The explosive science of volcanoes and peat bogs

Can you name a volcano? For many people, their answer to this question may be Mount St Helens. The eruption that took place there on the 18th May 1980 was actually relatively small as eruptions go, but as it ripped through the Douglas fir forests of southwest Washington State, USA, just 50 and 95 miles from the major cities of Seattle and Portland, respectively, it erupted into the public consciousness.

I recently visited Portland, Oregon, for a conference on tephra (a.k.a. ‘volcanic ash’, but see below!) and its potential in multidisciplinary science. For many of the volcanologists and other earth systems scientists vising the Pacific Northwest that week, the conference fieldtrip out to Mt St Helens was somewhat of a pilgrimage.

As we entered the blast zone, about ten miles away from the volcano, the devastation from the thirty four year old eruption was still clear. Within 30 seconds of the eruption starting, 230 square miles of forest had been destroyed by a pyroclastic flow, a scorching current of gas and rock, travelling at speeds of up to 670 mph.

Many trees still lie on their sides, long dead, arranged in eerily straight lines caused by the direction and force of the blast. The few trees that are still standing following the eruption have been stripped of their branches and are scattered across the landscape like abandoned telegraph poles. If it weren’t for the relatively modest regrowth, the landscape would look like the surface of the moon.

But what am I doing here? As a Bogologist I am primarily concerned with the development and (changing) ecology of peatlands, so why am I visiting a volcano that erupted many thousands of miles from my study sites? In fact, many Bogologists possess a keen interest in volcanoes, although perhaps for less obvious reasons than their volcanologist, seismologist and geologist colleagues.

Ask someone to sketch an erupting volcano and they will probably draw a triangular shaped mountain with a large cloud of gas, rock and ash rising ominously over the landscape. This popular image is based on the well-known stratovolcanoes, such as Mts Fuji, Krakatoa, Stromboli and Vesuvius – the famous eruptions of which have burnt themselves on to our collective memory.

When certain volcanoes erupt they eject many tonnes of material into the atmosphere and as with many things in science, this material can be classified in categories. Volcanic ‘bombs’ or ‘blocks’ are larger than 64 mm; ‘lapilli’ are between 2 and 64 mm; and ‘ash’ is anything below 2 mm, but collectively, this airborne material is known as ‘tephra’, a term coined by Icelandic volcanologist Sigurdur Thorarinsson in 1954.

This ‘cloud’, also known as the eruption column, can reach heights of up to 45 km, pumping tephra and aerosols at high-speed in the earth’s atmosphere. The pattern in which tephra is deposited and how far it travels from its volcanic source are specific to each eruption, but generally volcanic material in the troposphere, the lower 7-20km of the atmosphere, falls out relatively quickly via rain or merely gravity. It often hangs around just long enough to make a nuisance of itself, however, and you probably remember the ash cloud from the 2010 eruption of Eyjafjallajökull in Iceland causing travel chaos across Europe and beyond – Matt actually got stranded in India for two weeks as a result of this tephra!

Of most interest to Bogologists, however, is the tephra that breaks through into the stratosphere, the second major level in the earth’s atmosphere, where temperatures are lower and conditions are generally calmer than in the underlying troposphere. As a result, volcanic material entering this section of the atmosphere has the potential to travel much further. Typically, tephra entering this zone is much finer and less dense than the coarser fragments and it often possesses a series of cavities and bubbles across its surface, known as vesicles, which form as volcanic gas passes through the magma from which the material was formed. Together these characteristics make the tephra far more buoyant and conducive to long-range dispersal.

The primary role played by tephra in the study of peatlands and past climate change is in helping to work out exactly how old our peat sequences are, answering the age old scientific question, ‘what happened when?’ We call this ‘tephrochronology‘.

Eventually, tephra in the stratosphere descends and lands on the earth’s surface, including its peatlands. As we’ve read elsewhere, bogs are excellent at preserving things and these layers of tephra are no different. Whilst we often can’t see them with the naked eye, careful analysis of our peat sequences can reveal the presence of microscopic tephra shards in well-defined, undisturbed layers.

Generally speaking, the tephra shards in each of these layers, which should correspond to a single eruption, have their own chemical signature. Over the years, researchers have built up extensive databases of this chemical information and we can now identify which layers come from which eruption.

Radiocarbon dating of the organic material in and around the tephra layers has also given us a pretty good idea of when most of the eruptions occurred. Of course, in places like Iceland and Japan, where people have been keeping written records of these eruptions for many centuries, we can assign an even more exact date to our tephra layers. It is quite strange to be able to say that a tephra, nearly a metre or more below the surface of a bog came from an eruption which took place in Iceland on a sunny Thursday morning in the back end of June many hundreds of years ago!

But these tephra layers don’t just help us build our age-models. We can say with some certainty that tephra layers deposited in different bogs that belong to the same eruption, were deposited at the same time. This give us a pinning point between two or more sequences, which allows us to correlate more accurately between the ecological and chemical conditions that were occurring in the bogs at that time.

Of course, the wider the tephra from any given eruption was dispersed, the more bogs we can include in our correlations. This allows us identify regional patterns of change which may be related to broader regional or even global changes in climate.

It has generally been assumed that tephra distribution is largely limited by proximity to the volcanic source and direction of the prevailing weather systems. As a result, Icelandic tephras were generally though to be limited the peat records of northern Europe, Alaskan tephras to those of northwest North America, Andean tephras to Southern South America on so on. However, a number of recent studies have made us rethink this – the tephras present in our peat records may have travelled rather further than we had first thought.

When a research team, led by Dr Sean Pyne-O’Donnell of Queen’s University Belfast, began looking for tephra in a peat bog on the island of Newfoundland on the far east coast of North America, they would be forgiven for not expecting to find much. The nearest volcanoes are c. 1750 miles away in Iceland and ‘upwind’ of the dominant westerly winds. There are plenty of volcanoes in North America but they are exclusively located in the far west of the continent. A few are sprinkled through the Rockies but the majority are located along the Cascade, Coast and Aleutian mountain ranges that fringe the North Pacific Ocean from California to Alaska, where the Pacific and North American plates heave and grind against one another, sandwiching the Juan de Fuca plate in between for good measure – part of the famous Pacific ‘Ring of Fire’. Although abundant, these volcanoes are well over 3000 miles from the bog in Newfoundland – surely too far for their tephras to be found? Well, it appears not.

All in all, twelve tephras were found in Newfoundland, greatly exceeding expectations and including material from the older Mt St Helens and Crater Lake (formerly Mount Mazama, before it collapsed following a large eruption approximately eight thousand years ago) eruptions, but also further afield from Mount Aniakchak in the heart of the Aleutian archipelago, incredibly over 4000 miles away.

The majority of peat-based tephra research was pioneered in northern Europe. As a result, the tephra record (also known as a tephrostratigraphy) preserved in the region’s many bogs is arguably the best developed of its kind in the world. When this work began a few decades ago, the first place many researchers looked to as the sources of their tephras was Iceland. The westerly airflow that is prevalent in the climate system of the North Atlantic ruled out many other potential European sources, such as the Eifel Volcanic Field in Germany, or the numerous volcanoes distributed throughout Italy, Greece and Turkey. This assertion was largely correct and the vast majority of tephras present in mainland Europe are of Icelandic origin. But as with anything in science, there have always been a few question marks.

One such uncertainty was a tephra horizon given the catchy moniker of ‘AD860B’, which was a nickname associated with its estimated age. The chemistry of this tephra did not match anything from any known Icelandic eruption and remained a mystery from 1995 when its discovery in Ireland was first reported by a team from Queen’s University Belfast. It is now one of the most widely distributed tephras in northern Europe, with a confirmed presence in over 20 bog and lake records. More recently, the tephra was found in the Greenland ice cores and perhaps this is what prompted a search for the tephra’s source from further afield.

In a forthcoming paper in the journal Geology, led by tephra expert Dr Britta Jensen of Queen’s University Belfast and the University of Alberta, Canada, the unexpected origin of the AD860B tephra was finally revealed. Comparison with an extensive database of North American tephra chemical data revealed the closest match to be the Alaskan White River Ash. This represents a journey of approximately 4500 miles from the volcano to the bog surfaces of northern Europe.

The extreme distribution demonstrated by this tephra has got the Bogology community thinking in a different way about the unidentified layers in their peat sequences. It looks almost certain that more long-distance tephras will be found in the coming years.

In addition to the North America tephras identified in Sean’s peat record from Newfoundland, a number of layers remained unidentified. Further work has since been done in the region and, as Britta presented at the recent tephra conference in Portland, evidence suggests that tephras from the Kamchatka Peninsula in eastern Russia are reaching the area. This pan-hemispheric distribution could potentially increase our ability to correlate between climate records derived from peatlands over similar spatial scales, significantly enhancing our capacity to identify regional patterns of past climate change – something which is crucial in our understanding of the ever-changing global climate system. The perspectives of Bogologists interested in these diminutive volcanic ash layers have been blown, almost literally, wide apart by these recent discoveries. The next few years will be an exciting time to be involved in tephra research!

Further reading:

Coulter, S.E. et al. (2012) Holocene tephras highlight complexity of volcanic signals in Greenland ice cores. Journal of Geophysical Research, v. 117, doi:10.1029 /2012JD017698.

Jensen, B.J.L. et al. (in press) Transatlantic distribution of the Alaskan White River Ash. Geology.

Lawson, I. T. et al. (2012) The spatial distribution of Holocene cryptotephras in north-west Europe since 7 ka: implications for understanding ash fall events from Icelandic eruptions. Quaternary Science Reviews, 41, 57–66.

Pilcher, J. R. et al. (1995) Dates of Holocene Icelandic volcanic eruptions from tephra layers in Irish peats. The Holocene, 5(1), 103–110. doi:10.1177/095968369500500111

Pyne-O’Donnell, S. D. F. et al. (2012) High-precision ultra-distal Holocene tephrochronology in North America. Quaternary Science Reviews, 52, 6–11. doi:10.1016/j.quascirev.2012.07.024

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