27 March 2013 Four ways to be killed by a volcano
Pyroclastic flow boiled the brains and vapourised the flesh of Herculaneum's inhabitants Active volcanoes are dangerous places. They can wipe out whole cities and kill large numbers of people.
The ghost-like casts from the Roman city of Pompeii are a reminder of the lethal eruption of Mount Vesuvius in AD79, which killed thousands and preserved their bodies in the position of their death. But it wasn't red-hot lava or suffocating clouds of ash that killed them, it was something far more unusual. Lava flows, or the molten rock that oozes from shield volcanoes moves far too slowly to be truly deadly. The real killers are much more frightening.
Here are four ways a volcano can kill:
1: Cooked by super-hot waves of gas
Volcanologist Brittany Brand explains how a volcanic eruption can produce lethal waves of hot gas and ash
The Roman cities of Pompeii and Herculaneum were annihilated on 24 August AD79 when Mount Vesuvius erupted explosively, sending fast-moving waves of superheated gas down the sides of the volcano at hurricane speeds. These 'pyroclastic flows' contain gas, ash and rock and can travel up to 450mph (700km/h).
The first wave hit the nearby Herculaneum with temperatures as high as 500 degrees Celsius. The searing heat was enough to boil the brains and instantly vaporise the flesh of its victims so that only blackened skeletons remained.
But how the people of Pompeii died has remained a mystery for many centuries. Volcanologists have now discovered they were killed by a later wave of pyroclastic flow.
Pompeii's wave was significantly cooler than the one that swept through Herculaneum, so although the victims bodies remained intact, the heat 'cooked' their flesh instantly. They were preserved by the falling volcanic ash and some of these can still be seen in Pompeii today.
Pyroclastic flows are arguably the most deadly volcanic event because they can travel for miles and are impossible to outrun.
They are produced by explosive 'composite volcanoes', which are made up of alternating layers of lava, ash and rock. When a composite volcano erupts, the rock layer is smashed into tiny dust particles. These particles mix with the hot ash and gases to form a giant mushroom cloud.
As the eruption weakens, this cloud can collapse under its own weight. It then cascades down the side of the volcano as a pyroclastic flow - destroying everything in its path. But that's not the only way they can cause big problems...
2: Buried by fast-flowing mud
In Columbia, 1985, a volcano called Nevado del Ruiz erupted. As pyroclastic flows exploded from the volcano they melted the glaciers on the mountain.
The melted water mixed with the volcanic ash, mud and rock, causing four enormous hot 'lahars' to speed down the mountain at 40mph (60km/h).
Armero: Only a few houses were left standing after hot lahars swept through the town Lahars are concrete-thick mixtures of mud and water that slide down mountains like avalanches. They can be extremely destructive because they travel with enough force to carry huge boulders at high speeds for up to 50 miles (80 km).
The lahars from Nevado del Ruiz flowed into the six major rivers at the base of the volcano before engulfing the town of Armero - killing more than 20,000 of the people that lived there.
3: Suffocated by poisonous gases
Pyroclastic flows and lahars are dramatic events, yet volcanoes can also be silent killers.
When a volcano sits beneath a lake, gases from the magma can filter through cracks in the Earth and become trapped under the water as carbon dioxide.
Silent but deadly: Lake Nyos in Cameroon, Africa Violent movement, from an earthquake or landslide for example, can cause the carbon dioxide to rise rapidly to the surface of the lake - a rare but deadly event called a 'limnic' eruption.
One of only two limnic eruptions ever recorded happened in Cameroon in 1986. A landslide disturbed a deep lake called Lake Nyos which sat within the crater of an inactive volcano.
More than 80 million cubic metres of carbon dioxide was released and surged into nearby villages; suffocating more than 1,700 people and thousands of animals and livestock.
4: Annihilation from ash clouds?
Poisonous volcanic gases, lahars and pyroclastic flows are deadly to almost anyone who gets in their way. But the devastating effects of a volcanic eruption can be even more far reaching.
In the Philippines in 1991, Mount Pinatubo exploded in a cataclysmic eruption, blasting 22 million tonnes of ash particles and sulphur dioxide 12 miles (19 km) into the atmosphere. Fierce winds from a passing typhoon blew the ash in all directions before it fell like thick snow on nearby buildings. Many roofs collapsed under the sheer weight; killing 300 people in their homes.
Ash rises as a plume from Mount Redoubt in Alaska Explosive eruptions like this are so powerful that lightweight ash particles can be lifted as high as the lower stratosphere by convection currents. When ash and volcanic gases spread across the globe after Mount Pinatubo's eruption, they reflected some of the Sun's radiation back into space.
This caused the global temperature to drop by 0.6 degrees Celsius. The cooling effect only lasted for two years. But some scientists think that volcanic activity like this may have caused a change in climate which contributed to the Permian mass extinction - an event which saw a staggering 96% of all the species on Earth perish.
Could a new giant volcanic eruption produce enough ash and gas to trigger extreme climate change that might threaten all life on Earth?
