Mantle convection drives plate tectonics. Hot material rises at mid-ocean ridges and sinks at deep sea trenches, which keeps the plates moving along the Earth’s surface. The outer limbs of the convection cells plunge down into the deeper mantle, dragging oceanic crust as well. This takes place at the deep sea trenches.

Plates sliding past each other cause friction and heat. Subducting plates melt into the mantle, and diverging plates create new crust material. Subducting plates, where one tectonic plate is being driven under another, are associated with volcanoes and earthquakes.

The Mid-Atlantic Ridge includes a deep rift valley that runs along the axis of the ridge along nearly its entire length. This rift marks the actual boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava and producing new crustal material for the plates.

The theory of plate tectonics is what brings together continental drift and seafloor spreading. Plates are made of lithosphere topped with oceanic and/or continental crust. The plates are moved around on Earth’s surface by seafloor spreading. Convection in the mantle drives seafloor spreading.

Earth’s Crust. “Crust” describes the outermost shell of a terrestrial planet. Earth’s crust is generally divided into older, thicker continental crust and younger, denser oceanic crust.

Earth’s oceans might not exist if water were not periodically subsumed by the planet’s mantle and then released. Without plate tectonics driving the creation of coastlines and the motion of the tides, the oceans might be barren, with life-giving nutrients relegated forever to the stygian depths.