Volcanic hazards Flashcards
What is a volcano?
A vent in the Earth’s crust where molten rock, gas and heat can escape a magma chamber
Lava
Molten rock above or on the Earth’s surface, forms extrusive igneous rocks
Magma
Molten rock below the Earth’s surface, forms intrusive ingneous rock
Magma types
Mafic
Ultramafic
Intermediate
Felsic
Mafic magma
Rich in Fe and Mg
Dark colour
Basalt (fine), Dolerite (intermediate) or Gabbo (coarse)
Intermediate magma
Grey colour
Andesite (fine), Microdolerite (intermediate), Diorite (coarse)
Felsic magma
Rich in silica
White/pink colour
Rhyolite (fine), Microgranite (intermediate), Granite (coarse)
Sill
Part of sub volcanic plumbing system that follows sedimentary units and bedding planes
Dyke
Part of sub volcanic plumbing system that cuts across sedimentary units and bedding planes
Batholith
Part of sub volcanic plumbing system, crystallised magma chamber
Volcanic neck
Part of sub volcanic plumbing system that connects magma chamber to surface of the Earth
Intrusive igneous rocks
Cooled slowly, therefore, deeper in the Earth and more insulated
Produces coarser crystals
Extrusive igneous rocks
Cool quickly
Produce fine crystals or glass
Volcanic Explosivity index, VEI
- Measures how explosive a volcanic eruption is
- Based on volume of explosive products erupyed and height of eruption cloud
- Logarithmic scale of 0-8
Lake Toba
Crater lake in Indonesia
- VEI of 8
- Dust and sulphurous gases in the atmosphere cooled the Earth
- Humans almost wiped out
Super-volcano affects
Severe burning and death Ash collapsing house roofs Suffocation Crop failure Volcanic winter
Vesuvius
- VEI of 5
- Deadliest in European history
- Petrified the city
Mt Tambora - Year without a summer
- VEI of 7
- Global temperature dropped by almost 1 degree
- Caused major food shortages and famine
Krakatau
- VEI of 6
- Volcano tore itself and the island apart
- Landslide exposed magma chamber and water mixed w/ it and generated a huge explosion
- Produced massive pyroclastic density currents
Living near a volcano - benefits
Energy (e.g. geothermal)
Tourism
Nutrient rich soil
Mineral resources
Geothermal energy process
- Pump cold water down
- Heat from magma warms the water and produces steam
- Steam turns turbine to generate electricity
How do volcanoes assist in fertilising soil?
- Volcanic ash and lava weather away and erode
2. Products of erosion introduced key nutrients for plants into the soil (k, Fe, P)
Mineral resources provided by volcanoes
- Diamond
- Copper
- Nickle
- Aluminium
- Gold
- Lead
- Zinc
Fissure volcanoes
- Long linear vent
- Effusive eruption producing basaltic lavas
- Fed by dykes that connect the magma chamber to the surface
- Found in rift zones and spreading centres
Shield volcanoes
- V. shallow slopes
- Basaltic magma w/ low gas content
- Made of lava flows from effusive eruptions
- Lava is v. hot and runny and so spreads over large distances
- Found at spreading centre and intraplate hotspots
Dome volcanoes
- Formed by relatively small bulbous masses of lava
- Viscous basaltic-rhyolite lava that cant travel far from vent as it is cooler
- Grows by slow expansion
- Found on convergent plates
- Mostly effusive but some explosive eruptions
Ash-Cinder volcanoes
- Small, grows v. quickly and steep
- Often near or attached to larger volcano
- Gas rich magma
- Made of unconsolidated ash, pyroclastic material, scoria and ejected rock fragments
- Cone-shaped, circular base and crater with a vent
Composite (stratavolcano) volcanoes
- Consists of altering layers of lava and pyroclastic material
- Felsic-intermediate magma composition
- Explosive type eruptions
- Steep, cone-shaped and w/ crater
- Lava flows from craters or from fissures
Caldera volcanoes
- Large volcanic eruptions that empty their magma chamber
- Ground above then falls into empty magma chamber to form a caldera
- Steep basin shaped depression
Effusive eruptions
- Produces lava flows that ooze out of vent
- Magma has low viscosity
- Typically Fe-Mg rich
- Low water/gas content
Explosive eruptions
- Produces tephra
- Magma has high viscosity
- Typically Si rich
- High water/gas content
- Gas expands rapidly generating an explosion
Hydrothermal eruptive material
- Just hot water, no magma
- Explosive pulverized rock producing ash
- Generally v. small and short
Phreatic eruption
- Heat from old magma interacts with water
- No new magma
- Explosive, producing ash
Phreatomagmatic
- New magma and water
- V. explosive
Icelandic (fissure) eruptions
- Effusive eruption
- Flows out of long parallel fissures
- Basaltic lava
- Builds large plateaus of lava
Characteristic of basaltic lava
Hot
Gas poor
Low viscosity
Hawaiian eruptions
- Effusive eruption
- Lava flows out of central vent and radial fissures
- Basaltic lava
- Builds a shield volcano
Strombolian eruption
- Explosive, mild sized, regular eruptions
- Magma is more viscous
- Basaltic or andesitic magma
- Caused by bursting slugs in volcanic conduit
- Produces ash and pyroclastic density currents
What is a slug?
Large bubble of gas
Vulcanian eruption
- Explosive eruption
- Short, violent and small explosion of viscous magma
- Andesite or rhyolite magma
- Caused by fragmentation of plug in volcanic conduit
- Ejection of cinders and lava bombs
- Produces cinder cones
Pelean eruption
- Viscous magma produces lava domes that then collapse
- Adesitic or rhyolite magma
- Produces many pyroclastic density currents
Plinian eruption
- Largest and most explosive
- Fragmentation of gassy and v. viscous magma
- Rhyolitic magma
- Produce large columns of ash and pyroclastic density currents
- Magma chamber completely emptied
Characteristics of flood basalt eruption
- Large scale effusive, short and silent
- HUGE lava volume
- Produces thick lava flows that cover huge areas
- Basaltic magma is v. hot and runny causing the lava to flood
- Create lava plateaus and mountain ranges
Formation of flood basalt eruptions
Large mantle plumes produce them by
- Hot materials from core-mantle boundary rising
- Plume hits base of the crust and raises the temp.
- Huge amount of melting occurs
Central Atlantic magmatic province
Flood basalt eruption 200 million yrs ago
Covered 10 million km2 of the Earth’s surface
Split the super continent of Pangea
Flood basalts on the moon
Marias:
The dark patches on the lunar surface, these are evidence for huge basalt eruptions
Deccan traps, India
- Successive layers of thick basalt rock from lava flow
- Caused by a deep mantle plume in the reunion island hotspot
- Produced a rift valley
Deccan traps and the dinosaurs
- Release of large volumes of volcanic gases, like sulphur dioxide, caused an intense climate change
- Global temp. dropped by 2 degrees
- V. toxic world was produced
Siberian traps, Russia
- 250 million yrs ago
- Most likely caused by large magma plume
The great dying, Permian-Triassic mass extinction
Most devastating mass extinction, caused by the volcanic gases, emitted by Siberian traps, causing severe climate change
Hot water springs and what can they help predict?
- When H2O comes into contact w/ hot rocks heated by magma underground
- Can change temp. or chemistry quickly, CAN signify an impending eruption
New thermal areas and what do they indicate?
- New areas of hot spring activity or reactivation of old hot springs
- indicate movement of magma
Geysers
- Underground plumbing systems where a cavity of H2O is heated by magma and hot rock
- This vaporises the H2O and generating a pocket of steam
Steam rapidly rises to surface, displacing H2O on the surface above
Geysers and predicting eruptions
Geysers can be regular and therefore not a clear sign of an eruption. However, change in geyser behaviuos can be.
Acidified H2O before an eruption
- Lakes and seas near volcanoes become saturated w/ CO2 and SO2, producing sulphuric and carbonic acid
- pH 0.1 can be reached, causing severe burns and death
- Lake ecology is killed
- Rapid change CAN signify an eruption
Drying up of springs/wells before an eruption
Movement of magma produces cracks, causing H2O to drain away
Fumaroles and relation to an eruption
- Heavy and toxic gases released in large amounts from vents
- Sink to ground forming a toxic pocket of gas that travels downhill to into valleys/depressions
- Can be regular, but a change in behaviour could signify an eruption
Effects of fumaroles
Humans and animals - asphyxiation, respiratory diseases and skin burns
Famine due do crop and vegetation death
Lake Nyos: Consequences
- Many people and livestock died
- Thousands made ill w/ respiratory diseases, lesions and paralysis
- Lake dropped 1m, turned red and knocked down bank trees
Lake Nyos: What happened?
- Volcanic gasses became trapped at lake bottom
- Earthquake or landslide released the heavy gases into atmosphere
- Gases hugged the ground and travelled dowm=n valleys and smothered villages
Lake Nyos: Precautions now takes
- Degassing station penetrates lake bed and releases any trapped gases
- Prevents gas build-up
Plant dieback in relation to an eruption
- Affect of toxic gases/liquids released from volcano
- Trees/plants yellow and die as a result of increased CO2 in the soil
- New areas of dying plantlife CAN be a sign of an eruption
Landslides in relation to an eruption
Volcanoes are weak structures and so landslides near summit, w/ no other explanatory cause, could be due to movement of magma FRACTURING EARTH SURFACE or GAS RELEASE
Soil temp. in relation to an eruption
- Ground near volcanic systems can be v. hot
- Soil heating up/cooling down indicates activity below surface
Ground uplift in relation to an eruption
- Ground level rising/falling
- Can indicate magma, H2O and gas is building up underground causing an uplift
Volcanic earthquakes in relation to an eruption:
VOLCANIC TECTONIC EARTHQUAKES
Caused by regional tectonic forces OR injection of new magma
Good indicators of an eruption
Volcanic earthquakes in relation to an eruption:
LONG PERIOD EARTHQUAKES
Caused by vibrations from movement of magma/fluid OR pressure build-up causing rocks to break
Good indicators of an eruption
Harmonic tremors in relation to an eruption:
- Specific type of long-lasting rhythmic seismic signal
- Caused by magma moving through fractures in rock
- V. good indicator of an imminent eruption
Changes in wildlife in relation to an eruption:
ANECDOTAL
Reported that animals behave strangely prior to an eruption and migrate away from volcanic area
What is ash fall
- Rain of volcanic ash
- Small abrasive particles of glass
- Travels v. far
- Severity depends on layer thickness & grain size
Ash effects on humans
- Can be irritating or deadly
- Small particles, < 4 micrometres, can be breathed into the lungs alveoli
- Mixes w/ H2O to form a type of concrete in the lungs, resulting in a respiratory disease
Ash effects on plants
- Ash cloud can block out sun for days-years
- Layer of ash covers leaves
- Plants die
Ash effects on buildings
- Layers of ash can become v. heavy
- Ash is twice as heavy when wet
- This can cause roofs to collapse
Ash effects on electrical systems
- Wet volcanic ash is conductive
- Shorts out electrical systems and damages power lines
Ash effects on air travel
- Scratches windows obscuring visibility
- Shuts engines down by fusing w/ turbines
- St Elmos fire
What is a St Elmos fire
- Type of lightening
- Electrical field is caused by static build-up in a volcanic ash cloud
- Lightening caused by ionisation of the air around sharp objects in the electrical field
- N and O in atmosphere cause blue fluorescence
Ash effects: Lahar’s
- Volcanic mudflow w/ consistency similar to wet concrete
- Consists of H2O, ash, pyroclastic material and rock
- Solidifies once it stops moving
- Fast moving, can travel up to 10s of m/s
Lahar causes
- Melting snow/glaciers come in contact w/ lava or pyroclastic material
- Rainfall or ash deposit after eruption
- Breakout of a crater lake
- Landslides
- Can form during/after an eruption
Volcanic explosion aftermath: Sulphurous gases
- Air had eggy smell
- Leaves/fruit fell from trees
- Persistent fog
- Respiratory problems, struggling to breath and death
Volcanic winter
- When enough SO2 and ash is released into upper atmosphere and the albedo of the planet increases
- Reflects sunlight back to space
- Cools the planet down (climate change)
Volcanic explosion aftermath: Erosion of the volcano
- Volcanic structures are not particularly strong, the shape is maintained by volcanic activity
- Once volcanism has ceased, the volcano begins to rapidly erode
- Eventually, only the plug remains
