If anything is characteristic of the world of rocks, it is change. Mountains are away and ocean basins are slowly filled over long periods of time. Changes that affect the features of the earth affect the rocks also. Rocks which have been changed so that their characters are altered are known as metamorphic rocks.
All rocks change after they are formed. The atmosphere, circulating water, the pressure of overlying rocks - all have some effect. But when these processes continue for a long time, or when they cause marked changes in the rock, then metamorphic rocks are formed. Some metamorphic rocks have been changed so much that they are completely different from the rocks from which they were formed. Unless these rocks are studied carefully, geologists cannot be sure of their origin.
Many forces in the crust of the earth change rocks. The most important of these forces are heat and pressure. Often heat comes from intruded magma. Magma at a temperature of 2,000 degrees or more may find its way into the overlying rock. The heat of the magma bakes and alters the nearby rock. If the mass of magma is large, the rate of cooling is slow. Then the effect of heat may be pronounced.
When hot, intruded rocks alter the rock on either side, the effect is described as contact metamorphism. The adjoining rocks are baked. Their mineral content may be changed, but the changes are usually limited to a narrow border zone, a few inches or a few feet.
Metamorphism is not only due to hot rocks, but to hot gases and hot liquids which flow from them. The hot gases move up through cracks to make a closer contact with nearby rocks and minerals. These volatile deposits may produce many new minerals. Hot solutions do the same thing and are likely to transport even more new minerals than hot gases. Heated waters have a much lower temperature than magma and bring their own kinds of minerals with them. The zeolites and arsenic minerals are examples of low-temperature deposits.
The effect of heat and hot chemical solutions is sometimes called local metamorphism in contrast to regional metamorphism which affects large areas. Regional metamorphism usually involves movements with the crust of the earth. The origins of these movements are hard to explain. They are probably related to a shifting in the earths crust as rocks and minerals are moved from one part of the earth to another by erosion. Regional metamorphism can raise or lower the level of rocks. Rocks may be tilted, folded, stretched or broken. Great masses of rock may be pushed over one another, forming zones of crushed rock. Sometimes these actions are slow and gentle, taking place over many thousand of years. Then very little change in the rock takes place.
At other times metamorphism is more rugged and the rocks are altered very much. Layers of soft coal are transformed into anthracite. The folding and squeezing these layers of coal remove most of the remaining gases and squeeze out any traces of water. This increases the percentage of carbon in the coal. Similar movements apply pressure to oils in shale or sand, and form where the oil and natural gas may be concentrated.
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Metamorphic rocks are hard to describe and harder to classify. Their appearance depends on the kind and the degree of change. One example of metamorphism is the alteration of soft sandstones to quartzite. This is a hard, tough, metamorphic rock - so tough that it breaks through the grains of sand as well as through the cement. Quartzite is harder, tougher, and more durable than the sandstone from which it was made.
Limestones are affected by heat, pressure, and circulating liquids to produce marble, a metamorphic rock. Some limestones are only slightly metamorphosed and the changes in them are difficult to see. Crystals and fossils in the rock are not altered much, if at all. While some of these slightly altered limestones are beautiful, they are not true marble. A more thorough metamorphism is needed.
Shale, formed from mud and silt, becomes metamorphosed into slate. Shale itself tends to break in flat layers. This is even more true of slate. However, slate breaks along lines that are usually at an angle to the original beds of the shale. Since slate splits so easily, it was once widely used for shingles, blackboards and paving. If the pressure that forms slate continues to act, a chemical reaction sets in, causing mica crystal to form. This new rock is called phyllite. It is a finegrained slate, glittering with almost microscopic flecks of mica. If the process continues further, the grains of mica grow larger and the result is a rock that is called schist.
All kinds of rock can be metamorphosed - even metamorphic rocks. Some volcanic rocks have been changed into schists. Quartz sandstone may be metamorphosed into quartzite, and in turn this may be altered into a quartz schist. Finally, granite, an igneous or metamorphic rock, may be changed into gneiss, a coarse rock which contains a good deal of mica. Hence, gneiss is not as strong a rock as the granite irom which it was made. Other kinds of rock may be altered into gneiss, too.
Some of the changes in the crust of the earth and in the rocks have been so complex that geologists are not sure just what has happened. Granite, for example, is sometimes an igneous rock, coming from a magma rich in silica and aluminium. It may also be a type of metamorphic rock so altered by invading materials that there is little or no trace of what the original rock might have been. It is possible to find a whole series of rocks grading from normal sedimentary kinds through schists and gneisses, which show an increasing amount of mica and feldspar, into crystalline rocks which clearly look like granite.
The changes in the crust of the earth producing the different kinds of rocks are all parts of a great cycle in which mountains are built up and mountains are worn down; in which the land is raised and the land is lowered. As mountains are worn down over periods of millions of years, the debris finds its way into the ocean basins, increasing their weight, while the weight of the continent is lightened. This puts a strain on the crust of the earth. The strain adjusts itself by means of movements which cause earthquakes and which, over long periods of time, elevate new mountains. Volcanoes form a great circle around the deep Pacific Basin.
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These earth movements are on such a large scale and involve such long periods of time that it is difficult to observe them first hand. It is not likely that continents have been lost in the Atlantic or that large islands have suddenly appeared. Most of the continents have occupied the same relative positions in the crust of the earth for millions of years. During some eras, shallow seas invaded the continents and sedimentary rocks were deposited. But sooner or later the continents emerged and have continued to be areas of uplift. At other times the continental shelf (the shallow ocean bottom surrounding the continents), now submerged, has been raised, and the continents were much larger than at present. At one period Alaska and Siberia formed a land bridge between Asia and America, and Australia joined Southern Asia.
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NOTES:
volatile , ;
anthracite - ;
silt , , , , ;
shingle , , .
Text 15. ROCK CYCLE
Rock Cycle is the evolution of rock material under changing physical conditions at or beneath the earths surface. In the rock cycle, each of the three principal types of rock, sedimentary, metamorphic, or igneous, can evolve into either of the two other types of rock or even into other rocks of its own type. For example, a rock may change into a sedimentary rock by eroding, and then accumulating as sediment in a new place, and then cementing into rock. Metamorphic rocks form when rock is heated or subjected to high pressures, or both, without melting, and igneous rocks form when rock melts and then cools again.
The rock cycle transforms rocks from one type to another, or to a new rock of the same type, depending on the temperature and pressure where these transformations occur. These conditions, in turn, depend on the depth at which the transformations occur. Weathering and erosion, which break down rocks and create sediments, occur at or near the earths surface. Compacting and cementing of sediments to form sedimentary rocks usually starts at depths from about 1 to 3 km. Most metamorphism, the process of forming metamorphic rocks, occurs at depths of between about 10 and 30 km. Magma, or molten rock, often forms at depths below 30 km, but magma may then rise before it solidifies. Igneous rocks can form at any depth, even at the earths surface. Igneous rocks that form at the earths surface are termed volcanic.
Any rock transforming into a sedimentary rock would begin its transformation by cropping out on the earths surface, where it would weather and erode into sediment. Gravity, water, glacial ice, or wind would eventually remove the sediment from the outcrop and deposit it somewhere else. The sediment would likely be eroded and deposited again and again until it ended up in an environment that was accumulating sediment instead of eroding. There, the sedimentary particles would be buried by subsequent accumulations of sediment. When the sediment becomes buried under about 1 to 3 km of overlying sediment, the pressure of the overlying sediment compacts the sediment and causes minerals to precipitate, which cements the sediment into sedimentary rock. In this way, any of the three types of rock could evolve into a sedimentary rock.
If enough sediments accumulate on top of a rock, it takes a different path through the rock cycle and becomes a metamorphic rock. This metamorphic rock forms as the original rock is pushed deeper into the earths crust and becomes hotter. The sedimentary rock shale, for example, consists primarily of clay particles. During metamorphism, these clay particles recrystallize into microscopic-sized crystals of mica, a plate-shaped mineral. If metamorphism stops at this point, the resulting rock is a slate. With increasing temperature, however, mica crystals increase in size to form the metamorphic rocks phyllite and schist. Additional heat can cause other minerals to form and segregate into different colored bands, resulting in the metamorphic rock gneiss.
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A rock undergoing metamorphism may experience even higher temperatures and begin to melt. If the resulting magma makes its way to the earths surface in a volcanic eruption, it will form an extrusive igneous rock that will be susceptible to weathering and erosion. Alternatively, the magma might cool and crystallize beneath the surface as an intrusive igneous rock. This igneous rock, in turn, could follow one of three paths in the rock cycle: It could experience another increase in temperature and pressure conditions and form a new metamorphic rock; it could remelt, and on cooling, form a new igneous rock; or it could be brought to the surface and exposed to erosion, leading to the formation of a new sedimentary rock.
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NOTES:
to erode , , ;
weathering - .
Text 16. VEIN
Vein, in geology, is a tabular mass of mineral matter, deposited in the fissure, crack, or crevice of a body of rock, and differing in composition from the substance in which it is embedded. Most veins are the result of the gradual precipitation of substances, which were carried by underground waters or gases after the formation of the enclosing material. Veins vary in size from tiny streaks, which may be entirely contained in a small rock specimen, to masses thousands of feet in extent. Veins of quartz and other minerals may also form when magmatic fluids are injected into fissures opened by intrusion of large bodies of igneous rock. In closely spaced stratified layers, this formation is known as a lode.
Many valuable metals and minerals occur in veins of igneous and sedimentary rock. Within a vein the ore may follow certain streaks, known as shoots, or be restricted to pockets of extreme richness. The nonvaluable minerals associated with the ore in a vein are called gangue. A variety of metals are found in veins, usually in chemical combination as minerals, but also, in the case of gold and platinum, in their pure or native form. In California, for example, gold is often found in veins of quartz, the most common gangue mineral.
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NOTES:
tabular , , , ;
lode , , ;
gangue mineral .
Text 17. GROUNDWATER
Groundwater is the water found below the surface of the land. Such water exists in pores between sedimentary particles and in the fissures of more solid rocks. In arctic regions, groundwater may be frozen. In general such water maintains a fairly even temperature very close to the mean annual temperature of the area. Very deep-lying groundwater can remain undisturbed for thousands or millions of years. Most groundwater lies at shallower depths, however, and plays a slow but steady part in the hydrologic cycle. Worldwide, groundwater accounts for about one-third of one percent of the earths water, or about 20 times more than the total of surface waters on continents and islands.
Groundwater is of major importance to civilization, because it is the largest reserve of drinkable water in regions where humans can live. Groundwater may appear at the surface in the form of springs, or it may be tapped by wells. During dry periods it can also sustain the flow of surface water, and even where the latter is readily available, groundwater is often preferable because it tends to be less contaminated by wastes and organisms.
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The rate of movement of groundwater depends on the type of subsurface rock materials in a given area. Saturated permeable layers capable of providing a usable supply of water are known as aquifers. Typically, they consist of sands, gravels, limestones, or basalts. Layers that tend to slow down groundwater flow, such as clays, shales, glacial tills, and silts, are instead called aquitards. Impermeable rocks are known as aquicludes, or basement rocks. In permeable zones, the upper surface of the zone of water saturation is called the water table. When heavily populated or highly irrigated arid areas withdraw water from the ground at too rapid a rate, the water table in such areas may drop so drastically that it cannot be reached, even by very deep wells.
Although groundwater is less contaminated than surface waters, pollution of this major water supply has become an increasing concern in industrialized nations.
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NOTES:
aquifer , ;
water table , .
Text 18. EROSION
Erosion is the removal of rock and soil material by natural processes, principally running water, glaciers, waves, and wind. Erosion transports rocky material after the process of weathering has broken bedrock down into smaller, moveable pieces.
Through erosion the surface of the earth is constantly being sculptured into new forms. The shapes of continents are continuously changing, as waves and tides cut into old land while silt from rivers builds up new land. As rivulets, streams, and rivers cut their channels deeper, gullies become ravines and ravines become valleys. The Grand Canyon, more than 1500 m deep, was produced by erosion probably within the past 5 million years. The overall effect of the wearing down of mountains and plateaus is to level the land; the tendency is toward the reduction of all land surfaces to sea level. Opposing this tendency are volcanic eruptions and movements of the crust of the earth that raise mountains, plateaus, and new islands.
Water Erosion. Water plays an important role in erosion by carrying away material that has been weathered and broken down. When an area receives more water (in the form of rain, melting snow, or ice) than the ground can absorb, the excess water flows to the lowest level, carrying loose material with it. Gentle slopes are subject to sheet and rill erosion, in which the runoff removes a thin layer of topsoil without leaving visible traces on the eroded surface. This erosion may be balanced by the formation of new soil. Often, however, especially in arid areas having little vegetation, the runoff leaves a pattern of gullies formed by rivulets. Water can even erode solid rock, especially along streambeds where the stones that are carried with the current scour and abrade. Every year rivers deposit about 3.5 million tons of eroded material into the oceans.
Glacial Erosion. Glaciers are important agents of erosion. Although a glacier moves slowly, it gradually removes all the loose material from the surface over which it travels, leaving bare rock surfaces when the ice melts. Besides removing loose material, glaciers actively erode the solid rock over which they travel. Rock fragments that become embedded in the bottom and sides of the moving ice mass act as an abrasive, grinding and scouring the bedrock which forms the walls and floors of mountain valleys.
Coastal Erosion. Coastal erosion of rocky cliffs and sandy beaches results from the action of ocean waves and currents. This is especially severe during storms. In many parts of the world the loss of land due to coastal erosion represents a serious problem. The action of waves, however, does not extend to a great depth, and the sea tends to cut a flat platform, characteristic of marine erosion, into coastal rocks.
Wind Erosion. Wind is another active agent of erosion, especially in arid climates with little vegetation. Wind blowing across bare land lifts particles of sand and silt but leaves behind larger pebbles and cobbles. Eventually, a surface layer of closely packed stones, called a desert pavement, is formed as the sand and silt is removed. The removal of large quantities of loose material is called deflation. Deflation lowers the landscape slowly, usually less than a meter in a thousand years. Also winds may sometimes deposits sand in large piles, known as sand dunes.
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NOTES:
bedrock - , ;
rill - , , ;
rivulet - , ;
scour - , , ;
silt , , ;
deflation , .
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Text 19. WEATHERING
Weathering processes, in geology, are the processes of physical disintegration and chemical decomposition of solid rock materials at or near the earths surface. Physical weathering breaks up rock without altering its composition, and chemical weathering decomposes rock by slowly altering its constituent minerals. Both processes work together continuously to produce debris that is then transported away mechanically or in solution. Weathering processes also aid in the formation of soil.
Physical weathering results primarily from temperature changes, such as intense heat; the action of water freezing in rock crevices; and living organisms, such as tree roots and burrowing animals. Temperature changes alternately expand and contract rocks, causing granulation, flaking, and massive sheeting of the outer layers. Frost action and organisms widen cracks, exposing deeper layers to chemical weathering.
Chemical weathering alters the original mineral composition of rock in a number of ways, such as by dissolving minerals by water and weak soil acids; by oxidation; by producing a reaction with carbon dioxide; and by hydration, which is a process in which water chemically combines and reacts with minerals. Plants, such as lichens, also decompose certain rocks by extracting soluble nutrients and iron from the original minerals.
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Text 20. ORE
Ore is a naturally occurring rock containing high concentrations of one or more metals that can be profitably mined. Ore minerals are the minerals within ores that contain the metal. Ores occur as large bodies of rock called ore deposits, which are metal-bearing mineral deposits.
The criteria for judging that a rock is an ore cover economic and legal issues as well as geological ones. Economic issues include mining costs and the price of the extracted metal. Legal issues include land ownership, land use restrictions, and environmental regulations.
Some important ores are galena (lead sulfide, which is mined for lead), sphalerite (zinc sulfide, which is mined for zinc), chalcopyrite (copper iron sulfide, which is mined for copper), and hematite and magnetite (iron oxides, which are mined for iron).
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NOTES:
galena , ;
sphalerite - ;
chalcopyrite .
