By Ed Yong
Around 74,000 years ago, the Toba supervolcano erupted on the Indonesian island of Sumatra. It was the biggest volcanic eruption of the last 2 million years, unleashing 2,800 cubic kilometers of magma. That’s enough to bury the entire United States in a foot-thick layer of ash and rock.
In the 1990s, several scientists argued that Toba’s unprecedented outburst radically changed the world’s climate, blocking out sunlight and lowering global temperatures by several degrees for many decades. This “volcanic winter,” it is said, almost drove humans to extinction, leaving behind a measly group of a few thousand survivors, from whom we today are descended. The “Toba catastrophe theory” is highly controversial, and other researchers have argued that it greatly overestimates both the degree of climate change that the volcano inflicted, and its effect on our ancestors.
Now, into the fray comes a new study from an unlikely location. In a cliff near Mossel Bay, a town on South Africa’s south coast, scientists have discovered a layer of microscopic glass shards. Known as cryptotephra, these shards are the products of Toba’s wrath, created when the volcano superheated the silica within its expunged rock. They drifted in the air over 5,500 miles and fell on southern Africa as the sparsest of drizzles. And they settled among bones, tools, and other signs of human occupation.
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