В Focus on structures.• Ultramafic nodules are thought to come up from a great depth. • The origin of kimberlite matrix is more obscure than that of the ultramafic nodules. • The interpretation of kimberlite and nodules it contains would be more secure if their history were less complicated.
Explain and expand. Explain and expand.
• Kimberlite pipes are the ultimate source of diamonds. For the geologists, however, kimberlite pipes supply gems of different kind. II I ■ Give the Russian for: to derive from; to be complicated by; eventful history; to be obscure; to be secure; to be confusing; thereby ■ Give the English for: основные компоненты; химические соединения; другими словами; инородные тела; включения; материнская порода; имеющиеся в наличии источники информации; в конце концов; очевидно; широко; вероятно, легко; повторно kimberlite — silicate — ______________ serpentines — inclusions — diamond — ______________ solid solution — hydration — ultramafic nodules —
The eruptions of the Hawaiian volcano leave pools of molten basalt that can take as long as 25 years to solidify. They provide a natural laboratory for studying the nature of magma from the earth's mantle. Magma—molten rock—from the interior of the earth is responsible for a host of phenomena at the earth's surface. The flow of magma out of the mid-ocean rifts adds to and pushes apart the rigid plates that make up the earth's surface and carry the continents on their backs. All igneous rocks are by definition formed by congealing of magma. If the magma is erupted at the surface as lava, it forms extrusive igneous rocks such as basalt; if it slowly crystallizes below the surface, it forms intrusive igneous rocks such as granite. In spite of the importance of magma, however, there is much that is not known about it. Most studies of the cooling crystallization and other properties of magma have centered on the laboratory analysis of small samples and on theoretical extrapolation from already solidified lava. A different approach is the studying of molten and solidifying lava in situ by examining three lakes of lava left in the wake of eruptions of the volcano Kilauea on the island of Hawaii. The three lakes are filled with basaltic lava. Basalt is the commonest rock formed by the solidification of magma extruded to the surface of the Earth, the Moon and perhaps other bodies in the solar system. Basalt is found on all the continents and covers huge expanses of land. Basaltic lavas, erupting from the mid-ocean rifts to create the floor that underlies the sediment of the ocean basins, have poured forth throughout geologic time from the early Precambrian to the present. Although basalt vary significantly in chemical and mineralogical composition, they have all formed at high temperatures. In principle the high temperature of the molten rock makes it attractive as source of energy, although in practice numerous obstacles stand in the way of tapping its heat. Most of the more than 500 active volcanoes on the earth are entirely or predominantly basaltic, including the active volcanoes that make up the southern two-thirds of the island of Hawaii. The basaltic magma that feeds the eruptions comes from the earth's mantle at depths of at least 50 kilometers below the surface. Geological and geophysical data suggest that magma rising from these depths is stored in an irregularly shaped reservoir. In the formation of a lava lake lava from the reservoir erupts to the surface and flows into a depression. Eventually the natural dikes that channel the lava into the lake collapse, and so the lake is cut from a source of lava and starts to solidify. Like a freezing lake of water, a lava lake solidifies from the top down. The surface of a lava lake is cooled by air and particularly by rain, which falls copiously on Hawaii. Unlike a water lake, however, a lava lake also solidifies from the bottom up. That happens because the rock under a lava lake is cooler than the molten lava. As a result the molten lava is sandwiched between two layers of solidified crust and takes the shape of a lens. The crust at the bottom of the lake is always thinner than the crust at the top because the rock under it has a low thermal conductivity and is not rapidly cooled by rainwater. The molten lens decreases in thickness as the lake solidifies from the top down and from the bottom up. A lava lake, like a lake of water, is more shallowed at the edges than it is at the center, so that the top and bottom crusts fuse at the edges of the lake, separating the molten lens from the rock enclosing the lake basin. As the top and bottom crusts continue to thicken the lens decreases in diameter and thickness until it disappears and the lake becomes a single body of solidified lava.
The ice on a lake of water is quite distinct from the water below it. That is not the case with a lake of basaltic lava, which consists not of one chemical compound but of different minerals that crystallize at different temperatures and rates. The solidified top crust grades slowly downward into the fluid lava through a region of partially molten crust several tens of centimeters thick. Within this region the temperate and the ratio of crystal grains to melt increase smoothly with depth. There is an interface, however, across which the physical properties of the partially molten lava change sharply. Above the interface the lava is solid enough to be drilled. Below the interface the lava is a fluid that yields like taffy when a drill probe is pushed into it. At the interface, whose temperature is 1,070°C, crystal grains and melt are equally abundant. At 980°C the lava is entirely crystallized except for a small fraction in the glassy state: a supercooled liquid in which the silicon oxide molecules have not been organized into crystals. The rate of thickening of both the top and the bottom crust decreases with time because the solidified material is a poor conductor of heat and so acts as an insulator. As molten lava cools, gas in the melt is driven out of solution. The gas either escapes into the atmosphere or remains in the lava as vesicles, or bubbles. The vesicles that were frozen into the crust at shallow depths are chiefly spheres as much as a centimeter in diameter. They were apparently created at high temperatures when the lava became supersaturated with gas on the reduction of the confining pressure above it, just as bubbles appear in a bottle of soda water when the cap is removed. With increasing depth in the lava the confining pressure increases, and so the vesicles become smaller and scarcer. Below six meters in lava lake most of the vesicles are minute angular pores less than a millimeter in diameter. They were apparently created when gas was driven out of solution by crystallization of the cooling lava. The composition of the gases expelled from the lava also changed as the lake cooled: water vapour increases in abundance at the expense of the more rapidly exsolved gases of carbon and sulphur compounds. Although a solidifying lava might be expected to sink because its crystals are denser that the melt, the lava in some lakes became more buoyant as it solidified because it was filled with gas vesicles. Therefore as the lens of molten lava below the surface at the center of the lake solidified it pushed up the surface above it. At the edges of the lake, where there was no solidifying lens because the top bottom crusts had fused, the surface subsided as the cooling lava thermally contracted and became denser; since this lava had already solidified, no bubbles were being formed in it that would make it more buoyant. The entire surface of other lakes, on the other hand, has generally subsided as the lakes have cooled. Since these lakes are deeper, the higher pressures within them have hindered the formation of vesicular lava that would have pushed up the surface. Another phenomenon that alters the cooling crust of the lava lakes is that large cracks developed in the crust as cooling basalt contracts. Such cracks open a minute or so after incandescent lava appears at the surface in the course of an eruption. As the crust continues to cool and thicken, the cracks propagate downward by further fracturing and new cracks open that divide the surface of the crust into polygons. Within a few hours the polygons become deformed as their centers are elevated by vesicular expansion of the solidifying lava under them. Then still more cracks open, the rate of cracking being greatest at times when the crust is being chilled by heavy rains.
I ■ Give the Russian for: to provide a natural laboratory for studying; to be responsible for; a host of phenomena; in situ; in principle; in practice; to feed the eruptions; to be stored; eventually; at last; at least; the least important; on cooling; when cooled ■ Give the English for: добавлять; отталкивать друг от друга; по определению; на поверхности; под поверхностью; несмотря на; иной подход к; подстилать; значительно изменяться; при высокой температуре; стоять на пути (препятствовать); на глубине; несмотря на важность...; подобно; в отличие от; при охлаждении; при затвердевании Ш Focus on structures. • Basaltic lava, erupting from the mud-oceanic rift to create the floor of the ocean basins, have poured fourth... • Rising magma is stored in a reservoir. • On rising magma is stored in a reservoir. ■ The text aims at discussing. a) the origin of magma. b)the composition of basalts. c) the distribution of basaltic volcanoes. d) the process of solidification of molten magma Ф* Х5Г Questions to discuss. • What are the advantages of studying lava lakes in situ? • What conclusions can be drawned from the fact that basalts are found on the moon and other bodies in the solar system? I П Ш Give the Russian for: that is not the case with; to grade slowly downloward; within this region; reduction of the pressure; at the expense of; to be buoyant; within a few hors; to provide a good opportunity; in the course of eruption; on the other hand И Give the English for: Отличаться от; расплав; плавиться; плоскость; над поверхностью; под поверхностью; полностью кристаллизоваться; плохой проводник тепла; пузырьки
• Within this region the temperature and the ratio of crystal grains to meet increase smoothly with depth. • As molten lava cools, gas is driven out of solution. • Although a solidifying lava might be expected to sink..., the lava in some lakes became more buoyant as it solidified... • Since these lakes are deeper, the higher pressures within them have hindered the formation of vesicular lava that would have pushed up the surface. • Eventually the natural dikes that channel the lava into the lake collapse.
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Text 9 THE LAVA LAKES OF KILAUEA
