Download Lecture 6- September 26

Volcanoes
GE 4150- Natural Hazards
Kelly Durst- Mt. Rainier
Earth’s Structure
Plate Tectonics
Plate Tectonics
• Divergent margins: Plate separation and decompression
melting -> low volatile abundance, low SiO2 (~50%), low
viscosity basaltic magmas (e.g. Krafla, Iceland)
• Convergent margins : Mixtures of basalt from the mantle,
remelted continental crust and material from the subducted
slab. High volatile abundance, intermediate SiO2 (60-70%), high
viscosity andesites and dacites (e.g. Montserrat, West Indies)
• Intraplate `Hot-spot` settings:
A. Oceanic: Mantle plumes melt thin oceanic crust
producing low viscosity basaltic magmas (e.g. Kilauea,
Hawaii)
B. Continental: Mantle plumes melt thicker, silicic
continental crust producing highly silicic (>70% SiO2)
rhyolites (e.g. Yellowstone, USA)
Kelly Durst- Colima, Mexico
Volcanic Settings
Where do volcanoes occur?
Ring of Fire
http://www.worldbook.com/wb/Students?content_spotlight/earthquakes/studying_predicting
Volcanic Rocks
http://www.teara.govt.nz/EarthSeaAndSky/NaturalHazardsAndDisasters/Volcanoes/1/ENZ-Resources/Standard/3/en
Igneous Rock Classification
A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750.
Oxford University Press.
Viscosity
• Viscosity- fluidity of a liquid, it’s internal
resistance to flow.
• Magma viscosity is affected by:
– Temperature, higher temperature lowers
viscosity
– Silica (SiO2) increase viscosity because
stronger bonds are formed
– Increasing content of mineral crystals
increases viscosity
Kelly Durst- Mt. St. Helens
Volcanic Rocks
Natural Disasters Sixth Edition by Patrick L. Abbott
Magma
Generated by:
-Lowering pressure
-Raising Temperature
-Increasing water
content (volatiles)
As magma rises, pressure
decreases, and gases
come out of solution,
forming bubbles that
expand with decreasing
pressure
Natural Disasters Sixth Edition by Patrick L. Abbott
Eruption Styles
Explosive
Gas-particle dispersion flows out of the vent
Extrusive
Lava flows or domes
Explosive
volcanic
eruptions
Strombolian
Vulcanian
Plinian
Increasing SiO2, increasing volatiles (mainly water), increasing explosiveness
Extrusive
eruptions
• Lava flows
• Lava domes
Photo: Copyright Marco Fulle - Stromboli OnLine - http://stromboli.net
Types of Volcanoes
• Shield Volcanoes
• Flood Basalts
• Scoria Cones
• Stratovolcanoes
• Lava Domes
• Calderas
Kelly Durst- Mt. Hood
Shield Volcanoes
Low viscosity
Low Volatiles
Produce large volume from lava
flows
Basaltic
http://volcanoes.usgs.gov/Products/Pglossary/ShieldVolcano.html
Shallow slopes
Fissure eruptions
http://mahi.ucsd.edu/Gabi/erth10.dir/shield.iceland.jpeg
http://volcanoes.usgs.gov/Products/Pglossary/Fi
ssureEruption_examps.html
Flood Basalts
Largest volcanic events on Earth
Erupt extremely large amounts of magma in a
relatively short time of 1 million years.
Occur on all continents and on all ocean floors,
but none has occurred in historic time
Although lava flows are the main hazard, the
large amount of CO2 and SO2 released is able
to modify our climate
Kelly Durst
Scoria Cones
Conical Hills, Low Height
Basaltic-Andesitic
Produced during a single
eruptive event (usually
Strombolian eruptions), lasting a
few hours to several years
Once activity ceases usually
never active again
http://www.geology.wisc.edu/courses/g112/Images/paricutin.jpg
http://static.howstuffworks.com/gif/volcano-scoria-cone.jpg
Stratovolcanoes
Steep-sided, symmetrical peaks
Built of alternating layers of pyroclastic
deposits and capped by andesitic-rhyolitic
lava flows
Eruption styles vary from Vulcanian-Plinian
Highly viscous lavas, Usually explosive
eruptions
Comprise 60% of the Earth’s volcanoes
http://en.wikipedia.org/wiki/Image:FujiSunriseKawa
guchiko2025WP.jpg
Kelly Durst- Mt. Rainier
Produce pyroclastic flows, lahars, ash
plumes and lava flows
http://vulcan.wr.usgs.gov/Imgs/Jpg/Shasta/Images/Shasta84
_mount_shasta_with_shastina_1984_med.jpg
Lava Domes
Domes are built after large eruptions, expelling all
of the high volatile magma. Leaving behind high
viscosity magma with low volatiles, which in turn
forms a plug or dome
Can form in a few hours or grow over decades
Pose many hazards such as: pyroclastic flows,
lava flows and lahars.
Bill Rose- Santa María
Mont Pelée
Kelly Durst- Mt. St. Helens
Calderas
Caldera-forming eruptions are the
largest of the violent, explosive
volcanoes
Large volcanic depressions formed by
roof collapse into partially erupted
magma chambers
Calderas range from 2-75 km in
diameter and are formed by inward
collapse
http://en.wikipedia.org/wiki/Image:Santorini_Landsat.jpg
Collapse occurs following large Plinian
eruptions producing pyroclasts. Void
spaces are left causing the mountain to
collapse into the magma chamber
Three different Caldera settings:
1. Calderas in the summit of
shield cones (Mauna Loa)
2. Summit of stratovolcanoes
(Crater Lake Oregon)
3. Giant continental calderas
(Yellowstone)
Giant Continental Calderas
Yellowstone
Yellowstone sits above a hotspot composed of rhyolitic
magma
Three Ultra-plinian eruptions have occurred in the past 2
million (2500 km3), 1.3 million (280 km3) , and 0.6 million
years ago (240 km3)
Deposit very high volumes of rhyolitic magma mainly in
the forms of pyroclastic flows.
An eruption of 1000 km3 would cover an area of 30000
km2, with a mass of pyroclastic debris ranging from a
few to more than 100 m thickness
http://geology.com/news/labels/Volcanoes.html
http://www.yellowstonegis.utah.edu/maps/locations/CalderaRim.html
http://people.uwec.edu/ERICKSKM/hazards.html
Volcanoes on Other Planets
http://apod.nasa.gov/apod/ap970321.html
http://www.nasa.gov/centers/goddard/news/topstory/2
006/vesper.html
Volcano Monitoring
Kelly Durst- Mt. St. Helens
Seismic Monitoring
Magma rises into reservoir
beneath volcano
Rising magma and volcanic
gases exert pressure
High pressure causes rocks to
break, triggering earthquakes
Volcanoes first warning sign is usually
a seismic signal
Low magnitude, Occur in swarms
Need a network of seismometers to
determine the location of the
earthquake
http://volcanoes.usgs.gov/About/What/Monitor/Seis/SeisMonitor.html
Ground Deformation
• EDM electronic distance measurement,
measures the distance between 2 benchmarks
on the volcano
• Tiltmeters- is placed in the ground with a
container filled with fluid and a bubble which
measures changes in slope.
• GPS-global positioning system, measures
horizontal and vertical movement, replacing the
EDM
• Satellite Interferometry- Satellite sensor uses
RADAR to obtain elevations, when compared
with other images can determine deformation
Gas Monitoring
• Gases dissolved in
magma are driving forces
for an eruption
• By monitoring specific
gases, coupled with other
monitoring data, hazard
warnings can be issued
• Many gases are
monitored to determine
changes in concentration
with a focus on CO2 and
SO2
Hydrology Monitoring
• Lahar detection using sensors
• Monitor streams around the volcano to
determine if changes in water chemistry
• Measure sediment and river discharge
• Mt. St. Helens - After 20 years, the
average annual suspended-sediment yield
in the Toutle River downstream from the
1980 landslide deposit was still 100 times
above typical background level.
http://volcanoes.usgs.gov/About/What/Monitor/Hydrologic/HydroMon.html
Kelly Durst- Mt. Rainier Lahar Deposit
Remote Sensing
• Allow scientist to monitor dangerous
volcanoes remotely
• Allow world wide monitoring over small
temporal scales (minutes-hours).
• Many sensors which monitor visible, near and
mid infrared, as well as thermal infrared
allowing us to determine temperature changes
Homework
• For Monday
– Read Sensing Remote Volcanoes (3 pages),
located at:
http://earthobservatory.nasa.gov/Study/monvoc/