Magazine article UNESCO Courier

Drilling into the Unknown

Magazine article UNESCO Courier

Drilling into the Unknown

Article excerpt

Drilling into the unknown

THE structure of the Earth reflects the long history of its evolution. By reconstrucing this history scientists hope to find the key to such processes as the formation of the Earth's crust, volcanism, and the upheavals, subsidences and foldings which led to the development, on the Earth's surface and in its sub-stratum, of conditions which favoured the formation, accumulation and preservation of useful minerals.

In the Soviet Union, study of the complex physical and physico-chemical processes which take place within the Earth's hard crust and the upper layers of the mantle is undertaken within the framework of a vast, integrated programme for the exploration of the "basement' of the country using geological, geophysical and geochemical methods as well as deep and very deep drilling.

The particularity of this programme is that it focuses on the study of the Earth's crust within the limits of the continental zone where most of the planet's useful minerals are concentrated. Geophysical explorations are being conducted along the system of profiles traversing the entire territory of the USSR, and deep and very deep drilling is being undertaken at their points of intersection.

In this way it has been possible to explore the Mohorovicic Discontinuity (the boundary between the Earth's crust and the mantle) and to obtain new data on the structure and physical properties of the upper mantle, to identify zones where important fractures occur in the Earth's crust and determine their extent, and to pinpoint the boundaries and structure in depth of major tectonic elements which may be the site of concentrations of mineral ores, oil or gas.

At the heart of the programme is the exploration of the deep structure of the continental crust, on whose layers is imprinted, as on the pages of a book, the whole story of its formation. The first drilling was made in the Kola Peninsula, on the fringe of the Baltic Shield, which is composed of ancient crystalline rock dating back to Precambrian time. The Kola drilling has thrown some new light on the evolution and structure of the early continental crust of the Earth as a whole, since similar formations are widely distributed in other parts of the globe--in India, North America, South Africa, Western Australia, Antarctica and Greenland. Very deep drilling explorations being carried out in the USA, Canada and the Federal Republic of Germany are also contributing to our knowledge of the deep horizons of the Earth in areas potentially rich in mineral raw materials.

The direct observations made possible by these drillings have provided the foundations of the first factually-based model of the continental crust and have led to a revision of earlier notions about the evolution and structure of the Earth's depths.

The Kola drillings have resulted in a series of unexpected and very interesting discoveries.

One of our objectives was to bore through the so-called granitic layer (the upper part of the consolidated crust) to reach a basaltic layer whose existence had been deduced from geophysical data. Geophysicists had observed sharp variations in the speed of seismic waves at great depths, and, since these waves travel faster through granite than through basalt, these variations were thought to indicate a transition in the Earth's crust from a granitic to a basaltic layer. But this was no more than conjecture because, unlike the granitic layer, identified with the Archaean granitoid gneiss which is widely distributed over the surface of the continents, the basaltic layer does not emerge on the surface.

The Kola drilling was the first ever to bore through the line where the seismic waves undergo a sharp change of speed. But contrary to expectations, no basaltic layer was discovered. It became apparent that the variations in the speed of diffusion of the seismic waves was related, not to a transition from a granitic to a basaltic layer, but to the decompaction of the rock which occurs at great depths. …

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