Volcano zones

Volcanologists study and classify volcanoes according to their activity. With special instruments they can determine if pressure in the volcano is building up. They check to see if the top of the volcano is plugged with cooled lava. There are three different classifications of volcanic activity: active, dormant or extinct. An active volcano is one that has erupted recently or on volcanologists might erupt soon. Active volcanoes are watched very closely so people and animals in the area can be moved to a save place. A dormant volcano or sleeping volcano is a volcano that has been quiet for a long time, but still has signs it may erupt again. A volcano becomes dormant when the vent is blocked by hardened lava, called a plug, or if the magma seeps back under the earth’s crust. Volcanoes can be dormant for hundreds of years. Then suddenly a volcano will erupt again. The eruption is usually very violent. The plug of a dormant volcano stops the magma from rising. Then pressure under the plug builds up, so the plug gives away and a large eruption is caused.

Worldwide there are over 1500 volcanoes; 500 of these are active. A volcano can have a lifespan of a million years and not erupt for several centuries.

Volcanoes are said to be active if they have erupted recently. The official Smithsonian Institute list of active volcanoes includes any that have erupted in the past 10,000 years. Extinct volcanoes will never erupt again.

Volcanoes occur either along the margins of tectonic plates, or over hot spots in the Earth's interior.

Some volcanoes erupt where the plates are pulling apart, such as under the sea long mid-ocean ridges. Some volcanoes lie near subduction zones, forming either an arc of vulcanic islands or a line of volcanoes on land, called a volcanic arc.

Subduction zone volcanoes are explosive, because the magma gets contaminated and acidic as it burns up through the overlaying plate. Acidic magma is sticky and gassy. It clogs up volcanic vents then blasts its way out.

Around the Pacific there is a ring of explosive volcanoes called the Ring of Fire. It includes Mt Pinatubo in the Philippines, and Mt St Helens in Washington State, USA.

Away from subduction zones magma is basaltic. It is runny and low in gas, so the volcanoes here gush lava. Effusive volcanoes pour out lava frequently but gently.

3D radar interferometry from satellites may pick up the minutest swelling on every active volcano in the world. In this way it helps to predict when eruptions may occur.