The lithosphere is the outer layer of Earth that includes the crust and upper mantle. This layer changes with the activity of geologic processes. The lithosphere is where tectonic plates move over, under, and past each other. Tectonic plates can be composed of both continental and oceanic crusts. Continental crust is composed of relatively lightweight minerals. Oceanic crust is composed of denser and heavier materials. When plates collide, the denser oceanic plate slides beneath the continental crust and is re-melted and recycled into Earth's mantle. This can result in magma rising and forming continental volcanoes.
The asthenosphere is the layer directly beneath the lithosphere and contains the upper layer of Earth's mantle. The constant churning and recycling of material in this layer is called a convection current. In convection currents, less dense, hot magma will move upward while more dense, cooler magma will move downward. Convection currents drive the movement of the tectonic plates. In places where convection currents rise toward the surface, cracks form in the crust. Hot magma pushes up and out through these cracks, creating new crust. These locations are known as mid-ocean ridges.
The diagram shows the processes of plate tectonics that have shaped Earth's surface.
Multiple Choice Question
Which statement BEST explains why most continental rocks are older than oceanic rocks?
A.
Oceanic crust is not affected by convection currents.
B.
Oceanic crust is constantly eroded by ocean water.
C.
Continental crust is not a part of the lithosphere.
D.
Continental crust is less dense and not recycled as often.
1 answer
The correct answer is:
D. Continental crust is less dense and not recycled as often.
This statement best explains why most continental rocks are older than oceanic rocks. Continental crust, being less dense, does not subduct as easily as denser oceanic crust, which gets recycled back into the mantle at subduction zones. As a result, continental rocks tend to remain intact longer than oceanic rocks, leading to their greater age.
