New research from North Carolina State University finds that major volcanic1 activity on the planet Mercury most likely ended about 3.5 billion years ago. These findings add insight into the geological evolution of Mercury in particular, and what happens when rocky planets cool and contract in general. There are two types of volcanic activity: effusive2 and explosive. Explosive volcanism is often a violent event that results in large ash and debris3 eruptions5, such as the Mount Saint Helens eruption4 in 1980. Effusive volcanism refers to widespread lava6 flows that slowly pour out over the landscape -- believed to be a key process by which planets form their crusts.Determining the ages of effusive volcanic deposits can give researchers a handle on a planets geological history. For example, effusive volcanism was active a few hundred million years ago on Venus, a few million years ago on Mars, and it still takes place on Earth today. Until now, the duration of effusive volcanic activity on Mercury, made of the same materials as these other planets, had not been known.NC State assistant professor and planetary geologist7 Paul Byrne and colleagues determined8 when the bulk of Mercurys crust-forming volcanism ended by using photographs of the surface imaged by NASAs MESSENGER mission. Because there are no physical samples from the planet that could be used for radiometric dating, the researchers used crater9 size-frequency analysis, in which the number and size of craters10 on the planets surface are placed into established mathematical models, to calculate absolute ages for effusive volcanic deposits on Mercury.According to their results, major volcanism on Mercury sTOPped at around 3.5 billion years ago, in stark11 contrast to the volcanic ages found for Venus, Mars and Earth.There is a huge geological difference between Mercury and Earth, Mars or Venus, Byrne says. Mercury has a much smaller mantle12, where radioactive decay produces heat, than those other planets, and so it lost its heat much earlier. As a result, Mercury began to contract, and the crust essentially13 sealed off any conduits by which magma could reach the surface.These new results validate14 40-year-old predictions about global cooling and contraction15 shutting off volcanism, Byrne continues. Now that we can account for observations of the volcanic and tectonic properties of Mercury, we have a consistent story for its geological formation and evolution, as well as new insight into what happens when planetary bodies cool and contract.The research appears in Geophysical Research Letters, with co-authors from the Carnegie Institution of Washington, Mount Holyoke College, the University of Georgia, Southwest Research Institute and Brown University. The MESSENGER mission provided substantial funding for this work.
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1 volcanic adj.火山的；象火山的；由火山引起的 参考例句： There have been several volcanic eruptions this year.今年火山爆发了好几次。 Volcanic activity has created thermal springs and boiling mud pools.火山活动产生了温泉和沸腾的泥浆池。
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