Looking is never just looking. When we gaze at something, we are not passive recipients of an image, instead our brain is constantly looking for patterns. If you are drifting over the earth, whether as an astronaut or via Google Maps then a simple shape such as a circle will ‘jump out’ at you. It turns out there are many different types of circle on the earth. They can be born in seconds or slowly, be mysterious, sinister, large or small. All are round.
A classic volcano has an elegant cone1 and round crater at the top. If you’re lucky you’ll get multiple circles, like on Mount Kilimanjaro.
Or here in Italy, Mount Vesuvius gives us three circles. The first is the volcano itself, its steep sides rising above the towns surrounding it. The highest portion is relatively bare of vegetation and makes a smaller grey circle for the crater to sit in.
Craters form in different ways. Some are holes made by volcanic explosions. Crater Lake in the US is a glamorous example of this – what was once a volcano is now just a hole in the ground.
Calderas are round structures formed when a magma chamber (the big pool of molten rock deep under the volcano) empties. The rocks above collapse down into the now empty space.
Circular structures under volcanoes too. A volcano is typically a single point source of magma. With no real reason to be asymmetrical, any structures that form tend to be circular. For example here in Ardnamurchan in Scotland.
The volcano here was active 60 million years ago, so we are looking at the eroded roots of it. The rock structure used to be interpreted as a ‘ring dyke’ a circle of rock that filled the vertical cracks within a caldera. It’s now thought instead to be a saucer-shaped sheet of rock called a lopolith.
Volcanic craters are common on the earth, so scientists long assumed that the many craters on the moon were volcanic in origin. In fact they are impact craters, not planetary acne as volcanoes are, but bullet holes – scars that show we are living in a dangerous neighbourhood.
The most elegant scar is in Quebec in Canada, lake Manicouagan.
Like Ardamurchan above, this is not a fresh structure, but the eroded roots of an ancient impact. With it’s old rocks and ancient surface, Canada is rich in impact craters. Australia, another craton is good too, such as here at Gosses Bluff.
The biggest impact crater know is the Vredefort structure in South Africa.
Over 2 billion years old, this was once a huge hole in the ground – up to 9 km of rock were instantaneously removed when a huge rock from space hit the ground.
The Meteor/Barringer Crater in Arizona is small but much fresher, a mere 50,000 years. Here we are looking at the original hole in the ground, rather than deep structures now brought to the surface by erosion.
Once we saw impact craters on the moon and thought they were volcanoes. Now we see round structures on the earth and assume they are impacts. Here are a few that formed in other ways.
The Richat Structure “eye of the Sahara” is formed of domed sedimentary rocks, perhaps uplifted by volcanic activity (volcanic rocks sit in the middle). Scientists have searched for ‘shocked’ minerals and other signs of impact and found none.
The Kondyor massif in eastern Russia is beautifully circular, but is not a crater. It’s an eroded volcanic structure.
Into the Anthropocene
Let’s end with a type of crater scoured by a massive fireball created by the same process that powers the sun. They appeared all over the earth for a few brief decades but (hopefully) they’ve stopped forming for good.
This sinister pair of holes were caused by nuclear testing on Enewetak Atoll in the Pacific. The left hand one was an island called Elugelab, until the explosion of the world’s first hydrogen bomb on the 1st November 1952.