Bedrock

Bedrock is the solid rock that underlies the layers of looser, weathered materials that are deposited on the Earth’s surface, such as soil, subsoil, or alluvium. Because these unconsolidated layers can be very thick, bedrock may lie at the surface or hundreds of meters below it, depending on local geology. In some places, however, weathering and erosion have removed surface deposits and exposed the underlying bedrock, or "outcrops." Bedrock is harder and more compact than the surface layers. It need not be homogeneous and can consist of many different types of rock.

Overview

Some types of bedrock, especially sedimentary bedrock, form through the process of lithification, which comes from ancient Greek words meaning "rock making." In lithification, surface material called "sediment"—any rock that has been broken down into particles, such as by weathering and erosion, and then transported and deposited through the actions of water, wind, or glaciers—turns into solid rock. As layers of sediment pile up, the bottom layers are compacted by the weight of the material above them. The constant pressure gradually compresses the sediment, reducing its porosity.

First, the pressure squeezes out liquids that were trapped in the sediment when it was first formed. These liquids, called "connate fluids," are typically a water-based solution in which mineral components have dissolved. This part of the process can help geochemists understand the historical origins of different layers of bedrock: trapped bubbles of connate fluid act as chemical fingerprints, providing clues about the conditions under which the bedrock formed and the thermal and mineral history of the host rock.

As fluids are squeezed from the pores, the pore spaces decrease, and individual sedimentary grains fuse together into solid sedimentary rock. Ions carried in the connate fluids precipitate out and form solid crystalline bridges between grains of sediment, binding them together in a process called cementation. Compaction and cementation are the processes by which grains of sand are bound together to form sandstone, for instance.

Much cementation occurs near and below the water table. As a result, the type of cement found in the bedrock on the sea floor is distinct from that in the saturated zone (the region beneath the Earth’s surface where water completely fills pore spaces and fractures in soil and rock). There, cementation is a constant, ongoing process. Fractures in bedrock can also influence how groundwater and dissolved materials move through underground rock layers.

Outcrops can form through various processes. In most of the world, vegetation and soil keep deposits in place, not only covering bedrock but replenishing sediment. However, glacial activity can create outcrops, and rocky ocean shores remain so because ocean waves scour outcrops and prevent the buildup of deposits. Studies of bedrock beneath ice sheets also help scientists measure changes in glacial water storage and ice movement. Steep hillsides, riverbanks where fast-flowing currents cause rapid erosion, and mountain ridges at high elevations with little vegetation are all typical sites for outcrops.

Bedrock is important to engineering geologists because of its stability and its importance in infrastructure design, groundwater studies, mining, geothermal energy, and energy-resource exploration. In engineering terminology, the surface of the bedrock where it meets unlithified deposit layers is called the "rockhead," and the overlying deposit layers are called "drift."

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