Sedimentary Rocks

Sedimentary rocks are a common type of rock found on earth’s continental regions and ocean basins. The other two rock types are igneous and metamorphic rocks. Sedimentary rocks and sediment comprise around 75% of the earth’s land surface and over 90% of the ocean basins. Overall, sediment and sedimentary rocks make up 80-90% of the earth’s surface area. Igneous and metamorphic rocks are usually located below sedimentary rocks.

Sedimentary rocks are derived rocks because they form from pre-existing rock fragments. On the other side, igneous rocks are often considered primary rocks because they crystallize from a liquid.

Rock Formation

Sedimentary rocks are formed from the deposition or accumulation of mineral or organic particles. The rock-forming process is divided into 4 steps. These steps are mentioned and explained below.

Weathering: Rocks at or near the earth’s surface are exposed to weathering events. Weathering is of two types; mechanical and chemical. Mechanical weathering breaks rocks into smaller pieces but doesn’t alter the minerals.

Chemical weathering changes minerals into new minerals and some byproducts. The result of chemical weathering will depend on the nature of the chemicals to which the rock is exposed.

A typical weathering of silicate rocks yields solid materials like clays and dissolved minerals like quartz, silica, and metal cations.

Transport: The weathered rocks are transported to deposition sites by agents like wind, water, glaciers, and landslides. Water is the primary transporting agent. It can move the smallest particles to large boulders. Water carries particles downstream as well as in the groundwater. The wind carries small particles and dust, glaciers can move large boulders and small particles, and landslides can also move sediments of all sizes. Transportation decreases the particle size and also sorts the sediment into similar-sized particles.

Deposition: The deposition depends on how powerful the transportation medium is. Large boulders are often deposited near their source. Only severe floods or glacier melting can carry large particles for large distances and downstream. Smaller particles like silt, sand, and gravel travel more and are often deposited away from their source. Sediment is deposited in horizontal layers.

Cementation/Compaction: The deposition process continues, and the lowered layers are exposed to increased pressure and temperature. It is compacted, and pore spaces are decreased. The water in the pores is also expelled due to this pressure.

These rocks can also change to metamorphic rocks if they are exposed to regional or contact metamorphism.

Types of Sedimentary Rocks

Sedimentary rocks are usually divided into 3 categories based on how the rocks were formed. Some geologists divide sedimentary rocks into 4 categories in this division.

Clastic: Clastic sedimentary rocks are made of rock fragments that have been cemented together. These clasts are usually individual grains of minerals like quartz, clay, mica, feldspar, or any other mineral.

Clastic sedimentary rocks are further subdivided based on the dominant particle size. Common examples are conglomerate, breccia, sandstone, siltstone, and shale.

Biochemical (Organic): These rocks are formed from the accusation of animal or plant debris. The formation process is the same, but the formation sites may vary because of different rock-forming needs. Common examples include chalk, limestone, and coal.

Chemical: Chemical sedimentary rocks form dissolved materials in a solution that come out and precipitate to form a rock. This happens only under some specific conditions. Examples are chert, some limestones and dolomite types, flint, iron ore, and rock salt.

Other Sedimentary Rocks: This is the 4th type of classification that a few geologists use. It includes sedimentary rocks formed from the deposition and cementation of lava fragments and impact events. Examples of these rocks are tuff, volcanic breccias, and impact breccia.

Depositional/Formation Environments

These are the places where sedimentary rocks are deposited. Every environment is different, with a characteristic combination of geological processes and conditions. Hence, the rock formed in one environment is different from another.

The common environments where sedimentary rocks form are marine, continental, swamps, and aeolian.

Sedimentary Rocks Uses

Sedimentary rocks have various uses depending on rock composition. Most rocks have high economic value and reveal important information about the earth’s history.

Sedimentary rocks hold economically valuable oil, natural gas, uranium, and coal reserves. They provide sand and gravel for construction projects. Sandstones and limestone are used as building stones and dimension stones. Salt is another highly valued sedimentary rock with numerous uses.