In an article published today in the journal Science researchers led by Dr
Gordon Walkden of Aberdeen University have reported the discovery of a 214
myr old impact layer in the rocks of the west of England. The 2cm thick
layer consists of millimetre-sized green spherules that were formed as
molten droplets of rock in the impact of a large asteroid or comet with the
Earth. The droplets formed by condensation from gases generated by
vaporisation of rocks at enormous temperatures and were scattered over the
entire Earth surface.

Speaking to the NEO Information Centre, Walkden said “I discovered the layer
in 1983 but didn’t realise it was an impact layer until the late 1990s.” Mr
Julian Parker, from Aberdeen, studied the spherule layer with Walkden and
discovered quartz grains that had been deformed by intense
pressures. “The orientation of the distorted planes through the grains
showed they had been shocked,” said Walkden, “and prove the layer was formed
as debris thrown out from a giant collision.”

Dr Simon Kelly, from the Open University, measured the age of the west
country spherule layer using the decay of radioactive potassium, that is
found in all potassium-bearing minerals. The age of 214 myrs is the same as
the 100 km wide Manicouagan Crater in Canada which is, therefore, the likely
source of the impact layer. Kelly, however, suspects that a number of
craters that have similar ages may have formed at the same time as a string
of impacts.

At the time, 214 myrs ago, early dinosaurs and mammals lived in, what was
then, the red arid deserts of Southwest Britain. The fall of the spherules,
which may have still been molten, and the other effects of the impact did
not, however, cause a mass extinction. The animals of the Triassic period
were not affected and survived until 13 million years later when many became
extinct for an, as yet, unknown reason.

“The lack of a mass extinction,” says Gordon, “suggests that in large
impacts, where the collision occurs is just as important as its size.” The
impact at Chicxulub 65 myrs ago, that is widely believed to have caused the
extinction of the dinosaurs, occurred into carbonate and sulphate rocks and
it is thought that the gases generated by the impact caused climatic
changes that resulted in the extinction.

The location of the impact layer is currently being kept secret in order to
protect it. Specimens of the layer, however, are being put on public
exhibition at the Natural History Museum in London, at the NEO Information
Centre at the National Space Centre in Leicester, and the National Museum of
Scotland, Edinburgh. Dr Gordon Walkden is also taking part in a public
discussion on impacts and mass extinctions, together with Dr Matthew Genge
from Imperial College, at 2.30 pm GMT on Nov 15 at the Darwin Centre of the
Natural History Museum. This discussion will be broadcast live on the
internet and at the National Space Centre, Leicester and the National Museum
of Scotland, Edinburgh.