Sedimentary Rock Facts

The Earth, similar to every other thing, is under constant change. The tremendous pressure and heat present at the core of the Earth, changes the rocks present at that level. The new rocks that are formed out of this process are called the metamorphic rocks. Oldest ones are called igneous rocks, like quartz, that formed when the earth started to cool down approximately, six billion years ago. The process of formation of the rocks can also be seen in action on the surface of the Earth and the rock formed is called sedimentary rock. The following are the facts related to this rock type, which may help you in understanding, how the process works.

Sedimentary Rock Formation

The Earth's surface is exposed to the natural forces such as the sun, wind, rain, cold, ice and rivers. These agents work round the clock, to erode the earth's surface. The rocks exposed to these forces expands under the glaring heat of the sun and contracts under the onslaught of ice and cold. The cycles of contraction and expansion that continues on seasonal as well as daily basis, cause the small bits of rocks, such as igneous, to break away from the mother rock. The wind, that blows against the tops and the peaks of mountains erodes them dust particle by dust particles.

The rainwater pouring down on these mountains carries the small breakaway particles and bits of rocks, pebbles, gravel and sand to the valleys and then to the rivers, finally depositing them in the lakes or on the seabed. The debris starts accumulating in convenient places. It piles up and eventually, build pressure on the lowermost layer. The combined pressure of the water and debris, compresses and brings materials in that layer together to form the lowermost layer of the sedimentary rock. The process takes a considerable amount of time. It is believed that sedimentary rocks started forming approximately, 3.6 billion years ago.

These rocks cover a considerable part of the earth's surface. The strata of the earth's crust in which the sedimentary rocks are present is called bedding. Heavy materials, such as sand, sink to the bottom faster than the medium and lightweight debris. The minerals present in the accumulated debris and the water, in addition, to the detritus, also play an important role in binding the materials of the layer together. Depending on the materials that gets deposited, this process gives birth to different types of sedimentary rocks. During excavation one can observe different layers constituting the sedimentary rock. It can be seen in places such as Ordovician Lexington Limestone, near Lexington, Kentucky.

Dead animals and plants are also carried and deposited by the water at the pile ups. They become a part of the layer. The sedimentary rocks layers often contain fossilized dead animals and plants, that roamed or inhabited the earth during the formation of that particular layer. Events such as an earthquake or movements of the tectonic plates can cause the seabed to rise or fall. One such event that formed the Arbuckle Mountains, near Ardmore, Oklahoma, also disturbed the limestone rock beds in the area. The same rock beds now, stands vertical as a part of the mountain, exposed for anyone to examine. Such exposed sedimentary rock beds yield fossils of different life forms giving us valuable information dating back to millions of years.

Sedimentary Rock Types

These rocks are often known as the secondary rocks because of the way they are formed. These rocks and free sediment cover the earth's crust, that is mostly made up of igneous rocks. Sedimentary rock identification is based on their composition. Texture of these rocks is used to identify the origin of materials found in them. Based on these materials, there are three basic types of this rock.

Clastic
This type of the sedimentary rocks, such as Breccia and Conglomerate, are made up of the small bits and pieces that broke away from their mother rock. The break away material is referred to as clast which eventually pile up and in the process of lithification turns into a sedimentary rock. During lithification, the clast are compacted under the pressure exerted by the weight of the debris on the top. At the same time, the layer under pressure loses fluids in it and gradually, turns in a rock layer. Shale is a product of sedimentation process that occurs inland, away from the shore. Its main constituent is clay. Sandstone is mainly composed of sand, and the likely place of its formation is shore.

Chemical
Chemical sedimentary rocks form, when stagnant water evaporates, leaving the dissolved minerals in it behind, to impregnate the slug at the bottom. Sedimentary rock formations of this type are commonly observed in arid lands. In these places, seasonal playa-lakes occur in depressions in the land. Each time the water evaporates, a sedimentary rock layer composed of minerals is formed. Rock salt and Gypsum are examples of this type of sedimentary rock.

Organic
Organic sedimentary rocks formation occurs in places where the debris caused by the organic processes accumulates. Seashells, bones, and teeth of animals contain calcium which gets deposited on the lake or sea floor. Over a period, debris pile up to form this type of sedimentary rocks. This type of rocks are also referred to as biologic sedimentary rocks. Limestone largely contains minerals calcite, aragonite, ooids, peloids, intraclasts, and extraclasts which are crystal forms of calcium carbonate. The particles of debris, shell, coral, algal, fecal debris and precipitation of calcium carbonate from lake or ocean water come together to form limestone.

Study of Sedimentary Rocks

Sedimentology is a branch of science devoted to the study of the origin and the properties of the sedimentary rocks. The discipline comes partly under geology and physical geography. It also borrows from the earth sciences such as pedology, geomorphology and geochemistry. The information obtained is of importance to civil engineering and to paleogeography and paleoclimatology, and in knowing about the prehistoric lifeforms on the earth.

The forces that are involved in the formation sedimentary rocks, work patiently. Bit by valuable bit they accumulate the material for creating this informative rock. For a layman, it is only a heap of dirt, but for a learned one, it is a storehouse of information. Layer-by-layer it gives information about the climates of different times, organisms and the plants that flourished or perished because of the changes. Fossils preserved in the sedimentary rocks of different era let us guess, how organisms evolved with the time. Scientists use it as a journal to know what mother nature has recorded in it. Just imagine what we will find, if we could locate and excavate the first sedimentary rock bed that formed and hopefully, kept on growing.