Common Rock Types Found in a River

Rivers contribute to shaping physical landscapes. Rocks are deposited in rivers via processes such as erosion and weathering. Rock material originates from various sources, including volcanic activity, the Earth's crust and even the fossils of living creatures. Three common types of rocks found in rivers are sedimentary, igneous and metamorphic rocks.

Rivers and Rocks

• Depending on energy generated from the flow rate between the source and mouth, rivers erode riverbanks, causing rocks to separate and enter into the water. Rocks also find themselves in rivers through physical, chemical or biological weathering, where rocks are broken into smaller pieces and fall into the river. Larger rocks are transported along the river bed via a process known as traction, while smaller rocks move in the water as a suspension.

Sedimentary Rocks

• In calm rivers, material deposits on the river bed as sediment. As sediment layers become compressed and thicken over time, water is forced out and layers solidify to form sedimentary rocks. Examples of sedimentary rock include sandstone, which is made from grains of sand; mudstone, which contains layers of mud; and limestone, which consists of skeletons and dead tissue of aquatic organisms.

Igneous Rocks

• Igneous rocks originate from molten rock or magma found beneath the Earth's surface. There are two key methods in which igneous rocks enter rivers. One is via a volcanic eruption, where magma cools to form small crystals such as basalt, which is deposited in rivers. Another method involves the cooling of magma within the Earth itself, rising to the surface as large crystals such as granite.

Metamorphic Rocks

• Rocks buried underground due to movements of the Earth's surface undergo high temperature and pressure exposure, which alters their crystal structure. These are called metamorphic rocks. The sedimentary rock limestone forms marble subsequent to this process, while schists are formed from the igneous rock basalt. Metamorphic rocks are usually located near mountain belts as these conditions are associated with mountain formation.