Rocks: A Very Short Introduction (Very Short Introductions) by Jan Zalasiewicz

Author:Jan Zalasiewicz [Zalasiewicz, Jan]
Language: eng
Format: epub, azw3, azw
ISBN: 9780191038310
Publisher: OUP Oxford
Published: 2016-12-01T07:00:00+00:00

Chapter 5

Rocks in the deep

We only really know the exterior of our planet, rather like the microbes that cling only to the surface of the skin of an apple, oblivious to the flesh of the apple that lies within. The Earth is a very large mass of rock, being over 6,000 km to its centre. Our direct experience of it goes to almost 4 km below the surface—in the deepest mines on Earth, the South African gold mines. The crust there is comparatively cool, which allows humans to penetrate that far, albeit aided by excellent cooling and ventilation systems, and skilled engineering to counter the ever-present danger of rockbursts and cave-ins associated with those crushing pressures. Indirectly we can penetrate further by drilling, and oil boreholes commonly extend to 5 km and more. However, the record depth attained is still the 12 km reached in 1989 in the Kola Super-Deep Borehole in Russia, which, terminating in rocks at 180°C, only penetrated to about one-third the thickness of the local continental crust.

To go deeper we have to cheat, and exploit the rock fragments brought from deeper levels by tectonic or volcanic processes. Alternatively, we can probe the Earth by means of analysing the patterns of change in its gravitational and magnetic fields; or by detecting seismic waves that have travelled through the Earth. Or, we try to re-create the conditions of the deep Earth in the laboratory (astonishingly, with some success).

It is now quite clear that the depths of the Earth are nothing—alas!—like the vistas dreamed up by Jules Verne in his Journey to the Centre of the Earth, in which tortuous caves lead to underground lands with dinosaurs and giant humans, and to seas where ichthyosaurs do battle with plesiosaurs. The depths of the Earth are the province of rock that becomes progressively more different from that of the oxidized, weathered, crumbled material at the surface. It is where the real stuff of a planet resides.

The Kola borehole itself was expected to pass from granite to that denser rock, basalt. However, that never happened, and the granite persisted to the point where drilling was abandoned. To get to the denser rocks of the interior—indeed to the ultrafic rocks from which crustal basalts are derived—one needs to go through the crust and penetrate the Moho boundary (see Chapter 2) that separates it from the mantle. The Moho typically appears as a sharp increase in rock density when detected by analysis of seismic waves. This has never yet been drilled through in present-day crust, not even in the oceans where the Moho lies at a depth of only between 5 and 10 km.

However, nature has already done the job for us, as tectonic displacements have, here and there, dislocated large slabs of lithosphere (most typically as the detached oceanic slivers termed ophiolites) and pushed them up into higher crustal levels—in places exposing ancient examples of the Moho boundary in the process. This shows that the sharp boundary seen by geophysics is more complex and gradational in the rock—but no less real for that.

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