IT IS a mystery which has puzzled generations of geologists – the origins of a layer of stratified rock trapped in the sediments which now form part of the coastline of north-west Scotland.
Scientists suspected for years that the curious seam could have been formed by volcanic activity millions of years ago.
But it was revealed yesterday that the 40-mile long rock layer was formed when the biggest meteorite ever to strike what is now the British Isles hit the Earth close to present-day Ullapool with the force of a 145,000 megaton bomb.
Geologists at Aberdeen and Oxford universities, who made the discovery, believe that the rock was formed by ejecta – material which was forced out of the Earth – by the meteorite, it hit 1.2 billion years ago.
The meteorite, which is thought to have measured up to 1km across, would have formed an impact crater up to 10km in diameter, but the material ejected by the impact spread out for at least 50km.
The rock layer, which stretches from Gairloch in the south to the Sutherland village of Stoer to the north, is sandwiched between sedimentary rocks which form part of the Torridonian sandstones of Sutherland.
Ken Amor, one of the leaders of the team of geologists involved in the discovery, said: "This is the most spectacular evidence for a meteorite impact within the British Isles to date, and what we have discovered about this meteorite strike could help us understand the ancient impacts that shaped the surface of other planets, such as Mars."
Mr Amor, a geologist at Oxford University's department of earth sciences, added:
"The massive impact would have melted rocks and thrown up an enormous cloud of vapour that scattered material over a large part of the region around Ullapool. The crater was rapidly buried by sandstone, which helped to preserve the evidence."
Scott Thackrey, a PhD student in geology and petroleum geology at Aberdeen University and an expert on impact craters, said:
"Ejecta deposits like this are extremely rare. These layers usually get eroded away very quickly, but this layer has been pretty well perfectly preserved.
"This deposit had originally been thought to be volcanic. But the problem was it is the only instance of this deposit in about a couple of kilometres of sediment. And, if it had been created by volcanic activity, we would have expected to see quite a few of these layers occurring, but this was the only one.
"We found indicators which prove that the layer was formed by a meteorite rather than a volcanic event."
The layer of rock varies in thickness from six to 22 metres and is composed of both melted rock fragments caused when the meteorite struck the Earth and by "shock mineral" debris formed by the ground surge as the force of the crater's impact spread across the landscape.
Mr Thackrey added: "The meteorite would have struck with the force of 145,000 megatons.
"The bomb that was dropped on Hiroshima, by comparison, was only a 16 kiloton explosion. If it happened today, we would get winds over 260mph in Aberdeen and a shock blast that would have flattened every tree in the area.
"We would probably survive – but only just."
Meteorites making a mark across the planet
THE Ullapool crater may be biggest yet discovered in the British Isles, but it is completely dwarfed by the world's largest known meteorite crater, the 300km wide Vredefort crater in South Africa.
The massive crater is believed to have been created when a 10km wide meteorite, larger than Table Mountain, slammed into the Earth some two billion years ago, vaporising about 70 cubic kilometres of rock.
The impact was so powerful that the rock beneath the impact point rebounded, creating a raised dome at the centre of the crater and generating a ripple of ringed crater rims which radiate out from the centre.
Its original impact may have created a crater up to 380km across, which consisted of three concentric circles of uplifted rock.
The Vredefort crater is far older than the Chixculub crater in Mexico which, with an age of 65 million years, is the site of a meteorite impact, which some scientists claim led to the extinction of the dinosaurs.
However, the best-known crater on Earth is the so-called "meteor crater" in Arizona, which is believed to have been created a mere 50,000 years ago when a mass of iron smashed into the present-day United States.
The meteorite impact created a 1.2km-wide hole in the ground, 170 metres deep.
It is the most visible space debris impact site on the planet.
To date, more than 170 meteorite impact craters or their remnants on Earth have been identified by scientists.