The viscosity of lava depends on several factors. One of the most important factors is composition, especially the amount of silica (SiO₂ or quartz) melted in the lava. Silica molecules have the tendency to stick together to form long chains. These long chains literally get tangled together and make it difficult for molecules in the melt to slide past each other. This slowing of movement at the molecular level makes the whole lava less fluid. A small difference in silica content can make a huge difference in viscosity: lavas with about 70% silica (called rhyolites) are about 10 billion times stiffer than lavas with about 50% silica (called basalts).

The other important property of lavas governing the type of eruption is the amount of gases dissolved in the lava. The most important gases in lavas are water and carbon dioxide. (Although water is a liquid at room temperature, it is definitely a gas at typical lava temperatures of 900 C [1600 F] to 1200 C [2200 F].) Very little gas can be dissolved in lava at the low pressures (around one atmosphere) found at the surface of the earth. We say the solubility of gases in liquids is low at low pressures. On the other hand, deep in the earth where lavas are formed the pressures are high (thousands to millions of atmospheres), and significant amounts of gas can be dissolved in the lavas (up to several per cent by weight). We say the solubility of gas in liquids is high at high pressures. If we dissolve lots of gas in a lava at high pressure and high solubility and then lower the pressure to get low solubility, the gas will "undissolve" (or "exsolve") from the liquid to form bubbles.

So what happens to the gases in lava when it rises from the deep interior of the earth to the volcano on the surface? Let's consider a couple of everyday examples. Look at a clear plastic bottle of soda pop that hasn't been opened or recently shaken. Most soda pop contains a few per cent by weight of dissolved carbon dioxide. Even if you look carefully at the liquid, you can't see any gas inside (remember, it is dissolved) except perhaps a few small bubbles and a tiny air pocket at the top. If you carefully open the bottle, many small bubbles quickly form and rise to the top and escape. If enough bubbles form, they take part of the liquid with them as a foam that flows out of the top. If you shake the bottle vigorously just before opening it, you trigger the formation of so many bubbles so quickly that the soda pop bursts explosively from the top, and you and everyone around you are sprayed with fine droplets of liquid. On the other hand, if the bottle of soda pop is left open for a long time, virtually all of the dissolved gas escapes, and no amount of shaking can cause the remaining liquid to spray out. It can only be poured out.

This shows the effect of dissolved gases. What about viscosity?

Think about making bubbles by blowing air through a straw into a glass of liquid. If the liquid is water (low viscosity), the bubbles you form escape easily. If you blow slowly (representing small amounts of escaping gas), the water is little disturbed. Even if you blow vigorously (representing large amounts of escaping gas), the water froths and bounces around a lot, but very little gets out of the glass. If you try the same thing with an ice cream soda or malt (representing high viscosity), bubbles escape only with difficulty. Even small bubbles bursting from a malt can get blebs of ice cream on your clothes. Blowing vigorously can launch the entire malt across the room!

[ Types of Volcanoes ] [ Types of Lava: page 1 / page 2 / page 3 ]
[ Sizes of Eruptions ] [ Volcano Eruption Animations ]
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