The Explosive Earth: Volcanoes
Magma Formation
Begins at depths under high heat
Rocks from earths interior are heated above melting temperature
Rocks rise and are subjected to less pressure, causing them to melt, becoming magma
Decreasing pressure causes gas bubbles to form, propelling magma upwards
gas bubble volume overwhelms magma and breaks it into pieces which burst out as a gas jet when it reaches the surface
Called lava once reaching the surface
Magma
Viscosity: resistance to flow(opposite of fluidity)
Viscosity depends on composition and temperature
Higher silica content, higher viscosity
Lower temperature, higher viscosity
Basaltic: highest temperatures, lowest silica content= lowest viscosity, peaceful eruptions
Rhyolitic: lowest temperature, highest silica content=highest viscosity, explosive eruptions
Gas Content: mostly water vapour, some carbon dioxide, trace sulfur, chlorine, fluorine gases
Water vapour gives magma their explosive character because it expands as pressure is reduced.
Low viscosity= effusive eruption
High viscosity= explosive eruption
Volcanic Explosivity index (VEI): logarithmic and ranges from 0-8
Volcanic Hazards
Lava flows: how low to moderate viscosity lavas flow down volcano slopes (rhyolitic cannot produce lava flows)
Pyroclastic flows: mixtures of hot, dry, rock fragments and hot gases that move away from the vent at high speeds (50-500km/hr)
Tephra: rock fragments ejected into the atmosphere during eruption. Ash < Lapilli < Bombs
Eruption column: When clouds of gas and tephra rise above a volcano
Pumice: form of volcanic glass full of cavities created when rock is ejected from a volcano
Lahars: hot, volcanic mudflows that occur during an eruption, where ash mixes with local water
Three V's of Volcanology
Viscosity: resistance to flow
Volatiles: gases(water and carbon dioxide) that form bubbles in the magma and can burst out quietly or explode out
Volume: Total volume of lava erupted over the active life of a volcano
Volcanic Eruption Types
Hawaiian
Low viscosity, basaltic magma
gas discharge can produce a fire fountain
lava flows down slope as a lava flow
Very little pyroclastic material produced
Icelandic
Eruptions of low viscosity magma
Fissure eruptions, because linear vents
"Curtain of Fire"
Strombolian
Distinct blasts of basaltic to andesitic magma
incandescent bombs
Effusive eruptions
Effusive eruptions
Mildly explosive eruptions
Lava flows from vents low on flanks of small cones
Low elevation eruption columns and tephra fall deposits
Vulcanian
Sustained explosions of solidified or highly viscous andesitic or rhyolitic magma
Eruption columns can reach several km above vent, and collapse to produce pyroclastic flows
Widespread tephra falls
Very explosive eruptions
Pelean
Collapse of andesitic or rhyolitic lava dome, with or without a directed blast
glowing avalanches
Violently explosive eruptions
Plinian
Sustained ejection of andesitic to rhyolitic magma
Eruption columns up to 45 km above vent
Violently explosive eruptions
Plinian ash clouds can circle the earth in a matter of days
eruption column might collapse, producing pyroclastic flows
Types of Volcanoes
Shield Volcanoes
gentle upper slopes, steep lower slopes
Thin lava flows
low viscosity basaltic magma
Mauna Loa (Hawaii)
Stratovolcanoes
Steeper slopes
Effusive eruptions
lava flows and pyroclastic materials, andesitic to rhyolitic
Higher viscosity, more explosive
Mount St. Helens, Mount Fuji
Cinder Cones
Small volume, predominantly of tephra
basaltic to andesitic
internal layered structure
Usually occur around summit vents of stratovolcanoes in groups of tens to hundreds
Mauna Kea , Paricutin
erupt during one episode lasting months to years, explosive
Lava Domes
High viscosity and low gas content andesitic and rhyolitic lava come out of a volcano, and lava piles up over vent
Lava dome at Mount St Helens
Craters: circular depressions, that form as a result of explosions that emit gases and tephra
Caldera: much larger depression, form when the. underlying magma chamber collapses. Crater lake Caldera, Mount Aso, Yellowstone
Resurgent Domes: When magma is reinfected back into the area below the caldera
Geyser: Hot spring has a plumbing system that allows for the accumulation steam from boiling water. Steam moves rapidly to surface, causing eruption of water
Fumarole: vent where gases emerge from surface if the earth, only visible if the water condensates
Hot Springs: Areas where hot water comes to the surface the Earth.
Plateau Basalts/Flood Basalts: large volume outpourings of low viscosity basaltic magma from fissure vents
Volcanoes and Plate Tectonics
Convergent zones
Most active and dangerous volcanoes are at subduction zones (10%), explosive, high viscosity (rhyolite) eruptions
Pacific Ring of Fire, volcanoes are parallel to subduction zones/trench
No magma formation at collision zones
Transform plate boundaries: no magma formation
Divergent Zones
lots at mid-ocean ridges , no threat to humans (80%)
Iceland is only place where mid-ocean ridges cause volcanism that can be seen by humans
Andesitic and rhyolitic magmas, eruptions tend to be explosive, with common Strombolian, Vulcanian, pelean and plinian. Repose periods tend to be hundreds of thousands of years, giving false security
Hot Spots
When volcanism occurs in the middle of plates rather than the plate boundaries (10%)
Magma created by deep mantle plumes, hot spots are fixed in position but the plate moves over the top, producing new volcanoes or seamounts (former volcanic islands that have eroded below sea level)
Primary and Secondary Effects
Oceanic eruptions create peaceful and build shield volcanoes (Hawaii)
Continental hotspots create explosive eruptions and form Calderas (Yellowstone)
Primary Effects
Secondary Effects
Lava flows
Common in Hawaiian and Stromblian eruptions
most are slow, some can travel up to 64km/hr
Most damaging to property, destroying anything in their path
Pyroclastic Activity
Example: Heimaey, Iceland 1973
cause death by suffocation and burning, usually travel so rapidly they are inescapable
Lateral blasts knock down anything in their path, drive debris through trees
Ash falls: collapse roofs, destroy vegetation and livestock
Example: Unzen Volcano, Japan 1993. Mount Pinatubo, Indonesia, 1991.
Poisonous gas emissions
Lake Nyos, 1700 died. Carbon dioxide in lake
Lahars (Volcanic Mudflows)
Mount St. Helens
loose tephra exposed to water
Debris avalanche and flows
Volcanoes become oversteepened because addition of new material and inflation.
Oversteepened slopes become unstable leading to slope failure and landslides, debris slides, avalanches
Flooding: melting of snow/ice, blockage of drainage systems
Tsunamis: debris entering water can create a tsunami
Atmospheric Effects: short term effect on climate, cooling due to reflection of solar radiation (Iceland 2010)
Benefits of Volcanism: renews soil, production of geothermal energy, rich ore deposits
Current Status of a volcano
Active
Has shown eruptive activity within recorded history (does not need to be in eruption)
Currently: ~600 active volcanoes
50-60 erupt each year
Extinct
Has not shown any historic activity, usually deeply eroded, no signs of recent activity
Yellowstone Caldera: 600 000 years old, deeply eroded, but fumarolic activity, hot springs and geysers all point to magma below the surface, so it is not extinct
Dormant (sleeping):
somewhere between active and extinct, has not shown activity within recorded period but shows geologic evidence of activity within geologic recent past
Can become active all of a sudden, so they are most dangerous
Mount St. Helens was dormant for 123 years, before erupting in 1980
Mount Pinatubo was dormant for 400 years before erupting in 1991
Mount Vesuvius was dormant before erupting in 79 AD
Long and Short Term Predicting
Long Term
click to edit
Studying past behaviour and deposits by ancient eruptions
Monitoring techniques
Ground deformation
Geophysical measurements
Remote sensing
Hydrology
Gas
Seismicity: earthquake activity measured by seismographs stationed on flanks of volcano
Short Term
Watching for magma to approach surface
if magma exists in a volcano, no S waves will travel through (can't travel through liquids)
Earthquakes usually precede and accompany a volcano
Difficult to predict, not enough time to take action
Deformation monitoring: tilt meters used to measure deformation of a volcano, slope of volcano
Precursors
Ground Deformation: volcano may inflate with magma
Changes in groundwater system: magma can cause water to rise or fall and cause the temperature of the water to increase
Changes in heat flow: magma causes surface to heat up, measured using infrared remote sensing
Changes in gas composition: often changes just prior to an eruption, increasing hydrogen chloride and sulfur dioxide
Strange animal behaviour: Cattle movement and uneasy dogs