Main Club March Meeting
Our speaker was geologist Mike Freeman. Mike graduated from Adelaide University and has 50 years’ experience in geology, including teaching the public about geology and landforms, and being involved in the Geological Survey of WA and land-use planning on the Swan Coastal Plain.
Mike talked to us about “a smorgasbord of earth wounds”, referring to 15 features in WA that show evidence of meteorite impacts. Only four of these are actual craters. The others are all eroded to a greater or lesser extent or have been geologically affected by later earth movements or have been buried in sedimentary basins. Overall in Australia there are 33 probable or confirmed impact sites. It is thought that there are quite a few more yet to be discovered. Those that have been discovered reveal a surprisingly wide range of features, stories of discovery, types of evidence, and implications.
Meteors hit the atmosphere at speeds between 10 and 70 km per second! That’s fast. And they may weigh many tonnes. Some explode in the atmosphere and some are deflected out into space again. Others penetrate the ground to a considerable depth and then explode, scattering not only parts of themselves but also fragments of the rock they penetrated. The object that created the Chicxulub Crater in the Yucatan Peninsula, whose impact is thought to have been related to the great dinosaur extinction of 66 million years ago, was probably an asteroid ten kilometres across, and sank to its own depth in half a second. Some impacts may have been created by comets, which have a much lower density. It is thought that meteor impacts were far more frequent in the early days of the solar system, with the “Late Heavy Bombardment” occurring about four billion years ago.
Yalallie is in the Perth Basin. The feature is 12 km across, but there is no visible crater. Oil drilling revealed a strange geology with magnetic anomalies showing a bullseye pattern 4 km across. This was controversial for years, but a NASA picture of a Martian crater showed a pattern of muddy sediments flowing outward from the impact site that matched the pattern at Yalallie. Since it is now recognised that the Yallalie impact occurred in the Cretaceous, when Perth Basin sediment was being deposited into a shallow sea, this site can be seen as proxy for the Martian impact.
Dalgaranga, near Cue, is the youngest and smallest of the WA sites. It has very little rim, just a slight raising of the peripheral rocks, but the Shire of Yalgoo has put in access roads and information signs and is promoting visits, so it is worth going.
The Hickman Crater near Newman is quite spectacular. It was found in 2007 by geologist Arthur Hickman, using Google Earth. A casual question by our speaker Mike Freeman resulted in a hole being drilled in its centre by Atlas Iron. WA Geological Survey bought the resulting core, to allow anybody to study it. It revealed a kind of glass that resulted from the rock and the meteorite that melted in the instant of impact.
Wolfe Creek is a highly spectacular crater in the Tanami Desert near the Bungle Bungles. It is now in a National Park, for its protection. At 880m across, it is the second biggest visible crater on earth. The object struck about 300,000 years ago, and at an angle, as shown by the crater’s asymmetry.
Mike also told us about the Yarrabubba Impact Structure near Meekatharra, where interestingly, dolerite dykes have intruded into the structure since the impact occurred onto a granitic landscape somewhere between 1.5 and 2.2 billion years ago. It has some spectacular evidence of the results of a major impact that might have produced a crater 50 km across.
The Shoemaker Impact Structure is 800km NE of Perth. It is named after Gene Shoemaker of the Shoemaker-Levy 9 comet that shattered and hit Jupiter in 1994. Gene died in an accident in 1997 in the Tanami and some of his ashes were deposited on the moon in 1999 by the Lunar Prospector probe. He is the only person whose ashes have been buried on any celestial body other than Earth.
Lastly, Mike described an impact structure that has immense significance. It is the Acraman Impact on the Eyre Peninsular in South Australia. Ejecta from this impact has been found hundreds of kilometres away in the Flinders Range and the NT. Evidence involving Carbon-12 has suggested a mass extinction of what were then very primitive life forms that happened after the impact, followed (a mere 35 million years later) by the spectacular flourishing of life forms that we know as the Cambrian Explosion, in which most of the major groups of animals we know today first appeared in the fossil record.
This was a fascinating glimpse into the history of the earth, as well as how geological evidence has been pieced together by perceptive geologists to explain strange features of the landscape. We even learned some strange new words like pseudotachylite*!
*Pseudotachylite is a cohesive glassy or very fine-grained rock that occurs as veins and often contains inclusions of wall-rock fragments. It is typically dark coloured and glassy and forms during meteorite impacts.