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The very fine particles of rock ejected during a volcanic eruption. These particles form part of the tephra, which is a term for all sizes of ejected volcanic material.
Andesitic Magma
Often found at subduction zones, thick (viscous) lava with increased levels of silica which can lead to stratovolcanoes (composite) with steep sides and fairly explosive eruptions.
The part of the mantle, below the lithosphere, where the rock is semi-molten.
Basaltic magma
High temperature with low viscosity (runny) with a lower silica content. Often found at constructive margins leading to shield volcanoes with low explosivity.
Benioff zone
The area where friction is created between colliding tectonic plates, resulting in intermediate and deep earthquakes.
Collision plate boundary
Where two plates move towards each other causing a very slow collision which is likely to cause folding and faulting of crustal rocks and the uplift of continental crust to form fold mountains.
Community adaptation
People within communities, either whole or parts of settlements, work together to change their way of life so that the impact of a tectonic hazard event is not as hazardous.
Community preparedness
People within communities, either whole or parts of settlements, work together to change their way of life so that the impact of a tectonic hazard event is not as hazardous.
Conservative plate movement
Where two plates meet and move alongside each other in a similar direction or opposite direction, usually at different speeds. Friction between the two plates is great and stresses and strains build up to create shallow earthquake foci.
Constructive plate boundary
Where two plates move in opposite directions, leaving a zone of faulting and a gap into which magma from the asthenosphere rises.
Convection currents
Hot, liquid magma currents moving in the asthenosphere.
Convergent plate boundary
Where two plates move towards each other and at the boundary the denser oceanic plate (basaltic) is subducted beneath the less dense continental plate (granitic), creating surface features such as a trench, and deep features such as the Benioff zone.
Crustal fracturing
When energy released during an earthquake causes the Earth’s crust to crack.
The point on the Earth’s surface directly above the focus of an earthquake.
Focal depth
The depth at which an earthquake starts (focus). It is divided into shallow, intermediate and deep. Shallow earthquakes have the greatest impacts, as the seismic waves have not lost as much of their energy by the time they reach the surface.
The point inside the Earth’s crust from which the pressure is released when an earthquake occurs.
Geological structure
The arrangement of rock in layers, or folds and the joints and bedding planes within them.
Provides a %age chance of something occurring, how likely it is to happen in a location.
Hazard-management cycle
A theoretical model of hazard management as a continuous four-stage cycle involving mitigation, preparation, response and recovery.
Hazard profile
An analysis of different types of hazard, or actual events, based on a range of physical criteria. (Magnitude, speed of onset, areal extent etc) This allows a useful comparison to be made.
Hot spot
Points within the middle of a tectonic plate where plumes of hot magma rise and erupt.
Hydrometeorological hazards
Natural hazards caused by climate processes (including droughts, floods, hurricanes and storms).
Intra-plate earthquakes
Earthquakes which occur far from plate margins.
A flood event caused by a volcanic eruption melting a glacier.
L waves
The slowest seismic waves, which focus all their energy on the Earth’s surface.
A mixture of meltwater from snow and ice on top of an active volcano and tephra (volcanic material such as ash) from eruptions that travels very quickly down existing river valleys, reaching some distance away from the volcano.
Land-use zoning
A process by which local government regulates how land in a community may be used.
A mass movement of rock and soil down a steep slope under the influence of gravity, perhaps triggered by an earthquake loosening material.
Lava flow
Molten magma that reaches the Earth’s surface is known as lava. It will flow down the sides of a volcano until it cools and solidifies. Basaltic lavas flow faster than andesitic lavas, for example, because of the different velocities.
When the violent shaking during an earthquake causes surface rocks to lose strength and become more liquid than solid.
The solid layer, made from the crust and upper mantle, from which tectonic plates are formed.
The amount of energy releases by a tectonic event. For earthquakes this is best measured on the Moment Magnitude Scale (MMS) and for volcanoes the Volcanic Explosivity Index (VEI).
Mass movement
The downward movement of material under the influence of gravity. It includes a wide range of processes such as rockfalls, landslides and solifluction.
When a major hazardous event becomes catastrophic and more than a disaster. For example, the scale of the impacts are unusually great or very sever with huge numbers of deaths, loss of buildings and infrastructure, or long-lasting impacts on normal social and economic systems.
Mercali scale
An earthquake intensity scale based on 12 levels of damage to areas.
Action to reduce the impacts of an event.
Modify loss
Reduce the impact of losses experienced from a tectonic hazard, for example by insuring belongings and property.
Modify the event
Alter the natural hazards itself in order to change its likely impacts. Earthquakes cannot be changed, but some volcanic activity can be modified, such as by diverting lava flows.
Modify vulnerability
Vulnerability is a key factor in determining the impact of a hazard, so making people less vulnerable will reduce the scale of a disaster.
Moment Magnitude Scale (MMS)
The most accurate earthquake magnitude scale, it measures the total energy released by an earthquake.
Multiple-hazard zone
An area that is at risk from multiple natural hazards such as hurricanes and earthquakes.
Natural hazard
A physical geographical event, tectonic, hydrological or meteorological, which has a negative impact on people through causing injury or deaths, loss of property, or disruption to the normal way of life.
P waves
The fastest seismic waves which travel through both solids and liquids.
The study of past changes in the Earth’s magnetic field.
Park model
Shows how a country or region might respond after a hazard event. Also known as Hazard Response Curve.
Knowing where and when a natural hazard will strike that can be acted on effectively.
Pressure and release (PAR) model
A tool used to work out how vulnerable a country is to hazards.
Rapid onset
A hazard that happens very quickly with no or little warning e.g. an earthquake.
The ability of a community to resist the impacts of a hazard by adapting and recovering.
Rhyolitic Magma
Viscous (very thick) lava with a lower temperature and high silica content can make this highly explosive. Made from continental crust. Often found at continental subduction zone.
Richter Scale
A logarithmic scale which measures the amount of energy released by an earthquake.
S waves
Seismic waves which only travel through solids and move with a sideways motion.
Sea floor spreading
The movement of the oceanic crust away from a constructive plate boundary, as recorded by the magnetic stripes in the basaltic rock (palaeomagnetism).
Slab pull
When newly formed oceanic crust sinks into the mantle, pulling the rest of the plate further down with it. Considered to be key factor in movement of tectonic plates
Slow onset
A hazard that happens very slowly with plenty of evidence and warning, such as a drought.
Sub-aerial processes
The processes of weathering and mass movement.
Subduction zone
The area in the mantle where a tectonic plate melts.
Transform fault
A fault created on a large scale when two plates slide past each other.
Volcanic Explosivity Index (VEI)
The scale used to measure the magnitude of a volcanic eruption.
Water column displacement
The movement of a volume of seawater above the point at which the seabed was moved up or down by an earthquake, such as a thrust.