For once, the scientists who ponder the collision of Manhattan-sized objects with Earth have some good news. Whatever else may befall the climate or local landscape, torrents of lava pouring from Earth are unlikely to be among the ills produced by an impact, according to a report in the October issue of Geology.
The notion that asteroid impacts could trigger massive lava flows on Earth and other planets has been bandied about for decades. The premise is fairly simple: Rock melts when it is heated or when the pressure bearing down on it eases. Because an impact carves a great big hole in the ground, pressure on nearby rock is lessened. If the unearthed rocks are hot enough and the pressure released great enough, the rock will melt and lava will flow.
"This idea of impact volcanism is deeply ingrained," says geophysicist H. Jay Melosh of the University of Arizona in Tucson. "But no one had presented a critical analysis of it." In a departure from the back-of-the-envelope calculations that had previously dominated the discussion, Melosh and Boris Ivanov of the Institute of the Dynamics of the Geospheres in Moscow used a numerical simulation to keep track of all the factors involved--everything from the detailed physics of crater formation to the melting requirements of mantle rock. Because the amount of lava depends on how hot the rock was to begin with, the researchers investigated two scenarios: impacts in a region about as hot as a volcanically active midocean ridge, and impacts in a region where hot rocks are buried more deeply beneath the ocean bottom.
The model suggests that even an impact that creates a crater 250 km across--larger than any crater yet found on Earth--would not dig deep enough to reach meltable mantle rock in either locale. And when the researchers looked at the number and size of craters on Venus and the moon, they found that larger craters are extremely rare: A mere handful occur every billion years. The chance of a large enough object hitting a hot spot and causing a surge in volcanic activity is vanishingly small, they argue.
The idea that impacts cause all of Earth's large-scale volcanism has always been a "fringe" idea among impact scientists, says planetary geophysicist William McKinnon of Washington University in St Louis, Missouri. Yet some questions about special cases remain open. For example, he points out, Ivanov and Melosh looked at a fairly tepid hot spot; hotter spots certainly exist. Modeling what happens if a large object smacks one of these active regions could be interesting, he says.