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154 Cards in this Set
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metamorphic rocks
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rocks that have undergone changes in mineralogy, texture, chemical composition, or all three.
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why do geologists study metamorphic rocks?
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to understand how Earth's crust has evolved over geologic time
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What are the basic 3 principal factors that drive metamorphism?
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the internal heat of the earth, its pressure, and its fluid composition
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low-grade rocks
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rocks formed at lower temps and pressures of shallower crustal regions
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high-grade rocks
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meta rocks formed at higher temps and pressures of deeper zones
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geothermal gradient
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the increase of temp with increasing depth. It varies depending on plate tectonic setting, but on avg is about 30 degrees C per kilometer of depth.
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What are the primary mechanisms that form most meta rocks?
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subduction and continental collision (transports rocks and sediments into the hot depths of the crust
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stress
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the force per unit area acting on any surface within a solid body. Stress consists of confining pressure and directed pressure
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confining pressure
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general force applied equally in all directions, like a swimmer emerged in a pool.
Ex: rock descending to greater depths in the Earth |
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directed pressure
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a force exerted in a particular direction, such as a ball of clay being squeezed between a thumb and forefinger. Also called differential stress.
ex: compressive force that occurs when plates converge - deformation of teh rocks near the plate boundary. |
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pressure is recorded in...
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kilobars (kbar)
1 kbar = air around surface of earth (like in the room right now) |
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two things that affect the pressure to which a rock is subjected
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thickness of overlying rocks and density of the rocks
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metasomatism
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change in rock's bulk composition by fluid transport of chemical substances into or out of the rock
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list the types of metamorphism
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shock, regional, contact, seafloor, low-grade (burial), High-Pressure and Ultra High Pressure
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regional metamorphism
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takes place at moderate to deep levels under moderate to ultra-high pressures and high temps.
convergent plate tectonic settings - island arcs and mountain belts rocks are typically transported to significant depths of the Earth's crust, then uplifted and eroded at Earth's surface. |
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contact metamorphism
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when the heat from igneous intrusions metamorphes surrounding rock.
Normally affects only a thin region of country rock along the contact (around magmas and molten rock) and occurs at high temperatures. it's limited to thin zones because lavas cool quickly so its heat has little time to penetrate deep into teh surrounding rocks and cause meta changes. |
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seafloor metamorphism
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aka metasomatism
often associated with midocean ridges hot, fractured basalts heat infiltrating seawater, which starts to circulate through the basaltic upper crust by convection. The increase in temp promotes chemical reactions between the seawater and the rock, forming altered basalts whose chemical compositions differ distinctively from that of the original basalt. In a nutshell: Intruding magma and hot rocks drive seawater circulation at mid-ocean spreading centers, where it metamorphoses extruded basalts. |
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Low-grade (burial) metamorphism
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occurs at lower temps and pressure (caused by progressive increase in pressure exerted by teh growing pile of overlying sediments and sed rocks and teh incease in heat associated with increased depth of burial in te earth) changes sed rocks.
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high pressure and ultra-high pressure metamorphism
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rarely exposed at the surface- form at great depths
most form in subduction zones where sediments scraped from subducting oceanic plates are plunged to depths of over 30 km, where they experience pressures of up to 12 kbr. can make eclogites- unusual rocks once located at the base of teh crust |
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shock metamorphism
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from heat and shock waves of meteorite impacts, it metamorphoses rocks immediately around the impact site.
country rock can be shattered or even partially melted |
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eclogites
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unusual meta rocks once located at the base of the crust that can sometimes be found at the surface. They indicate pressures of greater than 28 kbar, suggesting depths of over 80 km.
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relation of grain size to metamorphic grade
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grain size inceases, metamorphic grade increases
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granoblastic rocks
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aka nonfoliated
composed mainly of crystals that grow in equant (equidimensional) shapes, such as cubes and spheres, rather than in platy or elongated shapes. Deformation is absent in their metamorphism. Ex: hornfels, quartzites, marbles, greenstones, amphiboles, grandulite |
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foliation
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set of flat or wavy parallel planes produced by deformation
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foliated rocks
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slates (the lowest grade of foliated rocks), phyllite, schist, gneiss, migmatite
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porphyroblasts
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large crystals surrounded by a much finer grained matrix of other minerals.
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slate's typical parent rock....
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shale, sandstone
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quartzite's parent rock...
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quartz-rich sandstone
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marble's parent rock....
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limestone, dolomite
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greenstone's parent rock...
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basalt
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amphibole's parent?
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shale, basalt
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granulite's parent rock?
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shale, basalt
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slate to gneiss's parent rock (porphyritic rocks' parent...)
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shale
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Metamorphic facies
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groupings of rocks of various mineral compositions formed under different grades of metamorphism from differet parent rocks. Facies give us clues to teh tectonic process resonsible for metamorphism.
different kinds of meta rocks form from parent rocks of different composition at the same grade of metamorphism. diff kinds of meta rocks form at diff grades of metamorphism from parent rocks of the same composition. |
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metamorphic P-T path
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teh history of teh changing pressure and temp
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according to the theory of plate tectonics, the steady relative motion between rigid plates of lithosphere causes deformation at the plate boundary. What are the 3 basic types of plate boundary?
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spreading centers, subduction zones, and transform-fault boundaries.
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outcrop
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where the bedrock that underlies the surface is exposed (not obscured by soil or loose boulders)
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what are the 2 measurements that describe the orientation of a rock layer exposed at an outcrop?
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strike and dip
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strike
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the compass direction of a rock layer as it intersects with a horizontal surface.
basically where that horizontal spot on the rock is on the map. Is it kind of going from east to west? (EW) or Northeast to soutwest? (NE SW) |
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dip
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measured at right angles to the strike, is simply the amound of tilting-the angle at which the bed inclines from the horizontal.
basically when you put a horizontal line on the rock and measure the angle between that line and the rock |
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geologic maps
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represent the rock formations exposed at Earth's surface
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dip angle
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water trickles down slope parallel to dip
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geologic cross sections
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diagrams showing the features that would be visible if vertical slices were made through part of the crust.
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what are the kinds of horizontally directed tectonic forces that act on rocks in plate boudary zones?
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tensional forces, compressive forces, shearing forces
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tensional forces
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stretch and pull formations apart, dominate at divergent boundaries, where plates move away from each other
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compressive forces
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squeeze and shorten rock formations, dominate at convergent boundaries, where plates move toward each other.
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shearing forces
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which push two sides of a formation in opposite directions, dominate at transform-fault boundaries, where plates slide past each other
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brittle material
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undergoes little deformation under incr. force, until it breaks suddenly
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ductile material
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undergoes smooth and continuous plastic deformation under incr. force and does not spring back to its orig shape when the deforming force is released.
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a fault
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a surface across which rock formations have been displaced
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dip-slip faults
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relative movement of the blocks up or down the dip of the fault plane.
divided into NORMAL faults, THRUST faults, and REVERSE faults normal- rocks above the fault plane move down relative to the rocks below the fault plane, extending horizontally. Tensions is involved. There is extension (look at p. 157) reverse- rocks about the fault plane move upward in relation to teh rocks below, causing a shortening of the structure- the reverse of what geologists have chosen as "normal". Compression is involved. thrust- a reverse fault with a shallow dipping fault plane. compression is invovled. |
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strike-dip fault
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the movement has been horizontal, parallel to the strike of the fault plane.
caused by horizontal shearing froces that shear the body left-laterally or right-laterally. |
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oblique-slip faulting
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caused by a combination of forces
(weird one) |
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anticlines
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layers that upfold into arches
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synclines
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layers that downfold into troughs
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limbs
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the two sides of folds
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axial plane
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an imaginary surface that divides a fold as symmetrically as possible, with one limg on either side of the plane.
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fold axis
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the line made by teh lenghtwise intersection of the axial plane with limbs dipping symmetrically away from the axis. If its axis is not horizontal, it is called a plunging fold.
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assymetric folds
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duh
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over-turned folds
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duh
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dome
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an anticlinal structure, a broad circular or oval upward bulge of rock layers.
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basin
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a synclinal structure, a bowl-shaped depression of rock layers in which the beds dip radially toward a central point.
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joints
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a crack along which there has been no appreciable movement.
they're found in almost every outcrop. can provide channels through which water and air can reach deep into the formation and speed teh weathering and internal weakening of the structure. the circulation of hydrothermal fluids through joints can deposit minerals such as quartz and dolomite, forming veins. |
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what are the basic deformation structures that geologists observe in the field?
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folds, fractures (faults and joints), circular structures (domes and basins), and deformation textures caused by the shearing of rock formations.
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absolute age
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the number of years elapsed from the event until now
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relative age
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how old one event is in relation to another
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the principle that states that sediments are deposited under the influence of gravity as nearly horizontal beds is....
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the principle of orginal horizontality
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the principle that states that each sedimentary layer of an undisturbed sequence is younger than the one beneath it and older than the one above it is.....
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the principle of superposition
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stratigraphy
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the study of strata (layers) in rocks
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stratigraphic succession
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a chronologically ordered set of strata
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formations
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groups of rock layers that can be identified throughout a region by their physical properties.
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types of unconformities
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disconformity, angular unconformity
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parent v. daughter
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p- original atom
d- the decay product |
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a useful element for isotopic age dating is...
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rubidium
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how do we know whether one rock is older than the other?
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we determine the order in which rocks formed by studying the stratigraphy, fossils, and cross-cutting relationships of rock formations observed at outcrops. An undeformed sequence of sedimentary rock layers will be horizontal, which each layer younger than the layers beneath it and older than the ones above it. In addition, because animals and plants have evolved progessively over time, the fossils found in each layer reflect the organisms that were present when that layer was deposited. Knowing the faunal succession makes it easier to spot unconformities, which indicate intervals of time missing in the sedimentary record.
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volcanic geosystem
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a chemical factory that processes the input material (magamas from the asthenosphere) and transports the end product (lava) to the surface through an internal plumbing system.
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name the types of lava
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basaltic, andesites, and rhyolites
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basaltic lava
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most common
produced along mid-ocean spreading centers and continental rift valleys, as well as from hot spots in the mantle beneath oceanic plates. Hawaii black or dark grey, but at its eruption temp, it glows in reds and yellows extremely fluid, and can flow downhill fast and far rarely explosive. fluid magamas fill up the volcano's plumbing system and overflow, sending lava down the flanks of the volcano in great streams that can engulf everything in their path. take on diff forms depending on how they cool: Pahoehoe (ropy) or Aa (looks like clumps of moist, freshly plowed earth) high temps and low silica contents |
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andesitic lava
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lower temps than basalts and higher silica contents than basalts
flow slowly and lump in sticky masses. if one of the sticky masses plugs up the throat of the volcano, gases can build up beneath the plug and eventually blow off the top of the volcano. Ex Mount St. Helens in '80. erupt mainly in the volcanic belts along active continental or oceanic margins above subduction zones. |
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rhyolitic lavas
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produced at zones where heat from the mantle has melted large volumes of continental crust.
Yellowstone volcano produces huge amounts of rhyolitic magmas that are building up in shallow magma chambers for the next eruption. Light in color, often a pretty pink. lower melting point than other two types of lava the lava is the richest in silica, so it is the stickiest and least fluid of all. it moves really slow and tends to pile up in thick, bulbous deposits. Gases are easilt pent up beneath rhyolite lavas, and large rhyolite volanoes such as Yellowstone produve the most explosive of all volanic eruptions |
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pyroclasts
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fragmentary volcanic rocks ejected into the air
they are classified according to their size (the smallest size is volcanic ash). also includes volcanic bombs, volcanic tuffs, and volcanic breccias |
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pyroclastic flow
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devastating form of eruption
when hot ash, dust, and gases are ejected in a glowing cloud that rolls downhill at high speeds (pic of Japanese volcano and guy trying to outrun it on p.277) the solid particles are buoyed up by the hot gases, so there is little frictional resistance |
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opening on top of volcano is called..
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central vent
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kinds of volcanoes
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diatremes, shield volcanoes, volcanic domes, cinder-cone volcanoes, stratovolcanoes, craters, calderas
p.278 has good pics of these |
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shield volcanoes
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Basaltic lava!
built up by the accumulation of thousands of thin basaltic flows that spread as gently sloping sheets. Magma can erupt on the flanks of a volcano as well as from the central vent. Ex: Mauna Loa in Hawaii |
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volcanic domes
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felsic lava- can barely flow
rounded, steep-sided mass of rock ex: Mount St. Helens Domes look as though lava had been squeezed out of a vent like toothpaste, with very little lateral spreading. felsic lava often plugs the vents, trapping gases.. pressure increases until an explosion occurs, blasting the dome into fragments. |
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cinder-cone volcanoes
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formed when ejected material is deposited as layers that dip away from the crater at the summit.
the vent beneath the crater is filled with fragmental debris. ex: Cerro Negro |
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craters
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bowl-shaped pit found at the summit of most volcanoes, centered on the vent.
when it erupts, the lava overflows the crater walls after the eruption, the lava that remains in the crater often sinks back into the vent and solidifies, to be blasted out by a later pyroclastic explosion. |
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calderas
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ex: Crater Lake, Oregon
result when a violent eruption empties a volcano's magma chamber, which then cannot support the overlying rock. It collapses into the chamber, leaving a large, steep-walled basin. A lake can form in the caldera- called a crater lake |
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diatreme
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when hot matter from the deep interior escapes explosively, the vent and the feeder channel below it are often left filled with volcanic breccia as the eruption wanes. the resulting structure is a diatreme.
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the largest eruptions come from...
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fissure eruptions
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fissure eruptions
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large, nearly vertical cracks in the Earth's surface.
main style of volcanism along mid-ocean ridges, where new oceanic crust is formed. in a fissure eruption of highly fluid basalt, lava flows rapidly away from fissures and forms widespread layers, rather than building up into a volcanic mountain. |
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flood basalts
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immense basaltic lava plateaus
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ash-flow deposits
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extensive sheets of hard volcanic tuffs produced by fissure eruptions of pyroclastic material on continents.
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fumaroles
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after major eruptions, volcanoes continue to emit gas fumes, water, and steam through small vents called fumaroles.
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hot spots
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volcanic centers found at the beginning of progressively older aseismic ridges or within a continent far from a plate boundary. Hypothesized to be the surface expression of a mantle plume.
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mantle plumes
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a narrow, cylindrical jet of hot, solid material rising from deep within the mantle and thought to be responsible for intraplate volcanism.
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lahars
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the torrential mudflows of wet volcanic debris
one of the most dangerous volcanic events can occur when a pyroclastic flow meets a river or a snowbank; when the wall of a crater lake breaks, suddenly releasing water; when a lava flow melts glacial ice; or when heavy rainfall transforms new ash deposits into mud. Lahars are known to carry huge boulders. |
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how are the structure and terrain of a volcano related to the kind of lava it emits and the style of its eruption?
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the chemical composition and gas content of lava are important factors in the form an eruption takes. Basalt can be highly fluid. On continents, it can erupt from fissures and flow out in thin sheets to build a lava plateau. A shield volcano grows from repeated eruptions of basalt from vents. Silica-rich magma is more viscous and, when charged with gas, tends to erupt explosively. the resulting pyroclastic debris may pile up into a cinder cone or cover an extensive agrea with ash-flow sheets. A stratovolcano is build of alternating layers of lava flows and pyroclastic deposits. the rapid ejection of magam from a magma chamber a few kilometers below the surface, followed by collapse of the chamber's roof, results in a large surface depression, or caldera. Giant resurgent calderas are among the most destructive natural cataclysms.
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stream
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any flowing body of water, large or small
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river
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major branches of a large stream system
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channels
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streams have these...
they allow water to flow over great distances |
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stream valley
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the entire area between the tops of the slopes on both sides of the river
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floodplain
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flat area about level with the top of the channel
lies on either side of the channel it's the part of the valley that is flooded when the river spills over its banks, carrying with it silt and sand from the main channel. |
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point bars
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eroded, curved sand bars deposited along the inside banks where the current is slower.
The build up part of meandering streams |
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oxbow lake
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crescent shaped, water-filled loop
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braided stream
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interlacing network of channels, which then rejoin in a pattern resembling braids of hair.
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natural levees
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ridge of coarse material that confine the stream within its banks between floods, even when water levels are high.
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divide
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a ridge of high ground along with all rainfall runs off down one side of the rise or the other
p. 432 good pic |
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drainage basin
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an area of land, bounded by divides, that funnels all its water into the network of streams draining the area.
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dendritic drainage
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branches similar to the limbs of a tree
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rectangular drainage
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follows joint pattern
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trellis drainage
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valley and ridge terrain
rocks of varying resistance to erosion are folded into anticlines and synclines. |
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radial drainage
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develop on a single large peak, such as a large dormant volcano.
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antecedent stream
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existed before the present topography was created, and it maintained its original course despite changes in the underlying rocks and in topography.
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superposed stream
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flows through resistant formations because its course was established at a higher level, on uniform rocks, before the downcutting began.
tends to continue the pattern that it developed earlier rather than adjusting to its new conditions. |
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laminar flow
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straight or gently curved streamlines that run parallel to one another wihtout mixing or crossing between layers.
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turbulent flow
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streamlines mix, complex, forms swirls and eddies
fast-moving river waters |
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whether a river is laminar or turbulent depends on what factors....
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1. its velocity
2. its geometry (primarily its depth) 3. Its viscosity (a measure of a fluid's resistance to flow. more viscous (thicker), more it resists flow). The higher the viscosity, the greater the tendency for laminar flow. |
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suspended load
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all the material temporarily or permanently suspended in the flow
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bed load
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the material the stream carries along the bed by sliding and rolling
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capacity
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total sedimet load carried by a flow
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settling velocity
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the speed with which suspended particles of various weights settle to the bottom
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saltation
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how sand grains move in a flow
intermittent jumping motion along the streambed. |
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delta
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a depositional platform build of sediments deposited in an ocean or lake at hte mouth of a stream
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recurrance interval
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the avg time interval btwn the occurance of 2 ruptures on the same fault that produce earthquakes or approximately equal magnitude.
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longitudinal profile of a river
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concave upward
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base level
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the elevation at which a stream ends by entering a large standing body of water, such as a lake or ocean.
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graded stream
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a stream in which the slope, velocity, and discharge combine to transport its sediment load, with neither sedimentation nor erosion.
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alluvial fan
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a cone or fan shaped accumulation of sediment deposited where a stream widens abruptly as it leaves a mountian front for an open valley.
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terrace
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a flat, steplike surface that lines a stream above the floodplain, often paired one on each side of teh stream, marking a former floodplain that existed at a higher level before regional uplift or an increase in discharge caused the stream to erode into teh former floodplain.
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How does a stream's longitudinal profile represent the equilibrium between erosion and sedimentation?
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a stream is in dynamic equilib btwn eros and sedimentation over its entire length. Topography, discharge, velocity, slope, and sediment load affect this equilib. A stream's long prof., always concave upward, is a cross section of the stream's elevation from its headwaters to the base level at its mouth in a lake or the ocean. Uplift at the upper end of a stream and the rise and fall of sea level at its lower end will change the profile. Alluvial fans form at mountain fronts, primarily as a result of an abrupt widening of the valley and secondarily as a result of a change in slope.
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what event causes the most fatalities during volcanoes?
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volcanic mudflows
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What controls metamorphism?
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external
-temp - geothermal gradients (change in temp w/depth in earth) - pressure (in kbars) - fluids Internal controls - protoliths - decarbonation reaction (releases C) - dehyrdation reaction (releases H20) |
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differential pressure stress
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pressure from ONE direction
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stress v. strain
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stress- pressure of force/area
strain- the response to stress, change of volume or shape |
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tectonic stress leads to.....
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strain
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types of teconic forces
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compressive, tensional (pulling apart), and shearing
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ductile stuff can do what...
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fold, stretch, shear
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characteristics of index fossils
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widespread, common, restricted in age
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lateral continuity
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strata extend laterally in all directions until they pinch out or reach the edge of a basin.
(in notes, where there's a chunk missing, and the doted lines show where the rocks would continue through the empty space) |
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tributary
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smaller stream that leads to bigger stream (also called trunks)
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reverse faults are caused by...
and which side is higher? |
compression or collision
right |
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normal faults caused by...
which side is higher? |
extension or tension
left |
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parent of...
marble schist amphibole quartzite gneiss |
limestone
shale basalt sandstone granite/sandstone |
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isograds
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lines of equal grade
draw at first occurance of certain mineral |
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shield v. strata volcanoes
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sh- mafic, running, spread out
strata- pyroclastic, many layers |
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tension forces....
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pull things apart
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with ductile forces, you get folds, but with brittle, you get...
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reverse faults
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plunging folds?
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when the axis of a fold is not horizontal
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tension forces....
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pull things apart
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what are the index minerals??
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....
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metamorphic facies series??
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......
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with ductile forces, you get folds, but with brittle, you get...
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reverse faults
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