the great dying

ImagesJohn Bannigan at the Dublin Review of Books:

Catastrophic extinctions like storms or earthquakes vary in scale but of the twenty that have occurred since the beginning of the Palaeozoic five are singled out as massive because the extinction rate exceeded seventy-five per cent. The Worst of Times describes the eighty-million-year time span from the mid-Permian to the mid-Jurassic, during which two massive extinctions occurred as well as four of lesser magnitude. Wignall gives a detailed account of the most massive one of all, in which ninety-five per cent of all life perished 250 million years ago. This was the second in the series and occurred at the Permian-Triassic (P-Tr) boundary. The account is based largely on field studies in which he has had major involvement over twenty-five years.

The main argument is that the extinctions were due to enormous episodes of volcanism and that their severity was intensified by the peculiar geography of the planet at the time. Wignall begins with the geography. In the Palaeozoic nearly all the land of the earth was concentrated in a single super-continent called Pangea. Shaped a bit like an irregular fat letter C, Pangea stretched from pole to pole. The shallow concavity of the C was open to the east and enclosed the Thetys Ocean with the equator on its southern shore. The Panthalassa Ocean occupied the rest of the planet. Pangea was the last of a series of super-continents formed by cycles of coalescence and fragmentation resulting from movements of the earth’s tectonic plates. The volcanism was caused by columns of magma ascending from the core-mantle boundary of the planet.

more here.