Discovery Science: Earth – Sedimentary Rocks

Earth Science: Sedimentary Rocks

Sedimentary rocks are the most common type of rock on Earth. All sedimentary rocks are formed in a similar manner: by sedimentation, compaction, or cementation of many smaller particles of mineral, animal, or plant origin.

Sedimentary rocks are categorized into three groups: clastic, chemical, and bio- genic. Clastic sediments, such as clay and sand, originate from fragments of rocks that have eroded or disintegrated. Among other techniques, classification of clastic rocks is based on the size of their components, found using the grain size. Rock salt and gypsum are examples of chemical sediments, which form by evaporation of aqueous solutions.

Biogenic sedimentary rocks, such as coal, develop from animal or plant remnants. Petroleum originates from dead microorganisms that have been deposited, undecomposed and under the exclusion of oxygen, on the seafloor. There, they are modified through diagenesis. Lime- stone occurs through precipitation from solutions or from remnants of marine shellfish.

Rocks from sediments

Millions of years pass before sediments compact to form rocks. Following their deposition they compress under the increasing pressure of laminated layers above. Grains are densely packed together, becoming interlocked to form a massive unit.

However, only during the cementation phase are particles “baked” into rocks. Groundwater flowing through the sediments contains calcite leached out from calcareous rocks. The precipitation of dissolved calcite into gaps in the structure leads to grain cementation, forming sedimentary rock.

Sedimentation cycle

Weathering and erosion wear down rocks exposed to the atmosphere. Rock fragments are transported by wind, rivers, and ocean currents and deposited in layers. In time the sediment is covered over and compacts (diagenesis).

Later on, through large-scale uplifts and the formation of mountains, sedimentary rocks are pushed up to the Earth’s surface, and erosion starts the sedimentation cycle once again.


Most fossils can be found in sedimentary rocks such as clay, lime, or sandstone, where organisms became entrapped during sedimentation. Some sedimentary rocks are made
up almost exclusively of fossils.

The Solnhofen Limestone site in Bavaria, Germany is one of the most significant fossil deposits in the world. All known specimens of Archaeopteryx have been found there.


COAL formation began with primeval forest remains changing into layers of peat. Oceanic incursions then covered the peat with new deposits.

The ensuing rise in pressure and heat resulted in coal.