Volcanoes
Magma
Basic- partial melting of mantle peridotite beneath constructive plate margins. caused by the reduction of pressure on the mantle as the plate thins (decompression melting)
Intermediate- partial melting of mantle peridotite above a subduction zone. caused by the addition of water being released from the plate (flux melting/ wet partial melting)
Acid/ Granite- partial melting of continental crust at a subduction zone
Shield Volcanoes- very wide volcanoes with low slopes (<10 degrees). created by non-explosive volcanic eruption of fluid lava E.g Moana Loa in Hawaii
Fissure Volcanoes- outpourings of basic magma through a crack or fault E.g Katla, Iceland
Cinder Cone- small conical mounds created by the accumulation of basic lava blown from the vent E.g Hekla Iceland
Stratovolcano- large conical mounds with steep sides (>30 degrees). eject intermediate or acid magma E.g Vesuvius
Caldera- huge circular depressions caused by volcanoes collapsing E.g Toba, Indonesia
Basic- high temperatures, >1000 degrees, low viscosity, high velocity, up to 30mph, non-explosive
Intermediate- viscous, low temperature, <800 degrees, solidifies in vent, highly explosive with ash
Acid- highly gaseous, very viscous, low temperature, violent
Highly viscous magma has a high gas content which expands then blasts out the top
Hotspot-Basic Constructive-Basic Destructive(o-o)-Intermediate Destructive(c-o)-Acid
Hazards
LAVA- magma extruded from a vent, not as dangerous to people as they can move out of the way
BLAST- massive cloud of lava fragments, rock debris and hot gas that form via rapid expansion of gas in the volcanic vent E.g Mount St Helens that killed 60 people and destroyed more than 230 square miles of forest
SECTOR COLLAPSE- structure failure of a large part of a volcano, can result in avalanches and eruptions E.g La Palma (future eruption) could trigger a major tsunami
ASHFALL- blasting of rock creates suffocation, building collapse, destroys crops , pollutes rivers
LAHARS- ash and water combine and can travel at up to 50mk/hour E.g Nevada del Ruiz
GASES- H20, C02 and S02 E,g Lake Nyos which was a cloud of CO2
PYROCLASTIC FLOWS- dense fluidised clouds of ash, debris and gas up to 800 degrees at 100mph for 50 miles. formed by the collapse of the eruption column, collapse of the lava dome and lateral blasts. they create thick layers of rock e.g pumice or block deposits. E.g Mt Pelee which killed all 28,000 people in a single flow after hundreds of years of inactivity
PYROCLASTIC DEBRIS/TEPHRA- fragmented rock, ash and lava blasted out of the volcano during an eruption. Ash <2mm and can block sunlight and promote surface cooling. Lapilli 2-64mm. Volcanic bombs >64mm
Changes before an eruption
1) Co2 emitted
2) Steam may be caused by evaporation of groundwater
3) Groundwater rises
4) Increase in earthquakes
5) Faulting
6) So2 emitted
7) Gradient of volcano increases
8) Lava dome forms
9) Increase in gravity measured by a gravity metre
Monitoring
Monitoring steam eruptions as these indicate that water is being heated by conduction from hot new magma
Monitoring ground deformation as tilting or bulging can indicate that magma is moving upwards. Monitored using electronic mechanical tiltmeters of field tilt surveying techniques
Monitoring gravity anomalies as renewed volcanic activity can be marked by an increase in the gravitational field. Measured with gravity meters
Monitoring thermal anomalies as an increase in temperature suggests magma is rising. Measured at a safe distance using thermal imaging equipment mounted on satellites or aircrafts
Monitoring gas emissions as the composition of gas changes during renewed activity e.g an increase in the ratio of hydrogen sulphide to sulphur dioxide. Monitored through remote sensing by a spectrometer
Monitoring seismic activity as earthquakes increase when highly pressurised magma fractures rock. Monitored using a number of seismometers located within 20km of the vent or within the volcano itself
Monitoring the lava dome which can form several days prior to the eruption