Which statement best describes seafloor spreading?

• A. Magma from deep in the mantle moves upward, fills cracks in the seafloor, and forms new crust that moves away from the ridge
• B. Crust is destroyed at subduction zones and recycled into the mantle
• C. Continental crust becomes thicker and mountains form along plate boundaries
• D. Volcanic activity ceases at mid-ocean ridges and new crust does not form

Answer: (A)

Seafloor spreading is the process by which new oceanic crust forms at mid-ocean ridges as magma rises from the mantle, fills cracks in the seafloor, and solidifies to create fresh rock. As this magma continually wells up and erupts, the newly formed crust pushes away from the ridge, causing the ocean floor to spread outward. This idea is supported by evidence such as symmetric patterns of magnetic minerals recorded in the rocks and the increasing age of seafloor the farther you move from the ridge. The other statements describe different tectonic processes: crust is destroyed at subduction zones, continental crust thickening and mountain-building occur from collisions, and ridges remain volcanically active rather than ceasing activity.