Lava you than me.
For the origin of the word volcano we look to the tiny island of Vulcano in the Mediterranean sea near Sicily. Named by the ancient Romans, its outpouring of hot lava fragments and clouds of dust were believed to come from the underground forge of Vulcan, blacksmith of the Roman gods, as he beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, god of war. And in one sense, the Romans were right. Volcanos are sites of furious production, with their most obvious creations being themselves. For they are made of piles of their own eruptive products -- lava (molten rock), tephra (ash and dust) and crusted over ash flows.
Birth of a monster
If you'd been wandering near the Mexican village of Paricutin in 1943 you could have watched the birth of a cinder cone volcano. Below ground on a local farm, rapidly expanding gas sent molten lava exploding from a vent in the earth. As it fell back, the lava solidified in cinders around the vent, and if you'd had about nine years to spare you could have sat and watched as the cinders piled up to form a cone 365 metres high. With a final flourish, one last explosion created a funnel-shaped crater to top off the cone. Time to take a picture of the latest of the planet's 500 or so active volcanos.
For the main cause of all this heated activity we can blame the dynamic duo of plate tectonics (the movement of the major structural features of the Earth's crust) and magma (molten rock created within the Earth's crust by high temperatures and pressure). Where tectonic plates collide (convergent margins in geo-speak), magma generated by heat and stress in the plates is forced upwards with a pressure that often generates explosive eruptions. The resultant stratovolcanos, built from layers of erupted ash and lava, tend to be large, steep-sided, symmetrical cones. Most famous are Mount Fuji in Japan and Mount St Helens in the US.
Separating plates (divergent margins) are a tad less volatile, creating rift zones where lava rises more gently than is the case with colliding plates. The relatively calm, smooth flow of lava from vents associated with separating plates tends to create a low and wide volcano with gently sloping sides. Known as shield volcanos, this fuming family includes some of the largest in the world including the Kilauea and Mauna Loa volcanos in Hawaii. In fact, with around 4572 metres beneath the sea and 4168 metres above it, Mauna Loa is the world's largest active volcano.
Although it is a shield volcano, Mauna Loa actually owes its existence to `hot-spots' -- another method of volcano creation. When tectonic plates move over particularly hot regions of the Earth's mantle, volcanos spring up in line with the plate's movement. The result is often an island chain, of which Hawaii is an example.
There are arguably five major types of volcanic eruption. A volcano can, however, exhibit any number of these characteristics during a particular eruption. A Strombolian eruption -- named after the highly volatile Stromboli in Italy, known since ancient times as `the lighthouse of the Mediterranean' -- blasts huge amounts of lava out. A Vulcanian eruption tends to eject fragments of new lava, whereas Pelean eruptions blow the full range of volcanic products from a central crater, which then forms a high-speed `glowing cloud' or avalanche. Hawaiian eruptions are known for their fast-flowing rivers of lava. The most destructive tend to be Plinian, where the explosive ejection of relatively viscous lava sends ash high into the air and is closely associated with deadly pyroclastic flows.
So why get steamed up over lava, which although being a serious hazard doesn't go as fast or as far as you might think, when there are hot and speedy pyroclastic flows to worry about? Like some terrifying land version of a tidal wave, pyroclastic flows happen when an ash cloud collapses and in a glowing avalanche sweeps down the volcano slopes, causing damage not just by burial and incineration but also by impact damage and asphyxiation. An apparently safe `getaway' distance meant little for the poor inhabitants of Pompeii and Herculaneum when Vesuvius erupted in AD79. The pent-up gas, whose violent release drove this famous eruption, sent a mixture of very hot ash and rocks scorching towards the two towns at speeds of up to 200 metres per second. The worst loss of life from a pyroclastic flow occurred in the Caribbean in 1902 when nearly 30,000 died after Mount Pelee erupted.
Also run like hell if you see a lahar coming your way. If water from melting glaciers or nearby rivers gets mixed up with volcanic ash, a giant mudflow can thunder down a volcano's slopes and into nearby valleys. One of the worst causes of destruction after an eruption, lahars can be truly devastating. After the 1985 eruption of the Nevado del Ruiz volcano in Colombia, a lahar swept down the Lagunillas River valley burying the town of Armero and killing 22,000 people. By the 1980s, volcano monitoring had become a powerful life-saving tool. The tragic irony was that although officials had been warned of an eruption and a lahar, a failure in communications left Armero to perish.
So what should you look out for if your next-door neighbour is a bit fiery? Change in shape is one thing, as magma pouring into chambers deep beneath a volcano cause it to swell. Precise equipment, such as tilt-meters and lasers can detect the slightest change in the tilt of a slope. Volcanic earthquakes triggered by rising magma hitting obstacles in the Earth's crust are also warning of an impending eruption (earthquakes are linked to the same movement of tectonic plates that are central to volcanic activity), and by carefully mapping the locations and depths of the epicentres of these volcanic quakes, scientists can track the subsurface movement of magma both horizontally and vertically.
Changes in the composition of a volcano's gases, caused by rising magma, are also a sign of impending showtime. A ten-fold rise in levels of sulphur dioxide (SO2) emissions over a two-week period gave warning of the massive 1991 eruption which tore apart Mount Pinatubo in the Philippines, thereby saving around 200,000 lives. Curiously, the real sign that it's time to get the hell out of there is when the SO2 emissions suddenly drop again, as this means the gas passages within the volcano have become sealed by hardened magma, causing increased pressure in the volcano's plumbing system which just has to be released.
With volcanic eruptions taking place over a cycle of hundreds or thousands of years, research into a volcano's geologic past is vital to extend the database for estimates of future activity and also pinpoint particularly hazardous zones around a volcano. For example, the lahar that wiped out Armero followed the same route as one that occurred in a previous eruption in 1845. US scientists predicted the likelihood of Mount St Helens reawakening in 1978 so that when it erupted in May 1980, precautions had been taken, hugely minimising the loss of life which might otherwise have occurred.
So where might you want to avoid if you fancy seeing in the Millennium in one piece? Well, although Mount Popocatepetl near Mexico City has spluttered into life recently, my money on `Where To Think Very Hard About Not Going' is Naples. Not content with having wiped out Pompeii, Vesuvius has spent the last few centuries having major blow-outs every 50-80 years. If you took a neopolitan ramble up its slopes to talk to one of the resident vulcanologists they might tell you about changes in gas emissions and shape, then shake their head as they describe the local authorities woeful plans to evacuate the city. Sorry Naples, you're a lovely city but your volcano may be about to get as hotheaded as some of your inhabitants.
WANT TO KNOW MORE?
* US Geological Survey http://vulcan.wr.usgs.gov/home.html
* The Annenberg/CPB Project Exhibits Collection - Volcanoes http://www.learner.org/exhibits/volcanoes
* Smithsonian Institution Global Volcanism Program http://www.nmnh.si.edu/gvp/volcano