ch3 review questions

Radiometric dating of a magnetic anomaly stripe of rock that is 225 km away from the mid-ocean ridge axis gives an age of 4.5 million years. Assuming a constant rate, sea-floor spreading in this area is: 50 km per year. 1,012.5 km per year. 5 cm per year. 20,000 cm per year. Only A and B are true.

5 cm per year. 225 km 1,000 m/km 100 cm/m = 22,500,000 cm; 22,500,000 4,500,000 yr = 5 cm per year (Section refs: 3.6, p. 72)

Identify the FALSE statement. Many individuals were involved in developing the theory of plate tectonics, including: Vine, Matthews, and Morley, who interpreted the significance of marine magnetic anomalies. Arthur Holmes, who believed giant convection cells moved hot rock upward within the Earth. Harry Hess, who was the first scientist to notice the fit of the continents. Robert Dietz, who coined the expression "sea-floor spreading." Alfred Wegener, who jeopardized his professional career by proposing the idea of drifting continents.

Harry Hess, who was the first scientist to notice the fit of the continents. Harry Hess cautiously proposed that formation of new ocean floor causes the sea floor to spread and the ocean basin to widen. The fit of the continents was noticed by scholars in the 1500s. (Section refs: 3.1, p. 56)

Which of the following statements is NOT true about the ocean floor? It is covered by a layer of sediment composed of clay and plankton shells. The heat flow through it is greatest at the mid-ocean ridges. Oceanic crust contains granite and metamorphic rocks. Ocean crust contains basalt and gabbro. Oceanic crust is quite different from continental crust.

Oceanic crust contains granite and metamorphic rocks. The only rock types found in oceanic crust are basalt and gabbro. (Section refs: 3.4, p. 66)

Earth's magnetism changes through slight yearly shifting of the magnetic poles and occasional reversals of polarity.

True Earth's magnetic poles shift by a fraction of a degree yearly and the magnetic poles reverse suddenly and sporadically on a time scale of thousands of years, but the two phenomena are not related. (Section refs: 3.3, p. 61)

Earth's magnetism is due to the flow of liquid iron in Earth's molten outer core.

True The flow of iron alloy in Earth's molten outer core makes it an electromagnet. (Section refs: 3.3, p. 60)

Locations of past glaciers can be deduced by deposits of till.

True Till is a glacial deposit so its presence indicates past glaciation. (Section refs: 3.2, p. 57)

Military needs during World War II resulted in increased study of the sea floor.

True War-related action on and in the oceans prompted studies of the sea environment. (Section refs: 3.4, p. 65)

Deposits of coal, reef, sand dunes, and salt were found where Wegener predicted; this supported his ideas.

True Wegener believed certain land masses currently at northern latitudes had moved there from more southern latitudes. Discovery of these subtropical deposits supported his beliefs. (Section refs: 3.2, p. 59)

Identify the FALSE statement. Bathymetric maps show the following features of the sea floor: broad flat abyssal plains at depths of 4 to 5 km below sea level. elongate mountain ranges whose peaks lie 2 to 2.5 km below sea level. elongate trenches that reach 8 to 12 km deep. chains of seamounts (submarine mountains), some of which are active volcanoes. bands of vertical fractures (fracture zones) that run parallel to mid-ocean ridges.

bands of vertical fractures (fracture zones) that run parallel to mid-ocean ridges. The fracture zones run at right angles to the ridges and thus segment them into small pieces. (Section refs: 3.4, p. 66)

Earth's magnetic field is created by

flow of liquid iron in Earth's molten outer core

ocean crust

is covered by a thin blanket of sediment that thickens away from the ridge axis

Late Paleozoic glaciation can be explained by assuming that the southern part of Pangaea was located over the North Pole.

False It was at the South Pole. (Section refs: 3.2, p. 58)

Sea-floor crust is the same age and is made up of the same type rock as continental crust.

False Oceanic crust is basalt and is much younger than continental crust. (Section refs: 3.4, p. 66)

All mountains in a seamount chain are active volcanoes.

False Only one end—the youngest—is active. (Section refs: 3.4, p. 66)

Science had no knowledge of submarine mountain ranges and oceanic deeps before World War II.

False Soundings in the 1870s hinted at their existence. (Section refs: 3.4, p. 65)

Which of the following statements is false?

Ocean basins get narrower as a result of sea-floor spreading.

All the mountains of a volcanic arc are active.

The volcanic mountains are the result of a plate subducting along the entire length of the arc. (Section refs: 3.4, p. 65)

Only one supercontinent has existed and split apart in all of Earth's history.

There has been sufficient time for several cycles of splitting, and scientists are convinced that there have been several. (Section refs: 3.1, p. 56)

Plate tectonics theory took decades to be accepted because:

Wegener's mechanism was implausible

Which of the following statements is true? Ocean basins get wider as a result of sea-floor spreading. Ocean floor is destroyed by the process of subduction. To keep Earth's surface area constant, the amount of sea floor spreading must equal the amount of subduction. Subduction causes continents to move together; sea floor spreading moves them apart. All of the above are true.

All of the above are true.

Identify the FALSE statement. The time intervals between Earth's magnetic reversals are called polarity chrons. All polarity chrons are the same length of time; they differ only in the magnetic strengths they represent. The series of rock stripes parallel to and bilaterally symmetrical across the mid-ocean ridge record the sequence of Earth's magnetic reversals over time. The width of each rock stripe is a measure of how long the polar direction remained constant. Positive magnetic anomalies occur over areas of sea floor when the poles of Earth's magnetic field and the paleopoles preserved in the sea-floor basalt agree; negative anomalies occur when they are opposite each other.

All polarity chrons are the same length of time; they differ only in the magnetic strengths they represent. Time intervals between reversals differ, so the lengths of polarity chrons also differ. Polarity chrons have nothing to do with magnetic strength. (Section refs: 3.6, p. 69)

Identify the FALSE statement about paired stripes of magnetic anomaly rock across the mid-ocean ridge. Dividing the age of the rock stripe by its distance from the ridge axis gives the rate of sea-floor spreading. The oldest stripes are farthest from the ridge. The rock is rich in iron, enabling it to record magnetism. The wider the stripe, the longer the time interval between reversals. The stripes are slightly irregular in shape because sea-floor spreading occurs in sporadic bursts.

Dividing the age of the rock stripe by its distance from the ridge axis gives the rate of sea-floor spreading. Divide the distance by the age to get the rate of sea-floor spreading. (Section refs: 3.6, p. 72)

No one noticed the fit of the continents until computers were available to manipulate the images.

False (The fit of the continents was noticed as early as the 1500s. (Section refs: 3.2, p. 57)

High heat-flow values along the mid-ocean ridge are explained by hot molten rock (magma) sinking into the troughs on its return to the mantle.

False The magma here is rising, not sinking back into the mantle. (Section refs: 3.4, p. 66)

The sea floor is covered by a thick, uniform blanket of sediment.

False The sediment blanket is surprisingly thin and varies in thickness depending on the distance of the sea floor from the ridge. (Section refs: 3.6, p. 74)

Earthquake epicenters in oceanic basins are found only on the abyssal plains, never by deep-ocean trenches or mid-ocean ridges.

False They're found by deep-ocean trenches, along mid-ocean ridges, and along fracture zones, not on the abyssal plains. (Section refs: 3.4, p. 67)

Plate tectonics theory was widely accepted by scientists in the 1930s.

False This theory was not generally accepted until the 1960s. (Section refs: 3.1, p. 56)

The relative widths of magnetic anomalies on the sea floor correspond to the: intervals between magnetic reversals. relative durations of polarity epochs. magnetic strengths of the anomalies. all of the above Only A and B are true.

Only A and B are true. Choice B is just a more scientific way of stating choice A; choice C is wrong because the strength of the magnetic field has nothing to do with the width of the anomaly. (Section refs: 3.6, p. 72)

Plate tectonics theory took decades to be accepted because: most of Wegener's ideas turned out to be wrong. of pure stubbornness by the scientific community. Wegener couldn't explain how continents moved. climate, fossil distributions, and land shape offered conflicting evidence. communications were bad, so only German scientists were familiar with Wegener's theory.

Wegener couldn't explain how continents moved. Wegener could not adequately explain how or why the continents moved. Sea-floor studies following World War II supplied the needed proof. Contrary to answer A, Wegener's basic ideas were correct. Contrary to answers B and E, Wegener spoke and published worldwide, and the scientific community was reluctant but not totally opposed to listening. Contrary to answer D, these lines of evidence all supported plate tectonics theory. (Section refs: 3.1, p. 56)

Choose the FALSE statement. Pangaea: means "all land." started to break apart during the Mesozoic. broke apart due to the force created by Earth's rotation. is the name for the most recent supercontinent; others have existed. is Wegener's name for the supercontinent that split to produce our continents of today.

broke apart due to the force created by Earth's rotation. Earth's rotational force is a million times too small to cause Pangaea to break apart. (Section refs: 3.1, p. 56)

Choose the FALSE statement. Magnetic anomalies are: places where the magnetic field strength is either greater or less than the expected strength. termed normal if the atomic dipoles match Earth's current magnetic field and point from north to south. found only on the sea floor. measured with an instrument called a magnetometer. the sum of Earth's dipole field and the magnetism of near-surface rock.

found only on the sea floor. Magnetic anomalies are found on continents as well as on the sea floor. They create irregular patterns on continents and distinct alternating bands on the sea floor. (Section refs: 3.6, p. 68)

Choose the FALSE statement. Magnetic reversals: have occurred many times, but not at regular intervals. reflect changes in the direction of flow of molten iron in Earth's outer core. happen slowly as the magnetic poles wander and finally cross the equator. happen in a geologically short time, possibly as little as one thousand years. are a global phenomenon.

happen slowly as the magnetic poles wander and finally cross the equator. (The magnetic pole does wander slowly, but it always stays relatively close to the geographic pole; reversals are a separate phenomenon. (Section refs: 3.6, p. 68))

Choose the FALSE statement. The paleomagnetic record: is preserved in strips of rock parallel to the mid-ocean ridge. is preserved in basalt layers on land. is symmetrical across the mid-ocean ridge. is a puzzle; it contradicts the sea-floor-spreading concept. shows magnetic reversals over geologic time.

is a puzzle; it contradicts the sea-floor-spreading concept. The paleomagnetic record offers strong support for sea-floor spreading. (Section refs: 3.6, p. 68)

The rate of sea-floor spreading: is the same worldwide. varies between 1 and 6 feet per year. is faster along the Mid-Atlantic Ridge than along the East Pacific Rise. has definitely remained constant throughout Earth's history. is fast enough to account for the formation and destruction of oceans many times in Earth's history.

is fast enough to account for the formation and destruction of oceans many times in Earth's history. Regarding answers A and D, the rate varies around the world and can be confirmed as constant for only the last 4.5 million years. Regarding answer B, it ranges from less than an inch to a few inches per year. Regarding answer C it is slower along the Mid-Atlantic Ridge than it is along the East Pacific Rise. (Section refs: 3.6, p. 72)

Choose the FALSE statement. The age of oceanic crust: has been determined by radiometric dating. is oldest at the mid-ocean ridge and youngest farthest from the ridge. varies with location. correlates well with calculated rates of sea-floor spreading. has been confirmed by sea-floor drilling.

is oldest at the mid-ocean ridge and youngest farthest from the ridge. It is youngest at the ridge, oldest far from the ridge. (Section refs: 3.6, p. 74)

Choose the FALSE statement. Pangaea:

is the name for the most recent supercontinent; no others have existed

Cynognathus, Glossopteris, Mesosaurus, and Lystrosaurus are: the names of the most recent periods of magnetic reversal. land-dwelling species whose distribution suggested joined continents. names of supercontinents that existed before Pangaea. distinctive assemblages of rocks. scientists who supported Wegener's proposals.

land-dwelling species whose distribution suggested joined continents. These are the names of land-dwelling reptiles and plants whose distribution in late Paleozoic and early Mesozoic times suggested joined continents.

Choose the FALSE statement. Glaciers: are slow-moving sheets of ice on land. leave deposits of sediment called bathymetries. covered large areas of continents during ice ages. occurred during the late Paleozoic in places they cannot exist today. occur today at high altitudes or in polar regions.

leave deposits of sediment called bathymetries. Glacial deposits are called till; bathymetry is the shape (topography) of the sea-floor surface. (Section refs: 3.2, p. 57)

Which is NOT one of the ideas Wegener offered to support his theory? the good fit of the outline of the continents the matching of the distribution of similar fossils across oceans the existence of the mid-ocean ridge, where sea-floor spreading starts paleoclimatic evidence of extreme climate changes in some areas the matching of similar rock types and structures across oceans

the existence of the mid-ocean ridge, where sea-floor spreading starts Sea-floor studies didn't occur until after Wegener's death. (Section refs: 3.2, p. 57)

Which of the following is NOT on of the ideas Wegener offered to support his theory?

the existence of the mid-ocean ridge, where the seafloor spreading starts

Which of the following is NOT a line of evidence used by Wegener to develop his theory that continents drift?

the presence of earthquakes in seismic belts along trenches, ridges, and fracture zones

Choose the one type of deposit that is not typical of a tropical/subtropical climate. coal reefs sand dunes salt till

till Till is glacial sediment. (Section refs: 3.2, p. 57)