Scientists have become much better at predicting eruptions and how dangerous a volcano is. It turns out it all depends fundamentally on one thing - how sticky the magma is.

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As hot magma rises towards the Earth's surface, bubbles of gas begin to appear. If the magma runs like water, then the gas can escape easily and no explosion will take place. The volcanoes of Hawaii and at Etna fall into this category.

Their trademark is gushing torrents of red hot lava, flowing down the side of the mountain, like a river of fire.

These are the less dangerous of the two types of volcano, because the path of the magma flow can easily be predicted.

Boom and bust

In contrast, when the magma rising up inside a volcano is sticky like treacle, the gas bubbles cannot escape, pressure builds and eventually the hot magma explodes and fragments.

This type of eruption is much less predictable and therefore much more dangerous. The explosions at ancient Pompeii, Mount St Helens, Pinatubo, Montserrat and Mount Usu were of this type.

Because the sticky magma is stronger, it can build taller volcanoes with steeper sides - the classic volcano shape - sometimes with catastrophic results.

As magma pushes up, the whole side of a volcano can become unstable and fall off in a landslide. This is like smashing the top off a shaken champagne bottle, rather than teasing out the cork.

Explosions of enormous power can result, blasting ash high into the atmosphere and sending clouds of burning gas and rock shooting down the sides of mountains. These "nuee ardentes" incinerate everything in their path.

Predicting trouble

The stickiness of the magma depends on how much silica it contains. The rocks that make up the continents are more silica-rich than the ocean floor.

So, unfortunately, the sticky, explosive volcanoes are nearly always on land and often near human populations. The runny, less dangerous volcanoes tend to be safely tucked away under the seas or on oceanic islands.

Overall, one in 10 people on Earth live within range of an active volcano.

Burning ash flows can travel extremely fast