Answer:
(b) a descending convection current in the Earth's mantle.
Explanation:
We cannot see the deep interior of Earth, but we know from a variety of observations that it is in continuous motion. This is because the mantle convects. This fundamental planetary process has profoundly influenced the character and evolution of Earth.
Mantle convection is the main way heat from Earth's interior is transported to its surface, and this heat escapes principally through mid-ocean ridges. In fact, the connected mid-ocean ridge system is in essence a 80,000 km long volcano.
Heat in the mantle comes from the Earth's molten outer core, decay of radioactive elements and, in the upper mantle, friction from descending tectonic plates. The heat in the outer core results from residual energy from the Earth's formative events and the energy generated by decaying radioactive elements. This heat warms the base of the mantle to an estimated 7,230°F. At the mantle-crust boundary. the mantle's temperature is an estimated 392°F.
