This glossary describes terms referred to in the volcanoes
pages, the Science Alert bulletins and the Ashfall prediction maps associated
with these pages.
Andesite - A type of magma with intermediate viscosity
and silica content. Forms large, steep sided volcanic cones composed of
lava and pyroclastic material (for example, Mount Ruapehu). Andesite is
also the name given to the volcanic rock formed from andesitic magma.
Ash, volcanic- see Volcanic Ash.
Ballistic projectiles - Ballistic projectiles are pieces of rock thrown from a volcanic vent in an eruption. They are generally confined to less than 3 kilometres radius from the vent because of their size.
Basalt - A type of fluid magma with low silica content. Forms dark coloured rock (often red or black). Examples are the scoria cones of Auckland.
Bomb/block - See Lava bomb
Caldera - A large volcanic depression up to 50 km across,
formed by collapse during and after an eruption. The northern part of
Lake Taupo is a caldera.
COSPEC - COrrelation SPECtrometer, used
to measure a particular gas species, in this case sulphur dioxide in the
gases from erupting Ruapehu. High SO2 levels indicate actively degassing
magma at shallow depths.
Dacite - A type of volcanic rock intermediate between andesite and rhyolite. Mount Tauhara at Taupo and Mount Edgecumbe (Putuaki) near Kawerau are examples of dacite volcanoes.
Debris avalanche - A sudden catastrophic collapse of volcanic material from an unstable side of a volcano.
Dome - Gas-poor lava forming a steep-sided hill, usually close to an eruptive vent. Examples include Motutaiko Island (Lake Taupo), Mokoia Island (Lake Rotorua), the domes that make up Mount Ngongotaha (Rotorua), etc.
Earthquakes, tectonic
See Tectonic earthquakes
Eruption plume- cloud of volcanic ash emitted
from a volcanic vent or volcano.
Eruption, volcanic- see Volcanic Eruption.
Fall deposit
Mantling blanket of pyroclastic particles (ash, scoria, pumice, etc) erupted
explosively and transported through the atmosphere before falling back
to the ground.
Fumerole - a vent in the ground
in a volcanic area from which steam and volcanic gases escape; characteristic
of the later stages of volcanic activity.
Gas, volcanic - see Volcanic Gas
Hazard Zones - the GNS Hazard Zones are
a series of zones set up around Mt Ruapehu to indicate areas prone to
various hazards (lava bombs, etc).
Ignimbrite
A volcanic deposit formed by a pyroclastic flow. Ignimbrite layers can
be soft and full of pumice such as the land surface around Taupo (from
the Taupo eruption 1800 years ago); or hard rock where the deposit has
been thick and hot enough for the particles to fuse together (eg at Bulli
and Te Toki Points on Lake Taupo).
Lahar - mud flow of water and volcanic
material (volcanic ash, volcanic blocks (lava bombs), etc); deposits
from such are also called a lahar.
Lapilli - material from 4 to 32mm in
diameter blown out by a volcano.
Lava - Magma that reaches the surface; molten material that is thrown out or has flowed from a volcano. Material that has reached the surface and solidified is also known as lava.
Lava bombs - Blocks of lava greater than 32 mm in diameter thrown out of the volcano by an eruption.
Magma - Rock material in a molten state within a volcano/underneath
the earth’s surface. Once magma reaches the surface it becomes lava.
Magma chamber - A large body of magma below the volcano
that rises up through the column and feeds the volcano.
Magma column - Pipe of magma within the volcano joining
the magma chamber to the surface.
Phreatic eruption - A small steam-driven volcanic explosion. The driving force is the flashing of hot water to steam by magmatic heat, but fresh magma itself is not involved.
Phreatomagmatic eruption -A volcanic eruption of fresh magma that mixes with water. Falling ash from these eruptions is usually wet and sticky and often accompanied by rain.
Plume - See Eruption Plume.
Pumice - A frothy rhyolitic volcanic rock full of gas holes. Usually floats on water. Abundant around Lake Taupo.
Pyroclastic fall - Volcanic material that has been erupted explosively into the air and has fallen back to earth (for example, volcanic ash).
Pyroclastic flow - A surface-hugging eruption cloud of very hot gas and volcanic particles that moves rapidly (up to 100 km/hr) across the ground surface, away from the vent.
Pyroclastic material -Volcanic material erupted in an explosive volcanic eruption (for example, ash, lapilli, ignimbrite, etc).
Pyroclastic surge - Like pyroclastic flows, pyroclastic surges are very fast moving clouds of gas and volcanic particles, and are very destructive. Some differences between the two are that pyroclastic surges travel faster than pyroclastic flows, cover less distance and deposit thinner layers of volcanic material than pyroclastic flows do.
Rhyolite - A type of highly viscous magma with high silica content, it is found as pumice, ignimbrite, lava or obsidian. Rhyolite is also the name given to the volcanic rock formed from rhyolitic lava.
Scoria - Scoria is frothy basaltic rock, full of small gas bubbles. Often a black or red colour.
Scoria cone - A conical hill formed by weak explosive eruptions of generally basalt magma.
Seiche - A wave or series of waves formed on a lake by strong winds, an earthquake or eruption.
Seismicity - Seismic activity; earthquakes and other shaking (tremors).
Silica - Silica (Silicon Dioxide (SiO2)) is present
in magma and affects its viscosity. The greater the silica content, the
greater the viscosity of the magma.
Strombolian eruption - Volcanic
eruption characterised by small explosive eruptions due do the viscous
(thick) nature of the lava.
Tectonic earthquakes - Earthquakes occurring in the crust
of the earth and caused by movement on fault lines.
Tephra - A collective term for all unconsolidated volcanic material erupted explosively. Includes volcanic ash, pumice, scoria, volcanic bombs, some pyroclastic flows, etc.
Tsunami - Seismic sea waves of long period produced by either a submarine earthquake, a debris avalanche or an underwater volcanic eruption.
Tuff ring - A wide low ring of pyroclastic material, surrounding a volcanic vent.
Vent - Opening through which a volcano erupts
or volcanic material passes.
Viscosity - The ability of a liquid to flow. Basalt magma
has a relatively low viscosity making it runny, whereas rhyolite magma
has a high viscosity making the magma thick and sticky.
Vog - Smog of volcanic origin, composed of volcanic
ash and gases.
Volcanic ash - Material in the form of small fragments
from 0.06 to 4mm in diameter, blown out by a volcano.
Volcanic eruption - Material from deep within
the earth/magma chamber finds its way to the surface.
Volcanic gas - Gases released from a volcano or
vent that were previously dissolved in magma.
Volcanic earthquakes - Earthquakes (up to about Magnitude 5.5) that are caused by the movement of magma within the crust.
Volcano - A hole in the ground through which magma is erupted. More generally refers to a mountain or caldera that may have several vents. Not all volcanoes are cone-shaped mountains. (eg Lake Taupo).
Glossary written by Craig Elms.