pyroclastic flow

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.


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.