At A Glance
An estimated 500 million people live near active volcanoes – some even build homes on their slopes. Despite the danger, volcanoes provide some of the most fertile land in the world, a cooler climate because of their height and, in many cases, a lucrative draw for tourists.
About 20 volcanoes are erupting at any one time in the world. Although improved disaster planning and monitoring of many volcanoes have saved thousands of lives since the 1980s, the mountains of fire remain a constant threat to homes and livelihoods.
When the Tungurahua Volcano in Ecuador erupted in 2006, it killed just five people after thousands were evacuated from the area. But it also destroyed villages, buried homes and fields under tonnes of ash and rained lava and hot rocks on the surrounding region. In all 300,000 people were affected.
Most explosive volcanoes are on the margins of the Pacific Ocean in the so-called "Ring of Fire", which runs from Latin America up to Alaska, across to Russia and down through Japan and Indonesia.
What is a volcano?
A volcano is an opening in the Earth's surface that allows hot, molten rock, ash and gases to escape from deep below the surface.
Volcanoes come in many different shapes and sizes, but all have a central vent that is connected to a magma chamber where lava builds up. Most of these chambers are several kilometres wide.
The flanks of a volcano are unstable and often fracture, allowing lava and volcanic gases to escape out of the sides.
How many volcanoes are there?
More than 550 volcanoes have erupted since historical records began.
About another 1,000 dormant volcanoes have the potential to erupt again.
Some of these are sleeping giants, like the supervolcano beneath Yellowstone National Park in the United States that has not erupted for 640,000 years but is still considered active because of its functioning geothermal system and frequent earthquakes.
Yellowstone's magma chamber is about 40 by 80 km (24 by 49 miles) across. If the volcano erupts again, the consequences would be catastrophic.
Many volcanoes in densely populated areas are constantly monitored by scientists, but hundreds in remote areas are not.
What happens when a volcano erupts?
The signs of a volcano about to blow include an increased number of tremors near the volcano, new gas emissions and shifts on its surface.
An eruption happens when magma rises to the earth's surface, fracturing the surface rock. The volcano produces:
- lava (called magma when it is beneath the surface)
- hot gases
- ash and tephra (larger fragments, including lava bombs)
- pyroclastic flows (a mixture of hot gas and tephra that flow at high speed)
- lahars (mudflows of ash and tephra mixed with water)
Lava flows destroy everything in their path but usually move too slowly to kill people.
More dangerous are superheated clouds of ash and hot gas that can move at hurricane speeds, hugging the ground and destroying everything in their path. These pyroclastic flows killed all but two of the 29,000 inhabitants of St Pierre, capital of Martinique in the Caribbean, in 1902. They died within seconds of the eruption.
Volcanic ash is made up of tiny jagged pieces of volcanic rock and glass formed from magma during an eruption. Heavy ash falls can cause roofs to collapse, darken skies and make it difficult to breathe.
When ash and lava erupt into the air, they often cause intense thunder and lightning storms and heavy rainfall. This can trigger floods, landslides and rivers of mud that wash downhill covering towns and villages.
About 23,000 people died when Nevado del Ruiz in Colombia erupted in 1985 – not because of the eruption itself but because of mudflows caused by the sudden melting of the volcano's ice cap.
Another danger during an eruption is earthquakes caused when the lava breaks the surface rock. These tremors in turn can trigger dangerous landslides.
Most eruptions end within three months. Some have lasted 30 years, including volcanoes in Ethiopia, Papua New Guinea, Vanuatu, Indonesia, Guatemala, Costa Rica and Ecuador. Stromboli volcano in Italy has been continuously erupting for more than 1,000 years.
Where do volcanoes form?
The earth's surface is broken up into slow-moving tectonic plates, each about 100 km (62 miles) thick. Most volcanoes form on the edges of these plates where they are either pushing towards or pulling away from each other, as you can see from this map (click on 'Environments' under 'Eruption Dynamics' on left hand navigation bar). A few exceptions are some highly active volcanoes that have formed in the middle of a plate, in areas called hotspots.
Volcanoes can form on the bottom of oceans as well as on land.
Many explosive volcanoes make up the Ring of Fire, a chain of volcanoes that encircles the Pacific Ocean from South America up to Alaska, across to Russia and down through Japan and Indonesia.
But scientists say they may not yet have found all the world's volcanoes – even the most dangerous ones. Some volcanoes are hard to detect if they are covered by dense vegetation, glaciers or water, according to the Global Volcanism Program.
A good way to keep track of volcanic eruptions is to sign up to the Global Disaster Alert and Coordination System, which emails alerts about volcanoes worldwide.
NASA also keeps track of volcanic activity and the USGS/Smithsonian Institute Global Volcanism Program produces a weekly report.
For information about specific volcanoes, you can search the Global Volcanism Programs database by name of volcano, region or eruption date.
To find out about the humanitarian impact of a particular volcanic eruption, the International Disaster Database is the best place to search.
What do you do if a volcano erupts nearby? Here's some information produced by the American Red Cross, which has some useful general principles. For more detailed recommendations go to the USGS site.
If you're interested in the science, there are some excellent websites that will tell you all you need to know. These include the San Diego State University webpages and the U.S. Geological Survey, which also includes a good source of links.
To see some amazing images and videos of active volcanoes, check out these sites: VolcanoWorld and the USGS Hawaiian Volcano Observatory site.
Finally, if you want to know how to cook with lava, check out this recipe.