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Earthquakes and volcanoes

Viewed from a safe distance, and watched on TV, the spectacular and devastating natural events such as volcanic eruptions, earthquakes, tsunamis, and floods are awesome and exciting. Indeed they make great TV and news items, and people will willingly watch the repeats. Being there is of course another matter. The media presentations of such events are equally skewed by our inability to comprehend, or be deeply interested in, events that are far from home. An earthquake that kills a thousand people on the far side of the globe may, or may not, be reported on the inside pages of our local city paper. However, if two of the local community were injured in the event, then it will be front-page news. This is not a lack of humanity on our part, but a useful mechanism to cope with news items that do not immediately impact directly on us.

We live on a very dynamic planet so all such natural events are extremely frequent. In terms of location, both earthquakes and volcanoes tend to be primarily confined to tectonic plate boundaries. The plates are the blocks of land masses which slowly drift across the surface of the earth. Sometimes they slide past one another and cause earthquakes and very obvious fractures. These fractures between blocks can be extremely sharply defined; I have seen examples in California where there are side steps in field boundary fences! In the UK, a classic example is Loch Ness, which is bordered by two different plates drifting past each other. Looking at the rock structures on each side, it is obvious there has been a slide of some seven miles between the two banks.

Whilst we only see media reports of major events, with sensitive equipment, geological surveys detect more than a million earthquakes a year. So the level of activity is greater than most of us realize.

Alternatively, plates can hit head on, and then one is forced down underneath the other. Not only does this result in earthquakes, but it is a very favourable condition to allow volcanic activity. The entire rim of the Pacific is highly active geologically because of these plate collisions. The impacts are slow, but they involve immense quantities of energy, as seen from the spectacles of volcanoes and earthquakes. Effects are not just localized at the collision sites: volcanic eruptions eject massive quantities of rocks and dust into the upper atmosphere that circulate the globe for many years. Earthquake geology is not an exact science, but in the last 50 years we have gained knowledge about plate tectonics, and so now have a reasonable understanding of where such events are likely to occur. The very fact that they are at plate boundaries means they are likely to be in coastal areas (e.g. the Pacific Rim), and therefore are associated with ports and fertile regions that are heavily populated. Inevitably we want to farm in fertile regions, and if the good soil was created by volcanic activity, then we choose to assume the volcano will never kill us, despite the numerous historic examples of large populations being obliterated. Future geological events will certainly cause local extinctions, and with increasing human populations more people will be killed.

TV-worthy earthquakes happen perhaps 30 to 50 times a year, and the larger ones are often linked to volcanic activity. Volcanoes are visible spectacles; again we see around 50 or so a year. Unlike earthquakes, they are not sudden, short-lived events, and often continue for considerable periods. Therefore as many as 30 active systems are happening at any one time. Earthquakes vary in intensity; surprisingly, the majority are not obvious to us, as they do not make the ground shake under our feet.

In recorded history, the most devastating earthquake was in the Shaanxi province of China in 1556 when nearly a million people were killed (at a time when the world population was much smaller than at present). In the last 1,000 years, there have been a dozen other major earthquakes around the world with death tolls above 100,000, so our concern and fascination with them seems justified (especially from the relative safety of a region such as the UK, where major earthquakes are currently unlikely).

A spectacular volcanic event took place in the Mediterranean during the Bronze Age, which totally destroyed the Minoan civilization with the eruption of the volcanic island of Santorini. The explosion occurred around 1600 BC; it is the largest documented volcanic event in our history. From the human perspective, it was a particularly unfortunate eruption as there are traces of written records and paintings suggesting that the Minoans were a remarkably civilized and egalitarian society (not least in the apparent importance given to women). With such a large explosion, the effects of that particular volcano were not limited to the Mediterranean. Chinese records imply that the same event precipitated the demise of the Xia dynasty. Their political collapse was triggered by atmospheric debris that caused the climatic impacts of a yellow fog, summer frost, and famine for several years. The Chinese dating would place this near 1618 BC. The explosion may also have been the origin of the myth of the world of Atlantis, since Santorini dropped below sea level (except for the part that shot into the atmosphere).

In the spirit of the TV-type descriptions, I probably should quote how much material was thrown into the atmosphere. The island of Santorini was originally a modest-sized mountain that has been replaced by a residual sea-filled crater that is some 5 X 7 miles across. However, this crater volume only indicates how much island vanished, and we can only guess at the volume of magma that was also spewed out during the eruption. Some estimates suggest it was some 60 or more cubic miles of material. Imagining this amount is hard, but nevertheless it was ‘a lot’ (the precision of my scientific background is showing).

Genuine numbers do exist for other volcanic events. Within living history, the 1980 eruption of Mount St Helens in upper Washington state in the USA is estimated to have tossed out about 0.7 cubic miles of material. This was a modest amount and certainly not in the same league as the ejections from the Yellowstone volcanoes. The 45-mile-diameter Yellowstone National Park is in fact a mega-sized volcanic crater from which there have been regular eruptions on a massive scale. One of the largest of these blackened the atmosphere and left immense surface deposits across vast tracts of land, as it expelled some 2,500 times the material seen from Mount St Helens. This Yellowstone volcano was so phenomenal that it is hard to comprehend, but as a guide this is roughly equivalent to a column of material the height of Mont Blanc that would fill the area around London bounded by the M25 motorway. An equivalent estimate for reference to the USA would be Long Island buried to the height of Mount Washington.

Eventually, Yellowstone will offer a repeat performance; current estimates suggest the underlying magma chamber has spread out over some 20 X 50 miles. In terms of fallout of ash when it erupts, one can predict that the middle of the USA will be devastated, and even the eastern seaboard will be buried under centimetres of ash. So across the entire country, there will be certain death to exposed animals, total loss of crops and communications, and a human toll measured in many tens of millions.

We think of mega-scale volcanic explosions as only occurring rarely, on geological timescales of roughly 100,000 years. The big events have been spread at various locations around the world. By contrast, we do not think in terms of humans over such timescales. Nevertheless, we have found human cave paintings dating back 30,000 years, and hominid bones that date to even earlier. Therefore these two timescales are closer than we would normally consider.

A final slightly worrying fact is that the last three eruptions of Yellowstone were around 2.1, 1.3, and 0.64 million years ago. I plotted a graph of these events to try to estimate when the next one might take place (assuming there is an underlying pattern). The graph is remarkably smooth and it suggests the next eruption is overdue by maybe 20,000 years! Perhaps we do not need to worry, but in geological timescales it is imminent.

A second well-documented Mediterranean example was the eruption of Vesuvius in 79 AD. This was on a very much smaller scale, but it buried Pompeii and Herculaneum. Since I am mostly discussing information loss, we could view that eruption more positively, as for archaeologists it offered a buried time capsule of life from the period. An observer at the time, Pliny the Younger, gave a detailed account of the eruption, including a description of ashes moving like a flood at high speed down the mountainside. His report of high-speed flow was originally discredited, but the same pattern was recorded in 1980 from the Mount St Helens eruption. This flood of solid and expanding hot gases is now termed a pyroclastic flow and examples have been seen to move at extremely high speeds.

Vesuvius is still an active volcano that poses a threat to Naples. Perhaps less obvious is that it is a small part of a much larger system, and the entire Bay of Naples is actually a crater that is the remnant of a massive volcano that erupted at some earlier period.

No summary of major volcanic events would be complete in the popular press without mentioning the major 1883 event when the island of Krakatoa in Indonesia erupted with massive quantities of material flung upwards in a series of explosions over the course of several weeks. The known death toll was around 36,000 people from ash and related tsunamis. The main explosion was so loud that it is claimed the sonic boom was heard in Europe. The vast dust and ash cloud modified global temperatures and produced colourful sunsets for several years after the event. In fact, Krakatoa is part of a set of volcanic islands around a central caldera that has erupted many times previously, and indeed the site is growing again with a new central cone that will erupt again.

In the spirit of capitalizing on our penchant for disasters, a film was made in 1969 that used elements of the actual eruption as the basis for the plot. The film went under the name Krakatoa, East of Java. The film has a nice emotive ring to the title, but unfortunately Krakatoa is actually west of Java, in the strait between Java and Sumatra!

 
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