Floating solids displace water equal to their mass. An iceberg "sinks" until the mass of water it displaces is equal to the total mass of the iceberg. The crust is fragmented into ~20 tectonic plates. Plates more continuously at a rate of 1 to 15cm/year. This motion appears to be slow on a human time scale. Yet it is extremely rapid on a geological time scale. "black smokers" are found at some mid-ocean ridges. Water entering fractured rock is heated by magma. Hot water dissolves mineral and cycles back out of rock. When water reaches the sea, minerals precipitate quickly. One plate dives back into the mantle (subduction) Subduction plate descent is revealed as earthquakes deepen away from trench. The subducted slab will release water, causing the mantle above to melt (called magma) The magma will rise to the surface and form volcanic chains or arcs, like the andes. Age change marks direction of overriding plate motion.