On what bases can sedimentary rocks be identified?

Answer 1

Sedimentary rocks are those made of grains of preexisting rocks or organic material that, in most cases, have been eroded, deposited, compacted, and cemented together. They typically form at the surface of the Earth as sediment moves as a result of the action of wind, water, ice, gravity, or a combination of these. Sedimentary rocks also form as chemicals precipitate from seawater, or through accumulation of organic material such as plant debris or animal shells. Common sedimentary rocks include shale, sandstone, limestone, and conglomerate. Sedimentary rocks typically have a layered appearance because most sediments are deposited in horizontal layers and are buried beneath later deposits of sediments over long periods of time. Sediments deposited rapidly, however, tend to be poorly layered if layers are present at all.

Sedimentary rocks form in many different environments at the surface of the Earth. Eolian, or wind blown, sediments can accumulate in deserts. Rivers carry sediments and deposit them along their banks or into lakes or oceans. Glaciers form unusual deposits of a wide variety of sediments that they pick up as the glacier expands and moves; glacial deposits are well exposed in the northern United States. Sediments can travel in currents below sea level to the deepest parts of the ocean floor. Secretion of calcium carbonate shells by reef-building organisms produce large quantities of limestone. Evaporation of seawater has resulted in the formation of widespread layers of salt and gypsum. Swamps rich in plants can produce coal if organic material accumulates and is buried before aerobic bacteria can destroy the dead plants.

Sedimentary rocks are classified on the basis of the sizes of the particles in the rock and the composition of the rock. Clastic sedimentary rocks comprise fragments of preexisting rocks and minerals. Chemical precipitates are sedimentary rocks that form by precipitation of minerals from seawater, salt lakes, or mineral-rich springs. Organic sedimentary rocks formed from organic matter or organic activity, such as coal and limestone made by reef-building organisms like coral. Grain sizes in sedimentary rocks range from fine clay and silt to sand to boulders.

The sediment in a sedimentary rock reflects its environment of deposition. For example, wind-blown sand grains commonly display evidence of abrasion of their surfaces as a result of colliding with other grains. Sediments transported long distances tend to decrease in size and are more rounded than sediment deposited near their precursor rocks because of wearing against other sediments or rocks. Large or heavy sediments tend to settle out of water or wind if the energy of the water or wind is insufficient to carry the sediments. Sediments deposited rapidly as a result of slides or slumps tend to include a larger range of sediment sizes, from large boulders to pebbles to sand grains and flakes of clay. Such rocks are called conglomerate. Along beaches, the rhythmic activity of waves moving sediment back and forth produces sandstones in which the grains are well rounded and of similar size. Glaciers pick up and carry a wide variety of sediments and often scratch or scrape the rocks over which they travel.

Sedimentary rocks are the only rocks in which fossils can be preserved because at the elevated temperatures and pressures in which igneous and metamorphic rocks form, fossils and organic remnants are destroyed. The presence of fossils and the types of fossil organisms in a rock provide clues about the environment and age of sedimentary rocks. For example, fossils of human beings are not present in rocks older than approximately two million years because humans did not exist before then. Similarly, dinosaur fossils do not occur in rocks younger than about 65 million years because dinosaurs became extinct at that time. Fish fossils in sedimentary rock indicate that the sediments that make up the rock were deposited in a lake, river, or marine environment. By establishing the environment of the fossils in a rock, scientists learn more about the conditions under which the rock formed.

Spectacular exposures of sedimentary rocks include the Grand Canyon (Arizona), the eolian sandstones of Zion National Park (Utah), the limestones of Carlsbad National Park (New Mexico), and glacial features of Voyageurs National Park (Minnesota).

Source(s):http://science.jrank.org/pages/5919/Rock…