A to Z of... Volcanoes
A

A is for ash. The 1,000°C lava is not necessarily the most dangerous thing associated with a volcano. One of the most disruptive aspects of any eruption is volcanic ash, wich can be carried on the wind for thousands of kilometers. It poses a major problem for aircraft, because the ash rapidly clogs their engines. Dozens of planes have suffered damage or even crashed after flying into eruption columns. Apart from the problem of inhalation, accumulation on buildings, water contamination, distruction of crops and damage to power lines, ash can linger in the Earth's atmosphere and have a long-term influence on the global climate.
B
B is for bombs. Volcanic bombs are clots of lava ejected from a volcanic vent wich then solidify in flight. Sometimes measuring several metres across, they can take various shapes, including spindles (formed as they spin and cool in mid-air), fine ribbons, or even cow pats.
C
C is for cryovolcanism. It's hard to imagine cold volcanoes but, on some bodies in the outer Solar system, icy eruption are, or have been, the norm. Images sent back to Earth from the Voyager and Galileo probes have revealed smooth surface textures on moons such as Enceladus (Saturn) and Europa (Jupiter) that could only have been caused by volcanism. On such cold worlds, ice and slush would have been erupted, not lava.
D
D is for the Deccan Traps. The Deccan plateau in India covers 250,000 square kilometers and is a 65 million-year- old reminder of one of the most devastating eruptions ever. The basalt rock that covers it is the result of an eruption of extremely fluid and mobile lava from fissures in the planet's crust, called a flood basalt. The Deccan Traps would certainly have had a big impact on the Earth's climate, possibly contributing to the demise of the dinosaurs.
E
E is for eruptions. Eruptions are rated in order of explosivity, and are named after typical volcanoes: Hawaiian-type eruptions are characterised by very fluid lava; Strombolian ones are more gaseous; Vulcanian eruptions are viscous with large amounts of ash; the most feared are Plinian - viscous and gas-rich, ejecting a huge amount of pyroclastic material.
F
F is for fumaroles. These are vents in the crater on the flanks of a volcano, wich emit hot gases. Caused by magma heating groundwater, their temperature can reach 1,000°C. Volcanologists sometimes take samples of fumaroles gases, as the detection of a decrease in sulphur can be early warning sign that an eruption is imminent.
G
G is for Giant's Causeway. Northern Ireland's most spectacular tourist attraction is a prime example of "columnar jointing"' of a basalt lava flow. The mostly hexagonal columns were formed when lava erupted about 60 million years ago; it cooled, contracted and fractured. Around 37,000 columns extend from the cliffs of the North Antrim coast to the sea. But if you prefer to believe the legend, they were laid by the warrior giant Finn MacCool as a path to his lover, who lived on the Scottish Island of Staffa.
H
H is for hot spots. Hot spots are plumes of ultra-hot material in the mantle, from where magma flows into the crust. The motion of a tectonic plate over a static hot spot forms a chain of volcanoes, such as Hawaii or the Canaries. Volcanoes form as the plate passes over a hot spot and become inactive once it's clear of it.
I
I is for Io. Io, one of the Jupiter's largest moons, is the most volcanically active object in the Solar System. The huge tidal forces between this moon and its planet result in an ever changing landscape blanketed by eruptions. Io, wich id roughly the size of our moon, has around 300 volcanoes, some of wich spew sulphurous lava to height of nearly 300km.
J
J is for jökulhlaup. This Icelandic word means "glacier-burst" and is the result of a volcanic eruption beneath an ice-cap that lies in a crater. The result of such huge heat breaking up the ice causes a colossal deluge of water. When Katla volcano erupted in Iceland in 1918, the flooding had a peak of about 100,000 cubic metres per second. That's about the same rate of discharge measured at the mouth of the River Amazon.
K
K is for Krakatau. The tiny Indonesian island of Krakatau was in one of the great sea lanes of the 19th century (the Sunda Straits between Java and Sumatra) and many a sailor passed it by without much interest. But in 1883 it became much better known, when a series of huge explosions of the island's volcanoes sent vast columns of ash into the sky, blocking out the sun for nearly two days. About two thirds of the island collapsed into the sea, generating tsunami (tidal waves) that devastated the shores of Java and Sumatra, resulting in a death toll of 36,000.
L
L is for lava. To most it's just mlten rock, but lava comes in many consistencies and compositions. Lavas range from those poor in silica (wich are called basalts), to those rich in silica (rhyolites), with several phases in between. Two of the most common kinds of basaltic lava have the Hawaiian names aa and pahoehoe. The former is a jumbleof loose, irregular cindery blocks; the latter is smooth, ropy and resembles cow pats.
M
M is for magma. Magma is the name for molten rock below the Earth's surface, before ir erupts as lava. Magma is generated deep within the Earth's crust (or upper mantle) and can pool to form an underground reservoir inside the crust, wich may become a magma chamber beneath the throat of a volcano. The build up of pressure and gases in the magma chamber results ina an eruption.
N
N is for Nuée Ardentes. When a volcano collapses explosively, it may cause a nuèe ardente. This turbolent, incandescent gaseous cloud consists of a pyroclastic flow and ash. A notable example took place at Mt. Peleé, on Martinique, in 1902. The 200-450°C flow raced along at 500km/h, killing nearly 30,000 people.
Top 10 active volcanoes

Bill McGuire, professor of geohazards and director of the Benfield Greig Hazard Research Centre at UCL, lists the most 10 active volcanoes. (Danger is determined by the number of people likely to be affected by a big eruption).

1 Stromboli, Italy
O
O is for Olympus Mons. Towering 24km above the surface of Mars, this is the larfest volcano in the Solar System. It stretches 600km across. But unlike Earth, Mars has no plate tectonics so its lava built up on the same static sopt, during the course of aeons. Olympus Mons has lain dormant for 500 million years.

Continuos mild explotions and lava flows have been recorded here since Roman times. It's predictable, benign and attracts many visitors.

Danger Potential •
P
P is for plate tectonics. The Earth's lithosphere (crust and upper mantle) is made up of several plates, wich interact with each other. Volcanoes occur at subduction zones, where one plate plunges beneath another causing heat and friction, and at constructive margins, where the plates move apart and new crust forms.2 Kilauea, Hawaii
The most continuosly active volcano on Earth, Kilauea spews out five cubic metres of lava per second but is not considered that dangerous, having a hotel on the rim of one fo its craters.
Danger Potential ••
Q
Q is for quakes. Earthquakes generally take place in the same tectonically active areas as volcanoes. When magma moves closer to the surface beneath a volcano, the surrounding rocks are put under strain and fractures occur. This, in turn, is what leads to tremors.3 Etna, Italy
Etna has persistent eruptions from its summit and from fissures along its flanks. It's constantly monitored, but a big eruption could happen.
Danger Potential •••
R
R is for Ruiz. The eruption of Nevado del Ruiz in Colombia, in 1985, illustrates how a lack of monitoring and poor evacuation can lead to loss of life. The eruption was small and didn't kill anyone but disaster came when molten rock melted the ice cap and caused mudflows, which wiped out 23,000 people.4 Colima, Mexico
The Colima complex consists of two volcanoes, Nevado de Colima and Volcan de Colima. The latter has been the source of large debris avalanches and pyroclastic and lava flows.
Danger Potential ••••
S
S is for St Helens. In may 1980, Mount St Helens, in Washington State, US, decided to blow its top-big time. After erupting a huge column of ash, steam and pumice, which developed into a mushroom cloud 70km across, the northern flank of the mountain was blasted off, unleashing pyroclastic flows and mudflows as ice melted. The eruption destroyed 250 homes, 300km of roads adn devastated large areas of forest. On a more positive note, there were few casualties in the sparsely populated area and the region has become a living laboratory for ecologists studying nature's remarkable recovery since the eruption.5 Popocatépeti, Mexico
Popocatépeti is North America's second highest volcano. It is covered in ice and a large eruption could lead to mudflows that would engulf the heavily populated surrounding areas.
Danger Potential •••••
6 Tungurahua, Ecuador
T
T is for Tambora. The largest eruption on record took place in 1815, when the top blew off Tambora in Indonesia, spewing out 50 cubic km of pyroclastic material. The peak collapsed to form a 7km crater and 92,000 people died.Historical eruptions include sudden explosions and lava flows, mudflows and pyrocalstic flows that reached people at the volcano's base.
Danger Potential ••••
7 Soufrière Hills, Montserrat
U
U is for Unzen. Unzen volcano, in Japan, first made history in 1792, when the collapse of its dome resulted in tsunami that hit the shores of Japanese provinces killing around 14,500 people. It sprung back to life in 1991, when block and ash flowa hurtled from the summit and 10,000 people had to evacuated from the vicinity. This time, 42 people were killed, including several over-curious journalists.Soufrière Hills' first historical eruption did not take place until 1995. Ash and pyroclastic flows forced evacuation of the southern half of the island and destroyed the capital city of Plymouth.
Danger Potential •••
8 Merapi, Indonesia
V
V is for Vesuvius. Images of cowering bodies immortalised in stone make the eruption of Vesuvius in 79 AD one of the most famous events of history. Pompei, about 9km south of the volcano, was a busting town with a population of 20,000. But disaster struck when Vesuvius unleashed cloulds of ash, followed by pyroclastic surges (incandescent clouds of pumice and dust that race along the surface at up to 100km/h), which swept through the town. Pompei was destroyed and nearby Herculaneum suffered a similar fate.Merapi lies in one of the world's most densely populated areas. Pyroclastic flows and mudflows have devastated crops and caused many fatalities during historical times.
Danger Potential •••••
9 Semeru, Indonesia
W
W is for warning signs. Although volcanoes remain fairly unpredictable, scientists can use measures to help foresee an eruption. These include monitoring earthquakes, which can become more prolonged in the run-up to an eruption, and measuring ground deformation around the volcano.The highest volcano on Java has been erupting almost continuously since 1967. Frequent small eruptions and pyroclastic flows and mudflows have damaged villages and caused deaths.
Danger Potential •••
10 Karymsky, Russia
X
X is for xenolith. These chunks of mantle rocks can be brought to the surface via volcanic eruptions. Often sold in new-age shops, they are rich in green olivine and its gem-quality equivalent, peridot. Xenoliths can tell us the composition of the Earth's upper mantle.Karymsky began erupting 500 years ago, after a 2,300-year quiscence. Eruptions have been moderate with occasional lava flows.
Danger Potential •••••
Y
Y is for Yellowstone. Famous to most as a breathtaking national park in Wyoming, US, Yellowstone is a huge caldera, 70km across. It is the most powerful volcano ever to erupt on Earth: 600,000 years ago it spewed out 1,000 cubic km of material. And it's not extint.
Z
Z is for Zambales. This mountain range in the Philippines is home to Pinatubo, wich gave vent to the third largest eruption of the 20th century on the 15 June 1991. Ash rose 30km into the air, and about 500 died, many due to falling on homes which caused the roofs to cave in. The eruption coincided witha typhoon and the torrential rain and ash caused some of the worst mudflows ever. On the plus side, thousands of lives were saved due to prompt evacuations.