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

At first sight, what may seem to be a static expanse of ice, rapidly (in geological terms) flows down the mountainside. The photo above, of the Franz Josef glacier almost looks like a river, complete with rapids!

As Arthur Holmes stresses, the key is time. Holmes illustrates this with the example of a ball bearing. If one is dropped into a jar of water it would take only a second to sink to the bottom. If the jar was filled with oil it would take several minutes. If filled with wax it would take maybe a hundred years to reach the bottom and if filled with ice and kept at a constant temperature, it would take millions of years.

So it is with rock. If pressure is maintained for long enough, rock will flow and bend and in the mantle it circulates. However, when rock rises, the pressure upon it decreases, causing partial melting and a change to liquid form. This of course leads to the formation of igneous rocks by way of extrusion at the surface through a suitable fracture to form extrusive rocks such as basalt, or  cooling slowly below the surface to form intrusive igneous rocks such as granite.

I will get stuck into the subject of igneous rocks and volcanoes next time.


1 comment:

  1. Hi Allyn, I am in southeast asia now. since the Myanmar quake thatkilled 70+ last month, ppl here are discussing a very hot topic, the reactivation of the so called geologically and tectonically stable Sunda shelf, and years ago, there were quakes (strongest only at 3 rit scale) happened in Peninsular malaysia which there shouldn't be any earthquake. thats scary, reactivation of faults and all that , this topic is very much in discussion here.


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