Rocks on Earth repeatedly undergo a cycle of formation, metamorphism, weathering, deposition, and remelting. This process is closely linked to plate tectonics and is key to understanding Earth's material cycles.
Basic Knowledge
The rock cycle is the process by which igneous, sedimentary, and metamorphic rocks change into one another. Igneous rock is exposed at the surface → weathering and erosion → clastic material → deposition → diagenesis → sedimentary rock → subduction and deep burial → metamorphic rock → further heating produces magma → solidification → igneous rock. This cycle is repeated.
Relationship between plate movement and rock formation:
・Mid-ocean ridges (divergent boundaries): mantle rises and undergoes decompression melting → basaltic magma erupts → forms oceanic crust (basalt and gabbro)
・Subduction zones (convergent boundaries): seawater and sediment are added as an oceanic plate subducts → andesitic magma forms → volcanic rocks of island arcs and continental arcs; subducting rocks undergo high-pressure metamorphism (Sambagawa type) → deep parts of the overriding plate undergo high-temperature metamorphism (Ryoke type)
・Collision boundaries: continents collide → regional metamorphism (Himalayan-type metamorphic belts) and intrusion of granite batholiths
Oceanic crust subducts and disappears within several tens of millions to about 100 million years after its formation. Continental crust has a low density (about 2.7g/cm³) and is difficult to subduct, so its rock cycle proceeds slowly; rocks more than 3 billion years old have been preserved in cratons (stable continental blocks).
Compare what kinds of rocks form at mid-ocean ridges and in subduction zones, including the types of magma involved.
Answer: At mid-ocean ridges, rising asthenosphere undergoes decompression melting and produces basaltic magma. As it cools, it forms oceanic crust composed of basalt at the surface and gabbro at depth. In subduction zones, water released from the oceanic plate lowers the melting point of the mantle wedge, producing mainly andesitic magma and forming volcanic and plutonic rocks (diorite and granite) in island arcs and continental arcs.
- Rock cycle: mutual changes among igneous, sedimentary, and metamorphic rocks
- Mid-ocean ridges: decompression melting → basaltic magma → oceanic crust
- Subduction zones: water supply → andesitic magma → island arcs
- Subduction zones: high-pressure metamorphism (blueschist) + high-temperature metamorphism (such as migmatite)
- Collision zones: regional metamorphism and granite batholiths
- Oceanic crust: disappears in several tens of millions to 100 million years through subduction
- Continental crust: low density → difficult to subduct → old rocks are preserved
Points to Note
① Decompression melting at mid-ocean ridges occurs not because the mantle cools, but because a decrease in pressure lowers the melting point. ② In subduction zones, water lowers the melting point because water is supplied to the mantle. ③ Each step of the rock cycle takes a very long time (several million to several hundred million years).
Practice
- Explain how the mantle melts at mid-ocean ridges (decompression melting).
- How does magma form in subduction zones?
- Why is continental crust more difficult to subduct than oceanic crust?