The surface of Mars shows a wide range of geologic features, including huge volcanoes-the largest known in the solar system-and extensive impact cratering. Three very large volcanoes are found on the Tharsis bulge, an enormous geologic area near Mars’s equator. Northwest of Tharsis is the largest volcano of all: Olympus Mons, with a height of 25 kilometers and measuring some 700 kilometers in diameter at its base. The three large volcanoes on the Tharsis bulge are a little smaller-a “mere” 18 kilometers high.
None of these volcanoes was formed as a result of collisions between plates of the Martian crust-there is no plate motion on Mars. Instead, they are shield volcanoes — volcanoes with broad, sloping slides formed by molten rock. All four show distinctive lava channels and other flow features similar to those found on shield volcanoes on Earth. Images of the Martian surface reveal many hundreds of volcanoes. Most of the largest volcanoes are associated with the Tharsis bulge, but many smaller ones are found in the northern plains.
The great height of Martian volcanoes is a direct consequence of the planet’s low surface gravity. As lava flows and spreads to form a shield volcano, the volcano’s eventual height depends on the new mountain’s ability to support its own weight. The lower the gravity, the lesser the weight and the greater the height of the mountain. It is no accident that Maxwell Mons on Venus and the Hawaiian shield volcanoes on Earth rise to about the same height (about 10 kilometers) above their respective bases-Earth and Venus have similar surface gravity. Mars’s surface gravity is only 40 percent that of Earth, so volcanoes rise roughly 2.5 times as high. Are the Martian shield volcanoes still active? Scientists have no direct evidence for recent or ongoing eruptions, but if these volcanoes were active as recently as 100 million years ago (an estimate of the time of last eruption based on the extent of impact cratering on their slopes), some of them may still be at least intermittently active. Millions of years, though, may pass between eruptions.
Another prominent feature of Mars’s surface is cratering. The Mariner spacecraft found that the surface of Mars, as well as that of its two moons, is pitted with impact craters formed by meteoroids falling in from space. As on our Moon, the smaller craters are often filled with surface matter-mostly dust-confirming that Mars is a dry desert world. However, Martian craters get filled in considerably faster than their lunar counterparts. On the Moon, ancient craters less than 100 meters across (corresponding to depths of about 20 meters) have been obliterated, primarily by meteoritic erosion. On Mars, there are relatively few craters less than 5 kilometers in diameter. The Martian atmosphere is an efficient erosive agent, with Martian winds transporting dust from place to place and erasing surface features much faster than meteoritic impacts alone can obliterate them.
As on the Moon, the extent of large impact cratering (i.e. craters too big to have been filled in by erosion since they were formed) serves as an age indicator for the Martian surface. Age estimates ranging from four billion years for Mars’s southern highlands to a few hundred million years in the youngest volcanic areas were obtained in this way.
The detailed appearance of Martian impact craters provides an important piece of information about conditions just below the planet’s surface. Martian craters are surrounded by ejecta (debris formed as a result of an impact) that looks quite different from its lunar counterparts. A comparison of the Copernicus crater on the Moon with the (fairly typical) crater Yuty on Mars demonstrates the differences. The ejecta surrounding the lunar crater is just what one would expect from an explosion ejecting a large volume of dust, soil, and boulders. However, the ejecta on Mars gives the distinct impression of a liquid that has splashed or flowed out of crater. Geologists think that this fluidized ejecta crater indicates that a layer of permafrost, or water ice, lies just a few meters under the surface. Explosive impacts heated and liquefied the ice, resulting in the fluid appearance of the ejecta.
火星表面展现了广泛的地质特征，包括巨大的火山 - 太阳系中最大的火山 - 和广泛的撞击坑。在火星赤道附近的Tharsis隆起上发现了三座非常大的火山。萨尔西斯（Tharsis）西北部是所有地区中最大的火山：奥林匹斯山（Olympus Mons），高25公里，底部直径约700公里。塔尔西斯山上的三座大火山稍小一点，只有18公里高。
这些火山没有一个是由于火星地壳板块碰撞而形成的 - 火星上没有板块运动。相反，他们是盾牌火山 - 由熔岩形成的宽广，倾斜的幻灯片的火山。所有四个展示独特的熔岩通道和其他流动特征类似于在地球上的盾牌火山发现。火星表面的图像显示了数百个火山。大部分最大的火山都与萨尔萨斯山的隆起有关，但是在北部平原有许多较小的火山。
火星火山的高度是地球低表面引力的直接后果。随着熔岩流动并蔓延形成盾形火山，火山的最终高度取决于新山支撑自身重量的能力。重力越低，重量越轻，高度越高。在金星上的麦克斯韦·蒙斯和地球上的夏威夷盾牌火山在它们各自的基地上升高约相同的高度（大约10公里）并不是偶然的 - 地球和金星具有相似的表面引力。火星的地表重力只有地球的40％，所以火山大约高出2.5倍。火星盾牌火山仍然活跃吗？科学家对最近或正在喷发没有直接的证据，但如果这些火山最近活跃的100万年前（上次喷发的基础上冲击坑上他们的斜坡程度的估计时间），其中一些可能仍至少间歇性地起作用。但是，数百万年之间，可能会在火山喷发之间传递。
火星表面的另一个突出特点是陨石坑。水手飞船发现火星的表面以及两颗卫星的表面上都形成了由流星从太空中坠落而形成的冲击坑。就像在我们的月球上一样，较小的陨石坑往往充满了表面物质 - 主要是灰尘 - 证实火星是一个干燥的沙漠世界。然而，火星陨石坑的填充速度要比他们的月球快得多。在月球上，不到100米（相当于约20米深）的古环形山陨石主要被陨石侵蚀所消灭。在火星上，直径不足5公里的陨石坑相对较少。火星的气氛是一种有效的侵蚀剂，火星风将尘埃从地方运送到地方，并且擦除表面特征的速度比单独的陨石撞击要快得多。
enormous 巨大的，所以正确答案是B，extremely large。如果不认识，将答案代入原文， 原文说在T这个地方有三座非常大的火山，定语从句修饰说T是个什么样的地区，能容 下三座大火山的当然是很大的地方。A重要、C不寻常和D活跃都不靠谱。