Answer
Decompression melting is a process by which rocks melt due to a decrease in pressure without a significant increase in temperature. It occurs when rock material that was previously subjected to high pressure and temperature in the Earth's mantle is brought to shallower depths where the pressure decreases.
The process of decompression melting is closely associated with plate tectonics and occurs primarily at divergent plate boundaries and hotspots. At divergent plate boundaries, such as mid-ocean ridges, tectonic forces pull the lithospheric plates apart, creating a gap. As the plates move apart, the underlying asthenosphere, which is composed of partially molten and more ductile rock, rises to fill the gap.
As the asthenosphere rises, it encounters decreasing pressure due to the reduced overlying weight of the lithosphere. The reduction in pressure causes the mantle rocks to undergo decompression. Even though the temperature remains relatively constant, the reduction in pressure lowers the melting point of the mantle rock minerals. As a result, the rock begins to melt and forms magma.
The newly formed magma, which is molten rock comprising various minerals, is less dense than the surrounding solid rock. It continues to rise through the fractures and cracks in the lithosphere, eventually reaching the surface. At the surface, the magma may erupt as volcanic lava, forming new crust and contributing to the growth of oceanic crust at mid-ocean ridges.
Hotspots, which are areas of anomalously high heat flow in the mantle, can also trigger decompression melting. As the hotspot material rises towards the surface, the decrease in pressure induces melting of the mantle rocks, leading to the formation of volcanic features such as shield volcanoes and volcanic islands.
In summary, decompression melting occurs when rocks experience a decrease in pressure without a substantial increase in temperature. It is associated with divergent plate boundaries and hotspots, where the decrease in pressure allows the mantle rocks to melt and form magma, which can ultimately lead to volcanic activity.
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