Sedimentary rocks are the rock, which come into form due to the deposition of sediments and gravels. The layer of the deposited material is compacted due to the intense pressure exerted by the upper layer of sediments. Each of the above layer put pressure on the lower layer, thus compacting and strengthening the mutual bonding of the sediments.
How are the Sedimentary Rocks Formed?
Sedimentary rocks are also called secondary rocks as these are formed from eroded sediments of the igneous rocks. These rocks are also termed as stratified and aqueous rocks. According to Goth Cheng Leong,
“Sedimentary rocks are formed from the sediments accumulated over long periods of usually underwater. They are distinguished from other rocks by their characteristic layer formation and are termed stratified rocks. The strata may vary in thickness from a few centimeters to many meters. The rocks may be coarse or fine grained, soft or hard. The materials that form sedimentary rocks may be brought by streams, glaciers, winds and animals. They are non-crystalline and often contain fossils of animals, plants and other organism.”
The word sedimentary is derived from the Latin word “Sedimentum”, which means “Settling”. Thus sedimentary rocks are those, which result from the deposition and compaction of rock and mineral grains eroded from already existed igneous, sedimentary, or metamorphic rocks. According to H.J.de Blij,
“Many sedimentary rocks begin their existence as loose deposit of sand or gravel at the bottom of a sea or lake, on a beach or in a desert. Later the sediment is lithified-compressed into a rock. As successive layer of sediment accumulate the weight of the sediments expels most of water between grains. Pressure caused by the weight of overlying materials will compact and consolidate lower strata. The grains are squeezed tightly together, especially in fine grained sediments such as clays and silts. This is process of compaction.
Characteristics of Sedimentary Rocks:
The sedimentary rocks are different from other types of the rocks due to the following distinctive properties:
- The layered formation of the sedimentary rocks makes them different from the rest of the types of rocks. The layered deposition of the sediments is called “Strata”.
- The rock may be composed of fine, grained, soft, hard, or crystalline granules.
- The material/sediments of the sedimentary rocks are eroded by the rocks by means of streams, winds, or glacial movement.
- These rocks possess mud cracks or marks of ripples of water waves because they are formed underwater.
- The particle forming sedimentary rocks are the different sizes varying in thickness from a few centimeters to many meters.
- With the passage of time, these rocks may be folded, faulted, or deformed.
Classification /types of the Sedimentary Rocks:
According to their mode of origin and composition, the rocks are classified into three categories:
- Mechanically formed rocks
- Organically formed rocks
- Chemically formed rocks
(1). Mechanically Formed Rocks:
These rocks are formed when the materials derived from other rocks are cemented together and get a solid. The abundant and familiar types are the sandstone. They are composed of sand grain, similarly quartz piece form granites. The sandstone are many types such as grit, conglomerate and breecai. Out of these, the grit is a courser type but the conglomerate is composed of larger pebbles in rounded form. However, when the fragments are angular, the rocks is called breecai.
(2). Organically Formed Rocks:
These sedimentary rocks are formed from the hard remains of corals or shellfish. Major types of this category are the limestones. Similarly, the carbonaceous rocks are also formed organically but from vegetative forests and swamps. In this case, the pressure of sediments compresses the plant remains into compact carbon which later on turns into other forms such as peat, lignite or coal.
(3). Chemically Formed Rocks:
These rocks originate by chemical means from different solutions. For example, the saltish sea-beds or lake-bade give birth to rock salt. Similarly, gypsum (Calcium Sulfate) is formed as a result of evaporation of salt lakes such as the Dead Sea.
Classification Based on Place of Origin:
The sedimentary rocks are formed both on watery surfaces and on land. Thus according to their place of origin, they are classified into two types:
- Rocks of Marine Origin
- Rocks of Continental Origin
(1). Rocks of Marine Origin:
These sedimentary rocks were formed under sea or lake-waters which once covered the continental-shelf areas. Such rocks are of following three types:
These are made of sand grains or quartz fragments. These are of different composition and color and are categorized into grit ( Coarser types) and conglomerate ( Large pebbles). Some sandstones are very hard due hard to presence of silica whereas other are softer because these cemented by calcite or iron oxide. Sandstones provide key to unfold the secrets of the past.
- Clays and sand:
Finer sedimentary materials form clay which is used for brick-making. On the other hand, the un cemented form occurs in the shape of sand and gravel.
Limestone is formed from accumulation of marine shell-fragments on the ocean floor. It originates as a result of respiration and photosynthesis of marine organism. Thus calcium carbonate is formed and then settles on sea-floor.
(2). Rocks of Continental Origin:
These rocks originate on the continental surface. Major sedimentary rocks of continent origin include gravels, pebbles, conglomerates and breccias.
These comprised of sand and small pebbles which acquire round shape through the activity of river.
These are large piece of rocks thrown on beaches by waves and rounded by water.
Conglomerates take shape when gravel is cemented firmly. It is coarsest grained sedimentary rocks.
When pebbles sized fragments in a conglomerate are angular and jagged then these are called breccias.