Bedrock is the solid rock that underlies looser surface materials like soil and sediment. This bedrock forms the uppermost part of the Earth’s crust, which is the planet’s thin, rigid outer layer. To understand what lies deeper, we must journey beyond the familiar crust into the immense, layered structure below.
The Deep Structure of the Earth’s Crust
The crust extends for many kilometers and is primarily composed of silicate rocks. Its thickness and composition vary drastically depending on location. Continental crust, which underlies landmasses, is relatively thick, averaging 35 to 40 kilometers, and is made of lower-density, silica-rich rock like granite. Oceanic crust, found beneath the ocean basins, is much thinner, typically 5 to 10 kilometers thick. This oceanic layer is denser and composed mainly of basalt, an iron- and magnesium-rich volcanic rock.
Crossing the Boundary
Immediately below the crustal bedrock is the Mohorovičić Discontinuity, or the Moho. This is a geophysical boundary defined by a sharp increase in the velocity of seismic waves, indicating a sudden jump in rock density and rigidity. The Moho marks the transition from the less dense crustal silicate rock to the much denser rock of the underlying mantle. Under the oceans, this boundary is shallow, found at depths between 5 and 10 kilometers. Beneath the continents, the Moho is much deeper, often ranging from 30 to 70 kilometers below the surface.
Entering the Upper Mantle
Immediately below the Moho lies the upper mantle, a layer that extends to a depth of about 410 kilometers. This region is subdivided into two distinct layers based on their mechanical behavior. The uppermost part of the mantle is rigid and brittle, welded to the crust above to form a mechanically strong layer called the lithosphere. The lithosphere is the tectonic plate itself, with a total thickness that can reach around 100 kilometers.
The layer beneath the lithosphere is the asthenosphere, and its properties are fundamentally different. The asthenosphere is characterized by high temperatures that cause the rock to behave plastically, meaning it is solid but ductile and capable of slow flow over geological timescales. This weak layer acts as a lubricating zone, allowing the rigid lithospheric plates above it to move and shift, driving plate tectonics.
Geological Activity and Material Composition
The upper mantle is composed primarily of peridotite, a rock type denser and richer in iron and magnesium than the crustal rock above it. The main minerals in peridotite are olivine and pyroxenes, which give the rock a characteristic greenish hue when brought to the surface.
The slow flow within the asthenosphere is known as mantle convection, powered by heat escaping from the Earth’s deeper interior. Hotter, less dense material slowly rises, while cooler, denser material sinks in a cyclical process. This convection current drags the overlying lithospheric plates, causing them to converge, diverge, and slide past one another. This constant, slow movement of the mantle is the source of Earth’s surface geological activity, shaping the landscape far above the bedrock.