Sedimentary rocks hold important secrets about Earth's geological and evolutionary history and contain most of the fossil fuel deposits. All these factors make them an important subject of study.
What are Sedimentary Rocks?
They are types of rocks, created from deposition of layers upon layers of sediments over time. These types of rocks are formed on the Earth's surface, as well as underwater. Wherever sedimentation goes on, rocks are formed over time. The sediments that compose these rocks may be of organic, chemical, or mineral origin.
Although, these rocks constitute only 5% of the total crust volume, they extensively cover most continental surfaces. They form a thin cover over the whole crust, holding important geological history, locked within them. Most of the natural energy resources like coal and fossil fuels are contained within the layers of this rock type. Drinking water is extracted and minerals are also mined from these rocks, which are created by millions of years of sedimentation.
Depending on the nature of sediments that create them, these rocks can be classified into three prime types:
- Clastic: These are composed of broken down fragments of minerals and remnants of weathered rocks. One example of this type is sandstone. They are mostly made up of quartz, amphiboles, feldspar, clay minerals, and other minerals derived from weathering of igneous and metamorphic rocks.
- Biochemical: The deposition of the biological remains of various organisms can create certain types of sedimentary rocks. Some examples are limestone (from the deposition of calcium carbonate, containing remains of corals, foraminifera, and mollusks), stromatolites (formed from deposition of microorganism biofilms of blue green algae), oil shale, and coal shale.
- Chemical Precipitate: Sediments formed from deposition of chemical reaction precipitates of mineral solutions are called chemical sedimentary rocks. Their formation occurs when water dissolves many minerals and deposits them on evaporation. Examples of this type are gypsum, barite, rock salt, and sylvite.
The Formation Process Explained
For millions of years, the process of deposition and formation of these rocks has been operational in changing the geological structure of Earth and enriching it. Let us now see how they form.
The formation process begins with weathering of existent rock , that is exposed to the elements of nature. Wind and water are the chisels and hammers that carve and sculpt the face of the Earth through the process of weathering. The igneous and metamorphic rocks are subjected to constant weathering by wind and water.
These two elements of nature wear out rocks over a period of millions of years, creating sediments and soil from weathered rocks. Other than this, sedimentation material is generated from the remnants of dying organisms.
Transport of Sediments and Deposition
These sediments, generated through weathering are transported by the wind, rivers, glaciers, and seas (in suspended form) to other places in the course of flow. They are finally deposited, layer over layer, by these elements, in some other place.
Gravity, topographical structure, and fluid forces decide the resting place of these sediments. Many layers of mineral, as well as organic and chemical deposits accumulate together for years. Layers of different deposits called bedding features are created from them. Crystal formation may also occur in these conditions.
Lithification (Compaction and Cementation)
Over a period of time, as more and more layers are deposited, the process of lithification begins. Two sub-processes that are a part of lithification are compaction and cementation. Compaction is the compression of the sediments under overlying weight of sediments. Cementation is the process of filling in the gaps in sediments, with minerals that gel them together. Crystal formation may occur here too. With these two processes at work, what was a loose layer of deposits, hardens and solidifies to become rock. This is the conclusive stage.
Each layer of these rocks has an imprint or snapshot of Earth as it was at that moment of time. They are libraries of history.