These valuable sources of copper and zinc form in zones of submarine extensional tectonics where magma rises to the seafloor, erupting huge quantities of pillow lavas. The combination of extensional structures and focussed heat flow drives convection of seawater through the fractured oceanic crust. The water becomes heated, acidified and stripped of its oxygen by rock–water interaction, causing it to leach finely dispersed metals from the lavas. During pauses in the volcanism, focussed discharge of the hydrothermal fluid at the seafloor creates the famous "black smokers", i.e. chimneys formed by rapid precipitation of iron, copper and zinc sulfides (and sometimes gold) upon cooling of the fluid as it mixes with normal seawater.