Explain the processes that form mid-ocean ridges and subduction zones.

• A. Volcanism only at hotspots creates ridges; subduction zones do not produce earthquakes.
• B. Crust forms at subduction zones and is consumed at ridges.
• C. Upwelling magma at divergent boundaries creates new oceanic crust at mid-ocean ridges; one plate sinks beneath another at subduction zones, recycling crust and generating earthquakes and volcanic arcs.
• D. Subduction creates new crust; divergent boundaries erase crust.

Answer: (C)

The big idea is how plate tectonics builds new ocean floor at divergent boundaries and recycles crust at subduction zones. At mid-ocean ridges, tectonic plates pull apart, causing mantle rock to rise and melt as it decompresses. The resulting magma erupts and cools to form new oceanic crust, so the seafloor widens and the ridge sits as a topographic rise with a rift system. As this newly formed crust moves away from the ridge, it ages and cools.

At subduction zones, one plate sinks beneath another because it is denser. This recycling of crust into the mantle drives a deep trench and a complex mantle zone where melting occurs, producing magma that rises to fuel volcanic arcs on the overriding plate. The movement of slabs and the shifting boundaries also generate earthquakes, especially where the plate meets or sticks.

Other statements mix up these processes: crust is formed at divergent boundaries, not at subduction zones; crust is consumed at ridges, not created there; and subduction does not create crust. The chosen description accurately captures both the creation of new oceanic crust at mid-ocean ridges and the destruction and recycling of crust at subduction zones, along with the associated earthquakes and volcanism.