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Introduction: the Global Overview

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Introduction: the Global Overview Introduction 1 Introduction: the Global Overview I. Plate Tectonics and Geology of the Earth II. Overview of Northwest Geology III. Timeline for Northwest Geologic Events I. Plate Tectonics and Mohorovicic discontinuity (the Moho) occurs at the Geology of the Earth basethe ocean of the floor crust is where 3 miles there (5 km) is the below greatest sea level. change The in seismic velocity. The mantle is a dense hot layer of semi-solid rock about 1800-miles (2900 km) thick. It Earth’s Structure contains more iron, magnesium and calcium than the The Earth consists of three main layers: the crust, crust, and it is also hotter and denser than the crust. mantle, and core. The crust, which is the outer layer of The core is twice as dense as the mantle because it the Earth, is relatively thin and ridged. The continental consists of an iron-nickel alloy. It has two parts: a crust tends to be variable in thickness, averaging about 1360-mile-thick (2,200 km) liquid outer core and a 18-miles (30 km) thick; however, under mountain 775-mile-thick (1,250 km) solid inner core. The Earth’s ranges it may be 60-miles (100 km) thick. The oceanic crust is 3- to 6-miles (5 to 10 km) thick and most of core as the Earth rotates. magnetic field is created by spinning of the liquid outer Cross section showing the solid inner core, the liquid outer core, the mantle, and the crust. The asthenosphere is overlain by the lithosphere. 2 Idaho Geology Block diagram with east-west cross section across the the surface along the island arcs, it typically has the Japan Trench, Honshu and the Sea of Japan. At this composition of andesite. If the magma rises near the convergent plate boundary, Honshu is underlain by edge of the continent, it may form the great granitic continental crust. batholiths. Granite is less dense than basalt and can Earth’s Crust Oceanic Crust lining the ocean basins is made of easily float through it. basalt. The Continental crust is 15- to 43-miles (25 to 70 km) oceanic crust consists of olivine tholeiitic basalt and thick, ranging up to 43-miles (70 km) thick under isa dark,composed fine-grained of plagioclase volcanic feldspar, rock called pyroxenes and mountain ranges and high plateaus. One third of the olivine. The oceanic crust is 3.5- to 6-miles (5 to 10 Earth’s crust is continental because the ocean basins have very little continental crust. The upper 6 to 9 below sea level. The oceanic lithosphere is 30- to miles (10 to 15 km) of the crust is brittle and below 60-mileskm) thick (50 and to most 100 ofkm) the thick. ocean No floor ocean is 3 crust miles is (5 older km) that depth the crust is ductile. Continental crust is than 180 Ma. Basalt is derived from mantle material formed by the differentiation of from basaltic granite called peridotite that rises by convection cells or rock through the subduction process. As the basaltic mantle plumes to the Earth’s surface. As the material rises, the pressure drops because of less overlying gradually cools, becomes denser, and eventually sinks material. Pressure-release melting allows a lower- crust flows away from the ridge riding on the mantle, it as much as 435 miles (700 km) into the mantle at the density liquid (basaltic magma) to separate from the subduction zones. As the basaltic crust travels higher density mantle materials. So basalt, which is towards the subduction zone, the basalt absorbs richer in silicates than the parent peridotite, is also less water through the hydration of certain minerals. The dense. The basaltic magma is injected at the divergent newly hydrated silicate minerals have a lower melting plate boundaries (where two plates are pulling apart) temperature than the rest of the minerals. So as the and forms new crust. slab of basaltic crust is subducted into the mantle, Lithosphere and Asthenosphere. Because the the silicate-rich hydrate minerals melt. Because this upper mantle is cooler and more ridged than the deep melt is less dense than the surrounding material, it mantle, it behaves like the lower portion of the crust. Consequently, the crust and upper mantle together floats toward the surface. When the magma reaches Introduction 3 form the lithosphere or the tectonic plates. The than 120-miles (200 km) thick or twice the average thickness of the lithospheric plates ranges from 9 miles thickness of the continental lithosphere. Cratons have (15 km) for young oceanic lithosphere to more than a lower density than the oceanic crust so that they 120 miles (200 km) for old continental lithosphere. do not sink into the deep mantle. Cratons consist of However, the worldwide average is about 50 miles igneous and metamorphic rocks and range in age from (80 km) in thickness. The lithosphere overlies a hot, one- to four-billion-years old. These ancient crystalline mobile, semi-solid zone in the mantle referred to as the rocks are commonly referred to as the basement. At asthenosphere. The ridged lithosphere is believed to the central portion of the cratons, basement rocks are generally exposed but may be covered by much Tectonic plates or lithospheric plates are immense, younger sedimentary rocks and sediments around irregularlyfloat and move shaped on the slabs asthenosphere. of rock consisting of oceanic and continental lithosphere. Plate boundaries can covers the basement, it is called a platform. be accurately mapped from outer space by satellites. the margins. Where a thin layer of sedimentary rocks Theory of Plate Tectonics composed of granitic rocks, which consist of low- densityContinental minerals crust such floats as because quartz andit is feldspar.primarily By Since the 1840s, the geosynclinal theory was used to contrast, oceanic rocks are composed of denser explain the origin of mountain belts until the theory of plate tectonics gradually superseded it during the basaltic rocks. 1960s. Plate tectonics is now the great unifying theory of geology. Many geological and biological events, Cratons previously thought unrelated, can now be explained by The oldest and most stable part of continental plate tectonics. Although many geologists at the time lithosphere is referred to as a craton. Geologically more opposed the continental drift active rocks such as mountain belts typically surround (1915), there is now a consensus among geologists the cratons. Most cratons are situated in the interiors that the plate tectonic theory is idea valid. of Plate Alfred tectonics Wegener of tectonic plates and have survived many cycles of merging and rifting of islands and continents. Cratons amount of corroborating evidence from almost tend to have thick lithospheric roots and may be more everyhas been subdiscipline repeatedly of tested geology, and paleontology, confirmed by physics, a massive The major lithospheric plates and their movement directions. U.S. Geologic Survey. 4 Idaho Geology U.S. Geological Survey world map showing shield areas, platform areas and orogen areas. chemistry, and mathematics. This evidence includes accreted lithospheric fragments, which may be either of isotope dating, sediment-core analyses, deep-ocean continental or oceanic in origin, are called terranes if drilling, paleontology, seismology, paleomagnetism, andthey appear are of sufficient out-of-place, size theyand haveare referred consistent to as geological exotic heat-flow analysis, geochemistry, petrology, structural orattributes. suspect Where terranes terranes. Exotic do terranes not fit the are local transported geology continents,geology, acoustic-bottom geomorphology, profiling, and the seismic-reflection correlation of by tectonic plates and are accreted or attached to strata,profiling, fold gravity belts, andprofiling, mountain glaciation, ranges. fit Plate of the activities an existing continental landmass. As a result they have also had an enormous effect on the evolution of are geologically unrelated to the adjacent landmass life. along which they accreted. The geology of western North America is complicated by many exotic terranes Evidence of Plate Tectonic Activity Before consisting of volcanic island arcs, oceanic sediments, and oceanic lithosphere. Pangea Existed Ocean Magnetism tectonic processes since the breakup of supercontinent PangeaWe now somehave an200 excellent million understandingyears ago. Plate of processes plate Magnetometers are capable of sensing rock magnetism the oceanic crust is less than about 180 million years magnetism comes from the basaltic oceanic basement old.before Oceanic Pangea crust are appearsmore difficult to be recyclingto decipher about because every all becausemore than the a overlyingmile below sediments the ocean are floor. too Most weakly of the 150 million years. Consequently, all evidence of pre- Pangea plate movements must come from the existing The magnetic properties of the oceanic basement continents. There is now ample evidence that plate dependmagnetized on the to affectamount the of magnetometer magnetite, thickness significantly. of the tectonics have existed for most of the Earth’s history magnetized layer, and its depth below the surface. with several cycles of supercontinent formation. For Marine magnetic surveys are concerned only with example, it has been proposed that Pangea was formed from the breakup of parts of a 540-million-year-old direction of horizontal and vertical components. The measurements of the total field, not with magnitude or supercontinent called Rodinia. Along some continental border zones, geologists gammas at the magnetic equator to about 60,000 have recognized remnants of ancient volcanic arcs, gammasmagnitude at ofthe the magnetic total field poles. ranges from about 30,000 oceanic crust, and ocean sediments accreted onto In 1963 Fred Vine and Drummond Matthews the continental margins by plate tectonic processes observed that basalt along the central ridge of the that operated, in some cases, before Pangea. These Indian Ocean was magnetized with normal polarity. Introduction 5 “Magnetic stripes” on the ocean floor indicating reversals of the Earth’s magnetic field.
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