According to the National Oceanographic and Atmospheric Administration, seafloor spreading is caused by the flow of plastic mantle rock beneath the Earth's oceanic crust. These flows force the seafloor apart, allowing pressurized mantle rock to force its way upward, causing a chain of vents and volcanoes along the mid-ocean ridges. The flow of mantle rock is thought to be responsible for all movement of crustal plates and the resulting interactions.
Current resources and technology are unable to observe the interaction of crust and mantle directly, and the mantle itself is observed only indirectly through such instruments as seismic sensors. However, all movement of crustal plates as well as resultant phenomena such as volcanoes and earthquakes are attributed to mantle movement. The Pacific Ring of Fire, for instance, is a ring of volcanoes around the Northern Pacific Ocean caused by the subduction of spreading oceanic plates beneath continental plates. This subduction is also the source of deep-sea trenches, which are the lowest points of Earth's surface.
Unknown processes in the mantle are also thought to be responsible for volcanism away from subduction zones. In this model, hot magma from the mantle rises up in a plume in a seemingly random location. This causes the land above to swell, and if the pressure is sufficient, the magma erupts as lava, breaking through the surface.