Drilling into the unknown – earth’s crust
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.
This phenomenon is due to the fact that, under the effect of the high pressure and temperatures existing at great depths, water is released from the crystalline network of minerals and, because of the enclosed space, exerts pressure on the rock, leading to fissuring and consequently decompaction. It turned out that the decompacted zone had subsisted over a long period.
The discovery of the phenomenon of “hydrogenous decompaction’ not only makes it possible to explain the geological character of certain boundary zones which reflect seismic waves at great depths, the nature of hydrothermal fluids, and the mechanism of tectonic deformations, it also radically alters our ideas about the hydrological cycle in continental conditions and of the structure of the underground hydrosphere.
The drilling provided extremely interesting information not only for geologists but also for biologists. It was discovered that the deep horizons which had been thought to be “dead’ since the beginning of time had actively participated at some stage in the biological processes which took place in the depths of the Earth.
Isotopic carbon analysis revealed two sources of carbonic gas–the first connected with the mantle and found mainly in Archaean rocks, the second of biogenic origin and found predominantly in Proterozoic rock, in which petrified remains of micro-organisms (micro-fossils) estimated to be thousands of millions of years old, were also found.
This was not an isolated discovery. Seventeen species of micro-organisms were found, bearing witness to an extensive development of biogenic processes on our planet in earliest times. Thus biological life started on Earth much earlier than had previously been estimated.
Data supplied by direct measurement of temperatures at great depths have obliged us to revise our concepts regarding present and geohistorical variations of temperature in the bowels of the Earth. It was previously thought that in regions of slight tectonic activity temperature rise in relation to depth increase was insignificant. However, although it was expected during the Kola drilling that the temperature at a depth of ten kilometres would be about 100 degrees Centigrade, it turned out in fact to be almost double this estimate, at 180 degrees Centigrade. It was established that during the Proterozoic era the geothermic gradient (the temperature increase per 100 metres of depth) was five times higher than it is today.
Study of the thermic regime in the bowels of the Earth also provided an answer to a question that has long preoccupied scientists–what is the contribution of the mantle and of radioactive decay of elements in rocks to the overall heat flow in the interior of the Earth? It was established that the mantle was the chief source of ascending heat.
Finally, the existence of underground water inside the old crystalline massifs at practically all the levels reached was proved for the first time. Inflows of highly mineralized water saturated with bromine, iodine and heavy metals were discovered, as well as gases circulating through the crystalline rocks in the zones of tectonic deformation. At depths of 6.5 to 9.5 kilometres, zones of low temperature hydrothermal mineralization (copper, lead, zinc, nickel, silver), previously considered to be primarily nearsurface formations, were discovered.
The large number of mineral associations discovered in the rock strata traversed during the drilling confirms the possibility that mineral deposits may be present in industrially viable accumulations.
We were also led to revise our ideas about the physical state and properties of rocks at great depths. It emerged that rock fissures do not disappear as depth increases.
This journey towards the centre of the Earth–the first to such a great depth (15 kilometres)–was made possible by the employment of radically new types of drilling technology and equipment. This vast undertaking necessitated the creation of a unique type of drilling installation–the URALMACH 15000.
Drilling was effected by the open-shaft (untubed) method. This considerably improved the quality and accuracy of geophysical measurements. The reconnaissance operations were conducted, not with steel tubes, but with extra-strong heat-resistant tubes made from specially-developed light alloys capable of withstanding temperatures of 230 to 250 degrees Centigrade. The specially-designed, highly efficient rock-blasting and turbo-drills were powered by the energy of the drilling liquid pumped into the well. A completely new type of apparatus was built to draw rock samples from great depths intact and preserved in their original orientation.
The programme for the study of the subterranean structure in depth provides for exploration throughout the territory of the Soviet Union. Drillings at very great depth are at present being conducted in the Transcaucasus, the Muruntauski region of Central and the Muruntauski region of Central Asia. A drilling is being set up in the Tiumen district of Western Siberia.
The data provided by them will make it possible, firstly, to assess the oil, gas and ore-bearing potential of deep horizons and, secondly, to elucidate the essential questions relating to the tectonic evolution of the Earth in the light of the continental drift hypothesis. The programme provides for special experimental and theoretical studies concerning the elaboration of a general theory of the evolution and structure of the Earth’s tectonosphere. The journey into the Earth’s past, undertaken by scientists to decipher its evolution, will continue.
Photo: The Kola Peninsula in the far north of the USSR is the site of the first very deep drilling made as part of an ambitious programme aimed at exploring the deep structure of the continental crust and seeking areas potentially rich in mineral, oil or gas deposits. Left, the drilling rig and the accompanying buildings and workshops which house the industrial and technical services. The drilling rig is clad with corrugated iron sheeting to maintain a constant positive temperature in this bleak Arctic environment.
Photo: Right, the new sophisticated and fully automated URALMACH 15000 drilling plant, which incorporates the very latest technology, is capable of drilling down to a depth of 15 kilometres. The derrick is 86 metres high and has a hoisting capacity of 400 tons.
Photo: Top photo: (a) This pebble of volcanic ash, found near lsua in west Greenland, has been shown to have an age of 3,824 million (plus or minus 12 million) years and is the oldest known rock in the world. Other rocks close to this age in the same area are the Amitsoq Gneiss (b) and a banded iron formation (c), whilst the fourth rock (d) is an ancient Antarctic rock.
Photo: Bottom left: A stony meteorite from Barwell, England. Meteorites have been dated back to about 4,600 million years.
Photo: Botton right: A rock from the Moon. The oldest rocks brought back from the Moon have been dated at about 4,600 million years.
Photo: Above: One of the geological and scenic wonders of the Earth, the Grand Canyon is an immense gorge (in places over 1.5 kilometres deep) cut by the Colorado River into the high plateaux of northwest Arizona, USA. Its rocks, including granite and schist some 4,000 million years old, constitute a unique record of geological events. Over 300 kilometres long, the Canyon is at some points more than 20 kilometres wide.
Photo: Below: Irazu, a volcano in the Cordillera Central, Costa Rica, erupts in an inferno of flame and lava.
Photo: Above: Gases, ash and incandescent, semi-solid lumps of lava are hurled upwards during a volcanic eruption.
Photo: Below: The Hawaiian volcano Mauna Loa during an eruption in 1984. Rising to 4,169 metres above sea level, Mauna Loa (meaning “long mountain’) has a dome 120 kilometres long and 102 kilometres wide. It has averaged one eruption every three and a half years since 1832.
COPYRIGHT 1986 UNESCO
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