Looking up to find what’s beneath our feet

By Eleanor

Everyone loves a good asteroid impact… although preferably not on the Earth today anywhere near human civilisation. You’ve heard it all before – big explosion (Figure 1), shock waves, nuclear winter, mass extinctions, dying dinosaurs. Death and destruction, as Thomas mentioned about a few weeks ago.

impact1

Figure 1: Artist depiction of a big asteroid hitting Earth.
Source: Wikimedia

But have you ever wondered what happens to the rocks? Probably not, right? It sounds kind of dull. Who cares about the rocks? Dinosaurs are way cooler!

Perhaps that’s true for some… but the rocks are interesting too. Looking at rocks that have experienced an impact can actually tell us very fundamental information about our planet.

I’m not talking about impacts like the one that killed the dinosaurs… in these cases, the asteroid (and some Earth rock too) gets completely vaporised because of the massive energy of the collision. Let’s head out to the asteroid belt, which is between Mars and Jupiter. There are millions of asteroids here, and although they are pretty spaced out (no pun intended) sometimes they do collide (Figure 2).

When that happens, you get a lot of debris thrown out (the very low gravity on these small objects means that this stuff doesn’t fall back onto the surface), and a shock wave will pass through the rock that survives. The shock wave causes a short period of intense pressure and temperature, which can have a pretty strong effect on the rock.

impact2

Figure 2: Artist depiction of two colliding rocky bodies.
Source: Wikimedia

Smaller impacts might cause deformation in the minerals, like parallel sets of fractures in olivine grains (olivine is a common mineral in asteroids). Bigger impacts can cause parts of the rock to melt.

If you melt a rock, you make lava, and if you cool the lava down very fast, you make glass (think obsidian, which is a volcanic glass found on Earth). This is because the atoms don’t have time to rearrange themselves into the ordered crystal structures of minerals. If you cool down the lava a bit more slowly, there is time for a few crystals to form (think basalt, which can sometimes have small crystals of olivine surrounded by glass).

In asteroids, you often form melt veins – thin slivers of molten rock that are surrounded by solid minerals. The veins form because shock waves travel through each mineral differently, leading to localised spikes in pressure and temperature which creates pockets and veins of melt. These melt veins will cool down pretty quickly if the surrounding rock is cold, so they turn into veins of glass with small minerals that have crystallised, like the black vein in the meteorite in Figure 3.

meteorite

Figure 3: Meteorite Sahara 99898 with a black shock vein.
Source: Meteorites Australia

Sometimes, the melt can cool down before the peak shock pressure has dissipated. When this happens, the minerals that crystallise from the melt as it cools can be quite different minerals to those found in the rest of the rock. To accommodate the high-pressure conditions, these minerals have much denser, more compact structures. Olivine, for example, takes on a different structure under high-pressure conditions, and we call that mineral ‘ringwoodite’.

Okay, so why do we care about minerals that form under high pressure?

We live on the surface of the Earth under the pressure of the atmosphere. If you went out to space without a space suit, you would explode, because there is not enough pressure to hold you together. If someone dropped a boulder on you, you would be squished, because the pressure is too much for you to withstand. (Incidentally, you being squished is analogous to olivine turning to ringwoodite. ‘Squished-you’, is favoured at high pressure, because it’s more compact than ‘Non-squished-you’; same goes for ringwoodite and olivine.)

Obviously the Earth is very big, and so the pressures deep in the Earth are huge. Imagine being underneath hundreds or thousands of kilometres of rock. MASSIVE pressure. (You would be very squished.) What are the rocks like down there? We can only guess, because they are much too deep for us to get at, usually.

There are lots of ways to guess. We can make predictions based on looking at the way earthquakes travel through deep interior of the Earth. We can also do high pressure and temperature experiments. But there is really nothing like a natural sample.

Meteorites, which are fragments of asteroids that have fallen to Earth, provide one kind of natural sample. These rocks have experienced shock pressures equivalent to being thousands of kilometres under the surface of the Earth, and when minerals crystallise out of melt at high pressure, we can get a tantalising glimpse at the minerals that exist deep within our planet.

Ringwoodite, the high-pressure version of olivine, was predicted way back in the 1960’s, and expected to be a very common mantle mineral. The first natural ringwoodite was found in a shock vein of the Tenham meteorite (Figure 4), which fell over Australia in 1879 (Binns et al., 1969). And this year, real terrestrial ringwoodite was found as a little inclusion in a diamond from Brazil. Magma carried this diamond from >660 km depth, all the way up to the Earth’s surface (Pearson et al., 2014). Finding samples of rock that come from this deep is rare.

tenham_cottingham

Figure 4: Tenham meteorite with glass veins.
Source: encyclopedia-of-meteorites.com

Even deeper than this, in the lower part of the Earth’s mantle, another high-pressure mineral is thought to exist – and it’s probably the most common mineral on Earth. We’ve called this one ‘silicate-perovskite’ up until last month, when it was seen for the first time in the wild. This mineral, now known as ‘bridgmanite’, was also found in the Tenham meteorite’s shock melt veins.

So it seems that impact events are not only interesting because of death and destruction… rather, looking towards impacts in outer space is one way to study what is deep beneath our feet.

 


Binns, R.A., Davis, R.J., Reed, S., 1969. Ringwoodite, natural (Mg, Fe)2SiO4 spinel in the Tenham meteorite. Nature 221, 943–944.

Pearson, D.G., Brenker, F.E., Nestola, F., McNeill, J., Nasdala, L., Hutchison, M.T., Matveev, S., Mather, K., Silversmit, G., Schmitz, S., Vekemans, B., Vincze, L., 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature 507, 221–224.

 

One response to “Looking up to find what’s beneath our feet

  1. great post Eleanor,
    they’ve since found one more Ringwoodite crystal in a superdeep dimaond, however they lost the original inclusion

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