托福听力考试背景知识综合辅导(四十七)

Giant Shield Volcanoes

The giant shield volcanoes on Mars are truly huge. The largest are three times as high as the biggest Earth volcanoes. They also are bigger in diameter. Thus, the biggest volcano on Mars is comparable to a pile of nearly 100 Hawaiian volcanoes. Despite this difference in size, the Mars shields look a lot like shield volcanoes on Earth. Both have the same broad flat profiles, large central calderas, and similar lava flow features. The giant martian shields are also much older than any Earth volcano. The youngest lavas on the martian shields are about 20 to 200 million years old. The oldest lavas are near 2.5 billion years old. Thus, these giant volcanoes were active for billions of years. This may explain their large size. On Earth, plate tectonics is always moving volcanoes away from their magma sources.

Such movements are very slow, but they mean that most Earth volcanoes have distinct lifetimes. In the Hawaiian islands, for instance, volcanism lasts fo only a few million years on any given island. In contrast, the lack of plate tectonics on Mars allowed volcanoes to just keep growing. The only limit on their final size was the volume of lavas available.