rhyolite

Ignimbrite on Eigg

Today’s song is Turn to Stone by the legendary Joe Walsh

Artwork © Lauren Delora Sears

Wow, this is the penultimate blog for this semester!

I have to return to my favourite topic in geology. Volcanism!

Back in my second post,  “Not just another brick in the wall”, I discussed some of the rocks you can see in the buildings around Dunedin. One of my all time favourites is ignimbrite. There is something particularly spectacular about the idea of an incandescent cloud rushing down a slope at speeds of 300m per second.

The Taupo Volcanic Zone has the world’s highest output of rhyolitic rock.  Over the lat 2 million years a minimum of 10,000 cubic kilometres of magma has erupted from the zone and the signs, in the central North Island, are everywhere. Pyroclastic flows – the red hot mix of particles and air eventually come to rest, settle and weld together to form the ignimbrite.

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Landsat of Lake Taupo : credit Nasa World Wind

Volcanic rocks are classified according to the amount of silica they contain. The light-coloured, rhyolitic rocks erupt at around 600-800 º C . All rocks derived from rhyolite such as obsidian, pumice, ignimbrite and pitchstone have the same chemical composition. So obsidian which has cooled very quickly forms a glassy texture.  Pitchstone is very similar although it has a coarser texture as it cooled more slowly. This gives the crystals time to grow.

Over on the Isle of Eigg in Scotland a flow previously considered to be lava has been reinterpreted. The Sgurr of Eigg is famous for its pitchstone and now it is thought to be ignimbrite. Part of the difficulty is in distinguishing between the two in the field and under a microscope. The flow shows some evidence of rheomorphism which are flow structures common in ignimbrite.

Scientists from the University of Glasgow have examined the deposits and think that they may be part of a larger complex which extends out into the North Atlantic.

Having just spent six months over in the area I am excited about the prospect of going for a visit next time. The Sgurr of Eigg is a famous landmark and I’m keen to lay my hands on it.

Not just another brick in the wall

Today’s music inspiration is ‘Another Brick in the Wall’ from the inimitable Pink Floyd.

It’s always great exploring new places. Gradually, a picture emerges from the blur of unfamiliar sights and sounds as you collect clues for the mental map that builds over time. Navigational skills are an essential part of that explorer’s kit, along with a camera and, of course, chocolate!

Last Tuesday had been a particularly long day and a group of us, from the Science Communication Centre, wandered across campus to meet up with some music students.

I drifted along, in my own wee world, barely noticing the chatter cloud around me. We meandered along the cobbled paths that eased gracefully between the heritage campus buildings. Large rock samples, embedded on stubby concrete plinths, caught my eye and I perked up. They formed a line in front of an old building and I made a careful mental note of each one as I passed.

A voice from the cloud laughed. “Hey Janis, you can break in there!”.

I turned to look up at the open window on the second floor. The startled man who was gazing out of it immediately closed it. It could have been coincidence but I think he sensed I could very well be tempted!

It was the geology building – one of the university’s oldest buildings – smartly dressed in a dark grey suit with contrasting pale trim. The dark blocks are cut from a basaltic lava known as Leith Valley andesite which erupted from the Dunedin volcano around 12 million years ago. I had my first excursion up into the Leith Valley with a run up around Ross Creek Reservoir, last weekend, so it was another place I could add to the fringe of my map.

As I gazed at the construction, I recalled the very first essay I wrote, in my majoring subject, about the Taupo Volcanic Zone. It is where I first learned about volcanic rock types. I made it my business to find out more about it because at the time I lived only an hour away, in the geothermal wonderland of Rotorua. And yes, I did get used to the smell.

Volcanic rocks are classified according to the amount of silica they contain. Those with less than 52% silica are termed basic and include basalts and ultrabasic rocks. Basalt erupts at around 1100-1250 º C and is quite runny. When you see images of red-hot flowing lava, with people standing around watching it move, you can be pretty sure it’s basalt. The dark colour comes from the high magnesium and iron (Fe) content so they’re also called mafic. These are the ones that the geology building is made from but there are others.

At the other end of the scale, acid rocks contain more than 66% silica. These paler, rhyolitic rocks erupt at a much lower temperature than basalt at around 600-800 º C . It is extremely explosive because the reduced temperature increases the viscosity or stickiness and the gases can’t escape so easily. Smooth black obsidian, lightweight pumice (the original exfoliant), ignimbrite and rhyolite all have the same chemical composition! They just look different because of their cooling history. Obsidian cooled very quickly and forms a glassy texture. Pumice is just rhyolitic rock froth and you can see that it is full of holes where the gases were trapped.

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Ignimbrite was given its name by former geology professor at Otago University, Patrick Marshall. It is formed from the cooling of a hot, ground-hugging, incandescent cloud of lava and pumice which hurtles down the side of a volcano at enormous speed. Some can reach speeds of 300m per second! No wonder the residents of Pompeii didn’t stand a chance.

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Some New Zealand ignimbrite is quarried in the North Island near Lake Karapiro. It is a popular cladding for houses known as Hinuera Stone. You can often see flattened pumice lenses within the blocks. They were entombed and squashed as the cloud coalesced, settled and cooled to form a thick welded rock sheet.

Andesite is the piggy in the middle. It is aptly called intermediate since its composition, eruption temperature and behaviour falls between the other two. Andesite volcanoes are the ones which many people visualise when they think of a volcano. They are steep sided held up by a framework of alternating layers of lava and ash. Mts Ruapehu, Ngauruhoe, Tongariro and Taranaki are all andesite volcanoes.

Oamaru stone facings provided light relief against the roughly-hewn blocks. I had spent many years in Oamaru living in an house built from it. This rock is very different from the volcanic ones. It is a limestone – composed of the compacted skeletal remains of millions of marine organisms. It tied in quite nicely when we finally reached the music department and one of the talented music students introduced himself as having hailed from Oamaru.

Eventually, I hope to gather enough clues on my travels around the city and its environs to gain a greater sense of the geological history of Dunedin and find a few extra treasures along the way.