Impact Deposits Research

Spherules Aberdeen scientists provide direct evidence of major ancient asteroid collision
Summary of Main Points:
  • The first British impact deposit
  • Only the second sedimentary record of a global-scale impact in the Mesozoic
  • Discovered within late Triassic (pre- Rhaetian) sediments in SW England
  • Spherules (formerly glass microtektites) and shocked quartz prove the case
  • Bright green spherules in a layer up to 150mm thick
  • Impact linked to the giant (100km) Manicouagan Crater in Canada
  • Evidence for more than one impact at the time suggests a multiple event
  • No evidence for a related extinction or biotic collapse
  • Discovery and description by Gordon Walkden
  • Shocked quartz characterised by Julian Parker reveals high energy blast
  • Argon-Argon dating by Simon Kelley at the Open University gives c.214Ma Manicouagan Crater
Blast from the Past: General interest text based on a Press Release of November 2002
Explosion
Scientists at in the Department of Geology and Petroleum Geology at Aberdeen University have discovered evidence in South West Britain of a massive prehistoric explosion. The explosion sent debris right across the British Isles and was caused by a large asteroid that hit the Earth about 214 million years ago.
Professor Gordon Walkden, Head of Department and leader of the research group involved, said: "We have found evidence for a massive shock wave carrying molten rock and dust that left a thin layer of glass beads and shattered mineral grains across the ancient British land surface. This has been preserved in sediments of late Triassic age, currently exposed near Bristol. It is only a thin layer (on average less that one inch or 2 centimetres) but the glass beads and shattered grains represent stresses well beyond the reach of a nuclear bomb and could only have been produced by a massive explosion caused by an asteroid colliding with the earth". A similar catastrophe is believed to have killed off the dinosaurs just 65 million years ago.

Links to known craters
Impact deposits are very rare in rocks worldwide and this is Britain's first recorded example. Professor Walkden and his co worker Julian Parker at Aberdeen University, together with Simon Kelley at the Open University, believe that the new discovery can be linked to a giant impact crater 100 kilometres (65 miles) across at Manicouagan in Quebec, NE Canada that was recognised some years ago. Both the newly discovered impact deposit and the ancient crater have been scientifically dated and they come up with exactly the same age, 214 million years old. Another big crater, at Rochechouart in central France, some 25 km (15 miles) across has a similar age and might also be involved, but Walkden and his team have used mathematical models to predict which is more likely, and the bigger crater at Manicouagan comes out the best.


Continent positions during the Triassic		 manigougan crater - nasa image

Manicouagan Crater. NASA Shuttle image (Shuttle fin at lower left).
Professor Walkden said "Due to the way the earth's plates are moving, Manicouagan would have been much nearer to Britain in the Triassic period than it is today, just 2000 kilometers (1220 miles) away. Manicouagan is one of the best known terrestrial impact craters and is a silent witness to a major catastrophe. It has never before been possible to link it to an actual deposit. The object that created the Manicouagan Crater was probably a chunk of rock 5 km (3 miles) across, travelling at 17 kilometres per second (36,000 miles per hour). As it hit the surface it completely vapourised creating one of the largest natural explosions to affect the planet in the last 600 million years. The shock wave, 40 million times larger than the Hiroshima blast, carried molten rock and dust thousands of km (miles) and it is this material that we have now found. It is likely that the Rochechouart crater was made at the same time, as part of a multiple impact, a bit like the succession of Shoemaker-Levy cometary impacts on Jupiter a few years ago".

Absence of biotic effects
Despite the scale of the impact event, Professor Walkden says that there is no evidence of a major extinction of organisms at the time. "This impact event occurred just as the dinosaurs were first becoming widespread. Perhaps it demonstrates that major asteroid impacts do not necessarily wipe out large numbers of species, but it wouldn't have been a particularly nice day."


Manicougan crater

Click image for information on other impact craters
More details
More information about impact materials
Ongoing work

Professor Walkden was awarded a £30,000 grant from the Natural Environment Research Council to study and report on the timing, correlation and environmental effects of the impact.