Geography 1111 Exam 3 Study Guide
Geography 1111 Exam 3 Study Guide GEOG 1111
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Geography 1111 Exam 3 Study Guide Lecture 21: Introduction to Geomorphology: o Definition: the development and changes on the Earth’s surface over time o Basic Terms: Landform: an individual element of a landscape, a mountain, a valley, etc. Topography: refers to the elevation changes of the Earth’s surface over a given space or distance Ex: ridges, valleys, etc. Uniformitarianism: the idea that the process of change we see today are the same as they were in the past and will be in the future Ex: the way a volcano functions today is the same way that volcanoes “worked” in the past and will into the future Types of Landforms: o Tectonic: those developed by the rearrangement of Earth’s crust, driven by internal energy Earthquakes, volcanoes, etc. o Fluvial: refers to those developed by flowing liquid water (rivers) o Karst: is a type of landscape developed primarily by the chemical weathering (carbonation) of limestone o Glacial: refers to those landforms developed by glacial ice (solid water) o Eolian: landforms developed by wind processes o Coastal: landforms are the result by ocean waves and current processes o Geomorphic Processes are of 2 basic types: 1. External Processes: those that occur on the Earth’s surface, such as fluvial, glacial, wind, coastal, etc. 2. Internal Processes: those that occur or originate within Earth’s crust, volcanism, mountain building, massive crustal rearrangement (plate tectonics), earthquakes, etc. Remember the ideas of equilibrium state, steady-state equilibrium, dynamic equilibrium, negative and positive feedback mechanisms from Lecture 1 Earth’s Internal Structure: o Characteristics: From what we know, the Earth’s interior is composed of or arranged in concentric layers or spheres, with heavier elements having settled toward the center and is the result of cooling since formation of the Earth, some 4.6 billion years before present (BYBP) Each layer is distinct in chemical composition and/or temperature and is either solid, molten, or a combination of the two Our assumptions as to the composition and character of the Earth’s interior are based on analysis of the material in the crust and of molten material (magma/lava) which comes to the surface Analysis of the behavior of seismic waves (the energy waves produced by an earthquake) also helps us understand the interior by analyzing their speed and direction as they pass thru the Earth Seismic waves change in both speed and direction with changes in the temperature and density of material they pass through Cooler material yields higher velocities while hotter material yields slower velocities AND changes in density of the material may reflect or refract the waves Three basic types of seismic waves: 1. P waves or Primary waves, which are push or compression waves 2. S waves or Secondary waves, which are shear or shake waves 3. L waves or Surface waves, which move through the surface/ground and are the waves you feel in an earthquake The Earth’s Layers: o Inner Core: The material in this layer is in a solid state and consists primarily of iron (Fe) and Nickel (Ni) o Outer Core: This layer is in a liquid/molten state, but also consists of Fe and Ni The material in this layer is under less pressure since there is less material on top of it o This causes its melting temperature to be lower, thus the material is in a liquid/molten state The Outer Core also generates most (~90%) of the Earth’s magnetic field Because this layer is molten and the material moves within the layer, it causes fluctuations in the magnetic field over time Discontinuities: exist between layers and are broad, uneven transition zones from areas differing in chemical composition and/or density The discontinuity between the Outer Core and the layer above it (the Lower Mantle) is called the Gutenberg Discontinuity o Lower Mantle: This layer is primarily solid, but with molten material also present and it consists of Fe oxides, magnesium (Mg) and silicon (Si) o Upper Mantle: This layer is a mixture of molten and solid material and consists of silicate minerals The Upper and Lower Mantle comprise about 80% of Earth’s volume and are often together referred to simply as the Mantle o Asthenosphere: It is primarily molten, with some solid material It is the main source (but not the only) of magma, the molten rock or lava which erupts onto the surface o Lithosphere: Consists of two solid layers, the Uppermost mantle, and the Crust There are 2 types of crustal material, Continental and Oceanic which vary in thickness The zone of contact between the Uppermost mantle and the Crust is the Mohorovicic Discontinuity or Moho Continental Crust Oceanic Crust Material of lower density Material of higher density Sialic rock or sial (Si and Simatic rocks or sima (Si and Aluminum) Magnesium) Ex: granite, shale, marble Ex: basalt, lava The Crustal Surface: o Topography or Topographic relief is the vertical difference between the highest elevation and the lowest elevation over a given area, as measured above sea level EX: In Georgia the highest elevation is Brasstown Bald and the lowest elevation is Sea Level (0') o Each continent has a core or nucleus of crystalline rock, called a craton These are usually of low elevation and greater than 570 million years old o Earth’s overall relief as measured above and below sea level is a high at Mt. Everest, and a low of 36,168' below sea level (in the Mariana Trench, Pacific Ocean) This is ~12.34 miles in topographic relief. Lecture 22: Info to know: The age of the oldest rocks: 4 billion years old The oldest sea floor: 200 million The beginning and ending of each of the eras o 520- 250 o 250- 65 o 65- current Tertiary (2 million) and quaternary (currently in) The top two epoch’s Geologic Time Scale: a method or depiction of the Earth’s age and geologic history using both a sequence of rock strata (their relative age) and their absolute ages o It is thus both a relative time scale and an absolute time scale and is governed by uniformitarianism o It assumes superposition, the idea that younger rocks and sediments are near the top of a formation, and older material is near the bottom o Absolute ages are determined by methods such as radioactive isotope dating Geologic Cycle: the vast cycling of material that occurs in and on the lithosphere, encompassing the hydrologic, tectonic, and rock cycles o The building up and wearing down of landforms involving various processes and events o 3 components: Hydrologic cycle which is the movement of water (H2O) through all 4 of Earth’s spheres Evaporation, Condensation, Precipitation, etc. Tectonic cycle: the movement of large portions of the Earth’s crust Rock cycle: refers to the formation of the three basic types of rock, igneous, sedimentary, and metamorphic Rock Cycle: ***know the diagram of the rock cycle*** o Mineral is a combination of elements that forms an inorganic, natural compound which has specific qualities, such as a unique crystalline structure, hardness, etc. Ex: Silicates, based on silicon (Si); Carbonates, carbon (C) These together make-up 90% of all minerals 3000 minerals, only about 10-20 make up 90% of the crust o Rock is an assemblage of minerals bound together, usually 2 to 5 different minerals Thousands of rock types, but all are classified into one of the three basic rock categories: igneous, sedimentary, or metamorphic The interactions/interrelationships between the processes which form the different rock types can be expressed by the Rock Cycle Igneous Process and Rocks: o Igneous Rocks are formed by the Crystallization of magma/lava, the solidifying of molten material either below or above the surface o The material may cool fast forming a fine-grained structure (extrusive igneous rocks) or cool slowly forming a coarse- grained structure (intrusive igneous rocks) o Two basic types of igneous rock: 1. Intrusive Igneous rocks are those which form below the surface, within the crust, as the magma cools and crystallizes before it reaches the surface The masses of rock formed in this process are generally known as plutons Ex: granite, rhyolite; Stone Mountain is composed of granit 2. Extrusive Igneous rocks are those which form on the surface, on the crust or as new crust as the magma cools and crystallizes after it reaches the surface The ocean floor is primarily composed of basaltic igneous rock Ex: lava or basalt, volcanic ash, obsidian, pumice Sedimentary Process and Rocks: o Sedimentary rocks are formed by a two-step process in which existing material (rocks) are first weathered/eroded down into smaller pieces of the original rock called sediments (dirt, sand at a beach, mud in a river, etc.) These are often then transported and deposited some distance from the location of the original rock The second step involves these sediments being “put back together” in the process of Lithification to form new rocks Lithification: is a process of cementation, compaction and hardening of sediments to new rock o Ex: Material is deposited on a lake bottom, inland sea, etc. and then has more material deposited on top, then more and so on The material, layers at the bottom come under greater pressure and get “squeezed” back together into a hard mass of new rock 2 main types of Sedimentary rock: 1. Clastic sedimentary rock forms from bits and pieces of former rocks that are/were visible pieces, such as the sand at the beach, or mud on a lake bottom, or soil material Ex: shale, siltstone, claystone, etc. 2. Non-Clastic sedimentary rock forms from the minerals dissolved in solution (water) which were deposited as a solid after evaporation of the water The material is various salts and other minerals originally dissolved in a river, lake or ocean Ex: limestone, coal, salts, chalk, gypsum Metamorphic Process and Rocks: o The rearrangement of rock crystals in an existing rock is the process of Metamorphism This can be accomplished by either physical and/or chemical change, and is often done under high pressure (dynamic metamorphism) and/or high temperature (contact metamorphism) situations o The new material/rock is usually more compact, denser, than the original material and thus harder Contact metamorphism: is the rearrangement of crystals brought about when magma comes in contact with the adjacent rock, and slightly (but not completely) melts it o This causes the crystals to move and rearrange Dynamic metamorphism: is when the rearrangement of crystals in a rock is brought about by it coming under high pressure and thus compacted or squeezed together o This occurs at convergent plate boundaries 2 main types of Metamorphic rock: 1. Foliated Metamorphic rock: is formed when minerals/crystals of the same chemical composition within the original rock form distinct bands or striations after metamorphism The like minerals are essentially drawn together into more homogeneous groups Ex: gneiss, slate, schist 2. Non-Foliated Metamorphic rock: exhibits a more homogenous mixture of the crystals after rearrangement and few, if any, striations It often forms from more homogenous rock (few different minerals) Ex: quartzite, marble, greenstones Lecture 23: Plate Tectonics: A theory of global dynamics in which the lithosphere is believed to be broken into dozens of pieces or plates that move in response to convection and other forces in the upper mantle o History of the theory: Alfred Wegener, a German geologist and meteorologist, gave it serious consideration in 1912, and was the first scientist to suggest that continents were moving, and to supply some evidence for his idea in 1915 Evidence: similarities of fossil records, climatic records, and geologic records between distant continents, especially South America and Africa o Ex: the land dwelling, mammal-like reptile Lystrosaurus, whose fossils have been found in South America, Africa, Antarctica and Asia OR mountain belts of similar age, rock structure, fossils, and structural style appear on both sides of the Northern Atlantic, (Appalachians, Ireland, Scotland, and Norway) Wegener proposed that one landmass existed about 200-225 MYBP (Triassic-Jurassic Period) and called it "Urkontinent", German for "primal continent" and analogous to the Greek Pangaea o This Pangea began to split over the next 10-20 million years o By about 135 MYBP (Cretaceous Period) it had broken into 2 large pieces, Laurasia (basically present-day northern continents) and Gondwana (the southern continents) His Theory of Continental Drift took a while to be fully accepted by the established scientific community, however, because he could not explain a mechanism for moving the plates, but near the end of his life, a Theory of Thermal Convection was developed (1928), which added a mechanism for moving the continents, which Wegener had not done 1940-1960's, Atlantic Ocean floor mapping using sonar enhanced our knowledge of tectonic processes by showing a truer image, than had previously been known, of the sea floor topography In 1960, Harry H. Hess and Robert S. Dietz proposed a theory of Sea-Floor Spreading Sea-floor spreading: is the idea that the movement of oceanic crustal material is the result or a mechanism of upwelling magma along a system of mid-oceanic ridges and the pull of gravity at the other end of the plate o The upwelling pushes the crust away from the ridge forming spreading zones, while pulling of the tectonic plate by gravity at the other end forms subduction zones o Because of this, the oldest oceanic crust (seafloor) is furthest from the ridge, and the youngest crust is at the ridge o To date, the oldest seafloor has been dated to ~200 MYBP o The discovery of geomagnetic reversals, or paleomagnetism, first proposed in the 1920s and later connected to seafloor spreading in 1963, helped support the theory of sea-floor spreading and plate tectonics Geomagnetic reversals (paleomagnetism) refers to the magnetic alignment of charged particles (especially iron material) in a rock which exhibit a symmetrical pattern as you move away from the mid-oceanic ridge o The polarity, the orientation of positive (+) and negative (-) particles to the Magnetic Poles of the Earth, changes as you move away from the mid-oceanic ridge o This occurs because as the upwelling magma cools forming new sea-floor (basalt), the charged particles of the iron in the magma are aligned according to the Earth’s magnetic alignment o Different sections of magma cool at different times and thus the polarity changes as the magnetic alignment of the Earth changes o Remember that the Earth’s magnetic alignment changes because of the movement of magma in the Outer Core which forms the Earth’s magnetic field o Paleomagnetism does not cause the plates to move, it only shows that they are moving o By 1980's the term Plate Tectonics was developed, which combined the ideas of sea-floor spreading, continental drift, paleomagnetism and other ideas Plate Tectonics is the theory covering crustal movements and the foundation of crustal tectonic processes to include: upwelling of magma, plate movement, subduction of crust, folding, faulting, warping, fracturing, earthquakes, and volcanic activity o Distribution of Plates: Currently at least 15-20 plates are recognized, with 8 major plates Major plates: Pacific, North American, South American, Eurasian, African, Australian, Indian, Antarctic Smaller plates: Nazca, Cocos, Caribbean, Arabian, Somalian, Scotia, Caroline, Fiji, China sub-plate, and Philippine plates are the largest of these 1 small plate is of interest to the U.S., that being the Juan de Fuca plate which lies between the Pacific and North American plates in northwestern U.S./southwestern Canada How to tell a plate boundary? o This was originally done by mapping the distribution of volcanic and earthquake activity observed/recorded across the Earth’s surface The majority of this distribution is quite linear in shape and often yields the location of plate boundaries EX: “Pacific Ring of Fire” which is a pattern of volcanoes on the Western side of both North and South America, then the eastern side of Asia down to New Zealand (also a zone of high earthquake activity) o Mid-Atlantic Ridge and other spreading zones also map as linear features of high earthquake and volcanic activity Types of Plate Boundaries or Interactions: o Divergent plate boundaries: areas where the plates are moving apart from each other These areas are under tensional stress and formed by the upwelling of magma (ridge push) and the pull of gravity (slab pull) They are areas of abundant earthquake activity and relatively mild volcanic activity Seafloor spreading along mid-oceanic ridges is a prime example Ex: Along Mid-Atlantic Ridge, or the formation of the Red Sea as the Arabian plate spreads away from the African plate A spreading center (divergent zone) can also exist on a continent where it may form rift zones or valleys Ex: Great Rift Valley of East Africa, where the Somalian plate is pulling away (eastward) from the African plate o Convergent plate boundaries: areas where plates are moving toward each other These areas are under compressional stress and formed by gravity pulling the opposite end from a spreading ridge of a plate down toward the center of the Earth (slab pull) This causes the plate to collide with another plate, forming what is known as a subduction zone There are 3 basic types of convergent plate boundary: Continental-Oceanic convergent plate boundaries are areas where oceanic crustal material (which is of higher density material) and continental crustal material (which is of lower density material) converge The oceanic crust is subducted (pushed/pulled) under the continental crustal material and eventually remelted in the Asthenosphere This forms a deep underwater canyon or trench on the seafloor at the area of subduction along the plate boundary At the same time, continental crustal material is compressed, forming folded mountains, fault-block mountains and volcanic mountains as magma reaches the surface through cracks and weak points These are areas of numerous earthquake and volcanic activity Ex: Nazca and South American plates converge forming the Peru Trench and the Andes mountains (on the S. A. plate); also the Pacific northwest of the U.S. at the convergence of the Juan de Fuca and North American plates Oceanic-Oceanic convergent plate boundaries are areas where oceanic crustal material of one plate collides with oceanic crustal material of a second plate One plate will be subducted under the other and this is usually the faster moving plate or the one with more force moving it Deep trenches are formed at these boundaries and they are areas of high volcanic and earthquake activity The formation of a volcanic island archipelago (chain) is often common These are formed on the plate that is not subducted Ex: Philippine islands, Japanese islands, and the Aleutian Islands off of Alaska Continental-Continental convergent plate boundaries are areas where continental crustal material of one plate collides with continental crustal material of a second plate There is little or no subduction of either continental plate, but subduction of oceanic material earlier may occur These are areas of great compression and mountain building, both folded and fault-block mountains, often on both continental plates and this numerous earthquakes, but little or no volcanism Ex: The Appalachians formed some 250-300 MYBP when the North American, African and Eurasian plates converged, and the Himalayas currently being formed by the Indian plate converging with the Eurasian plate o Transform or Lateral plate boundaries These areas undergo shear stress as the plates slide laterally past one another They are areas of high earthquake activity and some crustal deformation (creation of hills and small mountains), but no subduction or volcanism Ex: San Andreas Fault Zone in Southern California What causes the plates to move? o Near the end of Wegener’s life, a Theory of Thermal Convection was developed (1928), which added a mechanism for moving the continents, which Wegener had not done This theory suggested that the continents were driven by huge convective cells within the asthenosphere o More recently another theory is that gravity pulls down on the leading, subducting edge of a plate, which drags the rest of the plate with it o Most likely plates move as a combination of these two actions OR a combination of the two ideas Lecture 24: Volcanism: the process by which magma and gases are transferred from the Earth’s interior to near or on the surface o 2 basic types: Extrusive and Intrusive Extrusive volcanic activity: o Volcano is a landform with a vent, or fissure (crack) on the surface which is the end of a conduit originating from below the crust in the Asthenosphere, or mantle o If the exit point is a vent, then usually a mountain landform is the result, but if the exit point is a fissure, then mountains usually do not form, only features of low relief o Magma often collects in a magma chamber below the vent or fissure before being expelled o Crater: the surface depression at the summit of the volcano, often the vent or opening is in the crater o Intrusive volcanic activity is when magma cools to form igneous rock below the Earth’s surface and thus is only exposed by erosion or uplift o These are known as Plutonic Landscapes A volcano's level of activity may be categorized as either: o Active wherein the volcano is currently erupting, has erupted during recorded history, or still has a high potential to erupt o Dormant refers to a volcano that is in repose (rest), but still has the potential to erupt in the future o Extinct refers to a volcano that has little or no potential to erupt again o Most convergent and divergent plate boundaries have high volcanic activity Hot spots are also areas of high volcanic and earthquake activity o Lava is the term used for the molten rock issued from a volcano that is now on the surface, as opposed to the term magma used for the molten rock that is still underground Two general types of material (lava): o Mafic or basaltic lava: high in Mg and Fe and composed of <50% silica which makes it less viscous, meaning it flows readily (more easily) This leads to less gas being trapped within the lava mass, leading to a less explosive reaction o Felsic: lava is richer in silica (>50%) which makes it more viscous, meaning it flows slower This leads to more gas being trapped within the lava mass and more explosive eruptions Pyroclastics or tephra are solid fragments of magma expelled explosively from a volcano o Quite common with felsic magma and explosive eruptions o Bombs (> 1 m), pumice, scoria, cinders (1-5 in), lapilli, ash, and dust (Based on size; bombs largest, dust/ash smallest) Two General Types of Eruptions: o Effusive: eruptions are relatively gentle, non-violent events because they are associated with mafic magma which flows more readily and gases can more readily escape, leading to less explosive eruptions If the eruption is through a vent, it may form a shield volcano Ex: islands of Hawaii, Galapagos islands and Iceland If the eruption is through a fissure, then it may form a plateau or flood basalt Ex: Columbia Plateau in the state of Washington (on land) and from mid-oceanic ridges under water to form sea floor o Explosive: relatively violent events because they are associated with felsic magma which flows more slowing often solidifying in the conduit and thus trapping gasses and increasing internal pressure Explosive eruptions are most commonly found along subduction zones and often have a high content of pyroclastics These eruptions are more commonly through a vent, and form a Ex: Mt. St. Helens, Mt. Rainier, Mt. Pinatubo, Mt. Vesuvius Other events that may accompany an explosive eruption: o Lahar is a special type of mudflow and the result of a volcanic eruption During the eruption of a volcano that is snowcapped, the heat of the magma melts the snow which mixes with any ash, soil, etc. on the mountain This mix of ash, mud, and water then comes down the mountain side and may travel for dozens of miles Ex: One formed by the November 13, 1985 eruption of Nevado del Ruiz in Colombia killed over 20,000 of the ~29,000 towns people Armero, by covering the town in up to 100' of mud o Nuée ardentes or Pyroclastic flow is a cloud of hot volcanic gas and ash that moves down the flanks of a volcano Ex: A flow created during the 1902 eruption of Mont Pelee on the island Martinique, West Indies annihilated the town of St. Pierre, killing approximately 30,000 people o Phreatic eruptions: violent, explosive eruptions made more severe by water entering the magma, increasing pressure from steam and increasing the explosive power of the erruption Ex: Krakatau (1883), Tambora (1815), Santorini (1645 BC) Other volcanic features: o Cinder cone: a volcano that consists of cinder-sized tephra, with little or no lava and are usually less than 200m high Ex: Sunset Crater (AZ) o Volcanic dome is a mound of lava which may form inside a crater and be a cap over the vent It may also be the only surface feature from an eruption o Caldera is the term given a large, more or less circular depression or basin associated with a volcanic vent, with a diameter often many times greater than the original vent(s) This may be the result of collapse or subsidence, or may also result from an explosive event Ex: Crater Lake (Oregon, USA), Yellowstone National Park (Wyoming, USA) Mantle Plumes and Hot Spots: o Mantle plume is a buoyant mass of hot mantle material (magma) that rises to the base of the lithosphere It commonly produces volcanic activity and structural deformation on the surface Current theory suggests these plumes may originate at the core-mantle boundary as mantle material becomes hotter than surrounding material and thus rises o Hot spot is the surface expression of a mantle plume that has either formed or found an opening in the crust, creating a conduit to the surface As the tectonic plate (surface) moves over this 'plume' it may form a series of features on the surface, sometimes including shield volcanoes Ex: Hawaii is an oceanic hot spot, while the Yellowstone National Park area is a continental hot spot; Iceland is also the result of a mantle plume associated with a mid-oceanic ridge (divergent plate boundary) Hydrothermal (Geothermal) Features: o These are found in areas where water below the surface has been heated to temperatures higher than commonly found on the surface o This may be the result of water flowing deep below the surface and heated as a result of the geothermal gradient (temperature increases with depth at approximately 3°/100m) o Or it may be the result of contact with a shallow magma chamber, sometimes associated with mantle plumes and hot spots o Types: hot springs, geysers, fumaroles, etc. Pluton: the exposed portion of a body or mass of intrusive igneous rock o Two Main Types: Discordant plutons: are those which disrupt or change, often melting, the existing geologic structure into which it intrudes; it often creates the space it then occupies Batholith: is a large discordant mass of intrusive igneous rock, measuring over an area of at least 100km2 o Ex: Stone Mt. (GA) is the exposed portion (pluton) of a larger mass; as are the Black Hills of South Dakota and the Sierra Nevada Mts. along the California/Nevada border One batholith in Idaho is nearly 41,000 km Dike is a relatively narrow mass of cooled magma that cuts across preexisting strata or other structural features of the surrounding rock in a vertical orientation o May be a few feet in diameter to several kms wide, long, and deep o The largest known dike is in Zimbabwe and is 600 km long and averages 10 km wide Volcanic neck: is the solidified magma that originally filled the vent and neck of an ancient volcano and that has been exposed by erosion o EX: Shiprock (New Mexico), Devil’s Tower (Wyoming) Concordant plutons: are intrusive magma feature that didn’t destroy (melt) the existing geologic structure; it may move the existing rock or simply conformed to it by filling existing spaces Laccolith: is a concordant pluton which has arched (pushed) up the strata into which it was injected, and so forms a lens-shaped body with horizontal floor but that is smaller than a batholith Sill: is a relatively narrow mass of cooled magma that was injected between preexisting strata of the surrounding rock in a horizontal orientation o It can range from a few cm to hundreds of meters thick and may extend for several kms Lecture 25: Earthquakes: a sharp release of energy resulting in a series of elastic waves propagated through the Earth at the moment of rupture along a fault o The rupture is initiated where stress along the fault exceeds the elastic limit of the rock so that sudden movement occurs They are also associated with volcanic activity Seismic waves: the pulses of energy generated by an earthquake that pass through the Earth as shock waves o Transmission speed and direction can vary according to the temperature and density of the various layers within the planet o Types of seismic waves: P wave (primary seismic wave) is a type of seismic wave, propagated like a sound wave, of short wavelength and high frequency in which the material involved in the wave motion is alternately compressed and expanded It is thus a compression or “push” wave which moves material parallel to its direction of movement P waves can pass through all internal layers of the Earth S wave (secondary seismic wave) is a seismic wave of short wavelength and high frequency in which particles of the material vibrate at right angles to the direction in which the wave travels It is thus a shear or “shake” wave which moves material at right angles to its direction of movement S waves cannot pass through the Outer or Inner core layers The P and S waves are generated at the focus of an earthquake L wave (surface wave) is a seismic wave of long wavelength and low frequency that travels along Earth’s surface, but not through the Earth, and at slower velocities than either P or S waves L waves spread out from the epicenter of an earthquake Some basic terms: o Focus: the place of origin of an earthquake and the point from which the P and S waves originate It may be at or near the surface, or deep within the crust or upper mantle o Epicenter: the point on the Earth’s surface directly above the focus of an earthquake and the point from which the L waves spread out (originate) o Fault: is a fracture in crustal rock involving the displacement (movement) of rock on one side of the fracture in respect to the rock on the other side o Fault plane: the surface of contact along which blocks on either side of a fault move o Fault scarp: the exposed cliff-like face of a fault plane formed by faulting with a vertical displacement component How are earthquakes measured? Magnitude vs. Intensity o Magnitude: measures the total amount of energy released by an earthquake by the amount of shaking of the ground during the event as measured by a seismograph or seismometer; it is a quantitative measurement Seismograph or seismometer is an instrument that measures seismic waves and earth vibrations It is measured on the Moment Magnitude Scale (formerly the Richter Scale), and considers the amount of fault slippage, the size of the area that ruptured, and the nature of the materials that faulted in estimating the magnitude of an earthquake o Intensity is a subjective measure of the size and damage of an earthquake as equated by its impact on the human landscape (structures and activities) It is measured using the Modified Mercalli Scale, and is a qualitative measurement Some major quakes: 1812 New Madrid, MO 1886 Charleston, SC (6.7) 1906 San Francisco, Ca (8.25) 1964 Alaska (8.6) 1976 Tangshan, China (7.6) 1989 Loma Prieta, Ca (7.1) 1994 Northridge, Ca (6.7) 2002 Denali, Alaska (7.9) 2004 Sumatra, Indonesia (9.3) 2010 Chile (8.8) 2010 Haiti (7.0) 2011 Honshu, Japan (9.0) Distribution: o Earthquakes may be classified as shallow (<63 mi depth), medium (63-150 mi depth), and deep (>150 mi depth) Some quakes have been recorded as deep as 400 mi below the surface o Circum-Pacific Belt is associated with the subduction zones around the Pacific Ocean 80% of shallow earthquakes occur in this belt and the vast majority of deep earthquakes occur at subduction zones o Trans-Eurasian Belt spreads from the Mediterranean Sea eastward through SW Asia, the Himalayas, to SE Asia o Mid-oceanic ridges and other divergent plate boundary areas o Intraplate earthquakes which are found not at plate boundaries This includes those which occur in Georgia, Missouri and other areas of the U.S. away from the West Coast These may be at continental hot spots like Yellowstone National Park Types of Faults: o Tensional or Normal Faults: are those which involve the stretching of crustal rock The two pieces or blocks of crustal material are being pulled apart which causes lengthening of the crust Normal faulting occurs along a steeply inclined fault plane in which one block, the hanging wall, has moved downward in relation to the opposite block, the footwall This movement results in the fault scarp of the hanging wall block not overhanging the footwall Hanging wall: is the block or surface of rock that lies above an inclined fault plane Footwall: is the block or surface of rock beneath the dipping fault plane This type of movement is analogous to what is seen at a divergent plate boundary o Compressional or Reverse Faults: are those which involve the compression of crustal rock The two pieces or blocks of crustal material are being pushed together which causes shortening of the crust Reverse faulting occurs along a steeply inclined fault plane in which the hanging wall has moved upward in relation to the footwall and thus overrides it along the fault plane This movement results in the fault scarp of the hanging wall block overhanging the footwall Thrust fault is a compressional (reverse) fault with a very low angle to the fault plane This type of movement is analogous to what is seen at a convergent plate boundary Echelon faults are a series of nearly parallel faults, either normal, reverse or a combination which forms a landscape known as horst and graben topography Horst: is the elongate fault block that has been uplifted in relation to the blocks on either side Graben: is the elongate fault block that has been lowered in relation to the blocks on either side o Shear or Strike-slip Faults: are those which involve the blocks of crustal material being moved laterally (horizontally) along the fault plane Strike-slip faulting involves movement that has occurred parallel to the strike of the fault plane In true strike-slip faulting this no vertical displacement only horizontal displacement The movement may be described as right-lateral or left-lateral This type of movement is analogous to what is seen at a transform plate boundary Tsunami is a Japanese term referring to a seismic sea wave which is the energy passing through the water, generated by an earthquake whose epicenter is on the sea floor These may travel at speeds up to 630 mph and form crested waves in shallow water as high as 200', sometimes thousands of miles from the epicenter Ex: Alaskan quakes of 1946 and 1964 caused tsunamis in Hilo, Hawaii with 100 ft. waves; the Indonesian quake of 2004 and the Japanese (Honshu) earthquake of 2011 also formed tsunamis Lecture 26: Structural Landscapes: o The relationship between geologic structure and the visible landscape, and how this is formed by various processes o Those landscapes formed by tectonic forces: compression, tension, and shearing o These forces/stresses create various features: folds, faults, fractures, and warping Folding and Folded Structures: o Folding: is the process that bends, warps, and deforms rock strata when subjected to compressional forces; commonly seen at convergent plate boundaries The result of folding is best seen in sedimentary rock To describe the nature and orientation of these structures, and deformed rock layers, we use the terms, strike and dip Strike: is the compass direction of the line of intersection between the rock layer and a horizontal plane (surface) o It indicates the direction of compression o Ex: N45E = a strike with an orientation roughly running NE to SW, or 45 between compass north and east This indicates the compression came from the NW and SE Dip: is the angle (in degrees) at which the rock layer tilts form the horizontal (which would be 0°) o It indicates the amount of pressure exerted during the compression process o Ex: A layer with a dip of 90º would be vertically straight up and down This indicates a great deal of compression to deform the rock layers from 0° to 90° The dip and strike are always at right angles to each other Primary features formed: o Anticline: in an arch-like upfold with limbs (beds) dipping away from its axis (or hinge) This often forms a mountain ridge-like structure o Syncline: is a trough-like downfold with the limbs dipping toward the axis This often forms a valley-like structure o Monocline: is a gentle bend with a step-like form in horizontally bedded rock strata The axis of the fold may be parallel to the surrounding terrain, or may plunge, dip (be tilted) along the axis In some areas and instances the rock strata may only be tilted and not folded Secondary features: o Anticlinal valley and Synclinal mountain: are the result of erosion of the original anticline and syncline o Hogback: is a narrow, sharp ridge formed on steeply inclined resistant rock; a sharp-crested, asymmetrical homoclinal ridge formed by differential erosion of a resistant bed of steeply dipping rock o Cuesta is an elongate ridge formed on the tilted and eroded edges of gently dipping strata; a symmetrical homoclinal ridge with a long, gentle slope corresponding to the dip of a resistant bed and a steep slope on the cut edges of the beds o Basin: is a circular or elliptical downwarp (syncline), with the younger strata exposed in the central portion after erosion o Dome: is a circular or elliptical unwarp (anticline), with beds dipping away in all directions from a central area Orogenesis (Mountain building): o The process of mountain building that occurs when large-scale compression leads to deformation and uplift of the crust, as seen at convergent plate margins o A mountain building episode, orogeny, occurs over millions of years o Orogenic belts (or orogens) are Earth's major chains of folded and faulted mountains Ex: Rockies (Laramide Orogeny, 80-40 MYBP; relatively young), Appalachians (Allegheny Orogeny, 325-260 MYBP), Fault-block mountains like the Grand Tetons (9-7 MYBP), Alps (~100 MYBP to present), Andes (~80 MYBP to present), Himalayas (~50 MYBP to present Sites of Orogenic events: o Primarily found at convergent plate boundaries Oceanic-oceanic: Japan, Philippines, Aleutians Oceanic-continental: Andes, Rockies Continental-continental: Himalayas, Appalachians o Isostasy: a state of equilibrium formed by the interplay between the lithosphere (landmasses) and the asthenosphere Isostatic rebound: is a process by which the crust is first depressed or sinks with added weight, thus pushing downward on the upper-most mantle and asthenosphere As the weight (load) is removed, say by erosion, the asthenosphere pushes upward lifting the crust back to its original position Ex: As a mountain range forms at a convergent plate margin it not only builds upward, but also downward pushing into the asthenosphere o As the range erodes, load is released and the asthenosphere pushes back, compensating for the lost material o This is also seen when a continental ice sheet forms, pushing down on the crust and then the crust rebounds as the ice mass melts o Over a broad area, say continental or plate size, this is called warping or broad warping
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