Chapter 4 - The Way the Earth Works: Plate Tectonics Flashcards
Earth’s lithosphere is divided into about ___ moving plates.
20
plate movement takes place at plate boundaries; this movement causes earthquakes, so the distribution of earthquake belts defines plate boundaries.
T or F?
T
geologists distinguish three types of plate boundaries based on the relative movement across the boundary. Distinct geologic features characterize each type.
True that.
we can directly measure plate motion by using ___?
GPS
lithosphere is?
consists of the crust plus the top (cooler) part of the upper mantle.
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The lith-osphere floats on a relatively soft, or “plastic,” layer called the ___?
Aesthenosphere. composed of warmer ( > 1,280°C) mantle that can flow slowly when acted on by force. As a result, the astheno-sphere convects, like water in a pot, though much more slowly
absolute plate velocity (p. 98)
if we describe the movement of both plates relative to a fixed location in the mantle below the plates, then we are speaking of absolute plate velocity
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accretionary prism (p. 86)
In the Peru-Chile Trench, as the downgoing plate slides under the overriding plate, sediment (clay and plankton) that had settled on the surface of the downgoing plate, as well as sand that fell into the trench from the shores of South Amer-ica, gets scraped up and incorporated in a wedge-shaped mass known as an accretionary prism ( Fig. 4.8c ). An accretionary prism forms in basically the same way as a pile of snow in front of a plow, and like the snow, the sediment tends to be squashed and contorted.
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active margin (p. 80)
Geoscientists distinguish 12 major plates and several microplates. Note that some plates consist entirely of oceanic lithosphere whereas some plates consist of both oceanic and continental lithosphere. Some plates have familiar names (the North American Plate, the African Plate), whereas some do not (the Cocos Plate, the Juan de Fuca Plate).Some plate boundaries follow continental margins, the boundary between a continent and an ocean, but others do not. For this reason, we distinguish between active margins , which are plate boundaries, and passive margins , which are not plate boundaries
asthenosphere (p. 78)
The lith-osphere floats on a relatively soft, or “plastic,” layer called the asthenosphere , composed of warmer ( > 1,280°C) mantle that can flow slowly when acted on by force. As a result, the astheno-sphere convects, like water in a pot, though much more slowly
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black smoker (p. 84)
Some magma rises all the way to the surface of the sea floor at the ridge axis and spills out of small submarine volcanoes. The resulting lava cools to form a layer of basalt blobs or “pillows” ( Fig. 4.5a ). We can’t easily see the submarine volcanoes because they occur at depths of more than 2 km beneath sea level, but they have been observed by the research submarine Alvin . Observers in the submarine have also detected chimneys spewing hot, mineralized water rising from cracks in the sea floor after being heated by magma below the surface. These chimneys are called black smokers because the water they emit looks like a cloud of dark smoke; the color comes from a suspension of tiny mineral grains that precipitate in the water the instant that the water cools
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buoyancy (p. 80)
Buoyancy is the upward force acting on an object immersed or floating in a fluid.
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collision (p. 91)
A convergent boundary ceases to exist when a piece of buoyant lithosphere, such as a continent or an island arc, moves into the subduction zone and, in effect, jams up the system. We call this process collision
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continental rift (p. 91)
A continental rift is a linear belt in which continental lithosphere undergoes rifting, or pulls apart ( Fig. 4.13a ). The lithosphere stretches horizontally, so it thins vertically, much like a piece of taffy you pull between your fingers. Near the surface of the continent, where the crust is cold and brittle, stretching causes rock to break and faults to develop. As a consequence of faulting, blocks of rock slide down, leading to the for-mation of a low area that gradually becomes buried by sediment.
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continental shelf (p. 80)
Along passive margins, continental crust is thinner than normal. (As we’ll discuss later, this thinning occurs when continents break apart. During this process, the upper part breaks into wedge-shaped slices.) Thick (10–15 km) accumulations of sediment cover this thinned crust; this accumulation is a “passive-margin basin” (see Fig. 4.1b). The surface of this sedi-ment layer is a broad, shallow (less than 500 m deep) region called the continental shelf , home to the major fisheries of the world. Some plates consist entirely of oceanic lithosphere or entirely of continental lithosphere, whereas some plates consist of both.
convergent boundary (p. 82)
A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary .
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divergent boundary (p. 82)
A boundary at which two plates move apart from each other is a divergent boundary .
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fracture zone (p. 88)
When researchers began to explore the bathymetry of mid-ocean ridges in detail, they discovered that mid-ocean ridges are not long, uninterrupted lines, but rather consist of short segments that appear to be offset laterally from each other ( Fig. 4.9a ). Narrow belts of broken and irregular sea floor lie roughly at right angles to the ridge segments, intersect the ends of the segments, and extend beyond the ends of the segments. These belts came to be known as fracture zones . Originally, researchers incorrectly assumed that the entire length of each fracture zone was a fault, and that slip on a fracture zone had displaced segments of the mid-ocean ridge sideways, relative to each other.
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global positioning system (GPS) (p. 99)
destinations, and geolo-gists use them to monitor plate motions. If we calculate care-fully enough, we can detect displacements of millimeters per year. In other words, we can now see the plates move—this serves as the ultimate proof of plate tectonics.
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hot spot (p. 90)
interior of the North American Plate. Worldwide, geoscientists have identi-fied about 100 volcanoes that exist as isolated points and are not a conse-quence of movement at a plate bound-ary. These are called hot-spot volcanoes, or simply hot spots ( Fig. 4.11 ). Most hot spots are located in the interiors of plates, away from the boundaries, but a few grew on mid-ocean ridges.
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hot-spot track (p. 91)
When the hot rock of the plume reaches the base of the lithosphere, it partially melts (for reasons discussed in Chapter 6) and produces magma that seeps up through the lithosphere to the Earth’s surface. The chain of extinct vol-canoes, or hot-spot track , forms when the overlying plate moves over a fixed plume.
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lithosphere (p. 78)
We learned earlier that geoscientists divide the outer part of the Earth into two layers. The lithosphere consists of the crust plus the top (cooler) part of the upper mantle. It behaves rela-tively rigidly, meaning that when a force pushes or pulls on it, it does not flow but rather bends or breaks
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lithosphere plate (p. 80)
The lithosphere forms the Earth’s relatively rigid shell. But unlike the shell of a hen’s egg, the lithospheric shell contains a number of major breaks, which separate it into distinct pieces. As noted earlier, we call the pieces lithosphere plates , or simply plates. 1 The breaks between plates are known as plate boundaries
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mantle plume (p. 91)
In Wilson’s model, the active volcano represents the present-day location of the heat source, whereas the chain of dead volcanic islands represents locations on the plate that were once over the heat source but progressively moved off. A few years later, Jason Morgan suggested that the heat source for hot spots is a mantle plume , a column of very hot rock rising up through the mantle to the base of the lithosphere
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passive margin (p. 80)
Geoscientists distinguish 12 major plates and several microplates. Note that some plates consist entirely of oceanic lithosphere whereas some plates consist of both oceanic and continental lithosphere. Some plates have familiar names (the North American Plate, the African Plate), whereas some do not (the Cocos Plate, the Juan de Fuca Plate).Some plate boundaries follow continental margins, the boundary between a continent and an ocean, but others do not. For this reason, we distinguish between active margins , which are plate boundaries, and passive margins , which are not plate boundaries.
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plate boundary (p. 80)
The lithosphere forms the Earth’s relatively rigid shell. But unlike the shell of a hen’s egg, the lithospheric shell contains a number of major breaks, which separate it into distinct pieces. As noted earlier, we call the pieces lithosphere plates , or simply plates. 1 The breaks between plates are known as plate boundaries
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plate tectonics (p. 78)
This idea, which we now call the theory of plate tectonics, or simply plate tectonics , required geologists to cast aside hypotheses rooted in the para-digm of fixed continents, and thus led to a complete restruc-turing of how geologists think about Earth history.
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relative plate velocity (p. 98)
Geologists use two different frames of reference for describing plate velocity (remember: velocity = distance/time). If we describe the movement of plate A with respect to plate B, then we are speaking about relative plate velocity .
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ridge-push force (p. 95)
Ridge-push force develops because the lithosphere of mid-ocean ridges lies at a higher elevation than that of the adjacent abyssal plains
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rifting (p. 91)
How does a new diver-gent boundary come into existence, and how does an exist-ing convergent boundary eventually cease to exist? Most new divergent boundaries form when a continent splits and sep-arates into two continents. We call this process rifting .
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slab-pull force (p. 95)
Slab-pull force , the force that subducting, downgoing plates apply to oceanic lithosphere at a convergent margin, arises simply because lithosphere that was formed more than 10 mil-lion years ago is denser than asthenosphere, so it can sink into the asthenosphere
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subduction (p. 85)
At convergent plate boundaries, two plates, at least one of which is oceanic, move toward one another. But rather than butting each other like angry rams, one oceanic plate bends and sinks down into the asthenosphere beneath the other plate. Geologists refer to the sinking process as subduction , so convergent boundaries are also known as subduction zones.
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transform boundary (pp. 82, 88)
Wilson introduced the term transform boundary (or transform fault) for the actively slipping segment of a frac-ture zone between two ridge segments, and he pointed out that these are a third type of plate boundary. At a transform boundary, one plate slides sideways past another, but no new plate forms and no old plate is consumed. Transform boundar-ies are, therefore, defined by a vertical fault on which the slip direction parallels the Earth’s surface.
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trench (p. 85)
Because subduction at a convergent boundary consumes old ocean lithosphere and thus closes (or “consumes”) oceanic basins, geologists also refer to convergent boundaries as con-suming boundaries, and because they are delineated by deep-ocean trenches, they are sometimes simply called trenches
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triple junction (p. 88)
Geologists refer to places where three plate boundaries intersect at a point as triple junctions , and name them after the types of boundaries that intersect.
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volcanic arc (p. 86)
A chain of volcanoes known as a volcanic arc develops behind the accretionary prism
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Wadati-Benioff zone (p. 86)
In fact, geologists have detected earthquakes within downgoing plates to a depth of 660 km. The band of earthquakes in a downgoing plate is called a Wadati-Benioff zone , after its two discoverers
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At a divergent plate boundary, plates move apart due to sea-floor spreading. As the plates move, new ocean lithosphere forms in between.
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yes
The youngest plate at a divergent boundary is along the ridge axis; the sea floor gets progressively older away from the axis.
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yes
Ocean crust forms from magma that rises along the mid-ocean ridge axis.
Copyright | W. W. Norton & Company | Earth: Portrait of a Planet (Fourth Edition) | charmsofgold@gmail.com | Printed from www.chegg.com
yes
As plates move away from the axis, they cool and the lithospheric mantle forms and thickens.
Copyright | W. W. Norton & Company | Earth: Portrait of a Planet (Fourth Edition) | charmsofgold@gmail.com | Printed from www.chegg.com
yes
Triple junctions exist where three plate boundaries intersect at a point. ● Hot spots are places where volcanism happens independently of plate motion. ● Most hot spots occur in the interior of plates, away from plate boundaries, but a few lie at mid-ocean ridges. ● As plates move over a hot spot, a hot-spot track of inactive volcanoes forms. ● Hot spots appear to be associated with mantle plumes, but this idea remains controversial.
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