Scientists deduce its inner layers from how these interfere with waves set off by earthquakes. The mantle is solid, but over very long periods of time most of it behaves like plasticine and flows. The temperature difference between the top and bottom of the mantle causes a circular flow called convection. The uppermost mantle and crust form an outer shell of rigid plates. The plates, and the continents they contain, move across the Earth's surface on the convection currents.
Heat generated by radioactive decay and gravitational friction keeps part of the mantle soft.
Convection currents stir this semi-fluid layer. On a smaller scale, convection currents can be seen in a cup of hot coffee as milk warms and wells upwards in billows, then cools and sinks again. Plates grow by the addition of new rock from the mantle at mid-ocean ridges, where the convection currents rise. The volcanic island of Iceland sits on top of the Atlantic mid-ocean ridge. Plates are destroyed when they are carried down into the mantle at subduction zones, such as the one off the coast of British Columbia.