Sedimentary Rocks

Formation of Sedimentary Rocks

• Sedimentary means settling

• Form when solids settle out of fluids like water or air

Weathering, Erosion, and Deposition

• First step in forming sedimentary rocks.

• Weathering breaks preexisting rocks into smaller pieces.

• Erosion removes the sediments or pieces of rocks.

• When the agent of erosion loses energy, the sediments settle out or are deposited.

Weathering

Physical Chemical•Also known as mechanical weathering•Rocks break apart into smaller pieces without changing their chemical composition

•Minerals in rocks change into new substances

Examples of Physical Weathering

• Abrasion—water or wind carrying rock fragments scours existing rock

• Ice wedging—water trapped in rocks or cracks freezes and expands, thaws and contracts

Examples of Physical Weathering

• Plant roots—as plant roots grow, they widen the cracks in rocks

Examples of Physical Weathering

• Pressure unloading—removal of overlying material allowing rocks to split along planes of weakness (jointing)

Examples of Physical Weathering

• Action of organisms—bacteria, mosses and lichens disintegrating rocks

Examples of Physical Weathering

• Oxidation—example: rusting• Acidification—example: water with

dissolved carbon dioxide forms a weak acid that dissolves limestone

• Spheroidal—example: water causes basalts and granites to weather more quickly on edges

Chemical Weathering

Chemical Weathering

• Erosion involves weathering and removal of rock

• Agents of erosion include water, wind, or ice

Erosion

• When an agent of erosion like water loses energy, sediments are deposited according to size.

• Large sediments like pebbles are deposited first.

Deposition

Compaction and Cementation

• Step Two• Sediments are turned into rock

(lithified) by the processes of compaction and cementation.

• Compaction is a process that squeezes sediments (removes water).

• Cementation is the process in which dissolved minerals are deposited between the sediments holding them together (microscopic glue).

Classification of Sedimentary Rocks

• Like igneous rocks based on how they form

Clastic Sedimentary Rocks • Rocks composed of weathered bits of

rocks and minerals• Most common minerals are clay minerals

and quartz• Grouped according to the size of the

sediments in the rocks—for example if the fragments are gravel size and rounded, the rock is called a conglomerate.

Clastic Sedimentary Rocks

Chemical & Biochemical Sedimentary Rocks

• Form when dissolved substances precipitate or separate from water solution.

• Generally occurs during evaporation• 90% of all limestones are formed from the

shells and skeletal remains of organisms that settle to the ocean floor.

• Examples: limestone, chert, flint, rock gypsum, rock salt

Clues to the Past

• Ripple marks• Mud cracks• Fossils• Each layer in a sedimentary rock

records a period of deposition, and in undisturbed layers the oldest layers are at the bottom.