A controversial idea which was published in 2007
suggested that a large space rock may have exploded over North
America around thirteen thousand years ago.
The blast may have wiped out one of America's
first Stone Age cultures as well as the continent's big mammals
such as the mammoth and the mastodon. The blast, from a comet or
asteroid, caused a major bout of climatic cooling which may also
have affected human cultures which were emerging in Europe and
Scientists outlined their evidence at a meeting
in Mexico in late May 2007. Their evidence came from layers of
sediment at more than twenty sites across North America. These
sediments contained exotic materials: tiny spheres of glass and
carbon, ultra-small specks of diamond - called nanodiamond - and
amounts of the rare element iridium that were too high to have
originated on Earth.
All of this, they argued, pointed to the explosion
of an extraterrestrial object of up to five kilometres in width a
few years either side of 10,900 BC. No crater survives today,
possibly because the Laurentide Ice Sheet, which blanketed thousands
of square kilometres of North America during the last ice age, was
thick enough to mask the impact.
Another possibility is that the comet or asteroid
exploded in the air.
The rocks studied by the researchers had a black
layer which, they argued, was the charcoal which was deposited by
wildfires which swept the continent after the explosion. The blast
would not only have generated enormous amounts of heat that could
have given rise to wildfires, but could also brought about a period
of climate cooling which lasted a thousand years - an event known
as the Younger Dryas.