Sedimentary Rocks

Sedimentary rocks form from the breakdown of pre-existing rocks at Earth’s surface.

They are the largest group of rocks on the Earth’s surface.◦Over 75% of all rock exposed

at Earth’s surface is sedimentary rock.

To form sedimentary rock a pre-existing rock must be subjected to the following four processes:

◦Weathering◦Erosion (or transportation)

◦Deposition◦Lithification

Weathering

Weathering is the breaking down of rock.This can occur through physical (or

mechanical) processes in which the rock is broken into smaller pieces of the same rock, called sediment.

Or it can occur through a chemical process in which components of, or all of the rock is dissolved by water or reaction with air.

Erosion

This is the transportation of the sediment or dissolved materials, usually by water

Rounding

As the eroded material is transported it is subjected to hitting and scraping against each other.

This results in having sharp edges and corners ground away.

The particles edges take on a rounded shape.

◦This may take place in a distance of less than 2kms.

◦The rounder the material the further it has traveled from its source.

Deposition

The sediment or dissolved materials is eventually deposited in an environment where, given the proper condition the making of a rock occurs

Sorting

Sorting is the process by which sediment grains are selected and separated according to grain size (or grain shape or specific gravity).

This is usually the result of the moving water slowing down, losing energy and thus losing its capacity to carry sediment.

The larger, heavier particle settle out first as it takes more energy to transport them

Lithification

Lithification is the process that convert loose sediment into sedimentary rock.

It consist of two action◦Compaction, and◦Cementation

Types of Sedimentary Rocks

Sedimentary rocks are classified as either

Clastic (detrital) or◦derived from sediments

Chemical◦derived from precipitation

Clastic Classification

Conglomerate – sediments of various sizes, with the majority of the sediments being larger than 2mm in length.

Sandstone – contains fragments or particles less than 2 mm in diameter but greater than 1/16 of a millimetre in diameter.

Siltstone and Shale – are much finer-grained and generally have a smooth texture. Particles are not visible to the naked eye.

Chemical Classification

Inorganic – sedimentary rock form from the evaporation of water. Materials often called evaporites.◦Gypsum and Limestone are good examples.

Organic – are rocks formed by the action of plants (coal) or animals accumulated materials (shells)