快猫短视频

IN 1980, Luis and Walter Alvarez and co-workers found irrefutable signs that a mighty asteroid impact occurred 66 million years ago. It coincided with the demise of the dinosaurs, among many other taxa, and with vast but short-lived magma outpourings called flood basalts in western India. Its crater survives off Mexico鈥檚 Yucatan peninsula.

Many Earth scientists greeted the causality the team ascribed to such coincidences with alarm: it threatened gradualist concepts in geology that had first appeared in Charles Lyell鈥檚 Principles of Geology in 1830. The theory of plate tectonics put forward in the 1960s 鈥� which envisioned continents shifting with toenail-growing slowness 鈥� was as revolutionary as most geologists were prepared to get.

Michael Rampino is different. He is enthralled by these awe-inspiring events, which show that Earth鈥檚 present can鈥檛 always serve as a key to understanding its past. Gigantic volcanic blurtings also piqued his interest. Such eruptions, he believes, may have shunted the biosphere suddenly and repeatedly in new directions. Rampino has spent his career following the threads of these destructive events. Cataclysms is his account of that journey.

Asteroids arrive with a speed of at least 13 kilometres per second and deliver colossal kinetic energy on impact. Little wonder, then, that Rampino has spent his time trying to track down impacts responsible for mass extinctions.

Sadly, continual processes on and below Earth鈥檚 surface work to obliterate the evidence. Despite four decades of optimism, only the dinosaur-felling end-Mesozoic mass extinction can be linked with an impact crater big enough to be responsible. Rampino discusses five or six instances where extinction events roughly coincide with a whiff of evidence for asteroid impacts, but these lack general acceptance.

Another feature of extinction events is that they seem to turn up on a roughly 27-million-year timetable. So, too, do flood basalts. These have coincided with extinction events eight times that we know of during the Mesozoic, and twice during the Palaeozoic.

In the 1920s, Milutin Milankovic predicted regular changes in Earth鈥檚 orbital and rotational properties from the fluctuating gravitational effects of other planets鈥� motions, which might explain climatic fluctuations, including ice ages. Convincing support emerged in 1976 from stable isotope studies of marine-sediment cores. Could some kind of astronomical forcing underpin the apparent cyclicity of impacts, flood basalts and mass extinctions?

The sun orbits the galactic centre roughly every 250 million years, and in doing so it wobbles through the galactic plane every 33 million years. Rampino thinks that the way mass 鈥� including stars, gas and maybe even dark matter 鈥� varies relative to the galactic plane might set up cyclical variations in the gravitational environment sufficient to perturb small objects orbiting the sun, triggering plunges into Earth. Not only might the impacts upset surface conditions to a deadly extent, but vast amounts of energy penetrating the interior could trigger mantle plumes to rise from Earth鈥檚 outer core too.

鈥淕igantic eruptions may have shunted the biosphere repeatedly in new directions鈥�

Lots of questions will be asked about the evidence and statistics involved in developing such a hypothesis. Nevertheless, Cataclysms is an easy, comprehensive account of how uniformitarianism and gradualism were challenged after 1980, and why many geologists began thinking differently about their discipline.

This article appeared in print under the headline 鈥淒rumbeats of destruction鈥�