VolcanicDegassing

Archives / 2013 / January

Field report: Pumice

Field report: Pumice

One of the most rewarding parts of fieldwork on volcanoes is when the parts start to fit together, and hunches turn into firmer ideas. When piecing together ancient volcanic eruptions, the process often starts with the discovery of the trace of a new deposit in a road cut section. This might be something as simple as the appearance of a scruffy yellow or orange band that catches our eye as we pass by (as, for example, in the photo below from an earlier blog post).

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit - the orange layer - in a road cut.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit – the orange layer – in a road cut.

A quick clean up of the surface with a trowel , and running the sample through your fingers, is usually enough to tell whether this really is a pumice deposit – and then the hunt is on. Where did it come from? How big was the eruption? How old is the deposit? Where does it fit into the record of eruptions we already know about?

We can usually get fairly good answers to most of these questions from the field, but to understand why we need to know a little bit about pumice. Everybody knows what pumice is: it’s that frothy rock that floats on water, and many people might even have a small lump of it in a corner of their bathroom. But to the volcanologist, pumice is the grail: a gobbet of magma, frozen in mid-flight that captures in its essence the story of the eruption: where the molten rock came from, how it matured before the eruption began, and perhaps even how the eruption started.

A block of pumice. This pumice sample had a chequered history even before it was erupted: it was broken apart while rising up through the plumbing system, and shortly afterwards healed up again, leaving the broken pieces of a pinkish pumice welded to the main white pumice.

A block of pumice. This pumice sample had a chequered history even before it was erupted. It broke apart, and then welded back together, as it rose up through the volcanic conduit towards the surface. The break separates the pink from the white areas. Coin is about 3 cm across.

Most explosive eruptions produce pumice, and it is this that gets carried high up into the atmosphere in the eruption plume and blown by the atmospheric winds, before raining down onto the surface below to form a pumice fall deposit.

Photo of pumice deposit

Pumice fall deposit, near volcan Mocho Choshuenco

These sorts of deposit are remarkably similar from volcano to volcano the world over. Typically, they are made up of angular fragments of white, cream, yellow or grey pumice that just pile on top of each other where they fell. Pumice blocks can range in size up to about 20 – 30 cm across, but are more usually centimetre to millimetre sized. And because these deposits are formed from strong eruption plumes – similar to that described by Pliny the Younger at Vesuvius in AD 79 – the pumice deposit covers the landscape like a carpet, gradually getting thinner and finer away from the volcano. We can use these changes in thickness and fragment size to work out the size and strength of the eruption; and we can also use the map of the deposit to work out not only where the eruption began, but also the wind direction at the time.

But we can also go further. Pumice is mainly volcanic glass: the molten rock, flash frozen as it erupted. This freezing process often catches the processes that actually cause the eruption in the act. Whether it is the expanding bubbles of volcanic gases that have caused the magma to froth in the first place,

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

or whether it is the droplet of once-hot basalt magma that might have stirred up the eruption in the first place.

Frozen droplet of basalt, on the edge of a large gas bubble, trapped within a larger block of rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption?

Frozen droplet of dark-grey basalt, on the edge of a large gas bubble, trapped within a larger block of cream coloured rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption? Bubble is 2 cm across.

The clues to both why an eruption happened, and its consequences at the time, can all be extracted from the buried remains of that ancient carpet of pumice.

After a day of successful pumice-hunting in the field here in Chile, we have returned to base camp (well, it’s actually a very well appointed wooden cabin) laden with samples, and many more questions to think about.

Field report: beware of the sealions

Beware of the sealions (‘cuidado lobos marinos’) declares the sign at Valdivia fish market, which stretches along the docks in this southern Chilean city.  And it’s no joke either, as a large clan of these creatures has set up stall in the estuary beneath the fish market, ready to feast on the daily pickings cast over the sea rail.

Valdivia fish market, with attentive sea gulls and sealions (not shown)

Valdivia fish market, with attentive cormorants and sealions (out of sight)

We are not here, however, to admire the sealife; or to see how the city has recovered from the devastation of past earthquakes. Instead, Valdivia is our gateway to the volcanoes of southern Chile; the chain of snow-capped mountains that adorn the landscape for over a thousand kilometers south from Santiago, like a trail of giant milk chocolate Hershey’s kisses.

Villarica, through the window glass

Villarica, through the window glass

For the past few years, we have been piecing together the trace of past eruptions from some of these volcanoes by scouring the landscape for layers of ash and pumice deposited long ago, and now buried deep in the soil. This time, the target is Mocho Choshuenco, a twin-peaked volcano that looms majestically over lakes Panguipulli and Rinihue.

Volcan Mocho Choshuenco, Chile. Choshuenco, to the left, is probably no longer active. Mocho, to the right, last erupted in 1864.

Volcan Mocho Choshuenco, Chile. Choshuenco, to the left, is probably no longer active. Mocho, to the right, last erupted in 1864.

Mocho, the younger part of the volcano, is last known to have erupted in 1864 and has shown no signs of activity since. This is not at all unusual for these volcanoes –which have lifetimes of hundreds of thousands of years, and where the intervals between eruptions can often stretch from decades to centuries. But it is the past activity of Mocho that we are looking for. Earlier work by colleagues from the Chilean geological survey suggests that there may have been two or three ‘large’ eruptions in the past 10,000 years, as well as numerous, undocumented smaller events. We plan to track down the deposits of these eruptions wherever we can find them – and this usually means buried deep within the rich soils of the temperate rain forests of the region. Rather than excavating pits ourselves,  we rely either on nature (erosion) or human activity to expose the pumice in road-side cuttings. A little bit like the sealions, perhaps, reliant on the pickings of the day.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit - the orange layer - in a road cut.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit – the orange layer – in a road cut.

This is only the first step in the detective work, because to identify which eruption the pumice has come from we need to collect samples for chemical analysis. In southern Chile, this is easier said than done – as the several metres of rain annually, and the warm summer temperatures, leave ten thousand year old pumices with the consistency of warm butter. Over the next few days of this field season, we will be completing the first stages of this work, and starting to put together a time-line of the past eruptions for which we can find a trace.

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