科技英语（地质英语）.doc

READING MATERIALThe quality of a body of water refers to its temperature and the amount and character of its content of mineral particles, solutes and organic matter (chiefly bacteria), in relation to its intended use. The most common source of pollution of water from wells and springs is sewage, and the infection most commonly communicated by polluted water is typhoid. Drainage from septic tanks, broken sewers, privies, and barnyards contaminates ground water. If the water contaminated with sewage bacteria passes through material with large openings such as very coarse gravel or the cavernous limestone, it can travel for miles without much change. If, on the other hand, it percolates through sand or permeable sandstone, it can become purified within short distances, in some cases less than 30m. The difference lies in the aggregate internal surface area of the material through which it percolates. The large aggregate force of molecular attraction holds the water and promotes its purification by (1) mechanical filtering-out of bacteria (water gets through but most of the bacteria do not), (2) destruction of bacteria by chemical oxidation, and (3) destruction of bacteria by other organisms, which consume and oxidize them. Purification goes on both in the zone of aeration and in the zone of saturation. Because clay particles are much smaller than sand particles, we might suppose that clay, with its much larger internal surface area, would be the ideal medium for purification. But it is not, because as we have seen, it is almost impermeable. Particles of sand are large enough to permit rapid percolation, yet small enough to permit purification within short distances. For this reason treatment plants for purification of municipal water supplies and processing of sewage percolate these fluids through sand.A substantial proportion of domestic sewage passes through septic tanks and then mingles with ground water. It percolates into streams, which it generally reaches in a pure condition. On the other hand the domestic water from many areas, as well as much industrial waste, is dumped unaltered into surface streams. Although purification can be accomplished during stream transport, the distances involved are much greater than those required for the purification of water in the ground, and the amounts of sewage in many rivers are far too great to be dealt with by natural processes. In densely populated industrial countries these facts constitute serious public-health problems.A dramatic illustration of the difference between surface and underground conditions is this. In some communities much-polluted water that has traveled tens of miles through a river is pumped into the ground, where it becomes a part of the local groundwater supply. In one city, percolation through a horizontal distance of 150m removed impurities from the sewage and made the water fit to drink.7 SNOW, ICE, AND GLACIERSGlacier ice primarily accumulates as snow on land where the mean annual air temperature is near freezing and where more snow falls in winter than can melt during the summer. The processes of converting snow to glacier ice include sublimation, melting and refreezing, and plastic deformation. Snow-flakes are well known to be platy, skeletal ice crystals. Fresh snow is full of entrapped air, and may have a bulk specific gravity even lower than 0.1. That is, a volume of loose snow might weigh only one-tenth as much as an equal volume of water.Snow that has survived a summer melting season is called firn. Firn is an intermediate step in the conversion of snow to glacier ice. It is granular and loose unless it has formed a crust. It represents the net positive balance between winter accumulation and summer losses.As successive annual layers accumulate, the deep firn is compacted. The individual ice grains freeze together and the included air is either expelled or becomes enclosed as bubbles in the ice. By definition, when grains of ice are frozen together so that air is prevented from permeating through the mass, firn becomes glacier ice. The bulk specific gravity is usually about 0.8 by this stage of consolidation. The remaining air can be expelled only slowly by shearing and breaking or by recrystallization. Therefore, most glacier ice is a polycrystalline mass of frozen water plus a variable amount of air. Other components are dust and rock fragments that fallen, washed, or been blown onto the ice surface, and rock that has been eroded from beneath the glacier.Ice is not a strong solid. Ice crystals begin to deform measurably under a unidirectional pressure of slightly less than one atmosphere. That is if a specimen of polycrystalline ice is subjected to a unidirectional differential or shearing pressure of about 14 pounds per square inch, it will slowly but permanently deform by internal adjustments in the crystalline grains of ice. In a year, a load of about 14 pounds per square inch will flatten a small column of ice by about 30 percent of its original length, with no melting involved. READING MATERIALIce is a rock, a mass of crystalline grains of the mineral ice. That idea should not be too surprising after all; it is a solid substance that occurs naturally on the Earth. It is hard like most rocks, but its composition makes it much less dense. Like igneous rocks it originates as a frozen fluid; like sediments, it is deposited in layers at the surface of the Earth and can accumulate to great thickness; like metamorphic rocks, it is transformed by recrystallization under pressure. Masses of ice may creep, flow, or slide downhill, and just like other masses, they may be folded and faulted. A large mass of ice that is no land and shows evidence of being in motion or of once having moved is a glacier. The motion of glaciers is the clue to the effective work they do in eroding the surface of the Earth into distinctive sculptural forms and in transporting rock debris and depositing it in various forms. The movement of glaciers is now invested with a new and practical interest for humans; early warning of global climatic changes may be indicated by advances or retreats of glaciers. Glaciers are abundant on todays Earth. It is estimated that there are between 70,000 and 200, 000 glaciers of all kinds and sizes in the world, covering about ten percent of Earths land surface.Much of the richest farmland in America is on glacier deposits. Much of the abundant good water supply of the glaciated terrain is from aquifers of glacial sand and gravel. Sand and gravel deposits are abundant. Lakes, on of our best recreational resources, are another of the gifts of the glacial epoch. But there are liabilities too. Any New England farmer can speak with feeling about all of the rocks in his fields of bouldery till, and geologists are sometimes frustrated by the glacial sediment that covers bedrock and prevents them from mapping geologic formations and mineral resources.8 MINERALSMost of minerals are chemical compounds; that is, they consist of two or more elements in combination. Of course there are exceptions, such as gold, copper, sulphur, and carbon, which may occur as elements by themselves as well as in chemical compounds. Minerals are naturally occurring substances. This statement rules out laboratory creations. Minerals have a reasonably definite chemical composition. Since they are naturally occurring substances, and not laboratory products, only rarely are they chemically pure compounds. For this reason, such properties as color may vary over a range as wide as from black to white, depending on the percentage of elements present for any mineral. Mineral also have certain physical properties, determined by their chemical composition and by the geometric arrangement of the atoms composing them. It is this atomic arrangement that determines the crystal form of a mineral. Other properties include such things as color, hardness, and specific gravity.In summary, then, a mineral may be defined as (1) a naturally occurring substance with (2) a fairly definite chemical composition and (3) characteristic physical properties by which it may be identified. In short, a typical mineral is a crystalline solid and is an inorganic substance. Most are chemical compounds, but a few, such as the diamond, may consist of a single element.Before we discuss the characteristics of individual minerals we should learn of the essential properties which are the chief means of their identification. Physical properties are the things we can see, or feel, or for such minerals as halite (rock salt), taste. True enough, the chemical composition is possibly the most diagnostic property a mineral possesses, but few of us are going to pack along a fully equipped chemical laboratory to be used for mineral identification on a field trip. Since one of the critical differences between minerals and rocks is that minerals are approximately homogeneous substances, and most rocks are not, this means that one piece of quartz will be about as hard as another piece, that it will have the same specific gravity, and if formed in a similar environment, it will have about the same crystal form. READING MATERIALAlthough some thousand mineral species have been described, the majority is comparatively rare, and it is necessary for the beginner to be familiar with only a limited number. These minerals he should be able to identify quickly and with certainty, either as hand-specimens or as they occur in their native habitat as constituents of rocks.The properties on which identification in the field depend include- luster, hardness, crystalline form, type of fracture, cleavage, color, streak, specific gravity (whether “light” or “heavy” when hefted in the hand), mode of occurrence, and associates (this is particularly valuable when dealing with ore-minerals). In addition, a few minerals are sufficiently strong magnetic to be picked out by means of a magnet, one only-namely, magnetite- shows polarity.Minerals are commonly classified according to their chemical compositions, which is not particularly useful to the field geologist, for it presupposes a chemical analysis. The most that the worker in the field can do is to make a few tests with the blowpipe to confirm the conclusions he has come to after examining the physical characters of a specimen.When identifying a mineral in the field, the following procedure may be followed:Note the kind of rock in which the specimen occurs and the associated minerals; this often gives valuable clue to its identity. Galena, for example, is often associated with fluorspar, chromite with serpentine, and wolframite with cassiterite.The general appearance may be very informative. Pyrite, for example, is often found in cubes or octahedra; the faces of the cubes are often marked by fine parallel striae, so disposed that those on any face are at right angles to the striae on contiguous faces. Calcite commonly occurs as rhombohedral or prismatic forms, with a perfect cleavage parallel to the unit rhombohedron.The specimen may feel decidedly heavy. Most of the ores of the metals, the metallic-looking minerals, and the compounds of barium and strontium feel distinctly “weighty” when hefted in the hand.The mineral may next be tested for hardness by means of a piece of quartz.9 COMMON MINERALSOn the basis of physical and chemical properties, some 3000 different minerals have been recognized and, described by mineralogists. However, this great number need not discourage us from attempting to understand the Earths materials. Most of the 3000 minerals are rare and of interest mainly to specialists. Actually, fewer than two dozen are abundant, and a knowledge of only ten mineral types is an adequate basis for a generalized understanding of the bulk of rocks which are most frequently encountered.Feldspar The most abundant mineral type, feldspar, composes over 60% of the rock materials, in the Earths crust. Strictly speaking, the term feldspar refers to a group of closely related minerals having generally similar composition and characteristics. They are alumina-silicates of sodium, potassium, and calcium, which explain why these elements, along with oxygen, silicon, and aluminum, are so abundant in the Earths crust. Potassium feldspar includes the minerals orthoclase and microcline. Plagioclase includes several sodium and calcium bearing feldspars.Quartz A very widespread mineral, quartz is the second-most abundant. Quartz is a specific mineral, the only common one of the silica group. Chemically, silica is SiO2; thus, quartz is a compound composed entirely of the two most abundant chemical elements. In large pure crystals, quartz resembles colorless glass. However, slight impurities may give it a variety of colors, and some minutely crystalline varieties, such as flint, may be opaque and of a waxy luster.Mica Mica is the name of a group of minerals which are readily split into thin flexible sheets. This distinctive property results from a single, perfect, cleavage plane, which is repeated throughout the mineral. The minerals are very complex potassium, alumina-silicates with added oxygen-hydrogen combinations. Two common minerals in this group are dark mica (biotite), which also contains iron and magnesium, and colorless or white mica (muscovite), in which these two elements are absent. The atoms of all these elements are arranged in a complicated manner to produce a loosely bonded sheet-like structure within the crystal. This is responsible for the characteristic cleavage.Amphibole and Pyroxene A number of silicate minerals and mineral groups are referred to as “ferromagnesians.” The name indicates that they contain appreciable iron and magnesium. Common dark micas are, therefore, in this general class, although the other micas are not. Amphibole and pyroxene are two common groups of related, ferromagnesian minerals. As with most iron-bearing minerals, they are mostly dark-colored and of comparatively high specific gravity. Both groups are rather similar in most properties, but can be distinguished in many cases by cleavage, which results from their somewhat different internal crystal structure.Olivine In hardness and luster, olivine resembles quartz, but the color and common occurrence in small grains are helpful for identification. Olivine is a ferromagnesian silicate characterized by a green (olive) color.Internally, olivine consists of separate silicon tetrahedral linked by iron and magnesium atoms. It is common in certain dark-colored igneous rocks and may be an important component of the rocks below the earths crust.Calcite Calcite is the only common rock-forming mineral which lacks silicon. All others are silicon and oxygen compounds in the form of silica or silicates. Calcite, however, is a carbonate, a class of compounds based on carbon-oxygen combinations. The “calc” in the word calcite comes from the Latin word calcis(lime), and indicates the presence of the common element calcium. Calcite has the simple formula CaCO3.At first glance, calcite might be confused with quartz where both are clear, colorless, and “glassy”. However, it is easily distinguished because, unlike quartz, calcite is quite soft, has excellent cleavage, and fizzes when drops of hydrochloric acid touch it. Calcite is a very widespread mineral in certain common rocks and in the shells of many organisms.Clay Clay minerals are important and abundant products of the slow chemical breakdown of rocks. Especially susceptible to this “rotting”, or weathering, are feldspars and ferromagnesian minerals in rocks exposed at the Earths surface. Pure clays, with some exceptions, are white or gray, but they usually contain impurities which stain them to blue, red, or other colors. Chemically, clays are complex aluminum or aluminum-magnesium silicates with added oxygen-hydrogen combinations. Clays, in bulk, have the property of being plastic and moldable when wet. This results because they consist of minute particles, less than 1/10,000 of an inch across, which resemble mica in form and cleavage. Structurally, these particles are made up of sheets of atoms which become plastic when water particles slip between them.READING MATERIALSPicking up pretty stones and showing or wearing them must go far back into human prehistory. The earliest records of practical use of stones, though, are of arrowheads and spear points made of flint (a sedimentary rock) or obsidian (volcanic glass), both of which are hard materials that break with sharp edges. From the practical use of individual stones as tools, weapons, and decorations, it was a big step to the wholesale mining or quarrying of rocks and minerals for building, for making clay for pottery, and then for the ores that contain metals. Today mining is done so expertly and intensively that geologists have come to concern themselves with the exhaustion of the worlds valuable mineral resources.Because of the many uses of rocks and minerals, we have a practical curiosity about where they are found and how they were formed: we want to be able to find more. Yet there are other reasons, too, for rocks, as we have seen, are the only records of how the Earth evolved, and they are an important guide to how the Earth works today. For this reason, mineralogy, the study of minerals, and petrology, the study of rocks, are important subfields of geology. Finally, there is the intrinsic interest in the extraordinary range of the mineral kingdom, with its immense variety of color, form, and texture. Minerals and rocks, after all, give us the marble and alabaster of sculpture, the jade of Eastern carvings, and the pigments used by Rembrandt. 10 THREE GREAT CLASSES OF ROCKSThe walls of many deep roadcuts show that surface soil and the unconsolidated mantle rock beneath it form only a thin veneer. At depths of a few feet, we pass through th