[meteorite-list] 6-22 m thick impact ejecta deposit 60 km wide via "pyroclastic density flow" in NW Scotland 1.2 BYa, 145, 000 MT: Rich Murray 2011.10.19
From: Rich Murray <rmforall_at_meteoritecentral.com>
Date: Wed, 19 Oct 2011 17:11:08 -0700 Message-ID: <CAHqJ8pYrhQR26BgCcz64Wu1hen4e4TEq5mw4Vyt718=81fFvew_at_mail.gmail.com> 6-22 m thick impact ejecta deposit 60 km wide via "pyroclastic density flow" at Ullapool, NW Scotland 1.2 BYa, 145,000 MT: Rich Murray 2011.10.19 http://rmforall.blogspot.com/2011/10/6-22-m-thick-impact-ejecta-deposit-60.html [ See also: impact ejecta [melt] emplacement on terrestrial planets, Gordon R Osinski et al 2011, 2 pages: Rich Murray 2011.10.19 http://rmforall.blogspot.com/2011/10/impact-ejecta-melt-emplacement-on.html ] "The meteorite, which is thought to have measured up to 1 km across, would have formed an impact crater up to 10 km in diameter, but the material ejected by the impact spread out for at least 50 km. 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." "This reveals that that the 10 meter-thick [33 feet] layer, which has been traced for over 50 km [31 miles] along the Scottish coast, was almost entirely emplaced as a devastating density current that sped outwards from the point of impact ? just like a density current from a volcano. Only the uppermost few centimeters actually fell out through the atmosphere," said study team member Richard Brown of the University of Durham. http://www.space.com/13325-gas-blast-meteorite-strike-resembled-volcanic-eruption.html Ancient Meteorite Blast Resembled Volcanic Eruption OurAmazingPlanet Staff Date: 19 October 2011 Time: 11:33 AM ET A billion years ago, a meteorite slammed into the Earth along the coast of what is now Scotland. A forensic investigation by a team of volcanologists has pieced together exactly how the debris from the impact devastated the surrounding region. The new research shows that some aspects of giant meteorite impacts may mimic the behavior of large volcanic eruptions. Meteorite impacts are more common than most people realize, but what happens when the meteorite hits? Direct observation is understandably difficult, but researchers can pick through impact debris that hasn't eroded away and then forensically reconstruct these catastrophic events. The volcanologists say that an improved understanding of what happens when large objects hit the Earth will help us understand how such events affect life on the planet. Volcanologists analyzed a layer of ejected debris from this huge meteorite impact and discovered that much of the debris moved across the ground as rapid, dense, ground-hugging currents of gas and debris, remarkably similar to the pyroclastic density currents ? fast-traveling streams of hot ash and rock ? that flow outward from explosive volcanoes. "In particular, the way that ash and dust stick together seems identical," said study team member Mike Branney of the University of Leicester in England. "Moist ash from explosive volcanoes sticks together in the atmosphere to fall out as millimeter-sized pellets. Where these drop back into a hot pyroclastic density current, they grow into larger layered structures, known as accretionary lapilli." The researchers studied the finely preserved deposit in northwest Scotland from the ancient impact. It shows both types of these 'volcanic' particles ? pellets and lapilli ? are produced. "This reveals that that the 10 meter-thick [33 feet] layer, which has been traced for over 50 km [31 miles] along the Scottish coast, was almost entirely emplaced as a devastating density current that sped outwards from the point of impact ? just like a density current from a volcano. Only the uppermost few centimeters actually fell out through the atmosphere," said study team member Richard Brown of the University of Durham. [ microphotos ] Meteorite impact ejecta (left) compared with volcanic deposits (right) showing closely similar structures made of dust particles. The top two photos show accretionary lapilli in density current deposits, whereas bottom two photos show pellets that formed when dust in the atmosphere clumped together and simply fell onto the land surface. CREDIT: From Michael Branney and Richard Brown 2011 (Journal of Geology 199, 275-292) View full size image http://www.theregister.co.uk/2008/03/26/uk_meteorite_impact/ UK's biggest meteorite impact rocked Scotland Prehistoric Ullapool enjoyed 'quite a show' By Lester Haines Posted in Space, 26th March 2008 13:00 GMT It's lucky for the good burghers of Ullapool in Scotland that they weren't around 1.2 billion years ago, because it was around then that the biggest meteorite ever to hit the British Isles would have made a bit of a dent in local house prices. That's according to the combined forces of the University of Oxford and the University of Aberdeen, who say that "unusual rock formations" previously thought to have volcanic origins are actually the debris ejected from a meteorite strike which threw material over an area 50 km across. The volcanic theory has always had geologists scratching their heads, since there are "no volcanic vents or other volcanic sediments nearby". The researchers moved in for the kill by taking rock samples in 2006, and have now published their revelations in the journal Geology. Ken Amor of Oxford Uni?s Department of Earth Sciences, explained: "Chemical testing of the rocks found the characteristic signature of meteoritic material, which has high levels of the key element iridium, normally only found in low concentrations in surface rocks on Earth. We found more evidence when we examined the rocks under a microscope; tell-tale microscopic parallel fractures that also imply a meteorite strike." Professor John Parnell, head of Geology & Petroleum Geology at the University of Aberdeen, chipped in with: "These rocks are superbly displayed on the west coast of Scotland, and visited by numerous student parties each year. We?re very lucky to have them available for study, as they can tell us much about how planetary surfaces, including Mars, become modified by large meteorite strikes. Building up the evidence has been painstaking, but has resulted in proof of the largest meteorite strike known in the British Isles." Amor added: "If there had been human observers in Scotland 1.2 billion years ago they would have seen quite a show. 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." The researchers hope that the evidence they've gathered will help them to "understand the ancient impacts that shaped the surface of other planets, such as Mars", Amor concluded. ? Wikipedia article: http://en.wikipedia.org/wiki/Ullapool_bolide_impact [ microphoto ] Accretionary lapilli from the Stac Fada Member Evidence for a bolide impact centered on Ullapool [ 57.899602 -5.157701 ] was published by a combined team of scientists from the University of Oxford and the University of Aberdeen, in March 2008.[1] he evidence is centred on Ullapool, a harbour town on Loch Broom in the Ross and Cromarty district of the Highland council area of northwest Scotland. This suggests it was the largest bolide impact ever to strike what are now the British Isles. The impact, which has been dated to 1177?5 million years ago,[2] melted rock at the site and left parallel shock fractures in quartz and biotite and a tell-tale trace of iridium. Centered on the impact crater, a wide ejecta field has been traced, some 50 km across, forming the Stac Fada member within the Stoer Group of the Torridonian. The affected layer of rock, which on land stretches from Gairloch [ 57.720780 -5.686293 ] in the south to Stoer [ 58.203414 -5.34027 ] in the north is six to 22 metres thick.[3] [ 60 km apart ] Until recently, these anomalous formations[4] were unsatisfactorily credited to an isolated instance of volcanism. The crater, preserved under sedimentary layers of sandstone, is currently presumed to lie under the Minch, the waterway that separates the Isle of Lewis in the Outer Hebrides from the north-west Highlands of Scotland. It has been estimated that the impact would have created a blast with the force of 145,000 megatons and that the shock wave would have created winds of 420 km/h as far away as the site of modern Aberdeen.[3] See also Silverpit crater, the only other proposed impact crater in or near the British Isles. Impact event List of impact craters on Earth North West Highlands Geopark Geology of Scotland References Kenneth Amor, Stephen P. Hesselbo, Don Porcelli, Scott Thackrey, and John Parnell, "A Precambrian proximal ejecta blanket from Scotland", Geology 36, 4, March 2008:pp. 303?306; DOI: 10.1130/G24454A.1. Notes ^ University of Aberdeen media release, 26 March 2008; Oxford University media release, 26 March 2008. ^ Parnell, J.; Mark D., Fallick A.E., Boyce A. & Thackrey S. (2011). "The age of the Mesoproterozoic Stoer Group sedimentary and impact deposits, NW Scotland". Journal of the Geological Society 168 (2): 349?358. doi:10.1144/ 0016-76492010-099. ^ a b Urquhart, Frank, (27 March 2008) "Discovery with deep impact on Scots coast". Edinburgh. The Scotsman. ^ The strata under study are part of the Stac Fada Member of the Precambrian Stoer Group of Scotland. http://geology.geoscienceworld.org/cgi/content/abstract/36/4/303 Geology; April 2008; v. 36; no. 4; p. 303-306; DOI: 10.1130/G24454A.1 ? 2008 Geological Society of America A Precambrian proximal ejecta blanket from Scotland Kenneth Amor 1, [ +44 (0)1865 272000 ken.amor at earth.ox.ac.uk ] Stephen P. Hesselbo 1, Don Porcelli 1, Scott Thackrey 2 and John Parnell 2 1 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK 2 Department of Geology and Petroleum Geology, Meston Building, King's College, Aberdeen AB24 3UE, UK Ejecta blankets around impact craters are rarely preserved on Earth. Although impact craters are ubiquitous on solid bodies throughout the solar system, on Earth they are rapidly effaced, and few records exist of the processes that occur during emplacement of ejecta. The Stac Fada Member of the Precambrian Stoer Group in Scotland has previously been described as volcanic in origin. However, shocked quartz and biotite provide evidence for high-pressure shock metamorphism, while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material. Thus, the unit is reinterpreted here as having an impact origin. The ejecta blanket reaches >20 m in thickness and contains abundant dark green, vesicular, devitrified glass fragments. Field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water-saturated sedimentary strata. The continental geological setting and presence of groundwater make this deposit an analogue for Martian fluidized ejecta blankets. Key Words: ejecta, impactites, shock metamorphism, PGE, suevite, Torridonian http://www.scotsman.com/news/discovery_with_deep_impact_on_scots_coast_1_1161196 "The meteorite, which is thought to have measured up to 1 km across, would have formed an impact crater up to 10 km in diameter, but the material ejected by the impact spread out for at least 50 km. 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." ______________________________________________ 10 m broken rock hill with black glazes, W of Rancho Alegre Road, S of Coyote Trail, W of Hwy 14, S of Santa Fe, New Mexico, tour of 50 photos 1 MB size each via DropBox: Rich Murray 2011.07.28 2011.08.03 http://rmforall.blogspot.com/2011/08/10-m-broken-rock-hill-with-black-glazes.html http://rmforall.blogspot.com/2011/08/35479730-106085926-1865-km-el-top-10-m.html photos 3-5 of 50 http://tech.groups.yahoo.com/group/astrodeep/message/92 ground views of over 100 .1-.5 km shallow (ice comet fragment bursts) craters, Bajada del Diablo, Argentina (.78-.13 Ma BP) [42.87 S 67.47 W] Rogelio D Acevedo et al, Geomorphology 2009 Sept: Rich Murray 2010.03.28 http://rmforall.blogspot.com/2010/03/ground-views-of-over-100-1-5-km-shallow.html http://tech.groups.yahoo.com/group/astrodeep/message/47 ______________________________________________ Rich Murray, MA Boston University Graduate School 1967 psychology, BS MIT 1964, history and physics, 1943 Otowi Road, Santa Fe, New Mexico 87505 505-819-7388 rmforall at gmail.com http://tech.groups.yahoo.com/group/AstroDeep/messages http://RMForAll.blogspot.com new primary archive http://health.groups.yahoo.com/group/aspartameNM/messages group with 118 members, 1,625 posts in a public archive http://groups.yahoo.com/group/rmforall/messages ______________________________________________ Received on Wed 19 Oct 2011 08:11:08 PM PDT |
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