Notes From Around The Sundial: Rocks From Space

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Gnomon's column image, showing a sundial surrounded with the words Notes From Around the Sundial'

And I think to myself, what a wonderful world!

Rocks from Space

Today I'm going to talk about astroblemes. These are geological features which are the remains of meteorite impacts. I'm occasionally asked why there are craters on the moon and not on the earth. In fact, there are craters on the earth if you know where to look, but there are mechanisms (erosion and plate tectonics) for wearing them away and removing them. On the moon, since nothing has changed for 3 billion years, when a crater forms, it sits there until it is obliterated by another crater.

On the earth, a crater is formed when a lump of rock flying through space hits the planet. We encounter such rocks daily, but the smaller rocks are heated up by friction with the atmosphere and burn up before they hit the ground. They appear as 'meteors' or 'shooting stars', a streak of light across the sky for a second and then gone. Bigger pieces which reach the ground are called 'meteorites'. There was a suspected meteorite strike in Ireland a few months back, and Astronomy Ireland, the Irish amateur astronomy association, is organising a countrywide search to try and find it. If you find a meteorite, you can probably sell it to a museum or science institute for a lot of money.

Really large rocks that hit the earth tend to explode on impact, leaving a crater. The classic example is Meteor Crater, Arizona. At about a mile wide and 550 feet deep, it is really impressive. The meteorite that caused it struck the earth about 50,000 years ago, when our ancestors were just thinking of leaving Africa. Getting further back into the past, we find traces of meteorite impacts going back right to the formation of the earth. The older they are, the more eroded they have become and it is often hard to recognise them, but it is fun to try. Geologists called them 'astroblemes' (literally 'star wounds'), and are interested in finding them even when there is no trace of a crater left.

Looking for Star Wounds

So get out your atlas or GoogleMap and start looking. The tell-tale sign is a very circular feature - it could be a perfectly circular lake, a ring of mountains or a section of coastline. Often the centre of the circle is raised. Don't be afraid to think big - some recognised astroblemes are hundreds of miles across. Here are a few to start you off:

  • Lake Manicouagan, Quebec, Canada - the shape of this crater has been accentuated by flooding it and creating a reservoir.

  • The Nordlinger Ries - a circular ring of mountains in southern Germany, just discernible from the pattern of forests on the map.

  • The southeast corner of Hudson Bay - the almost perfectly circular coastline on the east side is a dead give-away, and there's even the raised section in the middle.

Some astroblemes are not visible on the maps, but can be discovered by geologists due to the different structure of the rock underground. The Chicxulub ("cheek-shuh-loob") astrobleme in Yucat√°n, Mexico is more than 100 miles in diameter but is not visible from the air. Analysis of the rock is needed to prove that a meteorite struck the earth here. It is estimated that the rock was at least six miles in diameter and struck the earth about 65 million years ago. The current theory (as of March 2010) is that this impact triggered the most famous (although not the biggest) mass extinction in the prehistory of the earth, in which the dinosaurs were wiped off the planet.

For speculation on a really big impact, look at the coast of West Africa. Doesn't it look circular? We know that Africa used to be part of a giant supercontinent, and the the continent cracked and then separated to form Africa and South America, but why did it crack along a circular line? Could the whole of West Africa be a giant astrobleme? If so, if must have happened long in the past, as any such impact of that scale would have cataclysmic effect on the earth.


So could the earth be struck by another meteorite with similar effects? Up to recently, there wasn't much point in worrying about such a thing, because there was not a lot we could do about it. But with modern nuclear weapons and space technology, it is conceivable that we could detect a giant rock hurtling towards the earth and destroy or divert it before it hit us. The first steps in this direction were taken recently with the establishment of Pan-STARRS, the Panoramic Survey Telescope and Rapid Response. This uses a telescope in Hawaii and a 1,400 megapixel camera. The results are analysed around the world.

In September 2010, astronomers in Queen's University, Belfast used the system to detect its first 'potentially hazardous object', a lump of rock 150 feet in diameter which has been named 2010 ST3. The rock was 20 million miles away when they spotted it, proving that this system can see smaller objects further away than any other similar telescope. The rock came within 4 million miles of the earth in October, but was not in any danger of colliding with us this time around the sun. There is a 'very slight' chance of it hitting our planet in 2098, so it is 'worth keeping an eye on'. By then we may have figured out some way of dealing with it.

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