Why do some volcanoes explode into the air while others flow out laterally?
Kay
Webers
Kay,
The way a volcano erupts depends on a number of factors, but two of the
most important are how much gas is in the magma and how easily it can
escape. For many common magmas (that is, molten volcanic rock) there is
only about one tenth of one percent gas. That seems like next to nothing,
but as the magma rises through the ground toward the surface, the gas
greatly expands in volume. The explosions at some volcanoes are due to the
rapid escape of this expanding gas.
Volcanoes above subduction zones - where a 100 km thickness of the Earth's
crust and upper mantle sinks down under a continent - have much more than
0.1% gas because ocean water is also sucked down the subduction zone and is
mixed in the newly formed magma. When this magma erupts, the extra gas
makes much more powerful explosions.
If a magma is made of material that is very stiff and viscous (slow flowing
- like tar) the gas can't easily escape. The trapped gas accumulates until
there is enough to overcome the magma's viscosity, and then there can be a
very powerful explosion as the gas breaks through the magma.
On the other hand, if a magma is very fluid (more like syrup) then the gas
bubbles can escape one by one. In this case there is no explosion, and the
magma erupts as a lava flow.
Chuck Wood, University of North Dakota
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