Accretionary Wedge - Metageologist

In Accretionary Wedge #41 – “Most Memorable/Significant Geologic Event That You’ve Directly Experienced” Ron Schott asked us to relate the story of the most memorable or significant geological event that you’ve directly experienced.

Living far from a plate boundary, I have a problem. There are no volcanoes in the UK. We have feeble earthquakes every now and then, but I always seem to be away or asleep when they happen (that’s right, these are earthquakes you can sleep through). I’ve witnessed rock-falls and erosion, which are important, but really I’ve not experienced any significant geological events at all. Which, come to think of it, makes me rather like a lot of the geological record.

Consider a nunatak in Antarctica. This is a lump of rock sticking out of the ice-cap. It is surrounded by ice for miles around and nothing happens. Cosmogenic nucleide studies show that the rocks surfaces are millions of years old. So little has happened that blobs of glass that fell from the sky the best of a million years ago (tektites), can be easily found on them.

On a bigger scale, Australia has had a quiet time of it since the dinosaurs. Many land surfaces on that continent are dated to be 10s of millions of years old. Weathering and soil formation has happened, but little else of geological significance.

Absence of evidence is not evidence of absence, but I can’t talk about nothing and Geology without mentioning unconformities. James Hutton’s realisation that these surfaces between rock packages can represent gigantic periods of time led us to the recognition of Deep Time, one of the most profound insights we have.

Unconformities are the most dramatic reminder of the amount of geological time for which we have no record, but a simple outcrop of sediments can do the same. Most sediments don’t represent a continuous record of sedimentation. Turbidites tend to record only occasional dramatic episodes of sedimentation. The steady drip-drip (or is that drop-drop?) of pelagic sedimentation is only intermittently preserved in such rocks. Sometimes we admire only the products of dramatic events and forget the huge periods of time in between. Even a calm-looking sandstone might in fact be mostly made up of storm deposits. A package of conformable sediments can contain huge gaps, but only subtle hints such as beds with intense bioturbation give a sense that for great periods of time, nothing happened, for all we can tell.

We should think about nothing more often. Thinking of Geology as a series of dramatic events is all very well, but its the enormous chunks of nothing that are truly remarkable. The human brain isn’t equipped to understand quite how insignificant we are in terms of space or time. Perhaps we should think about this more often, while staring at nothing.

Accretionary Wedge #37 called for examples of ‘Sexy Geology’. Here’s mine:

You always remember your first time.

I was a young man, freshly graduated and I’d somehow persuaded the government to give me (just) enough money to spend three years studying for a geology doctorate. Once spring arrived I eagerly set off for my field area. I’d been here before, but chaperoned on a undergraduate field trip. Now it was just me and the field-area, alone at last.

I spent most of my first field season with a small hill called Currywongaun. She’s a lovely thing: a glacially scoured lump of gabbro facing the Atlantic in Connemara, Western Ireland. The view is beautiful, the rocks are beautiful and sexy.

Gabbro is lovely stuff, with a touch of the exotic coming from its origin in the mantle. Most gabbro intrusions are petrologically interesting but structurally simple. Not my Currywongaun, it has modal layering, sure, but its orientation varies wildly and sometimes it’s folded. It is also has a fabric defined by the igneous minerals, showing that the magma itself was deformed while it was still partially molten. It is a syntectonic intrusion, emplaced not into the edge of an opening ocean basin, like the Cuillins or Skaergaard but into an actively deforming mountain belt.

Magmatic fabric in syntectonic gabbro

The base of the hill covers the edge of the intrusion and the rocks beneath. These are the sorts of rocks that get fans of structural geology all hot and bothered: mylonites, with lots of consistent shear sense indicators. Two sets of fabrics too, high temperature ones related to thrusting parallel to the intrusion edge and lower-grade cross-cutting extensional shear zones recording orogenic collapse.

Fancy a bit of metamorphism? Me too. Rocks close to gabbro get very hot indeed and metasedimentary xenoliths on Currywongaun have corderite-spinel-orthopyroxene assemblages; virtually all water (found in minerals such as muscovite) has been driven out of the rock due to extremely high temperatures. There’s a nice aureole around the intrusions as well which is large and high temperature (sillimanite isograd) since the rocks were already fairly hot (500 °C).

Cordierite-spinel-orthopyroxene granulite facies xenolith

A bit of mystery can be sexy too. There was an odd lens of an off-white rock, kind of a streak across the hillside, that I couldn’t quite identify in the field. Once back in the lab, armed with thin sections, I could piece together a story to explain the mystery.

Responding to powerful forces from deep within, the mafic magma forced its way deep into already hot metamorphic rocks. It crystallised into gabbro, became rigid and immediately the contrast in rheology with the softer reforming rocks around it created a zone of thrusting along its edge (I found sheath folds). The host rocks quickly responded, they were heated and metamorphosed with reactions that produced granitic melt and lots of hot fluids. These were forced back into the intrusion, altering it, transforming it. My mystery sample? Well, the gabbro is noritic, rich in orthopyroxene which when moistened is altered to everyone’s favourite amphibole, cummingtonite.