Volcanoes
Some additional resources are listed below.
The distribution of volcanoes tells us where conditions for melting at depth have prevailed
divergent - where the crust is stretched and thinned permitting hot mantle rock to rise and melt as the pressure decreases
Examples - East Africa, Iceland
convergent - where crustal rock, that melts at lower temperatures than mantle rock and that contains water, is dragged down to depths where it can melt
Examples - Aleutians, Andes
Magma (molten rock) rises due to the force of gravity because silicate liquids are less dense than the solids from which they melt -
This can be expressed in the sentence: the pressure (which is a force) on a column of magma is determined by the density of the liquid, the height of the column of liquid, and the gravity constant. In the same way, the density of the rock, the height of the column of rock, and the gravity constant determine the pressure on a column of rock.
This can also be written as an equation: P (Pressure) = g (gravity constant) x h (height) x r (density).
The magma rises until the pressure of the liquid column and the pressure due to the surrounding solids are equal.
g*h*liquid density=g*H*solid density
where g= gravity acceleration, h = height liquid column, H = height solid column
- since g is the same for both columns of rock, this reduces to
h*liquid density = H*solid density
- since liquid density is less than solid density, h must be greater than H. Thus the volcano is higher than the surrounding land and the height depends on the difference in density between the liquid and the solid rock
Additional resources
Some animations from PBS Out of the Inferno. This includes quick time movies of some volcanic processes.