Final answer:
At a mid-ocean ridge, magma emerges, cools, and forms new oceanic crust, which then pushes the older crust into subduction zones where it is recycled. These processes are part of the dynamic cycle of plate tectonics, involving the creation and destruction of crust, typically involving mafic magma that forms basalt or gabbro.
Step-by-step explanation:
At a mid-ocean ridge, molten rock or magma emerges from a rift, cools, and solidifies to create new oceanic crust. This process is driven by upwelling currents in the mantle and is a feature of divergent plate boundaries. As new crust is added, the older oceanic crust is pushed away and can encounter a convergent plate boundary, where it's driven underneath another plate into a subduction zone. Often marked by deep ocean trenches such as the Japan trench, subduction zones involve one plate being forced down into the upper mantle where high pressure and temperature eventually melt it, recycling its material and balancing the flow of material that rises at rift zones.
Subduction zones and rift zones are critical components of the cycle of plate tectonics, where new crust is continually created, and the old crust is destroyed, maintaining a dynamic equilibrium.
The oceanic crust is primarily composed of mafic magma that becomes basalt when it erupts at the seafloor or cools down to form gabbro. When oceanic plates diverge, the space is filled with this magma, creating new crust. Conversely, when oceanic crust is subducted, it may contain hydrated minerals that release water under high temperature and pressure, contributing to the melting and recycling process in subduction zones.