ch4 review questions

Crust beneath mountains can be twice the thickness of normal continental crust.

False Due to the principle of buoyancy, the low-density continental crust that floats the highest must be balanced by the continental crust that reaches the deepest; thus the overall thickness of mountain crust exceeds the thickness of normal continental crust. (Section refs: 4.7, p. 94)

Movement along plate boundaries is slow but steady; this explains why earthquakes don't occur there.

False Plate movement is not slow and steady, and there are numerous earthquakes along plate boundaries. (Section refs: 4.2, p. 82)

Identify the False statement. Oceanic lithosphere:

gets warmer as it gets farther from the ridge axis

Oceanic lithosphere: gets older and thicker as it gets farther from the ridge axis. gets cooler as it gets farther from the ridge axis. attains its maximum thickness when it's about 80 million years old. that is 10 million years old is cool enough to be denser than the asthenosphere, so it can sink down through the asthenosphere. All of the above are true.

All of the above are true

Accretionary prisms form due to what process?

Subduction

Identify the FALSE statement

There are 20 major plates, and several micro plates

Pick out the FALSE statement. There are twelve major plates and several microplates. These plates may consist of all ocean floor or all land. These plates may consist of mainly ocean with just a little land, like the Pacific Plate composed of Pacific Ocean plus a small part of California and Baja. These plates move from one to fifteen feet per year. These plates shift the continents around as they move, so Earth's surface is constantly changing.

These plates move from one to fifteen feet per year. Plate movement is from one to fifteen centimeters per year. (Section refs: 4.8, p. 98)

A mid-ocean ridge is under water for most of its length.

True Except for Iceland, which is an island atop the Mid-Atlantic Ridge, essentially all mid-ocean ridge systems lie beneath the ocean surface. (Section refs: 4.3, p. 83)

Hot spots may occur under land, as illustrated by Yellowstone, or under the sea floor, as illustrated by Hawaii.

True Geothermal activity of Yellowstone today and its volcanic activity in the past happen because the region overlies a hot spot; the Hawaiian islands all formed as the Pacific Plate passed over the hot spot. (Section refs: 4.6, p. 91)

The Alps, Himalayas, and Appalachians are all the results of collision.

True The Alps and Appalachians were formed by continental collisions in a previous cycle of plate tectonics. The Himalayas continue to grow as the Australian-Indian Plate collides with the Eurasian Plate. (Section refs: 4.7, p. 94)

There is convective flow (with hotter rock rising, cooler sinking) within the asthenosphere.

True The asthenosphere is hot enough to flow; the lithosphere above it is not. (Section refs: 4.8, p. 95)

Slab-pull and ridge-push are the two processes that cause sea-floor spreading.

True Upwelling basalt does not push the sea floor apart; slab-pull and ridge-push are the mechanisms of sea-floor spreading. (Section refs: 4.8, p. 95)

The Himalayas are growing because: two plates are moving past each other there. a continental plate is colliding with another continental plate there. an oceanic plate is converging with a continental plate and subducting there. two plates are diverging there.

a continental plate is colliding with another continental plate there. India crashed into Asia 40 million years ago, and this continental collision produced the Himalayas. (Section refs: 4.7, p. 94)

Pick out the FALSE statement. According to present plate tectonics theory: plates move on the asthenosphere. plates move at speeds of a few centimeters per year. a continental plate will subduct under an oceanic plate when the two converge. plates are composed of the crust and the uppermost part of the mantle. all continental material started out as one supercontinent called Pangaea.

a continental plate will subduct under an oceanic plate when the two converge. Continental plates never subduct; any subducting plate is oceanic. (Section refs: 4.4, p. 86)

The global occurrence of earthquakes reveals that

they usually occur on the boundaries of plates or at hot spots

Pick out the FALSE statement: Plate boundaries where subduction occurs are also called consuming boundaries. Sea-floor spreading behind a volcanic island arc creates a small ocean basin called a back-arc basin or marginal sea. An accretionary prism is a wedge-shaped mass of sediment scraped off a subducting plate as it slides under the overriding plate. The triple junction north of San Francisco, where the San Andreas Fault heads to the sea, is the meeting of the Juan de Fuca, Pacific, and North American plates. A mid-ocean ridge is always in the middle of the ocean basin where it is located.

A mid-ocean ridge is always in the middle of the ocean basin where it is located. The Mid-Atlantic ridge is in the middle of its ocean, but this is not true for mid-ocean ridges in the Pacific and Indian oceans. (Section refs: 4.3, p. 82)

Pick out the FALSE statement. When an oceanic plate subducts under another oceanic plate, a volcanic island arc is formed. Oceanic lithosphere gets no older than 200 million years. All oceanic lithosphere eventually subducts into the mantle and probably melts there. Iceland will eventually split apart because it sits astride a diverging plate boundary. Kilimanjaro is a volcano because it is located on a hot spot.

Kilimanjaro is a volcano because it is located on a hot spot. Kilimanjaro is a volcano because it is in a continental rifting zone. (Section refs: 4.7, p. 94)

Pick out the FALSE statement. As the sea floor spreads, the asthenosphere rises, melts to become magma, and fills the space between plates. Some magma generated during sea-floor spreading spills out to produce a new layer of sea floor called gabbro. Some magma generated during sea-floor spreading erupts from submarine volcanoes. Observers in Alvin have seen submarine volcanoes. The sea floor is made of basalt.

Some magma generated during sea-floor spreading spills out to produce a new layer of sea floor called gabbro. Choice B is false; gabbro does form along the sides of magma chambers in the crust below the ridge axis, but the magma that rises to become sea floor cools to form basalt, as stated in answer E. (Section refs: 4.3, p. 84)

Pick out the FALSE statement. According to the mantle plume model, hot-spot plumes: stream upward slowly because their hot rock is less dense than the overlying rock. partially melt the base of the overlying lithosphere, and the magma thus formed rises toward the surface. produce volcanoes above themselves. all originate at the crust-mantle boundary. that exist for a long time spawn chains of volcanoes.

all originate at the crust-mantle boundary. This model proposes that plumes originate just above the core-mantle boundary. Alternative explanations of hot-spot volcanoes propose plumes at shallow depth or no plumes at all. (Section refs: 4.6, p. 91)

Pick out the FALSE statement. The rate of plate motion: can be determined to within millimeters by using the Global Positioning System (GPS). is faster along the East Pacific Rise than along the northern Mid-Atlantic Ridge. was first correctly determined by J. Tuzo Wilson in the 1960s. can be expressed as absolute plate velocity, which describes movement of a single plate relative to a fixed point. can be expressed as relative plate velocity, which describes movement of one plate in relation to another plate.

was first correctly determined by J. Tuzo Wilson in the 1960s. Wilson worked on transform faults and hot spots, not rates of plate motion. (Section refs: 4.5, p. 88)

The global occurrence of earthquakes reveals that: their locations are randomly scattered. they usually occur on the boundaries of plates or at hot spots. they usually occur at the center of plates. they prove that Earth is splitting apart (rifting) at all plate boundaries. they occur at divergent plate boundaries and hot spots only.

they usually occur on the boundaries of plates or at hot spots. Contrary to answers A and C, volcanic eruptions and earthquakes happen along plate boundaries, not in stable plate interiors. Contrary to answers D and E, the shifting of huge plates in any direction (past, into, or away from each other) creates major disruptions like earthquakes and volcanoes. (Section refs: 4.2, p. 82)

Earthquakes but not volcanoes occur in San Francisco because: two plates are moving past each other there. a continental plate is colliding with another continental plate there. an oceanic plate is converging with a continental plate and subducting there. two plates are diverging there.

two plates are moving past each other there. San Francisco is located along the San Andreas Fault, which is part of a transform plate boundary. Plates moving past each other generate earthquakes, but not molten rock and volcanics. (Section refs: 4.5, p. 88)

This diagram shows the South Island, New Zealand. The Alpine Fault cuts across the land in a northeasterly-southwesterly direction. Gold was discovered in 1861 near Queenstown on the east side of the fault. The same gold-bearing rock masses are found on the west side of the fault, but they are 480 km to the northeast. Geologists believe movement along the fault has been occurring for 23 million years. The average rate of motion along the fault is: 2.09 x 10-5 cm per year 2.09 cm per year 20.9 cm per year

2.09 cm per year 480 km x 1,000 m/km x 100 cm/m = 48,000,000 cm; 48,000,000 cm/23,000,000 years = 2.09 cm per year

It is jokingly said that in 100 million years Los Angeles, California, will have moved far enough north to become a suburb of Anchorage, Alaska. On the basis of this observation and knowing that the Pacific Plate is moving north at the rate of six centimeters per year, approximately how far is Los Angeles from Anchorage? 600 km or 372 mi 6,000 km or 3,720 mi 60,000 km or 37,200 mi

6,000 km or 3,720 mi 100,000,000 years times six cm per year (Section refs: 4.8, p. 98)

Hot spots result when columns of especially hot material move slowly beneath plates.

False Hot spots don't move; the plate moves across the stationary hot spot. (Section refs: 4.6, p. 90)

All islands along a hot-spot chain remain volcanically active for the lifetime of the mantle plume involved.

False Only the island currently over the hot spot (plume) is volcanically active. (Section refs: 4.6, p. 91)

Only one supercontinent has broken up during Earth's history.

False Relicts of ancient collision mountain ranges, like the Appalachians, predate the existence of Pangaea and demonstrate there were previous supercontinents. (Section refs: 4.7, p. 94)

The Mid-Atlantic Ridge began as a continuous mountain chain and has been broken into segments along its transform faults.

False The Mid-Atlantic Ridge began as offset segments linked by transform faults. (Section refs: 4.5, p. 88)

The San Andreas Fault is a long, narrow valley from which lava often oozes onto the surface.

False The San Andreas Fault is a transform fault that defines part of the plate boundary between the North American Plate and the Pacific Plate. Physically it is a valley, but transform motion doesn't create volcanic activity, and lava does not emerge from the valley. (Section refs: 4.5, p. 88)

The asthenosphere rising from the depths along a mid-ocean ridge causes the plates there to move apart.

False The asthenosphere passively fills the space created by the sea-floor plates moving apart; it doesn't push them apart. (Section refs: 4.8, p. 95)

Convective flow within the asthenosphere, which drags plates along, is the basic cause of plate motion.

False There is convective flow, but it cannot explain the motion of plates. (Section refs: 4.8, p. 95)

Earthquakes typically occur along passive continental margins.

False They occur along active continental margins. (Section refs: 4.2, p. 82)

Hot-spot tracks are always straight lines with the plume at one end of the track.

False This is often the case, but if the plate motion has changed, the track will have a bend in it, and if the hot spot is on a mid-ocean ridge, the track extends in two directions opposite each other from the plume location. (Section refs: 4.6, p. 91)

Pick out the FALSE statement. The direction of movement of the Pacific Plate changed about 40 million years ago, so that hot spot tracks bend. Plates move in response to two forces: the ridge-push force and the slab-pull force. The North American Plate is currently moving toward the northeast at about 10 feet per year. India crashed into Asia to create the Himalayas about 40 million years ago. Southwestern California and the Baja Peninsula are moving toward the northwest along the section of the Pacific Plate-North American Plate boundary called the San Andreas Fault.

The North American Plate is currently moving toward the northeast at about 10 feet per year. The North American Plate is moving toward the west/southwest. Even if you didn't know the direction of the plate's movement, you should have realized that answer C was false, because no plate moves faster than a few inches per year. (Section refs: 4.8, p. 98)

The chain of Hawaiian Islands extends northwest across the Pacific as shown in the figure. The island of Hawaii is the youngest, and the islands get progressively older to the northwest. Plate tectonic theory explains this as: a hot spot moves under the Pacific plate toward the southeast. a fixed hot spot is currently situated under Kauai across which the Pacific Plate is moving in a southeasterly direction. microplates break loose from the Pacific Plate as it moves, leaving this chain of islands in its wake. the rifting of the Pacific Plate, starting at the northwest end of the chain and extending southeast to Hawaii. a fixed hot spot currently situated under Hawaii across which the Pacific plate is moving in a northwesterly direction.

a fixed hot spot currently situated under Hawaii across which the Pacific plate is moving in a northwesterly direction. Contrary to answers C and D, rifting and microplates have nothing to do with the Hawaiian Islands. Contrary to answers A and B, since Hawaii is the youngest island, the stationary hot spot must be located there. The older islands to the northwest were created when their portions of plate sat over the hot spot. (Section refs: 4.6, p. 91)

The chain of volcanoes along the west coast of South America (the Andes Mountains) exists because: the Pacific Plate is sliding past the South American Plate and heading toward the northwest. the Pacific Plate is rifting apart from the South American Plate. an oceanic plate is subducting under the western edge of the South American Plate. a continental plate is colliding with the South American Plate and causing it to buckle. basalt is pushing up through the trench that parallels the west coast of South America.

an oceanic plate is subducting under the western edge of the South American Plate. Subduction at this location causes the plate to sink deep and melt. Contrary to answers A and D, neither transform motion nor continental collision produces melted rock and volcanics. Contrary to answer E, basalt does not push up through trenches. (Section refs: 4.4, p. 86)

Pick out the FALSE statement. A subducting plate: can be either continental or oceanic lithosphere. has a trench along its seaward edge. can be charted by noting its Wadati-Benioff zone. has a volcanic arc associated with it. has an accretionary prism associated with it.

can be either continental or oceanic lithosphere. A subducting plate is always oceanic lithosphere. (Section refs: 4.4, p. 86)

Pick out the FALSE statement. A mid-ocean ridge: is a submarine mountain range that winds through all the world's oceans. sits higher than surrounding sea floor because its rock is hotter and less dense than its surroundings. can be described as a continuously moving conveyor belt in which magma spills out at its center, cools, solidifies, and moves laterally away from the ridge. is covered by a layer of basalt lava blobs called dikes. is flanked by features called chimneys, which are vents that spew forth hot, mineralized, dark-colored water.

is covered by a layer of basalt lava blobs called dikes. The layer exists, but the blobs are called pillow basalt, not dikes. (Section refs: 4.3, p. 84)

Asthenosphere: is warm enough to flow slowly. subducts when it collides with continental lithosphere. is the lower layer of both continental and oceanic lithosphere. is the uppermost layer of Earth's core. contains the Wadati-Benioff zone where submarine quakes originate.

is warm enough to flow slowly Asthenosphere is part of the mantle, not the core. It supports the lithosphere, rather than being part of it or colliding with or subducting under it. (Section refs: 4.2, p. 78)

The center of a mid-ocean ridge is where: molten basalt subducts back into Earth. new oceanic lithosphere is being created. the Hawaiian Island chain formed. two continental plates converge. you find the oldest, densest, hottest oceanic crust.

new oceanic lithosphere is being created. Contrary to answer A, molten basalt is rising, not sinking, in the trough. Contrary to answers C and D, the ridge has nothing to do with the Hawaiian hot spot or with continental convergence. It's associated with an oceanic divergent boundary. Contrary to answer E, here the crust is the hottest, but it's also the youngest, least dense of all ocean crust. (Section refs: 4.3, p. 83)

Continents: plow their way through the sea floor. passively ride along as the sea floor spreads. may subduct if they are old and dense enough. consist of rock that is younger than sea-floor rock. have retained the same size and shape throughout Earth's history.

passively ride along as the sea floor spreads. Continents don't move through the sea floor but are passively moved by the sea floor. Contrary to answers C and D, continental lithosphere (continental plate) is always older and less dense than oceanic lithosphere, so continents never subduct under the ocean. Contrary to answer E, continents' shapes have changed as they have merged or split. (Section refs: 4.4, p. 86)