Bubbles shed light on exploding volcanoes
A study of bubbles trapped in pumice rock suggests it's a lot easier for volcanoes to erupt than we think, say Australian researchers.
"I'm talking about the type of eruption that makes a big cloud of ash and pumice," says Wright.
Geologists have long known that volcanic explosions are caused by fast-flowing magma.
Magma acts like silicone putty, says Wright. It is very viscous and can flow gently like a fluid but if it is pushed out of the earth under high pressure it can snap like a solid.
Scientists have developed models to help predict when a volcano will erupt. These models assume an eruption will occur when the magma flow reaches a certain critical threshold speed. The absolute speed depends on a number of factors, such as composition of magma.
But Wright and Weinberg's latest findings suggest the models are too simplistic.
Conventional volcano models assume that the magma behaves in a homogenous way, says Wright.
But Wright and Weinberg's findings suggest magma is much more complex and eruption can occur at much lower average speeds than previously thought.
Wright and Weinberg's study shows the speed of magma varies greatly within a volcano.
There can be rapidly flowing magma in one place and a millimetre away it can be flowing very slowly, says Wright.
She says this makes it is too simplistic to talk about "averages speeds".
"Nothing is behaving in an average way," says Wright.
She says as long as magma in one part of the volcano is flowing at the threshold speed, eruption will occur, even if the average speed is lower than this.
"Because things are so spatially variable, you can reach the critical threshold at a lower average speed," says Wright.
Wright and Weinberg based their study on magma flow from the Cerro Galan volcano in north-western Argentina, which exploded in a 'supervolcanic' eruption about 2 million years ago.
Close examination of the bubbles in the pumice (solid magma) from the eruption showed that when they are round, they rub against each other and slow down magma flow, says Wright.
She says, when the bubbles are elongated and flat, they slide against each other and speed up magma flow.
The complex pattern of round and elongated bubbles in the pumice reflects enormous variabilities in magma speed, says Wright.
She says it's now time for computer models to be updated to reflect this complexity.
"Even though that takes more computing power, this is what is necessary in order to be most accurate in terms of modelling behaviour."