface debris such as rocks fragments, minerals, shells, bones etc and PRECIPITATION of minerals.

THEY ARE CLASSIFIED THROUGH:

1. Texture
2. Compositions.

 TYPES OF SEDIMENTARY ROCKS:

1. Chemical
2. Clastic

1. Chemical

• They are formed by precipitation of minerals from cold saturated solutions near earth's surface.

• Interlocking crystals are visible/ microcrystalline.

• Individual grains are not visible.

• example: Calcite, Quartz.

• Halite and Gypsum precipitate on salt evaporates in ponds where climate is dry.

2. Clastic

• They are made from sediment that formed through physical weathering of rocks and transported as solid particles cementation and compaction of sediments.

• Individual grains are apparent.

• Mud - sized clasts are visible.

• example: sandstone, siltstone, and shale.

STAGES OF CREATING SEDIMENTARY ROCKS:

1. Weathering and Erosion.
2. Transportation
3. Deposition
4. Lithification.

1. Weathering and Erosion.

Weathering is the breaking down or dissolving of rocks and minerals on Earth's surface.
They are of 2 types:

• Physical weathering.

• Chemical weathering.

Physical weathering:

• It breaks down the rocks into smaller fragments of rocks.

• It is dominant in arid area with large changes in temperature.

Chemical weathering:

• It breaks down the internal structure of the minerals in the rock.

• The minerals are dismantled ion by ion.

• Water is important chemical weathering agent and transports the ions and molecules removed from the minerals

• It is dominant in humid area with high temperature as Alabama.

Erosion:

Erosion is the process by which the surface of the Earth gets worn down. Erosion can be caused by natural elements such as wind and glacial ice.

2. Transportation:

• Deposition is done by water or wind when water velocity decreases, sediments are deposited at the bottom.

• Chemicals in solution are precipitated into new minerals deposited at the bottom.

• Steno's law of original horizontality:

According to this law the sediment is deposited horizontally in parallel layers called strata/bedding.

• Principle of superposition:

According to this law the oldest sedimentary layer is on bottom and youngest layer is on top.

 

3. Deposition:

• Deposition is the geological process in which sediments, soil and rocks are added to a landform or landmass.

•  Deposition occurs when the eroding agent, whether it be gravity, ice, water, waves or wind, runs out of energy and can no longer carry its load of eroded material.

Environments of deposition:

i. Evaporite:

An area that has once filled with water and water evaporates leaving behind salt in mineral form ex Halite, Gypsum.

ii. Terrestrial

An area where deposition takes place in land
ex coal, breccia, sandstone, shale

iii. Shallow marine:

Large /Whole pieces of fossils are deposited in this area ex shells, corals

iv. Deep marine:

No whole pieces of fossils are found.
Deep marine limestone might not contain any fossils ex chalk (composed of microscopic marine shells).

4. Lithification:

• Conversion of soft sediments into solid rocks or converting newly deposited sediments into undulated rocks are called lithification.

• 1 stage of process is compaction.

• Compaction occurs as the weight of the overlying material increases. It forces the grains closer together, reducing pore space and eliminating some of the contained water.

• Some of this water may carry mineral components in solution and these constituents may later participate as new minerals in pore spaces this causes Cementation which binds the individual particles.

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