Written by: Arty Goodwin, University of Manchester
When a huge meteor or comet hits into Earth, the resultant impact is similar to a huge explosion! Meteor collisions carve out craters into solid rock, scooping out and ejecting material over the surrounding landscape. Events like this have happened since Earth first formed. Yet the marks they leave are often obscured from plate tectonics that perpetually reshapes the surface of our planet, destroying old rocks and creating new ones. This means that not everything from the past is preserved.
And for the case of the Stac Fada Member in Northwest Scotland, we have evidence of a meteor impact without knowing where the original crater was. About 1.2 billion years ago, long before complex animals and plants had evolved, the Bay of Stoer in Scotland would have been an otherwise arid environment. Imagine a sandy desert with rare flash floods and infrequent rivers. A meteor collided with the ground and exploded somewhere close by, throwing out a huge splurge of material that got hot enough to melt. Over the space of minutes to hours, a huge rush of debris swept across the ground and coated the ground — similar to what happens in a very violent volcanic blast. A steaming hot ejecta blanket was created, cooled, covered by sediment, and preserved as rock.
From our most recent work, we studied the composition of this ancient impact ejecta blanket. By looking at the minerals inside, we can estimate the high pressures and temperatures caused by a meteor collision. In such extreme conditions, a common mineral called quartz transforms to an unusual mineral called coesite — something we identified via a technique known as Raman spectroscopy. Coesite can only be formed in very violent explosions and is almost exclusively restricted to meteor impacts. By using a special property of reflected laser light, Raman spectroscopy can estimate the arrangement of atoms within different materials. It is very good at identifying when carbon is compressed to form diamonds. We discovered diamonds generated by the meteor impact both within the ejecta blanket as well as chunks of rock thrown out by the huge shockwave.
Of course, science is never done in isolation. Our new findings add more evidence, for example, to the initial proof that Stac Fada is an impact deposit (Amor et al., 2008). Additionally, we help constrain the ordering of events that happened during the impact. Building on Simms et al., (2015) by using diamonds to prove that blocks were thrown out of the impact faster than the ejecta blanket, which later coated the arid environment. This is another small piece in the puzzle and we are excited to continue working on understanding this awesome event.
Read “Provenance of altered carbon phases and impact history of the Stac Fada Member, NW Scotland” via open access, published (preprint) in Meteoritics & Planetary Science here LINK Authors: Arthur Goodwin, Romain Tartèse, Russell J. Garwood, Rhodri Jerrett, Katherine H. Joy
References:
Amor, K., Hesselbo, S.P., Porcelli, D., Thackrey, S. and Parnell, J., 2008. A Precambrian proximal ejecta blanket from Scotland. Geology, 36(4), pp.303-306.
Simms, M.J., 2015. The Stac Fada impact ejecta deposit and the Lairg Gravity Low: evidence for a buried Precambrian impact crater in Scotland?. Proceedings of the Geologists’ Association, 126(6), pp.742-761.
Earth & Solar System 