#10 UHP Metamorphism, The Eiffel Tower, Queen Elizabeth’s Face, and Quartz Grenades


We’re all used to the concept of pressure, but in many different ways. Some think of peer pressure, the pressure of working life, or maybe of your ears popping on an aeroplane. For geologists however, geological pressure along with it’s siblings geological time and temperature describe conditions in the Earth which are unimaginable to most people.

An illustration of the components which determine pressure.

Pressure is a measure of force per unit area, and it’s quite easy to calculate given just a few variables (i.e. area, force, maybe acceleration too). Beyond the base of the Earth’s continental crust, these geological pressures reach 2,500,000,000 pascals or 2.5 GPa, and continue to rise the deeper you go. From this point on, a rock experiences Ultra High Pressure (UHP) and may be metamorphosed (rather than melted). It is, however, difficult to preserve rocks which have been down so incredibly deep as there are very few processes which take rocks down below the base of the crust and bring them back up to the surface quickly enough so that they preserve information on their burial history. Therefore although UHP conditions prevail throughout much of the Earth, not many rocks on the surface ‘remember’ if they were ever there. More than likely they weren’t.

The majority of tectonic activity on Earth results in rocks being sent deeper in the Earth and new rocks being created in igneous processes at or near the surface. Only special circumstances result in rocks re-surfacing after being very deep in the crust or mantle.

But just how much pressure is felt by rocks when they reach UHP conditions? Well yes we know it’s 2.5 GPa or above, but how can we relate to that? Well, to generate a UHP of 2.5 GPa concentrated on the golden face of the queen on a standard first class stamp, you would need to load that stamp with over 76 million kilograms. That’s equivalent to balancing 10 full-sized Eiffel towers on Lizzie’s Chops! Even that kind of pressure is hard to imagine, but suffice it to say you’d not want to trap your finger under that!

To generate 2.5 GPa on a postage stamp you’d need to load it with over 10 Eiffel Towers!

The photomicrograph shows a garnet crystal with inclusions of various minerals. The yellow dots show the inclusions which were once coesite, but are now quartz. The expansion f the coesite/quartz has created radial fractures in the surrounding garnet.

In my eclogites the mineral garnet commonly contains inclusions of quartz. When the garnet bearing rock experiences UHP conditions for long enough, the quartz changes its structure to a more dense form we call coesite. For the UHP rocks in my field area, these rocks inevitably start making their way to the surface. As the pressure starts to drop, the coesite inside the garnet starts to expand. Like the casing of an imminently exploding grenade, however, garnet resists the expansion. Eventually, however, the coesite reverts back to the quartz structure which takes up a larger volume. This shatters the garnet in the surrounding space just like the explosives in a grenade must shatter the casing to continue expanding.

So when I look at my rocks in thin section, I really like to think that I’m looking at tiny quartz grenades exploding on the face of the queen under the weight of 10 Eiffel towers!

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2 thoughts on “#10 UHP Metamorphism, The Eiffel Tower, Queen Elizabeth’s Face, and Quartz Grenades

  1. 2.5GPa?!?!? I’ll trump that any day of the week with the piston cylinder…(well, going to 3GPa anyway!) Nice analogy though!

  2. Ha ha! I know that it’s not actually that high of a pressure in the grand scheme of things… It’s just so easy to get lost in the numbers. Did the napkin calculation and I thought it was going to be more to be honest. I’d love to see what 2.5+ GPa would do to ordinary household objects. Have you ever seen the program “Will it blend?” – a suitibly geologically themed version would be top notch!

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