August 23, 2002
Big Rock, Fall From Sky
Posted by scott at August 23, 2002 10:35 AM
BBCnews.com has this brief overview on evidence of a new giant impact discovery. Report says it's was at least twice the size of the one that killed the dinosaurs. Unfortunately no word on exactly when it was supposed to have happened. Could this have caused the Permian extinction?
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Actually they say they don't know where the impact occurred, the date is given at 3.4 billion years before present. (So unlikely to be related to the Permian extinction ('the day the Earth nearly died'), which was 250 million years ago).
There are so few rocks surviving from so far in the past that it's not surprising that so little can be determined about the nature of this impact.
What is the Permian Extection?
To start by putting this on a rough time-line, the earliest evidence of life on Earth dates from around the time of this impact - 3.5 billion years ago. Until about 550 million years ago life consistent solely of single celled bacteria, but during the 'Cambrian Explosion' (550 MA bp) the first multi-celluar forms appear and rapidly radiate to become all the body forms we see today (arthopods, worms, vertebrates etc), hence, 'explosion'. By the Devonian fairly modern fish are starting to appear and the colonisation of the land starts. Dinosaurs first appear about 250 million years ago and go on to do very well indeed. Mammals are kept very much in the shadow of the dinosaurs, but when they go extinct at the start of the Tertiary (65 MA bp) they rapidly radiate to fill all the niches vacated by the dinosaurs. The first hominids appear about 6 million years ago, the first modern humans about 150,000 years ago and the first civilisations about 10,000 years ago.
Anyway, throughout this time a series of 'mass extinctions' has caused large numbers of species to disappear very suddendly from the fossil record. The Cretaceous-Tertiary (K-T) extinction 65 million years ago is by far the most famous because it resulted in the death of the dinosaurs. (Mass extinctions often mark the boundaries between geological eras because the types of fossils are an easy and reliable way of establishing the age of a rock formation.) However, the K-T event was not a particularly severe mass extinction - only about 70% of species went extinct.
The most severe mass extinction ever marks the start of the Permian era, about 250 million years ago. Over 95% of the Earth's species went extinct and for several million years afterwards the only large terrestrial vertebrate was a therapsid called listrosaurus. Therapsids were an intermediate form between reptiles and mammals and listrosaurus is believed to have resembled a pig and lived by grazing the very poor vegetation that survived the extinction.
The causes of mass extinctions are the subject of considerable debate. It has been suggested that the Permian co-incided with one of the occasional gatherings of the continents into one giant landmass. This would have rendered the interior of the super continent an uninhabitable desert and messed up the chemistry of the oceans (life in oceans depends on minerals eroded off the land).
However, the Permian also coincides with a flood basalt eruption in what is now Siberia. Flood basalts are sort of super volcanoes. Millions of cubic kilometers of lava are erupted over a period of a few million years. All the Earth's flood basalts have been eroded and covered, but the dark 'seas' on the Moon are believed to be the result of flood basalt eruptions.
The K-T has been popularly declared to be the result of a Great Big Rock hitting the Earth. And it's true that a large object did hit what is now Mexico right at the time of the extinction, but there was also a flood basalt eruption going on forming the landscape known as the Deccan Traps in what is now India (the name Traps comes from a word meaning 'staircase'). The debris from the Mexican impact has been found in the middle of the Deccan Traps sequence. It is therefore uncertain what the relative impact of the flood basalt eruption and the impact were.
Although we should see a rock the size of the K-T impactor coming a long way off, flood basalt eruptions are much more poorly understood and I don't think anyone is claiming the ability to predict them.
Well thank you very much Robert UK! I have found someone who likes words almost as much as Scott.
Um... yeah... what he says :)
The Japanese have just discovered what really killed the dinosaurs. (Tip o' the hat to New Scientist).
I can't begin to start criticising this theory - for a start the 100-tonne dinosaurs lived happily for tens of millions of years (complaints about the fucking noise don't fossilise) and they went extinct long before the K-T event. The dinosaur species around then were more like T. Rex and hadrosaurs.
They changed the link! This is the correct link to the Japanese site.
Heh... and then there's the professor who did demonstrations with his wife as to how dinosaurs may actually have mated. No, really, I think it was on Nova or something. I can't remember if they were Brits or not, but it positively screamed "Meaning of Life" to me.
A scenario has been put forth recently...
-Meteor strikes the earth
-Causes global warming by cloaking the earth
in "greehouse gases".
-The ocean water warms up enough to release
hydrated methane which bubbles to the surface
and into the atmosphere
-Almost all life then perishes.
does the recent scenario suggest only a localized release of methane?i wonder how much needs to be released to raise the atmosphere, say, 5 degrees globally.further,is it possible that in shallow depths the release was more likely while the deepest deposits remained in hydrate due to less temperature fluctuation and higher unaltered water pressure?or did all of the deposits of methane release in a dominoe effect?if so,why do we have an abundance of it today?
Actually I think Joe's point is "we need to stop driving cars and live in mud huts or we'll all die, and I'm going to keep declaring all mass extinctions are the result of greenhouse gases somehow until you all agree with me."