Types of Lava
Volcanoes come in many shapes and sizes. Volcanic eruptions may be quiet outflows of lava which are so peaceful that one can stand close enough to toss in pebbles (or leis; see figure) or so explosively violent that they blow mountains apart and blast everything within a hundred miles to smithereens (not recommended for close observation). Both the shapes of volcanoes and the violence of volcanic eruptions depend on the same rather mundane thing: the physical properties of erupting lavas. Photo: Courtesy of Fabman, Life Magazine.
The two most important properties of lava are its viscosity, and the amount of gases dissolved in the liquid rock. Let's look at each of these separately.
Viscosity is a term that describes the fluidity or "runniness" of the lava. Some lavas are very "runny," not quite like water, but more like warm honey or hot wax. When these lavas erupt, they flow for large distances before cooling enough to turn solid. You can imagine what kind of volcanic mountain you could make with runny lavas by thinking about (or carefully doing) pouring hot wax on a large sheet of paper. The wax spreads out into a large, flat layer. Let it cool and harden and then pour another layer. The second "flow" of wax will partly pond on the first and partly run off onto the paper to form another flat layer. If you keep pouring more and more "flows," you will get a large, but almost flat pile of wax. Therefore, many small eruptions of runny lavas form large, almost flat mountains like shield volcanoes. Eruptions of huge amounts of really runny lavas form flood basalt type volcanoes.
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