Topsy turvy

STUDYING the seabed does not always mean penetrating the sea itself, even if that penetration is done using sound waves rather than submarines (see article). There are a few places where what a geologist would call the ocean floor is actually dry land. One such is the Danakil depression, which lies near the northern vertex of the Afar Triangle, a rift valley stretching from the Dallol volcano in Ethiopia past the salt plains of Lake Assal, in Djibouti, to the north-west tip of Somalia, and then inland to Awash. Millions of years ago, the Danakil was indeed covered by the sea—in its case, the Red Sea. But volcanic eruptions formed barriers of lava that isolated it from the ocean. What water remained evaporated in the intense heat, leaving brine lakes and saline flats. These are mined, and the resulting slabs of salt exported by camel, by nomadic Afars who are the nearest thing the depression has to permanent inhabitants.

Dallol, appropriately, means disintegration in Afar. For this is a place where Earth’s crust is, indeed, disintegrating. The triangle sits at the convergence of three tectonic plates, which are slowly separating. A glance at the map shows that, were the whole triangle to flood (not possible at the moment, because not all of it is currently below sea level), the African and Arabian coasts would run parallel, as they do farther north along the Red Sea. That sea is an incipient ocean. The continents either side of it are being pushed apart by basaltic eruptions along a line that will, in millions of years’ time, form a mid-ocean ridge.