Sunday, 20 February 2011
Volcanoes, Earthquakes and Tsunamis - a review of S186.
It always helps me to look back on what I have learned on completion of a Open University course, and I aim to take a look at a few key points from he last one - S186: Volcanoes, Earthquakes and Tsunamis. I'll make a start by looking at one misconception that I had held since my school days.
If you think about the prospect of great expanses of rock flowing like a river, one inevitably thinks of rock heated sufficiently below the surface to change it into a molten state. Thus, when a combination of circumstances allow it to erupt from a suitable opening at the surface a river of molten rock, or lava results.
One revelation produced by the course, was that the mantle was made up of solid rock, that circulates in a solid state - 'solid state convection' as it is known. Now just how can a solid lump of rock really circulate by convection? It defies logic surely? Generally speaking, a 'fluid' is anything that can 'flow', but in geologic terms it can include solids that when under specific conditions of pressure and temperature can also flow while maintaining their solid state.
Arthur Holmes, in his renowned book 'Principles of Physical Geology' explains the paradox of flowing solids wonderfully. He gives the example of pitch, which behaves like a brittle solid if struck with a hammer causing it to shatter, but under ordinary temperatures, will flatten out into a thin sheet simply under its own weight.
Another solid clearly seen to flow is ice.
Above: The Franz Josef glacier, New Zealand