EARTH STRUCTURE AND COMPOSITION The earth is divided into 3 parts. The core is Earth’s central zone, which begins about half the 6,330-kilometer (3,900-miles) distance from the surface to center. The inner core is solid made mostly of nickel/ iron. The outer core (liquid) surrounds the inner core. Earth’s core is surrounded by a thick, solid zone call the mantle, which begins at depths of 6-40 mi. The crust is the thinnest of Earth’s zones (2% of earth’s volume, 1% of its mass). Its thickness varies from 16-40 miles. Only eight, elements make up 98.5% of the weight of the earth’s’ crust. They are (by weight): Oxygen 46.6%, Silicon 27.7%, Aluminum 8.1%, Iron 5%, Calcium 3.6%, Sodium 2.8%, Potassium 2.6%, and Magnesium 2.1%. 71% of Earth’s crust is very dense oc is Continets consist of folded mountain belts at thes of oceanic crust are the mid-oceanic ridges, the abyssal plains, and the trenches. The oceanic ridge system has a total length of more than 50,000 miles, is Continets consist ofses 1-2 miles above the abyssal floor. The abyssal floor consists of deep-ocean basins. Trenches, the lowest areas of the earth’s surface, are about 5 miles below sea level. Continental crust is higher and thicker that oceanic crust. It is as much as (40 miles) thick. Continental crust has a lower density than oceanic crust, so it floats higher on the denser mantle. Continets consist of folded mountain belts at the edges and cratons (flat,ore ancient, stable areas) more towards the interior. The continetal shelf is part of the continent itself but is underwater.esRedidual heat from formation and cof radioactive The Geological changes originating from within the earth are called internal processesRediduallearth’stoS orThis motion is called,sseismometersTheis a logarithmic scale. For exampley the continental crust, adds to the flow of heat from within. The asthenosphere is the part of the earths mantle that flows plastically. The area above this including the crust is called the lithosphere. The earth’s crust is divided into plates. This theory replace the theory of continental drift. In some areas magma continually rises in a certain area which we call mantle plumes or hot spots. The Hawaiian Islands are examples of hot spot volcanoes. Internal Process: Plate Tectonics The earth’s crust is divided into plates. This theory replace the theory of continental drift. the process that occur at their boundaries is called plate tectonics. This theory replace the theory of continental drift. which : andContentscontinentalResidualearth’stoS orThisent, convergent, and transform fault. At a divergent plate boundary the plates move apart. Because most divergent plate boundaries are along the oceanic ridge system, it is here that new crust is formed along these spreading centers. At most convergent plate boundaries crust is carried downward (subducted) under the island arc or the continent at a subduction zone. A trench ordinarily forms at the boundary. As it descends material melts, producing volcanoes. The third type of plate boundary, called a transform fault, occurs where plates slide past each other along a fault. California’s San Andreas Fault is one example. Ordinarily volcanism does not occur at transform faults. External Processes: Erosion, Weathering And Mass Wasting Geological changes based directly or indirectly on energy from the sun and on gravity, instead of on heat in the earth’s interior, are called external processes. Water, wind, glaciers and gravity remove loosened material, as well as material not yet separated, and deposit it in other places, a process called erosion. Loosened material that can be eroded is usually produced by weathering. Weathering is a result of mechanical processes, chemical processes, or both. In mechanical weathering a large rock mass is broken into smaller fragments. An example is frost wedging, in which water collects in pores and cracks of rock, expands upon freezing, and splits off pieces of the rock. Chemical weathering is when a mass of rock is decomposed by one or more chemical reactions, resulting in products that are chemically different from the original material. Weathering is responsible for the development of soil. Rock masses newly detached from underlying material may move downslope in various ways under the influence of gravity. This transport of material is called mass wasting. The terms used to classify the types of mass wasting include: rockfall, rockslide, slump, creep, earthflow, and mudflow. Another agent of erosion is a glaciers. Glaciers erode by abrasion and by their own unique process, called plucking, in which glacial ice freezes to rock and pulls fragments out when the glacier flows. THE ROCK CYCLE AND MINERAL RESOURCES A mineral is an inorganic, naturally occurring, crystalline solid with a definite internal crystal structure. All of the earth’s crust, except the rather small proportion composed or organic material, is made of minerals. A rock is any combination of minerals. Some rocks contain only one mineral example Limestone (made up of calcite CaCO3) The Rock Cycle The interaction of processes that change rocks from one type to another is called the rock cycle. It is the slowest of Earth’s cyclic processes and is responsible for concentrating mineral resources. Geologic processes constantly redistribute the chemical elements within and at the surface of the earth. Based on the way it forms, rocks are divided into three categories: igneous, sedimentary, and metamorphic. Igneous rocks cooled and solidified from molten rock (magma) below the earth’s surface (intrusive or granitic) or from molten rock (lava) above the earth’s surface (extrusive or volcanic). Sedimentary rocks form from sediment mostly when preexisting rocks are weathered and eroded into small pieces, transported from their sources, and deposited in a body of water. As these deposited layers become buried and compacted, the resulting pressure causes their particles to bond together to form sedimentary rocks such as sandstone and shale. Metamorphic rocks are produced when a preexisting rock is subjected to high temperatures (which may cause it to melt partially), high pressures, chemically active fluids, or a combination of those agents. A mineral resource is an concentration of naturally occurring solid, liquid, or gaseous material, in or on the earth’s crust, in such form and amount that its extraction and conversion into useful materials are currently or potentially profitable. They include energy resources (coal, oil, natural gas, uranium, geothermal energy, metallic mineral resources (iron, copper, aluminum), and nonmetallic mineral resources (salt, gypsum, clay, sand, phosphates, water, soil). Identified resources are deposits that have a known location, quantity, and quality or that are estimated from direct geological evidence and measurements. Reserves are identified resources that can be extracted economically at present prices with current mining technology. Other resources are identified and unidentified resources not classified as reserves. An ore is a metal-yielding material that can be economically extracted at a given time. Copper, for example, makes up 0.0058% by weight of the earth’s crust, but copper ore must contain at least 0.5% copper; thus the concentration of copper in ore is at least 86 times (0.5/0.0058) its average crustal abundance. Aluminum ore must have 3.7 times the average crustal abundance of that metal, and iron 5 times. However, the gold in gold ore must be concentrated 1,600 times its crustal average, and mercury an astonishing 100,00 times. Sometimes water containing dissolved minerals can be released in the late stages of the cooling of magma. These waters, called hydrothermal solutions, may be injected into fissures and pores, where they form veins and other types of ore deposits. Lead, zinc, copper, tin, gold and silver are some of the metals found in such deposits. Sulfide ores of several of these same metals are produced by hydrothermal solutions at undersea hot springs, called black smokers because of the black specks of metal sulfides in the water. These vents, which form at divergent plate boundaries on the seafloor, also support marine organisms that produce organic nutrients by chemosynthesis. We currently get salt, bromine, and magnesium from the sea. These same materials are also obtained from natural brines--a concentrated solution of salts derived from seawater or desert lakes by evaporating some of the water. Weathering in the tropics produces residual soils known as Laterite. They are red from the iron oxide they contain and are usually rather hard. All of our aluminum and some iron, maganese, and nickel come from laterites. Solid grains and fragments of minerals sometimes are eroded, transported and deposited. This type of deposit, called a placer, may concentrate titanium, gold, diamonds or platinum. The gold the forty-niners panned for in the California gold rush was in placers eroded from veins, the “mother lode,” on the west slope of the Sierra Nevada Mountains. When water in marine lagoons or desert lakes evaporates partly or completely, with nearly constant replacements of lost water, various minerals, called evaporites, are deposited. Salt, gypsum (used in drywall and plaster of Paris), potassium minerals (potash used in commercial inorganic fertilizers), soda ash and sodium sulfate (used in glass, insecticides, paper, and other products) and borates (borax minerals used toS orThis motion isedicines and fiberglass) are evaporites. Organisms have also contributed to sedimentary mineral resources. Phosphate rock deposits, which are formed in the phosphorous cycle, are phosphate bones and teeth of fish and other organisms or chemically precipitated phosphate, or sometimes both. Peat, lignite, and bituminous coal result from the transformation of accumulated plant remains. Petroleum and natural gases result indirectly from sedimentary processes involving the remains or organisms. LOCATING AND EXTRACTING CRUSTAL RESOURCES Finding And Mining Crustal Resources After profitable deposits of minerals are located, deep deposits are extracted by subsurface mining and shallow deposits by surface mining. Subsurface mining disturbs less than one-tenth as much land as surface mining and usually produces less waste material. However, it leaves much of the resource in the ground and is more dangerous and expensive than surface mining. In surface mining, equipment strips away the overburden of soil and rock, and usually discards it, a waste material called spoil. Surface mining extracts about 90% by weight of mineral and rock resources and more than 60% by weight of the coal in the United States. In open-pit mining machines dig holes and remove ores such as iron and copper. This method is also used for sand and gravel and for building stone such as limestone, granite, and marble. Another form of surface mining is dredging, in which chain buckets and draglines scrape up underwater mineral deposits. Strip mining is surface mining in which the overburden is removed in strips. It is used mostly for removing coal. Environmental Impacts After extraction from the ground many resources must be separated. Ore typically contains two parts: the ore mineral, which contains the desired metal, and the gangue, which is the waste mineral material. Beneficiation, or separation in a mill of the ore mineral from the gangue, produces waste called tailings. Mining can affect the environment in several ways. S orThis motion istion of the land surface. Land above mines collapses or subsides, spoil heaps and tailings can be eroded. The air can be contaminated with dust and toxic substances, and water pollution is a serious concern. Acid mine drainage occurs when aerobic bacteria produce sulfuric acid from iron sulfide minerals in spoil from coal mines and some ore mines. Other harmful materials running off are radioactive uranium compounds and toxic metals such as lead orThis motion isout effective pollution control equipment, smelters emit enormous quantities of air pollutants such as sulfur dioxide, soot, and tiny particles of toxic elements and compounds, which damage vegetation and soil. Smelters also cause water pollution and produce liquid and soil hazardous wastes. NATURAL HAZARDS Earth’s internal and external processes cause natural hazards, events that destroy or damage wildlife habitats and kill or harm humans and damage property. Earthquakes and volcanoes are the results of internal Earth processes. Floods and mass wasting are the result of external Earth processes. Earthquakes Stress can cause solid rock to deform elastically until it suddenly fractures and is displaced along the fracture, producing a fault. This motion is called,sseismometersTheis a kinds of human activities have caused or increased earthquake activity. The added load of water on Lake Mead behind Hoover Dam caused numerous tremors in the early years after the dam was completed. Underground nuclear testing and deep-well disposal of liquid waste have also caused minor earthquakes. The epicenter of an earthquake is the point on the surface directly above the focus,sseismometersTheis a logarithmic scale. For example aeEalong but cannear St.uake is by its magnitude on the Richter scale. The magnitude is a measure of the amount of energy released in the earthquake, as indicated by the amplitude (size) of the vibrationsseismometersTheis a logarithmic scale. For example aeEalong but cannear St.Theis a logarithmic scale. For example aeEalong but cannear St. Louis600,rocks tomostly greEalong but cannear St. Louis600,rocks tomostly e is 100 times greater than a magnitude 4. The amount of energy released is 30 times greater for each higher step. Ealong but cannear St. Louis600,rocks tomostly s but cannear St. Louis600,rocks tomostly ay from plate boundaries (as in Hawaii). At other interplate sites, such as the New Madrid fault zone near St. Louis600,rocks tomostly ed the presence of old faults. Coastal areas can be damaged by large, earthquake-generated water waves, called tsunamis (misnamed “tidal wave”) that travel as fast as 600,rocks tomostly Precursor phenomena are events that precede earthquakes which : lude: andContentscontinentalResidualearth’stoS orThis motion isrical and magnetic properties, the amount dissolved gases in groundwater, and even the unusual behavior of animals. We can reduce earthquake hazards by 1) mapping out and not building along active faults, 2) map out areas where poor soil will amplify earthquake waves, 3) establish better building codes, 4) be able tomostly earthquakes. Volcanoes An active volcano occurs where magma reaches the earth’s surface through a central vent or a long crack (fissure). Volcanic activity mostly ccurs in the same areas as seismic activity. The volcanoes at convergent plate boundaries include Fujiyama, Mount St. Helens andContentscontinentalResidualearth’stoS orThisely and eject particles and gases. Volcanic eruptions at divergent boundaries, as in Iceland and the African rift valleys, are quieter. There lava typically just flows. Intraplate volcanism has an eruptive style related to the geologic location. Those in the ocean, as on Hawaii, erupt very quietly. The benefits of volcanoes are outstanding scenery, some lakes (such as Crater Lake in Oregon) and other landforms, geothermal energy and most importantly highly fertile soils produced by the weathering of lava. We can reduce volcanic hazards by 1) being able to predict eruptions, 2) develop evacuation plans and 3) improve land use planning. Stream Floods And Flood Control Natural flooding by streams is caused primarily by heavy rain or rapid melting of snow. The flat valley floor next to a river is called a floodplain. Valleys with floodplains are popular places for human habitation. In some cases the wide valley floor slopes gently down away from the channel, and the channel has a ridge, called a natural levee, on both sides. The levees are made of fine sand and coarse silt, deposited as sediment each time the stream overflows the channel. A flood here can easily inundate the entire floodplain because the stream is higher than most of the natural levee. On floodplains the soil is fertile and water is available for irrigation and for transportation. On marine coasts flooding is due most often to the wind-driven storm surges and rain-swollen streams associated with typhoons and hurricanes. Various ways have been developed to reduce the hazard of stream flooding. Channelization deepens, widens, or straightens a section of a stream. Sometimes the channel is lined with concrete to accommodate a higher discharge. A flood control dam retains flood water, releasing it downstream over tgeologic À! À! À! À! À! À! À! À! re are no natural ones and sometimes on top of the natural ones. If the levee breaks or theood spills over it , however, floodwater may be trapped between the levee and the valley wall long after the stream discharge has decreased. A 25-year analysis revealed that 39% of the deaths from natural hazards were caused by floods, followed by typhoons and hurricanes (36%), earthquakes (13%), gales and thunderstorms (5%), and volcanic eruptions (2%). Mass Wasting Frost action forces rock apart on the face of a cliff and causes a rockfall. Soil on a mountainside outside Los Angeles becomes saturated by an infrequent rain and, having lost its vegetative cover in a recent brushfire, becomes a mudflow. A large mass of rock is jarred loose by an earthquake and creates a rockslide. Frost wedging, geologic À! À! À! À! À! À! À! À! and lye the soil on a hillside to move slowly downhill in a process called creep. Several factors affect mass wasting. Steepness of the slope and soil type or earth material are the most common. Water frequently contributes to mass wasting. The effect of vegetation on mass wasting varies. In some instances vegetation contributes to mass wasting by slowing the fall of rain on slopes, which helps water to infiltrate and saturate the soil. In others, it inhibits mass wasting by removing water from the ground and putting it into the air through transpiration, and by holding the soil together with its roots. Subsidence is a vertical movement of large rock masses that is not slope-related. Collapse can occur in volcanic regions when magma underlying an area withdraws and the unsupported rock drops. Collapse can also occur in underground coal mines. Subsidence that happens rapidly, such as in sinkhole formation. Pumping of groundwater or oil has caused subsidence.
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