If all this seems a long way away from everyday life, think again. Richard Muller of the University of California at Berkeley points out that shifting material at the core-mantle boundary (the CMB, if you are meeting geophysicists you need to impress) could have effects at the Earths surface. There could be landslides there which might encourage volcanism, arguably the most spectacular of natural phenomena and definitely one of the deadliest.
Almost by definition, a volcano involves molten rock coming up from inside the Earth. This means that the place you are least likely to find one is somewhere where the Earths crust is at its thickest. So the Himalayas, with many tens of kilometres of mountain root below the surface, are no place to look for a volcano. Mostly, as we have seen, they are found where the crust is thin and energy and material are rising from the mantle, or where material is being subducted back into it.
So far, so simple. In both of these cases, we can see just why there would be a volcano in a specific place, whether it is in the Andes – behind which rock is being subducted – or along the Mid-Atlantic Ridge, where it is being brought from below.
But glance at the map of the Earths volcanoes on p.67 and you will see that things are not that simple. There are some volcanoes that just appear where they are not meant to. If you are puzzled, you are in good company because the best minds in geology and geophysics have been baffled as well. It is not as though the volcanoes in question are a few oddities that can be quietly ignored. They include one entire state of the US – Hawaii – which is the biggest active volcanic system on Earth. But tectonically, Hawaii is also in the middle of the Pacific plate, where no volcano has the right to be.
The plume theory
The classic explanation for these volcanoes is that they are produced by hot spots in the mantle. These melt the rock above them and burst out onto the Earths surface. And it happens on a grand scale. From the bottom of the Pacific near Hawaii to the peak of Mauna Kea, the highest point on the islands, is 10,200m of vertical climb.
This version of events is supported by determining the ages of rocks in the Hawaiian chain. The idea is that there is a “plume” of heat rising from the mantle. It stays still, but the slow movement of the Pacific plate pushes ocean crust over it through time. The result is a chain of islands getting steadily younger from northwest to southeast. Today, active volcanism is seen only on the south-easternmost of the islands. Big Island, but may revive on the next-oldest, Maui. To the northwest the islands get steadily older, with Kaui, last in the chain, 5 million years old. Beyond it come a string of seamounts, islands which once poked above the ocean and have now been eroded away.
They are known as the Emperor Chain and are mainly named after past emperors of Japan. To the southwest of Big Island is an active volcanic zone of sea floor, Loihi, which promises in a few thousand years to form the next Hawaiian island.
Who could possibly disagree? Plenty of people in recent years. The trouble started when it became possible to make seismic images of the deep Earth below possible plume sites such as Hawaii and Iceland. The image of a plume cutting through mantle and crust like a welders torch is appealingly simple, but it proved hard to detect plumes running to anything like the depth that the theory called for. They ought to start not far short of the Earths core but sceptics found them elusive, although plume supporters claimed to observe them in numbers.
Instead, doubters point to experimental work that suggests that continental plates may be less indivisible than had been supposed. They may crack open to allow material from the top of the mantle to reach the surface. This might explain volcanoes far from plate boundaries, such as Hawaii and Yellowstone in the US. It is also a possible explanation for Iceland, which, although it is at an active centre for ocean floor production, generates far more lava than anywhere else on the Mid-Atlantic Ridge.
Once you accept that the mantle is a complex place where subduction and other effects might produce large volumes of rock with low melting points – potential future volcanoes – it may not be necessary to have plumes from the very deep Earth as well. Either way, the plume argument shows how something in science that everyone thought was obviously true can become disputed overnight and may turn out to be false, or only part of the story.
Types of volcanoes
Whatever causes them, volcanoes are objects of awe and fear for humans, and with good reason. They come in several main types.
Those with the runniest lavas are least dangerous, as we have seen, because they produce the least violent eruptions. But they also cover the most territory. The lava they produce forms huge shield volcanoes like the islands of Hawaii, or builds up into massive areas like the Deccan Traps of India or the Colorado River plateau in the US. These are so unlike our conception of a typical volcano that they tend to be called Large Igneous Provinces rather than volcanoes.
When things get a little stickier, the result is a classic volcanic shape like the one a child would draw, exemplified by Mount Fuji in Japan, last active in 1708. These very beautiful mountains are subject to sudden change when they do erupt, as the removal of a large piece of Mount St Helens showed in 1980. They are called stratovolcanoes – layered volcanoes – and consist of mixed layers of ash, lava and other components.
Other types of volcanism can produce cones that consist almost entirely of ash, called cinder cones. They are produced by gassy lavas and are too weak to sustain a big lava lake like a classic volcano, so lava often runs out of the side instead. The best known, Paricutin, just appeared in a field in Mexico one day in 1943 and by the time it had finished, 25 square kilometres of land had a new lava coat.
The least familiar volcanoes are those that appear under the sea, typically at centres of ocean floor spreading. As well as fresh lava, these volcanoes, known as hydrothermal vents, produce a huge amount of hot or even boiling water, depending on their depth below the surface, and some are called “black smokers” in honour of their carbon-rich dark emissions. The energy released by these vents doesn't go to waste, as they are home to a lively ecology of fish, shellfish and other animals. Some live in ways that would not be possible elsewhere on Earth, such as bacteria which depend on sulphur emitted from the vents.
Volcanoes are beautiful but they are also hazardous. When Mount St Helens erupted in 1980, the wave of debris, steam and gas that surged out moved at up to 1 lOkph, and 540 million tonnes ol ash were emitted. Over the centuries, volcanoes have been responsible for many thousands of deaths. There are many ways a volcano can kill. For example, when Tambora in Indonesia erupted in 1815 it killed 12,000 people directly, but 80,000 more died subsequently from starvation. The most direct hazard from any volcanic eruption is being engulfed in a cloud of ash and air running downhill from the volcano. Known as nuees ardentes, these burning clouds can travel at speeds far too fast to escape. In 1902, a nuee ardente killed the entire population of St Pierre in Martinique – apart from one man in the safety of the condemned cell in the prison. These eruptions are called Plinian in honour of Pliny the Younger's description of the destruction of Pompeii by Vesuvius in 79 AD. We do not know what Pliny the Elder thought of it.
The eruption left no trace of him but his sandals.
But volcanoes offer plenty of other hazards. For those close to the volcano at the time of an eruption, volcanic tephra (chunks of ash or even full-scale rocks thrown out of a live volcano) are a danger. Likewise, eruptions often cause landslides and avalanches which can catch the unwary.
But the worst volcanic hazards are those that strike far away and with little warning. For example, over 23,000 people were killed by lahars (mudflows) in the eruption of Nevada del Ruiz in Colombia in 1985. Lahars can run for many kilometres, often outpacing warning systems as well as people. Although they come from volcanoes, they do not necessarily need an eruption to get going. Sometimes they happen when a volcano breaks open the flanks of a lake and releases the water, but they can also occur when its heat melts snow and ice. Indeed, a volcano can alter drainage systems over large areas by blocking rivers as well as by releasing water.
Of all the dangers, volcanic gases are the most insidious. Volcanic rock contains gas even at depth, but as it rises and the pressure falls, the gas expands and helps drive the eruption. Much of it is just carbon dioxide and water vapour, but some is toxic, including sulphur dioxide, which attacks the lungs and causes acid rain. Most pernicious is hydrogen fluoride, which has sometimes fallen on cropland near volcanoes in amounts sufficient to kill grazing cattle by poisoning their feed.
But carbon dioxide is the most dangerous of these gases despite being ostensibly harmless. It kills by replacing the oxygen in the air, because it is more dense, and is especially hazardous in narrow, steep valleys which trap both the gas and its victims. Even worse, volcanic carbon dioxide can kill without needing a volcano. Lake Nyos in Cameroon has repeatedly killed people – 1700 in one episode in 1986 – by absorbing volcanic carbon dioxide filtering into it from belowr before releasing it in a sudden explosion. People and animals were killed nearly 30km away, as there was no volcano to warn them of the hazard. In general, low-level volcanic activity is benign, and people have enjoyed the warm waters and fertile soils it produces for millennia, but its benevolence cannot be taken for granted.