Aa: Hawaiian word which refers to a certain type of lava flow with a very rough, broken surface.

Active volcano:A volcano which is erupting.

Aerosol:Particles which are suspended in a gaseous medium.

Ash:See Volcanic Ash

Ashflow:See Pyroclastic Flow

Astenosphere: Soft layer of the mantle, beneath the lithosphere.

Atmosphere:Envelope of gases surrounding the earth.

Basalt:One of the most volcanic rocks, on land it occurs in many cinder cones, lava flows, shield volcanoes and volcanic plateaux. The bulk of the ocean floors is formed by basalt.

Blast:A very explosive eruption producing clouds of hot ash and other volcanic material.

Block:A large angular lava fragment which is thrown out during an eruption. Usually 1m or more across.

Bomb:Bombs are large, rounded, lava fragments thrown out during an eruption. They are often modified in shape during their flight or upon impact.

Caldera: A huge bowl-shaped crater, often kilometres across, formed by collapse into the magma chamber or by explosive volcanic eruptions.

Cinders:Light fragments of lava, between 64mm and 30cm in size. Also named scoria.

Cinder cone: A steep conical hill formed above a vent, usually less than 250m high. Also named scoria-cone.

Complex Volcano: A volcano that consists of a complex of two or more vents, or a volcano that has an associated dome, either in its crater or on its flanks.

Composite Volcano: A steep volcanic cone composed of lava fragments as well as other volcanic products. Also called strato-volcano.

Compound Volcano:See Complex Volcano.

Conduit: The passage followed by magma in a volcano.

Continental Drift:Theory formed by the German scientist Alfred Wegener. According to the theory continents move in consequence of the movement of plates which move in consequence of convection currents.

Crater:A bowl-shaped, steep-sided, hollow in the summit of a volcano which lies above the vent from which volcanic material is expelled.

Crust:The thin outermost layer of the Earth, forming both the continents and the ocean floors.

Debris Avalanche:A rapid mass movement that may include volcanic rock, water, snow, glacier ice, trees and other pyroclastic material.

Dome: Domes form when a dome-shaped mass of volcanic rock is too viscous to flow far from the vent. They are often formed on composite volcanoes at the end of an eruption.

Dormant Volcano:A volcano which is presently inactive but which may erupt again.

Eruption:The way liquids, solids and gases are expelled on the Earth's surface by volcanic activity. This can range from explosive outbursts to quiet, noiseless lava-outcome.

Extinct Volcano:A volcano which is not currently erupting and which isn't likely to do so in the future.

Fissure: A crack or fracture in the Earth's crust or the slope of a volcano, up which magma may rise.

Flood Basalt: Large-scale outpourings of basalt lava producing thick accumulations.

Fumarole:A vent or opening giving off gases or steam. Fumaroles occur on both active and dormant volcanoes.

Gas:See Volcanic Gas.

Hot Spot:Fixed places within the mantle where rocks melt to generate magma.

Hydrovolcanic Eruptions: Eruptions highly associated with water.

Lahar:Mudflows formed by the mixing of volcanic particles and water.

Landslide:downslope movement of a mass of bedrock under the influence of gravity.

Lava:Magma or molten rock which reaches the Earth's surface exposed to air or water. Lava occurs as flows, domes, fragments within cones of volcanoes and as pillows formed on the ocean floor.

Lava Flow:Lava which is outpoured in a flow on the land from a vent or fissure. Examples of lava flows include aa and pahoehoe lavaflows.

Lithosphere:The solid outer shell of the Earth which is composed of the Earth's crust and the solid outermost layer of the mantle. The lithosphere lies above the asthenosphere. It is broken up into plates.

Maar:German word used to describe a crater which is almost circular. The crater is usually filled with (rain)water.

Magma: Very hot, molten rock which is formed within the earth by partial melting of the mantle.

Magma chamber:A chamber(cavity or reservoir) in the lithosphere where rising magma is "collected" and halts for varying lengths of time.

Mantle:The intermediate zones between the Earth's crust and core.

Nuée ardente:French word used to describe a glowing avalanche of hot gas and other volcanic fragments of all size(like ash, pumice, rock debris, etc.).

Oceanic Ridge:Ridge on the ocean floor where two adjacent plates diverge and where new crust is generated by volcanic eruptions.

Pahoehoe:Hawaiian term used to describe thin, smoothly flowing, lava flows with a glassy plastic skin.

Phreatic Eruption:A sudden violent eruption of steam which is caused when water and heated volcanic rocks interact.

Phreatomagmatic Eruption:Eruptions characterised by steam explosions caused by the interaction of rising magma and surface water.

Pillow lava:Interconnected lobs of lava formed underwater by submarine activity.

Plate:Plates are huge slabs into which the crust as well as the upper part of the mantle are divided.

Plate Tectonics:Theory dealing with plates and their activity.

Plinian Eruption:One of the most explosive types of eruptions, known for producing nuées ardentes and lahars.

Plug:Lava solidified in the conduit of an extinct volcano.

Pumice:Cinder so bubbly that it floats in water.

Pyroclastic:Adjective used to describe rock material formed by a volcanic explosion.

Pyroclastic Flow:Mixtures of hot gas and ash travelling very quickly down the slope of a volcano.

Ring of Fire:The belt of volcanoes that surround the Pacific Ocean. This belt contains about two-thirds of the world's active volcanoes. It is marked by the volcanic chains of Japan, Kamchatka, South Alaska, the Aleutian Islands, the Cascade Range of the US and Canada, Central America, the Andes, New Zealand, Tonga, Vanuatu, Papua- New Guinea, Indonesia, the Philippines, and last but not least the Mariana, Izu and Bonin Islands which complete the circle.

Seafloor spreading:The process in which the ocean floor is extended when two plates move apart.

Seismograph:An instrument that is used to record vibrations of the Earth.

Shield Volcano:Volcano which has a broad shaped cone with gently slopes. Shield volcanoes are built by runny lava flows.

Silica:Fundamental component of volcanic rocks. It is the most important factor determining the fluidity of magma. The higher the silica of a magma the greater its viscosity.

Solfatara:Italian word used to describe a fumarole that emits sulphurous gases.

Spatter:Lava fragments, between 64mm and 30cm in size. which are often emitted as hot clots. Spatter often welds together to form steep-sided cones.

Spatter cone:Steep-sided cone of spatter.

Stratosphere:The upper layer of the atmosphere.

Stratovolcano:See composite volcano.

Strombolian Eruption: Short-lived eruptions that throw out blocks and bombs. Lava flows also occur.

Subduction:The process in which one plate is pushed downward beneath another plate into the underlying mantle when plates move towards eachother.

Tephra:Collective noun for all sorts of volcanic material that are erupted from a crater or vent.

Tremor:Earthquake activity with a low amplitude associated with the movement of magma.

Tsunami:Japanese word that is used to describe huge sea-waves generated by earthquakes or violent volcanic eruptions.

Tuff Cone:Steep conical hill with a deep, wide crater. The cone is composed of thin layers of fine fragments. A tuff cone is smaller and steeper than a tuff ring.

Tuff Ring:A broad, circular, low-rimmed, well-bedded accumulation of fine fragments, often 1km in diameter, around a crater. The crater of a tuff ring is wider than the one of a cinder cone.

Vent:The vent is an opening at the Earth's surface through which volcanic materials are released.

Viscosity:Term to show how sticky a magma or lava is.

Volcanic Ash:Volcanic rock which is exploded from a vent in fragments less than 2mm in size. Ash may be solid or molten when first erupted.

Volcanic Gas:Volcanic gas is contained within magma. As the magma rises to the Earth's surface the gases are exsolved and can become a very important factor in the violence of an eruption. Because some gases are toxic they can suffocate people. Examples of volcanic gases include steam, sulphur dioxide and carbon dioxide.

Volcano:A cone which is formed above and around a vent by accumulations of erupted volcanic materials like ash, pumice,lava flows and other volcanic rocks. "Volcano" can refer to both the vent and the cone.

Volcanologist:A person who studies volcanoes and their behaviour.

Volcanology:The science of volcanoes.

Vulcan:God of fire of the ancient Romans. The volcano is named after Vulcan.

Vulcanian Eruption:A Vulcanian Eruption is similiar to hydrovolcanic activity. Much gas as well as ash, cinders and pumice are given off.