Martian Volcanoes
Earth is not the only planet to have volcanoes; in fact Mars-though all of its volcanoes have long been inactive-has the two largest ones in the solar system: Olympus Mons and Alba Patera. Mars has spectacular lava fields, large craters formed by a volcanic explosion, and shields (broad, low volcanoes shaped like shallow domes).
The most striking characteristic of Martian volcanoes is in fact their great size and longevity. The giant shields of Tharsis and to a lesser extent the volcanoes of Elysium and Hellas pack in many times more lava than the largest volcanoes on Earth, and yet Mars is a smaller planet. The reason lies in the planets’ internal makeup. Because it is larger and hotter, Earth has a nearly molten (hot liquid) mantle that is overlain by a very thin rock crust-less than 10 kilometers thick under the ocean basins. The hot mantle churns over in great loops, and rips the overlying rigid crust into great slabs, called tectonic plates, that shuffle around the globe. In those places where deep-seated “hot spots” send plumes of molten material to the surface, the magma (the trapped lava) starts piling up on the crust to build a volcano, but because the crust is drifting, the volcano eventually breaks away from its source and goes extinct. In its place, a new volcano starts to grow, so that a hot spot under a mobile plate creates a string of medium-sized volcanoes rather than one giant shield.
On the smaller, faster-cooling Mars, the crust has solidified and thickened to the extent that it remains stationary with respect to the underlying hot spots. When a plume of magma erupts at the surface, it will build a single volcano in one place for as long as the supply lasts-typically hundreds of millions of years. Not surprisingly, then, the volume of one Martian shield like Olympus Mons or Alba Patera is comparable to that of an entire chain of volcanoes on Earth, such as the Hawaiian Emperor chain that stretches across the Pacific and spans nearly 100 million years of hot-spot activity.
Another characteristic of Martian volcanoes is the large size of their craters and the long run-out distances of their lava flows. This is principally due to the lower gravity on Mars: 38 percent, or about one third, of Earth’s gravity. Gravity controls to some extent how solid rock behaves under stress. When the crust stretches and breaks in a low gravity field, as it does on Mars, the fissures that open up are wider than on Earth and can funnel larger amounts of magma toward the surface.
Magma chambers grow correspondingly larger and when they empty out and collapse, they yield larger craters. Eruption rates also tend to be higher on Mars, since greater quantities of lava can flow out of the wider fissures. These larger volumes guarantee a better retention of heat. Because it stays hot and molten for a longer period of time, lava on Mars travels greater distances than it does on Earth before cooling and slowing to a halt.
Explosive eruptions are also affected by the unique set of conditions that exist on Mars. The atmospheric pressure is so low that any gas bubbles trapped in the magma will undergo tremendous expansion upon reaching the surface. As a result, it will take lesser amounts of dissolved gases in Martian (versus terrestrial)magma for the mixture to foam and shoot out explosively from the vent. On Earth, the confining pressure of the atmosphere requires that magma contains close to 1 percent (by weight) of light gases for it to spray upward as a lava fountain. About 3 to 4 percent is needed for the bubbles to grow large enough to blow the magma to shreds and create an ash cloud. On Mars, thresholds for such disruptive behavior are much lower: respectively 0.03 and 0.2 percent gases by weight. Twenty to thirty times less gas is needed on Mars to achieve similar results.
1
The most striking characteristic of Martian volcanoes is in fact their great size and longevity. The giant shields of Tharsis and to a lesser extent the volcanoes of Elysium and Hellas pack in many times more lava than the largest volcanoes on Earth, and yet Mars is a smaller planet. The reason lies in the planets’ internal makeup. Because it is larger and hotter, Earth has a nearly molten (hot liquid) mantle that is overlain by a very thin rock crust-less than 10 kilometers thick under the ocean basins. The hot mantle churns over in great loops, and rips the overlying rigid crust into great slabs, called tectonic plates, that shuffle around the globe. In those places where deep-seated “hot spots” send plumes of molten material to the surface, the magma (the trapped lava) starts piling up on the crust to build a volcano, but because the crust is drifting, the volcano eventually breaks away from its source and goes extinct. In its place, a new volcano starts to grow, so that a hot spot under a mobile plate creates a string of medium-sized volcanoes rather than one giant shield.
The word “longevity” in the passage is closest in meaning to
Alength of life
Blevel of activity
Ccomposition
Ddistribution
2
The most striking characteristic of Martian volcanoes is in fact their great size and longevity. The giant shields of Tharsis and to a lesser extent the volcanoes of Elysium and Hellas pack in many times more lava than the largest volcanoes on Earth, and yet Mars is a smaller planet. The reason lies in the planets’ internal makeup. Because it is larger and hotter, Earth has a nearly molten (hot liquid) mantle that is overlain by a very thin rock crust-less than 10 kilometers thick under the ocean basins. The hot mantle churns over in great loops, and rips the overlying rigid crust into great slabs, called tectonic plates, that shuffle around the globe. In those places where deep-seated “hot spots” send plumes of molten material to the surface, the magma (the trapped lava) starts piling up on the crust to build a volcano, but because the crust is drifting, the volcano eventually breaks away from its source and goes extinct. In its place, a new volcano starts to grow, so that a hot spot under a mobile plate creates a string of medium-sized volcanoes rather than one giant shield.
In paragraph 2, why does the author provide the information that Mars is smaller than Earth?
ATo clarify a common misconception about Mars
BTo help explain the size of Earth’s volcanoes
CTo emphasize the surprising size of Martian volcanoes
DTo indicate essential differences between the two planets
3
On the smaller, faster-cooling Mars, the crust has solidified and thickened to the extent that it remains stationary with respect to the underlying hot spots. When a plume of magma erupts at the surface, it will build a single volcano in one place for as long as the supply lasts-typically hundreds of millions of years. Not surprisingly, then, the volume of one Martian shield like Olympus Mons or Alba Patera is comparable to that of an entire chain of volcanoes on Earth, such as the Hawaiian Emperor chain that stretches across the Pacific and spans nearly 100 million years of hot-spot activity.
Which of the sentences below best expresses the essential information in the hiahliahted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
AA comparison of the Hawaiian Emperor chain. Olympus Mons.and Alba Patera shows they are all about 100 million years old.
BThe volume, then. of one Martian shield is comparable to the entire number of volcanoes on Earth, spanning nearly100 million years
CLogically. then, the volume of a single Martian shield is about the same as a chain of volcanoes on Earth.
DA chain of volcanoes like Earth’s Hawaiian Emperor chain would take nearly 100 million years to form on Mars.
4
On the smaller, faster-cooling Mars, the crust has solidified and thickened to the extent that it remains stationary with respect to the underlying hot spots. When a plume of magma erupts at the surface, it will build a single volcano in one place for as long as the supply lasts-typically hundreds of millions of years. Not surprisingly, then, the volume of one Martian shield like Olympus Mons or Alba Patera is comparable to that of an entire chain of volcanoes on Earth, such as the Hawaiian Emperor chain that stretches across the Pacific and spans nearly 100 million years of hot-spot activity.
According to paragraph 3, why do volcanoes grow larger on Mars than they do on Earth?
AThe underlying hot spots erupt from much greater depths on Mars than on Earth.
BThe magma on Mars is not as thick as on Earth, allowing for faster expansion.
CThe interior of Mars has a larger supply of magma than does the interior of Earth.
DThe thicker crust on Mars allows volcanoes to grow in the same place for much longer periods.
5
Another characteristic of Martian volcanoes is the large size of their craters and the long run-out distances of their lava flows. This is principally due to the lower gravity on Mars: 38 percent, or about one third, of Earth’s gravity. Gravity controls to some extent how solid rock behaves under stress. When the crust stretches and breaks in a low gravity field, as it does on Mars, the fissures that open up are wider than on Earth and can funnel larger amounts of magma toward the surface.
According to paragraph 4, the low gravitational field of Mars causes
Afissures in the rocks to be quite large
Bmagma to flow more slowly to the surface
Cthe planet’s crust to break more often
Dmagma chambers to last longer
6
Magma chambers grow correspondingly larger and when they empty out and collapse, they yield larger craters. Eruption rates also tend to be higher on Mars, since greater quantities of lava can flow out of the wider fissures. These larger volumes guarantee a better retention of heat. Because it stays hot and molten for a longer period of time, lava on Mars travels greater distances than it does on Earth before cooling and slowing to a halt.
The word”retention” in the passage is closest in meaning to
Aintensity
Bdispersal
Cproduction
Dconservation
7
Magma chambers grow correspondingly larger and when they empty out and collapse, they yield larger craters. Eruption rates also tend to be higher on Mars, since greater quantities of lava can flow out of the wider fissures. These larger volumes guarantee a better retention of heat. Because it stays hot and molten for a longer period of time, lava on Mars travels greater distances than it does on Earth before cooling and slowing to a halt.
According to paragraph 5, why does molten lava stay hotter on Mars’s surface than on Earth’s?
AIt comes from hotter sources.
BIt has been under the crust longer.
CIt travels shorter distances.
DIt flows in larger amounts
8
Explosive eruptions are also affected by the unique set of conditions that exist on Mars. The atmospheric pressure is so low that any gas bubbles trapped in the magma will undergo tremendous expansion upon reaching the surface. As a result, it will take lesser amounts of dissolved gases in Martian (versus terrestrial)magma for the mixture to foam and shoot out explosively from the vent. On Earth, the confining pressure of the atmosphere requires that magma contains close to 1 percent (by weight) of light gases for it to spray upward as a lava fountain. About 3 to 4 percent is needed for the bubbles to grow large enough to blow the magma to shreds and create an ash cloud. On Mars, thresholds for such disruptive behavior are much lower: respectively 0.03 and 0.2 percent gases by weight. Twenty to thirty times less gas is needed on Mars to achieve similar results.
Assuming that terrestrial and Martian magma have similar levels of gas. which of the following can be inferred from paragraph 6 about Martian volcanic eruptions?
AThey are less affected by atmospheric pressure levels than by other atmospheric conditions.
BThey produce ash clouds more frequently than terrestrial eruptions do.
CThey include lava sprays 3 percent to 4 percent of the time
DThey produce lava fountains less often than they produce ash clouds.
9
The most striking characteristic of Martian volcanoes is in fact their great size and longevity. The giant shields of Tharsis and to a lesser extent the volcanoes of Elysium and Hellas pack in many times more lava than the largest volcanoes on Earth, and yet Mars is a smaller planet. The reason lies in the planets’ internal makeup. Because it is larger and hotter, Earth has a nearly molten (hot liquid) mantle that is overlain by a very thin rock crust-less than 10 kilometers thick under the ocean basins. [■]The hot mantle churns over in great loops, and rips the overlying rigid crust into great slabs, called tectonic plates, that shuffle around the globe. [■]In those places where deep-seated “hot spots” send plumes of molten material to the surface, the magma (the trapped lava) starts piling up on the crust to build a volcano, but because the crust is drifting, the volcano eventually breaks away from its source and goes extinct. [■]In its place, a new volcano starts to grow, so that a hot spot under a mobile plate creates a string of medium-sized volcanoes rather than one giant shield. [■]
Look at the four squaresthat indicate where the following sentence could be added to the passage
These series of volcanoes will continue to be created until the source of lava dries up.
Where would the sentence best fit？Click on a square sentence to the passage.
10
The unique characteristics of Mars have created the largest volcanoes in the solar system.
AAlthough it is smaller than Earth, Mars is hotter, and its mantle, the layer of molten rock beneath the crust, is larger.
BVolcanoes on Earth are much smaller than those on Mars because Earth’s moving crust causes the volcanoes to go extinct quickly.
CMars’s solid crust and low gravity and atmospheric pressure create large volcanoes that release large amounts of lava for a long time.
DMars’s volcanoes, like those on Earth, begin when hot spots deep in the molten mantle release streams of magma up through the surface crust
EThe many extremely large volcanoes on Mars have caused the planet’s crust to thicken and remain stationary.
FThe release of gases from magma on Mars causes the magma to cool down much more quickly than it does on Earth.
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