地质专业英语电子版
The Earth
From classical times it has been known that the earth is roughly spherical in shape. Actually the planet is shaped more like a slightly flattened ball whose polar radius is about 21km shorter than its equatorial radius. The average radius is 6371km. The earth’s specific gravity is 5.5. It is 5.5 times as heavy as an equal volume of water. The specific gravity is greater than that of any other planet in the solar system, but not appreciably different from that of Mercury, Venus and Mars. Because the average specific gravity of surface rocks is only about 2.7, the material existing deep within the earth must have a specific gravity well in excess of the 5.5 average. Very likely, the material at the earth’s center has a specific gravity as high as about 15.
The splendid photographs of the earth taken from space by Apollo astronauts remind us that our planet is more than a rocky globe orbiting the sun. The patterns of white clouds above the azure blue color tell us of the presence of an atmosphere and hydrosphere. Here and there one can even discern patches of tan that indicates the existence of continents. Greenish hues provide evidence of the planet’s most remarkable feature: there is life on earth.
THE A TMOSPHERE
We live beneath a thin but vital envelope of gases called atmosphere. We refer to these gases as air. “Pure air”is composed mainly of nitrogen (78.03%) and oxygen 920.99%). The remaining 0.98% of air is made of argon, carbon dioxide and minute quantities of other gases. One of these “other”components found mostly in the upper atmosphere is a form of oxygen called ozone. Ozone absorbs much of the sun’s lethal ultraviolet radiation, and is thus of critical importance organisms on the surface of the earth. Air also contains from 0.1 percent to 5.0 percent of water vapor. However, because this moisture content is so variable, it is not usually included in lists of atmospheric components.
Every day, the atmosphere receives radiation from the sun. This solar radiation provides the energy that heats the atmosphere and drives the winds.
Distribution of solar radiation is one of the most important factors in determining the various kinds of climate we experience on the earth.
THE HYDROSPHERE（水圈）
The discontinuous envelope of water that covers 71 percent of earth’s surface is called hydrosphere. It includes the ocean as well as water vapor. The water contained in streams and lakes, water frozen in glaciers, and water that occurs underground in the pores and cavities of rocks. If surface irregularities such as continents and deep oceanic basins and trenches were smoothed out, water would completely cover the earth to a depth of more than two kilometers.
Water is an exceedingly important geologic agent. Glacier composed of water in its solid form alter the shape of the land by scouring, transporting and depositing rock debris. Because water has the property of dissolving many natural compounds, it contributes significantly to the decomposition of rocks and, therefore, to the development of soils on which we depend for food. Water moving relentlessly down hill as sheetwash, in rills, and in streams loosens and carries away the particles of rock to lower elevations where they are deposited as layers of sediment. Clearly, the process of sculpturing our landscapes is primarily dependent upon water.
By far the greatest part of the hydrosphere is contained within the ocean basins. These basins are of enormous interest to geologists who have discovered that they are not permanent and immobile as once believed, but rather are dynamic and ever changing. There is ample evidence
that the sea floors move, and that these movements have a direct relation to the formation of mountains, chains of volcanoes, deep sea trenches and mid-ocean ridges. In the ocean are collected the layers of sediment from which geologists decipher earth history. Here also one finds mineral resources and clues to the location of ore deposits elsewhere on the planet. The ocean provides part of our food supply and has a pervasive influence on the climate we experience.
Lesson Two Common Minerals
（5学时）
] 石英)
QUARTZ([
] The mineral quartz is one of the most familiar and important of all the silicate ([ 硅酸盐) minerals. It is common in many different families of rocks. As mentioned earlier, quartz
] 临界的，根本的，最终的) in cross-linkage of silica represents the ultimate ([
tetrahedral ([
] 四面体的); it therefore will not break along smooth planes. In quartz, the tetrahedral are joined only at the corners and in a relatively open arrangement. It is thus not a dense mineral, but it is quite hard because of the strong bonding in its framework structure (架状结构). When quartz crystals ([
] 晶体)are permitted to
] 六边形的)prisms grow in an open cavity they may develop hexagonal ([
] 棱柱)topped by pyramids (棱锥) that are prized by crystal collectors. More ([
frequently, the crystal faces can not be discerned because the orderly addition of atoms had been interrupted by contact with other growing crystals.
] 碧Such minerals as chert ([
] 燧石,黑硅石), flint ([flint] 燧石), jasper ([
] 玛瑙)are varieties of a form of quartz called chalcedony 玉)and agate ([
] 玉髓). Chalcedony is composed of extremely small fibrous ([
([
] 含纤维的,质状的)crystals of quartz. The crystals are so tiny that their study often requires the use of an electron microscope. Spaces between the crystals are usually occupied
by water molecules ([
] 分子). Among the varieties of chalcedony, chert is exceptionally abundant in many sedimentary rock units. It is a dense, hard ,usually white mineral
or rock. Flint is the popular name for the dark gray or black variety of chalcedony much used by stone-age humans for making tools. Jasper is recognized by its opaque ([
] 不透明的)appearance and red or yellow color derived from ironoxide (氧化铁)impurities
杂质). The term agate is used for chalcedony that exhibits bands (impurity[ ]
(夹层,带)of differing color or texture (质地,纹理). There are many other varieties of quartz minerals than those briefly mentioned here.
] 长石)
THE FELDSPARS ([
Feldspars are the most abundant constituents of rocks , composing about 60 percent of the total weight of the earth’s crust. There are two major families of feldspars: orthoclase
] 钾)feldspar group which are ([
] 正长石)or potassium ([
] 铝硅酸盐，硅铝酸盐), the potassium aluminosilicates ([
] 斜长岩)group, which are the aluminosilicates and the plagioclase ([
]钠)and calcium ([
] 钙). Members of the
of sodium ([
plagioclase group exhibit a wide range in composition—from a calcium-rich end member called
] 钙长石)(CaAl2Si2O8) to a sodium- rich end member called albite anorthite ([
] 钠长石) (NaAlSi3O8).Between these two extremes, plagioclase minerals ([
containing both sodium and calcium occur. The substitution of sodium for calcium, however, is not random but rather is governed by the temperature and composition of the parent mineral. Thus,
] 熔铸的)rock it is possible
by examining the feldspar content of a once molten ([
to infer the physical and chemical conditions under which it originated. Feldspars are nearly as
] 浅蓝色的)gray. hard as quartz and range in color from white or pink to bluish ([
Silica tetrahedra in the feldspars are joined in a strong three-dimensional lattice ([ ]格子)that is characterized by planes of weaker bonding in two directions at (or nearly at) right angles (直角) to each other. Because of this, the feldspars have good cleavage (break along smooth
] 矩形的,成直角的) planes) in two directions. The resulting rectangular ([
cleavage ([
] 解理)surfaces and a hardness of 6 are properties useful in the identification of feldspars.
The plagioclase feldspars provide an example of the manner in which ions can be interchanged
in a mineral group. A chemical analysis of specimens ([
] 标本,样品)of plagioclase taken from several different rocks would very probably reveal that the proportions of
] 铝), and silicon (the principal cations calcium, sodium, aluminum ([
] 阳离子)in plagioclase) would differ among the specimens. This variability ([
] 可变性)occurs because some ions resemble each other in size and ([
] 可electrical properties (电性质) and are thus interchangeable ([ 互换的)in a given crystal. Calcium and sodium ions are large and nearly identical
] 同一的) in size. Both aluminum and silicon are small ions and not ([
greatly different in size. Thus, calcium ions might substitute for (代替)sodium ions freely if size alone were the only requirement. However, the electrical neutrality (电中性)of the crystal must also be maintained. The electrical charge (电荷)of the calcium ion is +2, whereas that of the
] 剩sodium ion is –1. To counteract ([
] 中和)the surplus ([
余的)positive charge, an aluminum ion (+3) may substitute for a silicon ion (+1) to maintain electrical neutrality. Thus, Ca2++Al3+ can interchange with Na+ and Si+. This process of
] 相互交换) is called solid solution (固体溶液). interchange ([
] 云母)GROUP
THE MICA ([
As noted earlier, mica is a silicate mineral having sheet structure (层状结构), and is easily
] 显著的)cleavage in one recognized by its perfect and conspicuous ([
] 定向的)plane. The two chief varieties are the colorless or directional ([
] 白云母)mica, which is a hydrous palecolored muscovite ([
] 含水的)potassium aluminum silicate (KAl2 (AlSi2O3 (OH)2) and the dark ([
] 黑云母)mica, which also contains iron and magnesium colored biotite ([
] 镁)(K(Mg, Fe)3AlSi3O2(OH)2). In muscovite mica, two sheets of ([
tetrahedra are strongly held together along their surface inner surfaces by positively charged ions aluminum. These sandwich like paired sheets are in turn weakly joined to others by positively charged ions of potassium. When muscovite is cleaved ([kli:v] 劈开)into paper-thin layers, the separation occurs primarily along the weaker plane where the potassium ions are located. In biotite, magnesium and iron ions hold the inner surfaces of the sheets together, but once again potassium ions serve to weakly join each basic set of paired sheets to its neighbor.
辨认,鉴定)of large specimens of mica is Identification ([ ]
rarely a problem because of its planar ([
] 平坦的)per feet cleavage and the way
] 薄片)snap back (迅速跳回)into place when they are bent and cleavage flakes ([
] 火成的)and suddenly released. The micas are common constituents of igneous ([
] 变质的)rocks, where they can be recognized by their metamorphic ([
shiny surfaces and the ease with which they can be plucked loose with a pin or pen knife. Before the manufacture of glass, one of the chief uses of muscovite mica was as window panes ([ ]
] 挖出,苦心找出)in Muscovy 窗格玻璃边,面). This clear mica was quarried (quarry[
] 俄国)(commonly name for Russia), and thus came to be known as “Muscovy ([
glass” and eventually muscovite. Today, mica is used in the manufacture of electrical insulators
] 石膏), ([
] 绝缘体)and as a filler (填充物) in plaster ([ roofing products and rubber.
角闪石)
HORNBLENDE ([ ]
] 玻璃质的)black or very dark green mineral. It is the Hornblende is a vitreous ([
] 闪most common member of a larger family of minerals called amphiboles ([ 石), which have generally similar properties. As can be seen from its chemical formula, NaCa2(Mg,Fe,Al)2(Si,Al)2O2(OH), Hornblende contains a relatively large number of elements. Because of the presence of iron and magnesium, hornblende (along with biotite, augite and olivine ] 橄榄石,黄绿) is designated a ferromagnesian [
] 铁镁矿物,含有铁与镁的)mineral. Crystals of hornblende tend ([
to be long and narrow. Two good cleavages are developed parallel to the long axis and intersect ([
] 横断,交叉)each other at angles of 56°and 121°. The cleavage is a reflection of the location of planes of weaker bonds that exist between the double-chain units of
] 原子的)lattice.
silica tetrahedral in the atomic ([
AUGITE ([
] 普通辉石)
Just as hornblende is only one member of a family of minerals called amphiboles, augite is an
important member of the pyroxene ([
] 辉石)family in which many other
] 种类)also occur. Its chemical formula Ca(Mg,Fe,Al)(SiAl)2O3 mineral species ([
indicates that it too is a ferromagnesian mineral and thus dark colored. An augite crystal is
] 短柱状的)in shape with good cleavages developed along typically rather stumpy ([
two planed that are nearly at right angles (87°and 93°). Thus, the cross section of a crystal
] 菱形的) as in hornblende). Unlike appears nearly square (rather than rhombic ([
hornblende which has a double chain silicate structure, augite is constructed of single chains, and
] 碎片,片
this accounts for its having differently shaped cleavage fragments ([
断).
] 橄榄石,黄绿)
OLIVINE ([
] 铁镁矿物)mineral, has been Olivine, another ferromagnesian ([
mentioned earlier as having isolated silicon-oxygen tetrahedral boned together by iron and /or
] 镁)ions. Its formula (Fe,Mg)2SiO4 indicates that it is a solid magnesium ([
solution mineral, containing variable proportions of iron and magnesium. The substitution ([
] 替代)of these ions for each other is facilitated (facilitate
] 助长,促进)by their having similar ionic radii ([
] 半径)and [
two electrons (电子)in their outer electron shell (电子壳层). The ions in olivine are so strongly held by ionic bonding (结合,粘合)that the mineral has a hardness of 6.5. As you might guess from
its name, this glassy-looking mineral often has a green color. Frequently, it occurs as masses of
] 熔岩,火山岩). It is small sugary grains or as tiny vitreous crystals in black lavas ([
also an important constituent of stony meteorites. If large unblemished crystals of magnesium-rich olivine are found, they may be cut and polished into attractive gemstones ([
]
] 贵橄榄石).
经雕琢的宝石)called peridot ([
Lesson Three Sedimentary Rocks
（5学时）
Once weathering products (风化产物) have been formed from pre-existing rocks (原岩，先成岩). The next stage in the sequence of events leading to (导致，产生) sedimentary rocks is the removal
] 剥蚀作and transport of those products. Many denudational ([
] 媒介), including running water, and moving ice, and
用的)agencies (angency[
] 帮助)in this removal. The wind is an effective agent in picking up and wind assist ([
] 冰川blowing away the smaller and lighter particles. Glacial ([
] 巨大的，极大的) ice can move very large pieces of rock and carry an immense ([
的)load (载重，负荷) of coarse sediment (粗粒沉积物). Streams are also exceptionally ] 格外地，异常地)effective in carrying not only solid particles of sediment ([
but invisible dissolved salts as well. Ultimately ([
] 最后，终于), sediment-laden (携带大量泥沙的，含沙量大的)streams flow into lakes or the sea and their load
] 粉砂质的)
of sediment is deposited. It may form sandy beaches (沙滩), silty ([
flood-plains (泛滥平原，漫滩，洪积平原), and sometimes muddy boggy ([
] 沼泽
] 三角洲).
] 河口湾)and deltas ([
的)areas of estuaries (estuary[
] 无论何The solid particles carried by wind or water will be deposited whenever ([
] 不足的) energy to carry them further. For 时)there is insufficient ([
]
] 速度，速率)of dust-laden wind abates ([ example, if the velocity ([
减少), there will be insufficient energy to carry particles of a given size ,and those particles will be
] 同样地，相似地), if a stream’s velocity is checked (受到dropped. Similarly ([
阻止，减弱), as when entering a standing (静态的) body of water (水体，储水池), the stream also loses energy and is unable to continue to carry the material formerly carried at the higher velocity.
A reduction in a stream’s velocity does not, of course, affect the dissolved materials as it does
] 悬挂，悬浮) solid particles. Material carried in solution suspended ([
([
] 溶液)is deposited by a process called precipitation
] 沉淀作用), in which dissolved material is changed to a solid and ([
separated from the liquid in which it was formerly dissolved. For example, calcium ([
]
] 碳酸盐), the principal ([ ] 钙) carbonate ([
] 分布广泛的，普遍的) sedimentary
主要的) substance in the widespread ([
rock known as limestone ([
] 石灰岩), may be precipitated from water that contains calcium in solution as indicated below：
Ca2+ + 2HCO3 CaCO3 + H2O + CO2
(dissolved (dissolved (calcium (water) (carbon
calcium bicarbonate([
]) carbonate) dioxide)
ions) ions) 碳酸氢盐
] 得The bicarbonate ions that participate in the above reaction can be derived ([
到)from the ionization ([
] 离子化，电离)of carbonic ] 碳的，含碳的)acid ([
] 酸). As indicated by the arrows（箭头）, the ([
reaction will proceed toward the right and calcium carbonate will be precipitated. If, however, carbon dioxide（二氧化碳）is added to sea water, then the amount of carbonic acid in the water would build, and the reaction would proceed to the left. This would result in (导致) a chemical
] 对...有益的，对...有帮助的) to (有益于) environment not conducive ([
calcium carbonate precipitation ([
] 沉淀(作用)), and one in which existing calcium carbonate might begin to dissolve. As is evident here, the precipitation of calcium
] 精密的，精细
] 复杂的)and delicate ([
carbonate is a complex ([
] 解的) process in nature. It is influenced by organisms that utilize or liberate ([
] 酸度) or alkalinity 放，释放)carbon dioxide, by processes that alter the acidity ([
] 有机
] 碱度)of the water, by the presence of organic ([ ([
] 磷),
] 硫), phosphorus ([
的) compounds, and by ions of sulfur ([
] 镁) which may be present.
and magnesium ([
Many changes take place in sediment after it has been deposited. Mineral grains (颗粒) may be dissolved away. Some may grow by additions of new mineral matter (矿物质), and the shapes of particles may be distorted ([
]
] 使...变形) by compaction ([
] 使...转变)
压实(作用)). The result of some of these changes is to convert ([
sediment into sedimentary rock (沉积岩). The conversion ([
] 转变，转
] 岩化). Cementation 换)process is called lithification ([
] 胶结(作用)，粘结), compaction and crystallization ([
([
] 结晶(作用)) are the principal means by which unconsolidated
] (使) ([
] 未固结的) sediment is lithified (lithify[
岩化).
Cementation involves the precipitation of minerals in the pore spaces between larger particles of sediment. The precipitated mineral, which most frequently is either calcium carbonate (CaCO3) or
] 胶结物). Cement ] 二氧化硅) (SiO2), is called the cement ([
silica ([
] 基质)
is added to a sediment after deposition. It differs from a rock’s matrix ([
] 碎屑状的) particle (often clay) that are deposited at which consists of clastic ([
the same time as the larger grains and help to hold the grains together.
The reduction in pore spaces in a rock as a result of the pressure of overlying (上覆的) rocks or pressures of earth movements is termed compaction. During compaction, individual grains are
] 排出) of water pressed tightly against one another, causing the expulsion ([
and rearrangement of particles. The result is often the conversion of loose sediment into hard
] 硬化的)rock. Compaction is greatest and most important indurated ([
as a lithification process in finer-grained sediments like clay and mud.
] 最初的，起始Lithification by crystallization may begin with an initial ([
] 沉淀物) in which the developing crystals grow 的)chemical precipitate ([
together to form a crystalline solid (结晶固体). The process of crystallization, however, may also result in a changing in the form of grains that have already been deposited. For example, quartz may be precipitated onto rounded quartz grains to form a strong interlocking
] 联锁的，镶嵌的)mosaic ([
] 镶嵌) of crystals. Clay may ([
]
] 缠结的，无光泽的，暗淡的)aggregate ([ be converted to a matted ([
] 骨骼的，骸骨的)
集合体)of tiny mica crystals or calcium carbonate skeletal ([
debris ([
] 岩屑，碎石) may be recognized into hard crystalline calcite ] 方解石). The migration ([
] 迁移，转移)of watery ([
solutions through sediment favors crystallization as does deep burial and consequent ] 作为结果的，随之发生的) increases in temperature and pressure.
([
Sedimentary rocks are identified and named according to their composition
] 结构). In regard to (关于) ([
] 组成，成分) and texture ([
]成分) of composition, the three most abundant mineral components ([
] 蒸发sedimentary rocks are clay minerals, quartz and calcite. Evaporite ([
岩)minerals and dolomite ([
] 白云石), although less abundant, nevertheless
] 然而，不过)form an appreciable ([
] 可估计的) ([
portion of many sedimentary sequences. Sedimentary rocks nearly always also contain variable
] 赤铁矿). amounts of limonite ([
] 褐铁矿) and hematite ([
] 概括，广义性) one can make about the
A generalization ([
mineral composition of sedimentary rocks is that most are mixtures of two or more components in
] 掌握，控制，支配，统治). Thus, which one mineral may predominate ([
sandstones composed mostly of quartz grains nearly always contain some clay or calcite, and limestones ([
] 石灰岩) made mostly of calcite neatly always are contaminated
]污染)with clay and quartz grains.
(contaminate[
Texture refers to the size and shape of individual mineral grains and to their arrangement in the rock. A rock that has a clastic texture ( from the Greek klastos, broken) is composed of particles of
] 砂砾，砾石), or fragments of parent clay, silt ([ ] 粉砂), sand, and gravel ([
] 化石) that have been moved individually from their place of origin. In rock or fossils ([
] 使与...对照) to (和...形成对比，对照)these clastic rocks, non-clastic contrast ([
rocks form by chemical or biochemical (生物化学的) precipitation within a sedimentary basin. Most non-clastic rocks are crystalline and include certain limestones and evaporates. Sedimentary rocks made of the remains of plants and animals are categorized as biogenic
] 源于生物的，生物成因的). Coal, for example, can be considered as ([
sedimentary rock derived from the accumulation ([
] 积聚，堆积物)of plant remains. Limestones composed predominantly of the skeletal remains of invertebrate
] 无脊椎的) animals would also be considered biogenic.
([
] 离Although it is convenient to separate sedimentary rocks into such discrete ([ 散的，不连续的)categories as clastic, crystalline and biogenic, it becomes quickly apparent ] 显然的) to anyone who enjoys collecting rocks that there are many gradational ([
([
] 有等级的)types, and some rocks can be placed in either of two
] 有效性，合法性，正确性). For example, many categories with equal validity ([
limestones are distinctive ([
] 与众不同的，有特色的)crystalline, but a limestone composed of fragments of a pre-existing limestone could be termed as clastic limestone.
Lesson Four Movement and Geological Structures
（8学时）
The eighteenth-and nineteenth-century scientists who laid the foundations of modern geology concluded that most sedimentary rocks were originally deposited as soft horizontal layers at the bottom of the sea and hardened over time. But they were puzzled that many hardened rocks were
tilted, bent, or fractured. They wondered, what forces could have deformed these hard rocks in this way? Can we reconstruct the history of the rocks from the patterns of deformation found in the field? The geologists of today would add, how do rocks of all types become deformed, and how does the deformation relate to plate tectonics (Press and Siever, 2001)？These questions are answered in the following.
1 ACTING FORCE AND DEFORMA TION OF ROCKS 作用力与岩石变形
All rocks in the crust of the Earth have been deformed, to some extent, by tectonics. We call the forces acting on rocks stresses（应力）.
There are two kinds of acting forces-static forces （静压力）and directed forces （定向压力）. Static forces or confining stresses （围压）make rocks contract their volumes and directed forces or stresses make the rocks change their shapes （Figure 1）.
Directed forces include three types：compression, tension and shear. Compressive forces squeeze the rocks and shorten the rock body; tensional forces stretch a body and tend to pull it apart; and shearing forces push two side of a body in opposite directions （Figure 1）.
Geological structures（地质构造）are the results of rock deformation.
Occurrence（产状）is the orientation of a structural plane or a structural line. It can be described by strike, dip and dip angle.
Strike（走向）is the direction or trend that a structural surface (such as bedding or fault plane) takes as it intersects with a horizontal surface. The direction is expressed with two azimuth values in the opposite directions.
Figure1 Stresses and rock deformation
(Adopted from http: //www.earthsci. org)
Dip direction（倾向）is the direction which points to the lower side of a structural plane and is perpendicular to the strike of the surface. The dip is measured at right angles to the strike in a horizontal surface, pointing to the lower side of the inclined plane. If a dip direction is measured, the strike of the surface can be calculated because strike is perpendicular to dip direction. However, if the strike is measured, the dip is still not determined because dip is a radial line.
Dip angle（倾角，dip）is the angle that a structural surface makes with the horizontal , measured perpendicular to the strike of the structure ( foreign geologists often use the term dip as dip angle ) . There are true dip and apparent dip, among all the angles the true dip is the largest one (Figure 2, right：δ). Strike, dip direction and dip angle are the three key elements of occurrences (产状三要素) of geological structures.
Figure 2 A sketch diagram showing true dip and apparent tip.
( Left：after Harwoods http：//https://www.360docs.net/doc/1f108805.html,/academics/science/harwoodr)
OS-strike; SS’-true dip; δ-true dip angle; ε0-apparent dip angle
2 FOLD AND FRACTURE 褶皱与断裂
There are mainly two kinds of deformation of rocks-fold and fracture, the most common forms of deformation in the sedimentary, metamorphic, and igneous rocks that make up Earth’s crust. Folds are the result of plastic deformation, while fractures are the result of brittle deformation.
2.1 Fold 褶皱
Fold (褶皱) is a bent or warped stratum or sequence of strata, which was originally horizontal, or nearly so, and was subsequently deformed. Folds in rocks are like folds in clothing. Just as cloth pushed together from opposite sides bunches up in folds, layers of rock slowly compressed by forces in the crust are pushed into folds. The geometry of a fold includes core(核) , limb (翼),crest (弧尖) , hinge (枢纽) or fold axis (褶皱轴) , axial plane (轴面) , axial trace (轴迹) ,fold length, width and height etc.(Figure 3)
Figure 3 Geometry of a fold.
( Modified after http：//www. earthsci. org/teacher/basicgeol)
The core is in central part of a fold in which the strata are folded more severely than that of the limbs. The crest is a point in a cross section in which the single stratum of a fold has its largest curvature. All the crests in the same stratum form a hinge or fold axis, and all the hinges of the whole beds in a fold form an axial plane. An axial plane is an imaginary surface that goes through the core of a fold and divides the fold as symmetrically as possible (Figure 3). The axial trace is the intersection line of axial plane with the ground surface.
The basic types of folds are anticline (背斜) and syncline (向斜). Folds which have both limbs dipping away from the fold axis are called anticlines ( Figure 4, left) ; folds which have both limbs dipping towards the fold axis are called synclines (Figure 4, right). When an anticline is uplifted and eroded, older rocks are exposed near the fold axis and younger rocks are exposed away from
the axis. When a syncline is uplifted and eroded, younger rocks are exposed near the fold axis and older rocks are exposed away from the axis.
Figure4 Basic types of folds: anticline (left) and syncline (right).
(Adopted from http：//www. earthsci. org/teacher/basicgeol)
Each type of fold can be further classified by the relationship of the axial plane to the limbs. For example, a symmetrical fold (对称褶皱) or vertical fold (直立褶皱) has a vertical axial plane and the two limbs form a mirror image of each other without being overturned in any side(Figure5) Asymmetrical fold (非对称褶皱)or inclined fold (倾斜褶皱) has an inclined axis with both limbs dipping steeper than the other( Figure5) . Overturned fold (倒转褶皱) also has an inclined axis with both limbs dipping to the same direction (i.e. one limb is normal and the other is overturned).
A recumbent fold (平卧褶皱) has an almost horizontal axis with both limbs being nearly horizontal too obviously, in a recumbent fold, one limb is overturned and the other is normal. Folds can also be classified according to the shapes of folds in cross section, such as fan fold (扇状褶皱，Figure6a) ，box fold (箱状褶皱, Figure6b), chevron fold (尖棱褶皱, Figure6c) and monocline(单斜,Figure 6d) etc.
Folds can also be classified into non-plunging fold and plunging fold according to the orientation of hinges. A fold with horizontal fold axis is called non-plunging （非倾伏褶皱）or upright fold(平轴褶皱), whereas a fold with tilted fold axis is called plunging ford（倾伏褶皱）and its plunge or pitch (倾伏角) is measured as the angle between the fold axis and a horizontal line(Figure 7). Plunging direction (倾伏向) is also important for analyzing the deformation mechanism.
According to their relative length and width, folds can also be classified into linear fold (线状褶皱), in which the ratio of length to width (L/W) is greater than 10, brachy-fold (短轴褶皱),in which the ratio is between 3 and 10, and dome (穹) or basin (盆), in which the ratio is less than 3 ( Figure 8).
Regional fold belts contain large anticlines and synclines, kilometers across, which are marked by the presence of reasonably systematically spaced smaller anticlines and synclines. The flank of an anticlinorium（复背斜）or synclinorium(复向斜) is typically marked by a set of approximately equal-sized second-order anticlines and synclines ( Figure 9, left). These in turn may contain sets of third-order folds, and so it goes. Apart from these very large-scale folds, there are some small-scale folds, centimeters across, often found in strongly deformed metamorphic rocks (Figure l0).The smaller folds are called parasitic folds (寄生褶皱, Figure 9, right).
Figure5 Sketch profiles showing types of folds according
to relationships of fold axis to limbs.
(Modified after https://www.360docs.net/doc/1f108805.html,/course-dev/explogeo/classl0/notes10.html)
Figure6 Sketch diagrams showing fan fold, box fold, chevron fold and monocline.
There are still more types of folds according to different criteria. However, the types mentioned above are the most common ones. More and detailed description of fold types can be seen in a classic textbook , Structural geology of rocks and, regions ( Davis and Reynolds, 1996).
Figure7 Sketch diagrams showing non-plunging fold (left) and plunging fold (right). (Adopted from http: //www.earthsci. org)
Figure 8 Sketch diagrams showing structural basin (left) and dome (right).
A-the oldest formation; E-the youngest formation
Figure9 Schematic rendering of an anticlinorium and synclinorium
(Modified after Davis and Reynolds, 1996)
Figure10 A photo showing small scale folds in a Precambrian metamorphic rock, Yixian County, west of Liaoning Province.
(Photo by Hongbo Lu, 2005)
2.2 The Recognition of Folds in the Field 褶皱的野外识别
Structure controlled landforms. The folds and faults produced by rock deformation often control the development of landforms.
In young mountains, during the early stages of folding and uplifting, the anticlines form ridges and the synclines form valleys (Figure 11, left). However, as tectonic activity moderates and erosion bites deeper into the structures, the anticlines may form valleys and the synclines form ridges or hills. This happens where the rocks (typically sedimentary rocks such as limestones, sandstone, and shales) exert strong control on topography by their variable resistance to erosion. If the rocks beneath an anticline are mechanically weak, as shales are, the core of the anticline may be eroded into anticlinal valleys, the syncline between the two anticlines would be a synclinal ridge or synclinal hill(Figure 11, right). This landform, due to the geologic structural control, is called relief inversion(地形倒置，地貌倒置) .
Anticlinal valley (背斜谷) is a valley which follows the axis of a breached anticline.
Synclinal hill (向斜山) is a mountain in which the geologic structure is that of a syncline. Cuestas (单面山) are asymmetrical ridges in a tilted and eroded series of beds of alternating weak and strong resistance to erosion. One side of a cuesta has a long, gentle slope determined by the dip of the erosion-resistant bed. The other side is a steep cliff formed at the edge of the resistant bed where it is undercut by erosion of a weaker bed beneath (Figure 12, left). Much more steeply dipping or vertical beds of hard strata erode more slowly to form hogbacks (猪背岭), which are steep, narrow, more or less symmetrical ridges ( Figure l2 , right) .
Figure11 Sketch diagrams showing the formation of synclinal ridge and anticlinal valley. In early stages, ridges are formed by anticlines (left); in later stages, the anticlines may be breached and ridges may be held up by caps of resistant rocks as erosion forms valleys in less resistant rocks( right).
(After Press and Siever, 2001)
Figure12 A sketch showing a cuesta(left) and a photo showing hogbacks( right).
(After Press and Siever, 2001)
Observations in the field seldom provide geologists with complete information. Either bedrock is obscured by overlying soils or erosion has removed much of the evidence of former structures. So geologists search for clues that they can use to work out the relation of one bed to another. For example, in the filed or on a map, an eroded anticline would be recognized by a strip of older rock layers forming a core bordered on both sides by younger rock layers dipping away. An eroded syncline would show as a core of younger rocks bordered on both sides by older rocks dipping toward the core. The followings are two principles used by geologists.
Symmetrically repeated strata(地层的对称重复性). If the core is older than the limbs, the fold is an anticline (Figure 8, right; Figure 13); and if the limbs are older than the core, the fold is a syncline (Figure8, left). In Chinese:内新外老—向斜；内老外新—背斜.
Figure13 A sketch diagram showing a plunging anticline.
For a plunging fold, the hinge is always plunging to the younger end of the fold. If the fold is a syncline, the hinge plunges to the open side (younger side); and if the fold is a anticline, the hinge plunges to the close end (still younger end, Figure 13). (倾伏褶皱的枢纽总是向相对新的地层一端倾伏)
Using the principle provided above, geologists typically work from available surface outcrops of rock formation to reconstruct subsurface structures. In fact, folds are typically found in elongated groups. A strip of country in which the rock layers are folded-that is, a fold belt (褶皱带)-suggests that the region was compressed at one time by horizontal tectonic forces.
2.3 Fracture 断裂
We have seen that the way in which rocks deform depends on the kinds of forces to which they are subjected and the conditions that prevail. Some layers crumple into folds, and some fracture. Fractures include faults ( 断层) and joints (节理) . A joint is a crack along which there has been no appreciable movement. A fault is a fracture with relative movement of the rocks on both sides of it, parallel to the fracture. Just like folds, joints and faults tell us something about the forces that
a region has experienced in the past.
2.3.1 Faults 断层
Folds usually signify that compressive forces were at work, whereas faults can be caused by all three types of forces: compressive, tensional and shearing. These forces are particularly intense near plate boundaries. Faults are common features of mountain belts, which are associated with plate collisions, and of rift valley, where plates are being pulled apart. Crustal forces also can be strong within plates and cause faulting in rocks far from plate boundaries.
Geologists define faults by the direction of relative movement, or slip, at the fracture. The surface along which the formation fractures and slips is the fault plane or fault surface (断层面). The two sides separated by the fault plane are called, respectively, hanging wall (above the fault plane, 上盘) and footwall (below the fault plane, 下盘, Figure 14).
Figure 14 A diagram showing the basic geometry of a fault.
The faults can be classified into three basic types：normal faults (正断层), reverse faults(逆断层) and strike-slip faults (走滑断层,平移断层). Normal faults and reverse faults are also called dip-slip faults (倾滑断层). A movement along the strike and simultaneously up or down the dip is described as a oblique-slip fault. In fact, there are a lot of oblique-slip faults (斜滑断层), suggesting the combinations of strike-slip and dip-slip movements.
In a normal fault, the hanging wall moved downward and the footwall moved upward along the fault plane. A normal fault is generally induced by horizontal extension in the crust (Figure15, left). The fault plane on a normal fault is generally very steep(>30°, most of them>60°). However, a fault plane with a very low angle(<30°, some geologists prefer <15°) on a normal fault may be present, and in this case the fault is called detachment fault (拆离断层,Figure 16).
Figure15 Block diagrams showing normal fault (left) and reverse fault (right).
(Adopted from http：//www. earthsci. org)
A reverse fault is one in which the rocks above the fault plane move upward in relation to the rock below, causing a shortening of the section (Figure 15, right). That is to say, reverse faulting results from compression.
Figure 16 Block diagram showing a detachment fault.
(After Harwoods, 2005)
A reverse fault at which the dip of the fault plane is small (usually <30°, but some geologists prefer <15°), so that the hanging wall is pushed mainly horizontally, is called a thrust fault (逆冲断层, Figure 17, left). Furthermore, a large-scale thrust fault, in which the thrust sheet(hanging wall) has been moved a great distance horizontally, is called overthrust fault (逆掩断层). In this
case, the large thrust sheet (逆冲片) having moved a long distance and resting on the autochthonous basement (本地基底,footwall) is called a nappe (推覆体). When the nappe (thrust sheet) has been eroded in some place and thus the basement has been exposed (in a small area relative to the nappe), the exposed area is called a structural window (构造窗). If most part of the nappe has been eroded and the basement around the nappe has been exposed, the residual nappe is called a klippe (飞来峰). In its restrict definition, a klippe is a large body or sheet of rock that has been moved a distance of about 2 km or more from its original position by faulting. A nappe , in other words , is an isolated residual part of the hanging wall of an overthrust fault, or even a part of a recumbent fold (www. britannica. com). The hanging wall above the fault surface is called allochthonous rocks (外来岩块), and the rocks below the fault surface are called autochthonous rocks (本地岩块,Figure 17, right).
Figure17 A block diagram showing a thrust fault (left), and a cross-sectional sketch showing nappe, klippe and window.
(After Harwoods, 2005)
Large overthrust faults occur in areas that have undergone great compressional forces. These conditions exist in the orogenic belts that result when two bontinental plates collide. The resultant compressional forces produce mountain ranges. The Himalayas, the Alps and the Appalachians are prominent examples of compressional orogenies with numerous overthrust faults.
A strike-slip fault (走滑断层) is one in which the movement is horizontal, parallel to the strike of the fault plane. When we face a strike-slip fault (i.e. we stand on one side of fault), we can find its movement direction relative to each side of the fault. If the block on the other side is displaced to the right, the fault is a right-lateral fault or dextral fault (右旋断层,Figure 18,right) ; if the block on the other side of the fault is displaced to the left, it is a left-lateral fault or sinistral fault (左旋断层,Figure 18, left). These movements result from shearing forces.
Figure18 Block diagrams showing sinistral(left) and
dextral(right) strike-slip faults.
(After Harwoods, 2005)
Cross sections of regions of normal faulting generally emphasized the presence of "horsts" and "graben". Horsts (地垒) are relatively uplifted, generally unrotated blocks bounded on either side by outward-dipping normal faults. Grabens (地堑), on the other hand, are relatively downdropped, relatively unrotated blocks bounded on either side by inward-dipping normal faults (Figure 19). Horsts and grabens are classical terms describing fault-bounded uplifted and down-dropped blocks, respectively, in extended regions.
Figure19 A block diagram showing graben and horst
(Modified after Harwoods，2005)
Half-graben (半地堑) is a normal fault that has a curved fault plane with the dip decreasing with depth can cause the down-dropped block to rotate. In such a case a half-graben is produced ( Figure 20) . It is called such because it is bounded by only one fault instead of the two that form a normal graben. It is also called a listric normal fault (犁式正断层) because the fault plane is curved or "spoon-shaped".
The evidences of faults (断层的证据). Direct evidences of faulting can often be difficult to locate due to the effects of weathering at the surface which tends to obscure the related features. Even though, geologists have found several types of features that provide direct evidence of faulting.
Figure20 A block diagram showing half-graben generated by horizontal tensile stress. (Adopted from http: //www. https://www.360docs.net/doc/1f108805.html,/ teacher/ basicgeol)
Slickensides (擦痕，摩擦镜面) are scratch marks that are left on the fault plane as one fault block moves relative to the other. A slickenside is a smooth, striated, polished surface. Slickensides can be used to determine the direction and sense of motion on a fault (Figure 21). Orientation of the scratching indicates the direction of the most recent movement of the fault, and so it does not tell the whole history of the fault movement. Thus, we should use this indicator with caution.
Figure21 A sketch diagram showing slickenside on a fault plane (left), and
a photo showing slickenside on a fault plane, Shandong Province, China
(Adopted from http：//homepage. usask. ca/~mjr347/prog/geoe118/geoe118.05.html;
photo by Hongbo Lu, 2005)
Faults are shear planes and commonly contain the debris from the frictional contact of the two surfaces. In strong rocks, material is fragmented to create a zone of crushed rock or fault breccia.
In weaker rocks, the material in the fault plane can be reduced to a very fine clay-size infill known as fault gouge.
Fault breccia (断层角砾岩) are the rocks or unconsolidated materials consisting of rock fragments (Figure 22,left; Figure 23, left). Occurs on either or both sides of the fault plane, breccia may form a zone which obscures the fault plane entirely.
Fault gouge (断层泥) is very fine-grained, unconsolidated material consisting of pulverized rock fragments (Figure 22, right; Figure 23, right). It is produced when fault blocks were passing each other. Thus, fault gouge looks like clays (in white, red, brown or gray and sometimes even black color) distributed along the fault belt.
Figure22 sketches showing fault breccia (left) and gouge (right).
(Modified from http://homepage. usask.ca/~mjr347/prog/geoe118/geoe118.052. html)
Figure23 Fault breccia (left) and gouge (right) in Aerjin Fault belt,
northwest of China.
(Photo by Hongbo Lu, 1996)
Apart from fault breccia and gouge that can be found on the fault plane, other marks on the sides of faults can also be used as some fault indicators, such as drag folds or joints. A drag fold (拖曳褶皱) is a fold induced by fault block movement , the crest of the fold points to the movement of the same wall (block) beside the fault plane (Figure 24).
Indirect evidence of faulting can also be present. This type of evidence may include the juxta-position (重叠，并置) of two map units which are usually not contiguous (相邻的), such as two sedimentary rock formations of different ages, or an intrusive in sharp contact with a country rock instead of containing a hornfels(角页岩) or skarn(矽卡岩) zone in between. Geologists also examine topographic maps and aerial photographs for linear features on the surface' Lastly' aeromagnetic anomalies (航磁异常) or other linear aeromagnetic features can be indicatives of large scale fault structures.
Apart from the evidences mentioned above, there are some marks in relief for identifying faults，such as fault scarp, valleys, rivers (Figure 25) and springs etc. All these are the relics or marks of faults exposed on the ground.
The dotted line is the fault trace and the lined areas are the fault scarps
Figure25 Fault scarps along Xilamulun River, north of Chifeng, China.
(Photo by Hongbo Lu, 2004)
In a geological map or a cross section, asymmetrically repeated identical strata or lack of some identical strata may be good clues for identifying a fault and its displacement.There are six different combinations as listed in the Table 1. A geologist can easily reconstruct the repeated or missing portions of the structure and the relative motion directions of the fault blocks.
In fact, faults (including jointing) and folds are often related, so there are fault-related folds (断层相关褶皱) such as fault propagation fold (断层传播褶皱) and fault bend fold (断层转折褶皱), etc.
Table 1 Relationship between the orientation and the fault results
2.3.2 Joints 节理
Joints, which can be caused by tectonic forces, are found in almost every outcrop. Like any other brittle material, brittle rocks break more easily at flaws or weak spots when they are subjected to pressure.These flaws can be tiny cracks, fragments of other materials, or even fossils. Regional forces (compressive, tensional or shearing) that have long since vanished may leave their imprint in the form of a set of joints (Press and Siever, 2001; Figure 26, upper left). Joints can also be caused by non-tectonic reasons, such as expansion and contraction of rocks or even contraction of