Published: 21 May, 2008

How do scientists determine the viscosity of lava?

How do scientists determine the viscosity of lava?

In a fluid, viscosity can be thought of as a measure of internal resistance to flow, We all have everyday experience with relative viscosity. Which has a lower viscosity, water or ketchup? The liquid with lower viscosity (water) flows faster down a slope (in a glass or bottle). We can't experiment with lava in the kitchen: (it's red hot to white hot), so we can let nature do an experiment for us. Lava flows on volcanic islands like Hawaii are rivers of basaltic lava that flow more slowly than water but about fast as a river of ketchup (imagine that!). Among lavas, basalt is relatively inviscid (low viscosity); other common lava types (andesite, dacite, rhyolite) have successively higher viscosity for two reasons: they have successively higher silica content (with more cross-linkages in the melt structure), and they are successively cooler when they exit the earth. As glass blowers know, viscosity decreases rapidly as temperature goes down. So much for relative viscosity, how about determining a value for viscosity (units pascal-seconds--PaS or poises--P, where 1PaS=10P). Several experimental methods are used, but for melts perhaps the simplest makes use of the fact that dense solid objects fall faster through low viscosity liquids than through high viscosity liquids (for instance, a dense object falls twice as fast through a liquid with half the viscosity) . From the fall rate one can calculate the viscosity. One experimental design uses a furnace to melt basalt in a sealed capsule. Encapsulated with the basalt is a small unreactive solid sphere (for instance, made of platinum); when the basalt melts, the sphere rests at the bottom. You turn the capsule over and the ball begins to fall. By cooling the capsule rapidly (quenching) you can stop the experiment at any time. After quenching, you cut open the capsule and see how far the sphere has fallen. From fall rate you calculate viscosity. For instance basalt at its eruption temperature (1200°C) has a viscosity of about 50 PaS, close to the ketchup value. An importantobservation is that when even a little water is added to the capsule, the viscosity is greatly reduced. Dry rhyolite at 900°C has the viscosity of pitch or asphalt (about 100 million PaS), and within the Earth it would not move toward the surface fast enough to avoid freezing. With 10% dissolved water, the viscosity is reduced to about 800PaS and then it will move upward rapidly. As the rising magma approaches Earth's surface, often within a volcano, the dissolved gas bubbles out (much as bottled soda pop does when the cap is removed, reducing the pressure). In the case of rhyolite, the expanding gas bubbles fragment the magma and cause an explosive eruption that frequently includes both a airfall ash and a fast moving (low effective viscosity) fluidized mixture of glass, pumice (think puffed rice) and gas. This is not a lava flow, but a pyroclastic flow. Caveat: for ketchup and pyroclastic flows the flow behavior is not simple ("Newtonian") and the apparent value of viscosity is depends on the flow rate itself.