Scars, acne and others: circles on the ground

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.

Categories: Great Geology in Google earth, impacts

Into the Third Dimension: using Google Maps to know what’s underground

Much of the earth’s surface is covered by sedimentary rocks. These form as sediment settles on the surface. As the types of sediment change – sand to mud to sand again – different layers are formed, some hard some soft. The patterns these layers make are responsible for some of the most interesting Great Geology in Google Earth.

Sedimentary layers start off flat1 but as plates collide and squash, they may be folded, like pushing the edge of a rug. The resulting 3-dimensional structures are later eroded and brought to the surface – itself a 3-D structure. The 2-D lines we see on the aerial images below are formed by the intersections of these different 3-D structures. This can make interpreting them a little difficult, as we’ll see.

Compare and contrast the next two images. First look at these smooth lines from Mexico.

Versus these zig-zag ones from Argentina.

Both of these patterns involve sedimentary rocks, but the causes of the wavy lines are very different. The secret is to work out the shape of land surface and then infer the shape of the sedimentary layers. Rivers are our friends here. In Argentina there is a clear relationship between them and the lines the layers are making on the ground. Every zig has a river in it and every zag is a little hill in between. The sedimentary layers are pretty much flat and the pattern of lines is caused by the shape of the land surface. Imagine cutting a wedge out of a layered cake – this is what it would look like.

In Mexico the pattern is mostly due to folding of the sediments. The beautiful curves and swoops are due to flat layers having been slowly buckled as the earth’s plates rearranged themselves.

Analysing these shapes and making sense of them is bread-and-butter for geologists. A bed-rock part of any geological education. Typically we use maps, but in desert areas photos tell everything we need to know.

One important trick geologists learn is to create cross-sections, drawing a slice through the earth to show how the folded rocks continue underground. One of the many types of sedimentary layer is a coal-seam, so you can see how this is not a purely academic exercise.

A good geological map will have symbols showing how many degrees tilt the layers have and in which direction they are pointing down. Without these we can’t do a proper cross-section. But using our friends the rivers and streams we can still tell a lot.

Here’s a nice fold. Imagine one of the layers is rich in valuable unobtanium and you want to mine it2. You can trace a line where the edge of it is, but which side of the line is the rest on? Think of it another way, are we looking down at an arch with the top sliced of (an antiform) or a basin (a synform)3 Should you sink your unobtanium mine-shaft in the middle of the fold or around the edge? If you get it wrong, you’ll choose the part where the valuable layer has already been eroded away from.

dipToTheNorthLook at the rivers (streams/creeks) that cross the layers on the top side of the fold. Notice a pattern? Every time the stream cross the layers, it makes a little V-shape, with the sharp bit of the V pointing North. A stream bed is always lower than its surroundings so it gives us a glimpse into the third dimension. Cut into the layer and it’s edge moves north – it’s deeper the further north you – it’s dipping to the north – it’s a tilted sheet that disappears under the ground to the north.

dipToTheSouthWe can check if we’re right because this a fold. For this trick to work the south side of the fold should have the opposite pattern. We are looking at a breached arch and the southern side should have the opposite pattern. It’s harder to see on this side (probably because the beds are tilted more nearly vertically and the effect is smaller) but indeed, our little V-shapes point the other way.

This 3-D stuff is hard work. Yet it’s something geologists have to be good at (maybe there’s some link with the strong anecdotal evidence that they tend to be left-handed). If you need some help, some of these Google Maps have good photos associated to help you get another view of the structures. Alternatively this post makes the same link.

Let’s leave you with some sheer aesthetic pleasure. Some totally flat layers turned into beautiful patterns by erosion.

Categories: Great Geology in Google earth, tectonics

Looking from the sky at diamonds

When a geologist looks at Google Maps images, we usually filter out any human activity. But in the case of mines, that would be a mistake – the holes humans dig can tell us about the geology.

What’s this? A big circular hole in the ground and large piles of bluish mine waste. The shadows are on the northern side, showing we are in the Southern hemisphere. This is the Letlhakane mine in Botswana.

And here’s another one, in the northern hemisphere. The hole here is half a kilometre deep (it’s an older mine, the Mirny mine in Siberia. They used jet engines to melt the soil so they could dig).

And again, the Diavik mine in Canada. Note that the hole is considerably deeper than the lake it sits in. They really wanted to dig that hole and yet most of the its contents are sitting in those big blue piles.

Finally the oldest example in the world, the Big Hole in Kimberley. The piles of dirt were put back in the hole, or covered in houses. Mining underneath here reached a kilometre depth.

We’ve been looking at some famous diamond mines. Diamonds form deep in the earth and those worth digging holes for only reach the surface via weird fizzy molten rock called kimberlite. This magma zips up from 100km depth to the surface in only hours. Travelling at depth along a narrow crack (or dyke in geo-speak1) when the magma reaches the surface it forms a carrot-shaped pipe. The magma solidifies, peppered with diamonds that formed at depth and were pulled up inside it.

The pipe is circular in cross-section, so as the miners dig out the kimberlite they leave a circular hole. The vast majority of what is dug out is waste – only the precious diamonds are extracted. Kimberlite is bluish in colour, as you can see from the piles of it above.

The Big Hole in Kimberley was the first kimberlite pipe ever identified2, in the 19th Century. Diamonds found before then were from placer deposits, river gravels that contain diamonds eroded out of kimberlite pipes. Diamond placer deposits were first discovered in India and then Brazil. But the biggest area for modern mining is on Namibia’s Skeleton Coast.

Southern Africa’s Orange river rises on the Kapvaal Craton, an area rich in kimberlite pipes. For 100 million years it has flowed across the continent into the Atlantic ocean, leaving thick placer deposits. These have since been pushed around since by wind and ocean waves to cover a wide area.

Starting at the beginning of the Twentieth Century, German settlers found diamonds and the government designated a huge area of land as Sperrgebiet – forbidden territory. This whole area remains closed, but active mining is concentrated the southern end, on the coast, where the diamonds are concentrated in ancient beach sands.

The mine here looks very different from the others, no round hole or blue kimberlite (but look for the regular patterns on the spoil heaps). Like the other mines, this one was caused by the desire for the beauty and strength of diamonds.

Categories: diamonds, Great Geology in Google earth

Great Geology in Google Maps: mapping from above

In most cases, geological maps are made by piecing together observations of hundreds of individual outcrops. Boundaries between types of rock are covered in grass and sheep1 and have to be traced on the map later as a line between rock outcrops, like a inverted game of dot-to-dot. In areas like Himalayas the same boundaries may be visible in an instant on a vast wall of rock. Quickly mapping vast areas of country by tracing features by eye across expanses of bare rock is a great way to do geological mapping.

It turns out you can have a go at this ‘Himalayan-style’ mapping at home. Make a cup of tea (add salted butter, or spiced sweet milk for authenticity) and fire up Google Maps. It works outside the Himalayas too – deserts are great for this.

This is an area of Namibia in SW Africa. Immediately you can see different areas of rock – the image itself is like a geological map, only without labels. Himalayan geologists would identify the different areas of rock by getting samples, but we’ll stick with Googling the geology of the area.

Let me take you through the geological components and their history.

First off, the orange lines that look like rivers. Ignore them: they are rivers and we are geologists, not geographers. Mentally filter them out of the image.

damaraNext look at the stripy area top left – we’ll call them the Damara sediments. They formed in the long-vanished Khomas ocean between the Congo craton (ancient piece of crust) and the Kalahari craton. The area is stripy because the sedimentary layers are not flat. They are now on edge and form ridges on the surface.
dgraniteThe light orange area far right is made of Damara granites (and modern sand, again filter it out, if you can). These formed when the Khomas ocean closed and Congo and Kalahari got stuck together as part of Gondwanaland. This happened about half a billion years ago.

KarooBottom left, the rusty brown area is sediments and volcanic rocks that are part of the Karoo Supergroup. These formed on Gondwanaland between the Carboniferous and the Jurassic when geological conditions in this part of Africa were relatively calm.


The really obvious round thing that I’ve perversely left until last is a circular granite intrusion called Brandberg that formed a little later, as Gondwanaland was ripped apart and the South Atlantic opened.


I’ve given you the relative ages of the different rock packages, but we could have worked this out from Google Maps. The observations I’ll show you are exactly the same as those used by field geologists. What follows are a sequence of close-ups. Some are outside of the original area, but none are far away.


We’ve got two areas of sediments, the Damara and the Karoo. How do we know one is older than the other?

Here we see the red-brown Karoo sitting in a blob in the middle, with stripy purple-brown Damara to the NE. Look carefully (zooooming helps) and you can see circular lines in the Karoo. Note that the rivers go round this blob.

We are looking at a hill of Karoo sediments. The lines are the edges of different beds and they are tracing contours around the hill. These are flat layers of underformed sandstone2.

Try tracing the lines in the Damara – these are also the edges of sediment beds but they have been tilted. Note that they are cut by the edge of the Karoo. The surface between the two rock packages is an unconformity. It represents the time when the Damara sediments were pushed into a mountain belt and then eroded into a flat surface.

Folding and cross-cutting granites

This is a view of the contact between the Damara granites and the Damara sediments, which have some zig-zaggedy folding here. The boundary between the two is fairly abrupt and cuts the folded bedding planes. Using the principle of cross-cutting relationships the granite is younger than the sediments and the folding (although the folding may be related to the emplacment of the granite). There are some fine white lines crossing the folded sediments that may be veins of magma that came out of the main granite – further evidence of what came first.

Scoot around a bit and you can see that there is an unconformity between this older Damara granite and the Karoo, which gives us the relative ages of the three.

Three-way contact

This is a view of three of our rock packages. There’s a prominent river forming a yellow bar across the picture. Apart from that, from left to right we have: Damara sediments; Karoo sediments, sitting above the unconformity; the Brandberg granite. The boundary between the Karoo and the Brandberg is straight and cuts across bedding, suggesting the intrusion is later.

Another possibility to consider is that this is an older granite and the edge is just the edge of a hill. Well, look a the patterns of the streams – they are flowing down from the centre of the granite (it actually forms the biggest mountain in Namibia). For it to be higher now than the flat Karoo sediments, it must be younger than them.

The boundary between the Brandberg and the Karoo is quite interesting3, with tilting of the Karoo and a zone where the sediments and granite are mixed. Apart from the small zone that is Karoo coloured but without clear bedding, I can’t make this out.

The geological history I started with is based on many things, including precise dating of events and detailed field work. However, the basic age-relationships between the rocks can be worked out using simple geological rules and good photographs.

These same principles are being used on other planets where geologists have never been. We know a lot about the geological history of Mars by mapping from space using just these techniques.

Damara granite next to folded Damara

Categories: Great Geology in Google earth, sediments