Pat James, age 11, of Peterborough, Ont., Canada, for his question:
What causes lava to form?
The seething rivers that flow from Hawaii's erupting volcanoes are made of molten minerals. The temperatures that liquefy these lavas must be very high for they contain such rocky minerals as silica and basalt. The volcanoes must provide as much heat as blast furnaces.
Rivers and floods of flowing lava may spread over the surface or ooze between layers of rock far below the surface. Earth scientists suspect that they spring from reservoirs of molten magma that form at the deep roots of volcanoes. But these experts are not so certain what causes these pools of buried magma. We know a lot about the structure of our planet, but not enough to explain everything that goes on deep below the surface.
Vibrations of Earthquakes indicate that its materials are arranged in layers that become denser toward the center. Other evidence suggests that the materials become hotter as we go down. This seems logical since the lower levels are under pressure from those above, and pressure tends to generate heat. More heat is generated by buried radioactive materials. The immense heat in the center of the Earth is sealed below a massive crust of cool solid rocks.
Volcanoes always occur in regions where the solid crust is cracked and weakened by growing mountains. Perhaps molten magma from the dense hot mantle below the crust oozes up through these cracks, and maybe volcanoes act as safety valves through which the seething core can let off steam. There are, however, facts against this theory.
Not all mountain making regions produce molten magmas and lavas, and the center of the Earth seems to be made from plastic rather than seething liquid materials.
A more likely theory suggests that magmas and lavas are caused by convection currents within the hot center of the globe. These complex motions of heat are believed to rise up from the depths in certain regions, much as the heat bubbles up around and down through a pot of boiling soup. Many Earth scientists suspect that mountains are caused by the Earth's complex convection currents.
Growing mountains are accompanied by earthquake upheavals that weaken the solid crust. The cracks and fissures provide vents and pipes that release the pressure that normally seals the heated layers below. The melting point is lowered with less pressure, and the compressed plastic material may become liquid. This may cause the buried reservoirs of molten magma from which lava erupts, but we are not certain.
No two volcanoes are alike and perhaps their eruptions of flowing lava are caused by several factors working together. But we know that they issue from reservoirs of foamy magma deep within the Earth. We are not surf how deep the roots of a volcano. Go down, but most experts say that they are at least 20 miles below the surface. Some suspect that the magma is in the solid crust, which is 10 to 20 miles thick. Others suspect that the liquid lava from an erupting volcano may issue up from between the solid crust and the plastic mantle rocks below it.