How Rocks are Formed: Sedimentary Most of Earth’s crust is made of igneous rock, but most of its surface is covered by sedimentary rocks.

Three Main Kinds of Sedimentary Rock Clastic: fragments of other rocks Chemical: mineral grains that come from dissolved solutions by evaporation or chemical action Organic: remains of plants and animals

Clastic Rocks Clastic sedimentary rocks come from the weathering of rocks that already exist Winds, waves, and glaciers pick up and move the particles Sediments are deposited when a stream slows down Ocean water, lake water, and groundwater contain natural cements that include silica(SiO2), calcite (CaCO3), and iron oxide(FeO) These dissolved minerals settle into the spaces between the sand grains or pebbles, binding them together into rock The pressure of overlying sediments can also make fine sediments stick together

Clastic Rocks Sandstone Conglomerate Shale

Chemical Sedimentary Rocks Dissolved minerals fall out of solution Evaporation or the combining of dissolved ions to form new minerals. The most common are limestone, rock salt, and rock gypsum Limestones are formed from tiny grains of calcite Rock salt is the natural form of table salt, it is almost pure halite Rock gypsum occurs in layers and is almost pure gypsum

Chemical Sedimentary Rocks Rock Salt Limestone

Organic Sedimentary Rocks Organic sediments come from the remains of animals and plants The most common are limestone and coal Organic limestone is mostly calcite, shell producing animals die and their shells pile up and are cemented Coal

Stratification Stratification is the arrangement of rocks in visible layers When there is a change in the type of sediments being laid down, new rock layers are formed The layers are called beds and are separated by bedding planes Cross-bedding may develop when beds are deposited by wind on dunes or by rivers on deltas or sandbars

Fossils in Sedimentary Rocks Animals and plants that die as sediments pile up are buried The hard parts may remain as fossils when the sediments turn to rock Fossils are the remains, impressions, or any other evidence of plants and animals preserved in rock

Ripple Marks and Mud Cracks Many sandstones show ripple marks that are formed by the action of winds, streams, waves or currents on sand. Mud cracks develop when deposits of wet clay dry and contract Mud cracks are later filled with different materials, and the clay becomes shale rock.

Nodules,Concretions, Geodes Nodules are lumps of fine-grained silica called chert in limestones Concretions are round masses of calcium carbonate in shale Geodes are limestones with hollow spheres of silica rock, the hollows can be filled with crystals of quartz or calcite Nodule