All the activity on Earth’s surface — erupting volcanoes, shifting tectonic plates, restless seas and myriad forms of life — depends on the two-part engine under the hood. Directly beneath Earth’s crust lies the mantle: rock that melts, churns and flows like putty, driving the volcanic and tectonic activity at the surface. And below that, there’s the outer core, a liquid-metal ocean whose swirling…
