s5 (a) Rock Layers
The sediment that forms sedimentary rocks is deposited in flat layers
one on top of the other. Over time, the sediment hardens and changes
into sedimentary rock. These rock layers provide a record of Earth’s
geologic history. In sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.
There are other clues besides the position of rock layers to the relative ages of rocks. To determine relative age, geologists also study extrusions and intrusions of igneous rock.
Igneous rock forms when magma or lava hardens. Magma is molten material beneath Earth’s surface. Magma that flows onto the surface is called lava.
Lava that hardens on the surface is called an extrusion. An extrusion is always younger than the extrusion below it.
Beneath the surface, magma may push into bodies of rock. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.
Igneous rock forms when magma or lava hardens. Magma is molten material beneath Earth’s surface. Magma that flows onto the surface is called lava.
Lava that hardens on the surface is called an extrusion. An extrusion is always younger than the extrusion below it.
Beneath the surface, magma may push into bodies of rock. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.
The geologic record of sedimentary rock layers is not always complete. Deposition slowly builds layer upon layer of sedimentary rock. But some of these layers may erode away, exposing an older rock surface. Then deposition begins again, building new rock layers.
The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.
The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.