By Mike

We’ve all been there. A trip to a restaurant or a dinner party where the company gets boring (sometimes conversation degenerates into talking about rocks), and there’s somebody eating really really slowly to the point where nobody can leave, or move onto dessert, until they’ve finished.

to be or not to be garnet? in this case, to be (it's made from garnet). source: www.skullis.com
to be or not to be…garnet? in this case, to be (it’s made from garnet). http://www.skullis.com

Anyone who has been stuck in a situation like this, forced to talk about different kinds of limestone or a similarly mundane topic, will be thankful that they didn’t make friends with garnet. First off, garnet is a mineral (crystal) so it doesn’t say much (it’s a great listener though). Secondly, it eats very slowly.

New research published in nature geoscience http://www.nature.com/ngeo/journal/v6/n5/full/ngeo1804.html sheds the light on exactly how slow.

In the interest of full disclosure, when I say eating, I do not mean eating. Rather, I mean that one mineral turns to a different mineral by ‘consuming’ something else that happens to be nearby. It’s sort of like some pastry ‘consuming’ some meat and forming a delicious pie.

basalt, the hot sticky poo of the rock world
basalt, the hot sticky poo of the rock world http://www.usgs.gov

So let’s start from the beginning. The ocean floor is made of a black, glassy rock called basalt. This basalt is continuously on the move, it moves at a rate comparable to fingernail growth away from the place its made (middle of oceans) towards the edge of the oceans. At this point, the basalt ocean floor is forced underneath the continent at the edge of the ocean, and sinks down into the depths of hell; that is, the Earth’s ‘mantle’. This is plate tectonics.

When the basalt is forced down into the mantle, it starts to get hot. And when I say hot, I mean hot enough to not only burn your pizza, but reduce your whole oven to a smouldering mess. It also starts to get squashed. Scuba divers will know that going underwater does things to your body which aren’t too pleasant. Imagine instead of going under 20m of water, being forced to dive under 20km of rock. At these intense heat and pressure conditions, individual atoms within rocks start to move around, and start to reorganize themselves into structures which are more suited to the new conditions, and in doing so, new types of crystal form. So, the basalt, which has no garnet in it, changes into a new rock, which does have garnet in it. This new rock is ‘eclogite’, a beautiful green and red rock made of garnet and another mineral called ‘pyroxene’.

eclogite: i told you it looked good. red stuff:garnet. green stuff: pyroxene. www.thinktank.ac
eclogite: i told you it looked good. red stuff:garnet. green stuff: pyroxene. http://www.thinktank.ac

Next, as the new eclogite keeps on sinking through the mantle, the garnet starts to get hungry. It starts eating things around it. Much like the relationship I have with my fridge, if the garnet sees food it will eat it. The food in this case is the pyroxene (which just formed as part of the eclogite). In the process of eating, the garnet+pyroxene forms a new mineral (the delicious pie I mentioned earlier) called majorite [as an aside, majorite is named after Alan Major, a former technician in the geology department at ANU]. The new research tells us that this process of garnet consuming pyroxene to form majorite happens really really slowly.

So the big question: why the hell does this matter? Well, without this change from garnet to majorite, the old slab of ocean floor would not sink through the mantle. If this didn’t happen, then the ocean floor wouldn’t be able to move (the ocean floor is still connected to the eclogite in the mantle), and plate tectonics would stop. If plate tectonics stopped, there would be nowhere for the heat from the inside of the earth to escape to, the planet would become a big pressure cooker, and it would periodically release bursts of heat in the form of melted rock which would kill us all.

plate tectonics. if you want to understand this diagram, come and do a geology degree
plate tectonics. if you want to understand this diagram, come and do a geology degree http://www.platetectonics.com

When the basalt turns to eclogite, it gets heavier. When the majorite forms, it gets heavier again. By understanding how fast this happens, we gain some more information about how plate tectonics works. We can’t speed it up, and we can’t make it go slower, but it sure is nice to know how fast it is going to go before you agree to take it out to a restaurant.