Подготовьте устное сообщение/презентацию на тему «роль плитотектоники в общем понимании геологических проблем».

 

Unit III. The Deep-Earth-Gas Hypotheses.

Focus on:

• общенаучная терминология
• независимый причастный оборот
• оборот «Complex Subject»
• способы выражения вероятности

 

Text Study:

There is much evidence indicating that earthquakes release gases from deep in the earth's mantle. Such gases may indicate methane of nonbiological origin, which could be a vast resource of fuel.

It is widely believed that the earth's supply of hydrocarbon fuels will be largely used up in the foreseeable future, the most desirable ones (oil and natural gas) within a few decades and coal within a few centuries. Diverse evidence leads us to believe that enormous amounts of natural gas lie deep in the earth and that if they can be tapped, there would be source of hydrocarbon fuel that could last for thousands of years. The hypothesis that there is much gas deep in the earth also provides a unified basis for explaining a number of otherwise rather puzzling phenomena that either give warning of earthquakes or accompany them.

The exact composition of the gas is not known, since the observational evidence is scattered and not easily interpreted. Volcanic eruptions bring gas out from the interior of the earth. It is not possible, however, to deduce from such observations the initial composition of the gas while it was still deep in the earth.

Gases released during earthquakes are probably more reliable samples of what resides in the deep crust and the upper mantle. The sampling of such gases is just beginning, and the data will not yet support confident conclusions. One can assume that the composition of the deep-earth gases varies from place to place, since the location of mineral deposits in the crust suggests that the underlying mantle is quite heterogeneous. For a variety of reasons we think methane of nonbiological origin is one of the principle deep-earth gases, and it will be the focus of our discussion here, although we do not mean to minimize the possible importance of other deep-earth gases in the phenomena associated with earthquakes.

The notion of nonbiological methane runs counter to the prevailing view in petroleum geology that virtually all the oil and natural gas in the earth is of biological origin. In that view the carbon in hydrocarbon fuels was originally derived from atmospheric carbon dioxide, and the energy to dissociate the carbon and the oxygen came from sunlight in the course of photosynthesis by green plants. The bural of some of these organic compounds before they could become oxidized would then have provided the source materials for oil and gas. It cannot be doubted that this process contributed to the genesis of much of the petroleum that has been recovered, but there may be more to the story.

The hypothesis that the earth contains much nonbiological hydrocarbon begins with the observation that hydrocarbons are the dominant carbon containing molecules in the solar system. The universe is made mostly of hydrogen, and the evidence of cosmochemistry suggests that the earth and the rest of the solar system originally condensed out of a hydrogen-saturated nebula. Most of the carbon in meteorites, which provide the best clues to the origin, composition of the inner planets, is in the form of complex hydrocarbons with some chemical similarity to oil tars.

The picture we favour is of dual origin, with some hydrocarbons derived from buried organic sediments and probably much larger amount added to those hydrocarbons by augmentation from a stream of non-biological methane.

Let us now examine some of the evidence for the escape of methane from the interior of the earth. A likely place to look is along the crustal faults and fissures of the tectonic-plate boundaries, which ought to provide the best access to the deep interior. Indeed, hydrocarbons appear to be clearly associated with such plates.

Another line of evidence connecting nonbiological hydrocarbons with such features is the striking correlation between the major oil and gas regions and the principal zones of past and present seismic activity. Oil fields often lie along active or ancient lines. Most of the known natural seeps of oil and gas are found in seismically active regions. The association suggests to us that the deep faults may provide a conduit for the continuous input of nonbiological methane and other gases streaming up from below. Moreover, the upward migration of methane and other gases in fault zones may contribute to the triggering of earthqnakes.

Seismologists have long recognized a difficulty in accounting for deep earthquakes. Yet earthquakes have been recorded from depth of as much as 700 kilometers and if the fracture is strong enough to fracture the ground up to the surface, the gas escaping may generate some of the peculiar phenomena that have been reported to accompany many major earthquakes. The phenomena include flames that shoot from the ground, "earthquake lights", fiece bubbling in bodies of water, sulphureous air and visible waves rolling slowly along alluvial ground. Tsunamis (large, earthquake-caused waves at the sea that are often highly destructive) may be an analogous phenomenon. It is usually assumed that they are generated by a sudden displacement of an enormous area of the sea floor over a vertical distance comparable to the height of the wave.

There is as yet no proof that any of the effects we have mentioned are caused by eruption of gas during earthquakes, but at least for the flame and bubbling water phenomena it is difficult to imagine a likely alternative.

Many of the precursory phenomena are detected only by instruments. Included in this category are changes in the velocity of seismic waves through the ground, in the electrical conductivity of the ground, in the tilt and elevation of the surface, in the chemical composition of gases in the soil and the ground water. The time between the onset of a precursor and the earthquake ranges from minutes to years.

Not all precursors of earthquakes can be detected only by instruments. Some are so obvious to the senses that they have been recognized since ancient times. We believe these effects too are caused by an increased flow of gas through the.ground. Among these "microscopic" precursors are dull explosive noises of unknown origin, the strange behaviour of animals, local increases of temperature, bubbling of water in wells and flames from the ground.

Many other lines of investigation can elucidate the degassing processes of the earth. Variations of the methane content of the atmosphere may be observable. Changes of fluid pressure in the ground can be monitored. No one has any firm evidence on the diverse gas regimes more than a few kilometers below the surface or on the quantity or frequency of the various gases emerge.

Our present attempt to formulate a relatively simple hypothesis to account for numerous previously unrelated facts will doubtless turn out to be in places oversimplified or overstated. We hope, however, that it will stimulate further research in this fundamental field of geophysics and geochemistry, leading perhaps to the discovery of large new sources of fuel and in any case to an improvement in the understanding of the earth and its resources.

Ex.1.

Опираясь на материал текста, дайте английские эквиваленты следующим высказываниям:

I.

· Существует большое количество свидетельств, указывающих на то, что…

· Существует широко распространённое мнение о том, что…

· Разнообразные свидетельства дают основание полагать, что…

· Данная гипотеза даёт общую основу для объяснения, в противном случае, довольно загадочных явлений.

· Те доказательства, которые можно наблюдать, представляют собой разрозненную картину, которую непросто объяснить.

· Из такого рода наблюдений невозможно сделать выводы относительно…

· Имеющиеся данные не позволяют сделать однозначные выводы.

· Можно предположить, что…

· По ряду причин мы полагаем, что…

· В центре нашего обсуждения будет…

· Данное представление вступает в противоречие с преобладающей точкой зрения.

· Согласно этой точке зрения…

· Не может быть сомнений в том, что…

II.

· Данные космохимии дают основание предполагать, что…

· Та картина, сторонниками которой мы являемся, имеет двойственную природу.

· Давайте проанализируем некоторые из имеющихся свидетельств.

· Обычно предполагают, что…

· Пока ещё не существует доказательств того, что…

· Многие другие пути исследований могут пролить свет на…

· Никто не имеет каких-либо твёрдых доказательств.

· Данная попытка сформулировать относительно того, что простую гипотезу для объяснения многочисленных, ранее несвязанных между собой фактов, будет несомненно способствовать дальнейшим фундаментальным исследованиям.

 

Ex. 2.