Sunday, February 11, 2007

Crater Watch

In 1492, a meteorite fall was witnessed in Ensisheim, Alsace, then a part of the Holy Roman Empire, but now a province of France. The stone was recovered and placed in the church with the following inscription:

Many know much about this stone,
everyone knows something,
but no one knows quite enough.


You may have seen the odd shooting star yourself: meteoritic material burning up on entry to the earth's atmosphere. Somewhere between 100 and a 1000 tonnes of meteoric material falls on the earth every day, mainly as dust.

Geology.com has a new web page which allows you to zoom and view fifty different meteorite craters scattered across the world. You can view it here.

As you probably know the moon is covered with meteorite craters that can be seen quite clearly with a telescope. There are few sedimentary processes on the moon which means that the craters are not destroyed by erosion and sedimentary infilling. The moon has eighty craters more than 300 km wide such as the Mare Orientale which is over 900 km wide. Even the craters of the moon have craters on them with over 10,000 craters ranging from 30 to 300 km wide. These craters all formed before 3800 Million years ago, and are still visible today.

It is estimated that at least 200 crater of large size (greater than 300 km) must have formed across the earth at the same period, but due to the presence of an atmosphere and active crustal tectonics, these have been buried and destroyed. You can see in the photographs on Geology.com the ongoing processes of deposition that are creating sediment fans around the edges of the craters, slowly filling them in or washing them away.

(Information in this post from the excellent 'Understanding the Earth' by Brown et al., published by Cambridge).

2 comments:

Clare Dudman said...

Very interesting - and thanks for the link to the map. I was disappointed to see that they'd missed out the one Wegener identified as a meteorite crater on an island in the Baltic Sea. Maybe it is a little small but it is perfectly formed!

Jonathan said...

Thanks Clare. There are a whole lot of craters that they didn't show. This site currently lists 172 confirmed craters on earth, some dating back hundreds of millions of years and still visible.