Why are large underwater mountain ranges found where crust spreads apart?

Why are large underwater mountain ranges found where crust spreads apart?

• A. Because the crust there is hot and buoyant
• B. Because it is older and cooler
• C. Because of subduction of oceanic crust
• D. Because of volcanic gas emissions

Answer: (A)

When oceanic plates move apart, hot mantle material rises toward the surface, melts, and feeds the creation of new oceanic crust. This fresh crust is warm and less dense than the surrounding older crust, so it sits higher due to buoyancy. That upward, buoyant layer forms the elevated underwater ridges we see along spreading centers. As the newly formed crust moves away from the ridge and cools, it becomes denser and gradually subsides, but the ridge itself remains a prominent, long underwater mountain range created by the upwelling mantle and the birth of new, buoyant crust. The other ideas don’t fit because subduction happens at convergent boundaries, older cooler crust would be heavier and lower, and volcanic gas emissions don’t create these large, persistent topographic features.