Snowball Earth (the Sturtian & Marinoan glaciations)

Somewhere south of the city, a few minutes off the freeway, there is a road cutting at Sturt Gorge where you can put your hand on a chaos of pebbles and boulders locked in a fine grey paste of mud. It looks like nothing much — a poorly sorted, slightly ugly rock a geologist would call a diamictite. It is some of the best physical evidence anywhere on Earth that the whole planet once froze solid.

Twice in the deep past, the ice went further than any ordinary advance from the poles: it appears to have reached the equator and closed over the entire globe, an idea now known, only half-jokingly, as Snowball Earth. The first and by far the longer of these was the Sturtian, which by current reckoning ran for something like fifty-seven million years — comfortably the longest glaciation the rock record knows of, on any continent. The second, the Marinoan, came later and ended more abruptly, around six hundred and thirty-five million years ago, its retreating ice-rubble buried directly beneath the first rock of the world the animals would inherit.

It was Douglas Mawson, the Antarctic explorer turned University of Adelaide geologist who had personally crawled across the genuine, present-day article, who mapped these beds, and who with his student Reg Sprigg gave both glaciations their names in 1950: the Sturtian, for Sturt Gorge, and the Marinoan, for the seaside suburb of Marino (Mawson & Sprigg 1950). Mawson read them, reasonably enough, as an unusually thick set of ordinary glacial beds. It took another seventy years of dating and stratigraphy — a volcanic ash layer pinning the Sturtian’s end to 663.03 million years ago almost to the day (Cox et al. 2018) — for geologists to grasp that these quiet suburban rocks were recording something closer to the coldest event the planet has ever staged.

Stand at Sturt Gorge, or drive past Marino on the coast road, and you are within a few kilometres of two places that lent their names to the deepest freezes the Earth has known. The book behind Adelaide opens with an ice age that makes the last one look like a light frost.