Aa Lava that carries lumps of solid rock called clinker.

Active volcano A volcano that has recently erupted.

Ash Lava that has been blown to powder by the force of an eruption. Ash clouds can reach up to 40 kilometres (25 miles) into the sky.

Basalt A dark grey igneous rock. The entire ocean crust and much of the Earth’s solid surface is made of basalt.

Batholith A large mass of igneous rock formed when an upwelling of magma spreads through rocks underground, but never reaches the surface. It may be revealed later when erosion wears away the rocks above it.

Caldera A large, deep hole formed when a volcano collapses in on itself after a violent eruption.

Crater A circular, funnel-shaped opening at the summit of a volcano.

Dormant volcano A volcano that has stopped erupting, but which may burst into life again in the future. Mount St Helens in the USA was a dormant volcano that came back to life spectacularly in 1980.

Eruption The blasting out of lava, ash or pumice from a volcano into the air or on to the Earth’s surface. As pressure in the magma beneath the Earth’s crust builds up, it forces its way up through cracks and erupts at the surface as lava. If the magma is quite runny, the gas contained in it can escape easily. The lava oozes out of the opening and flows down the sides of the volcano. If the magma is thick and sticky, the gas cannot escape, but builds up until it explodes. Huge clouds of gases and ash fly high up into the air.

Extinct volcano A volcano that has not erupted for tens of thousands of years.

Fissure A large crack or opening in the Earth’s crust. Lava may emerge through a fissure without an explosive eruption. Undersea volcanoes are fissures in the seabed.

Fumarole An opening in the Earth's crust, usually close to volcanoes, through which steam and gases are blasted out under pressure.

Geyser A natural fountain of hot water or steam that periodically ejects from an opening in the Earth’s crust. Geysers erupt when underground water comes into close contact with hot, volcanic rocks.

Granite A hard, pink-grey igneous rock, made up of large, clearly visible crystals.

Hot spot A weakness in the Earth’s crust where magma is liable to force its way through. Over time, solidified lava builds up and may eventually break the surface of the ocean. The Hawaiian Islands, for example, are the summits of volcanoes rising from the floor of the Pacific Ocean.

Igneous rock A type of rock formed when magma rises to the Earth’s surface and solidifies. Intrusive igneous rock, such as granite, is formed when magma solidifies beneath the ground. Extrusive igneous rock, such as basalt, is formed when it solidifies above the ground.

Laccolith A dome of igneous rock that lies between layers of sedimentary rock. It is formed when magma pushes between the sedimentary rock strata.

Lava Magma that has erupted at the Earth’s surface through volcanoes. It may flow as hot, molten rock for a while, but then quickly cools and solidifies. Lavas vary from thick to runny, according to the amount of gas contained within them and the type of silicates they are made of.

Lava dome A lava dome forms when very viscous (thick) lava squeezes out of a volcanic vent without exploding. The lava piles up into a dome, which grows a little like toothpaste coming out of a tube. Some lava domes grow in a matter of hours or day, while others may take years.

Magma Hot, molten rock beneath the Earth’s surface. Magma is formed in the Earth’s mantle. It is composed mostly of silicates, with gases contained in bubbles.

Magma chamber An underground pool of magma. Over time, pressure in the chamber builds up, forcing magma to push through cracks in the Earth’s crust. Many volcanoes are situated above a magma chamber.

Obsidian A dark, glassy rock formed when lava cools down so quickly that its minerals do not have time to crystallize.

Pahoehoe Very runny lava that flows beneath a hardening, rocky crust. On cooling, it has a smooth, billowy texture and rope-like appearance.

Pele's hair Volcanic glass fibres formed when particles of molten rock are erupted and spun out by the wind into long, hair-like strands. It is named after Pele, the Hawaiian goddess of volcanoes.

Pillow lava Runny lava that erupts under the sea and hardens into round, pillow-shaped lumps of rock.

Pumice A volcanic glass “froth” formed from cooling, gassy lava. It contains numerous bubbles.

Pyroclastic flow The extremely hot mixture of volcanic rock and gases that sweeps down a volcano’s slopes at more than 300 km/h (200 mph), destroying everything in its path. The residents of Pompeii were wiped out by a pyroclastic flow when Mt Vesuvius erupted in AD 79.

Ring of Fire An area surrounding the Pacific Ocean where large numbers of earthquakes and volcanic eruptions occur. There are 452 volcanoes in the Ring of Fire, about 75% of all the world's active and dormant volcanoes.

Shield volcano A large volcano with gently sloping sides. Shield volcanoes build up over time from repeated lava flows. They erupt thin, runny lava.

Silicates Types of mineral that contain silicon and oxygen. Silicates make up more than 90% of the rocks in the Earth’s crust. Thin, runny magmas consist of different types of silicates to those that make up thick, viscous magmas.

Stratovolcano A tall, steep, cone-shaped volcano. Stratovolcanoes are formed from layers of volcanic lava, built up by multiple eruptions over hundreds of thousands of years. They erupt ash, pumice and thick, viscuous lava. Stratovolcanoes are the most common type of volcano.

Supervolcano A volcano that erupts more than 1000 cubic kilometres of magma, devastating a very wide area. Supervolcanoes are extremely rare (the last one erupted at what is now Lake Toba in Sumatra, Indonesia, about 74,000 years ago). Some supervolcanoes come into existence when magma in the Earth’s mantle rises into the crust, but cannot break through. Pressure builds up in a large underground magma pool, until the crust is unable to contain it any longer. Supervolcanic eruptions cover vast areas with ash, destroying vegetation and may even change world climates.

Tephra Fragments or pieces of rock produced by an explosive volcanic eruption. Airborne fragments are known as pyroclasts. Tephra fragments are classified by size: particles smaller than 2 mm (0.08 inches) across are known as ash; pieces measuring between 2 and 64 mm (0.08 and 2.5 inches) are called lapilli or cinders; blocks larger than 64 mm (2.5 inches) in diameter are referred to as volcanic bombs.

Tuff A type of rock made of hardened volcanic ash. It was used by the Romans to make many buildings and bridges, and by the natives of Easter Island to make their famous statues.

Vent An opening in the Earth’s surface through which lava and gases seep out.

Viscosity A measure of the fluidity of magma and lava (and other liquids). Thick lava is more viscous than thin, runny lava.

Volcano A volcano is an opening, or vent, in the Earth’s crust through which magma, molten rock, erupts. The word volcano is usually used to describe a cone-shaped mountain with a central vent and a crater at the summit, but this is not always the case. Many volcanoes have gently spreading slopes. Others are simply cracks in the ocean bed. Most volcanoes are situated along the edges of the giant tectonic plates that make up the Earth’s surface. More than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the so-called “Ring of Fire”.

Consultant: Ian Fairchild