Natural Science: Volcanoes

The relentless motions of tectonic plates, the uplift and the erosion of mountain ranges, and the evolution of living organisms are processes which can only be fully appreciated across the deep time of geology.

But some of the processes at work in our planet can manifest all too suddenly, changing the landscape and destroying lives on a very human timescale: volcanoes. Superimpose a map of active volcanoes on a world map showing the boundaries of the tectonic plates and their association is obvious.

The ring of fire around the Pacific, for example, is clearly associated with the plate boundaries. But where is the molten rock that feeds them coming from? Why are volcanoes different from each other, with some producing gentle eruptions and regular trickles of molten lava, whilst others erupt in devastating explosions? And why are some volcanoes, such as those of Hawaii, in the middle of the Pacific, far from any obvious plate boundary?

The molten rock

The key to understanding volcanoes comes from understanding how rocks melt. For a start, they do not have to melt completely, so the bulk of the mantle remains solid even though it gives rise to a fluid, molten magma.

That means that the melt does not have the same composition as the bulk of the mantle. As long as the so-called dihedral angles, the angles at which the mineral grains in mantle rock meet, are large enough, the rock behaves like a porous sponge and the melt can be squeezed out.

Calculations show how it will tend to flow together and rise quite rapidly in a sort of wave, producing lava at the surface in the sort of quantities seen in typical eruptions (The University of Sydney, 2006).

Melting does not necessarily involve increasing the temperature. It can result from decreasing the pressure. So a plume of hot, solid mantle material will begin to melt as it rises and the pressure upon it reduces. In the case of a mantle plume, that can happen at considerable depths (McKenzie, 1984, p. 717).

Beneath the mid-ocean ridge system, the melting takes place at much shallower depths. Here there is little or no mantle lithosphere and the hot asthenosphere comes close to the surface.

The lower pressures here can result in a larger proportion of the rock melting, perhaps 20 or 25%, supplying magma at about the right rate to sustain sea floor spreading and produce an ocean crust 7 kilometres thick. Most of the ocean ridge eruptions pass unnoticed as they take place more than 2,000 metres underwater as rapidly quenched pillow lavas (Basin Topography, 2006).

But seismic studies have revealed magma chambers a few kilometres beneath the sea floor in parts of the ridges, particularly in the Pacific and Indian oceans, though there is also some evidence of magma chambers beneath the mid-Atlantic ridge. Where a mantle plume coincides with an ocean ridge system, as in the case of Iceland, more magma is generated and the ocean crust is thicker, in this case rising above the sea to form Iceland (Scarth, 1994, p.14-17).

Hawaii

The Big Island of Hawaii has welcoming people and friendly volcanoes. The town of Hilo is probably more at risk from tsunamis triggered by distant earthquakes than from the great 4,000-metre volcano of Mauna Loa that looms behind it.

To the north and west lie the other Hawaiian islands and the Emperor seamount chain, tracing the long journey of the Pacific plate across the hot spot of an underlying mantle plume (Scarth, 1994, p.42). To the south of the Big Island of Hawaii is Loihi, the newest of the Hawaiian volcanoes.

As yet it has not broken the surface of the Pacific, but it has already built a high mountain of basalt on the ocean floor and will almost certainly become an island above water before long (14). Because Hawaiian lava is very fluid, it can spread over a wide area and does not tend to form very steep slopes.

Such volcanoes are sometimes known as shield volcanoes, and they can flood basalt over a wide area. Often, a particular flow will develop a tunnel around it as the outer crust solidifies but the lava continues to flow inside. When the supply of lava ceases, the tunnel can drain and be left hollow (41-42).