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 PhysicalChemical 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

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.