volcanoes windows into the earth volcano – a mountain formed by lava & pyroclastics crater –...
TRANSCRIPT
•Volcano –
a mountain formed by lava & pyroclastics
•Crater –
opening at the top of a volcano
•Caldera –
summit depression >1 km-usually caused by collapsed magma chamber
Important Vocabulary
• Active – erupted in recent history (within the last
few 100 years)• Dormant –
no recent eruptions (not within the past several 100 years), not eroded or worn down
• Extinct – no recent eruptions (not within the past
1000 years) and eroded
Where are all the Volcanoes?
• 1,900 volcanoes are active today or are known to have been active during historic times.
• 90% of these volcanoes are on the Ring of Fire– a band of volcanoes circling the Pacific Plate
Practice Questions
• What is the difference between a crater and a caldera?– size
• Is the picture below a crater or a caldera?– caldera
http://earth.jsc.nasa.gov/EarthObservatory/CraterLake,Oregon_files/ISS006E15238_Crater-Lake.jpg
Practice Questions
• What is the difference between a dormant and an extinct volcano?– Time since last eruption & amount of
erosion
• Where are most of the volcanoes in the world located?– Ring of Fire– Convergent and Divergent Plate Boundaries
(not transform)
Magma vs. LavaBoth are molten (liquid)
rock with suspended mineral grains and dissolved gases
• Magma – interior of the earth
• Lava – magma that reaches
the earth’s surface
Magma and Viscosity
•Viscosity can be affected by many variables
Mineral compositionTemperatureDissolved gases
Viscosity is a fluids resistance to flow
Practice Question
• Rank the following substances from high to low viscosity.– Ketchup– Vitamin Water– Magma– Maple syrup
1. Magma
2. Ketchup
3. Maple syrup
4. Vitamin Water
Mineral Composition
• Remember silica?– Silica is a very common mineral composed
of silicon and oxygen (SiO2). Silicates make up about 95% of the earth’s crust
• A magma’s viscosity is directly related to its silica contentHigh silica magmas (granitic) are
highly viscous with short thick lava flows
Low silica magmas (basaltic) are more fluid and may travel up to 90 miles and have low viscosity
Temperature•High temperatures decrease
viscosity and make magmas and lavas more fluid= low viscosity
•As a lava flow cools it begins to thicken and movement decreases= high viscosity
Dissolved Gases• As magmas reach the surface, the pressure is
reduced and gases expand with explosive results
• Gas composition varies, but it usually consists of:– Mostly H2O (water vapor) & some CO2 (carbon
dioxide)– Minor amounts of Sulfur, Chlorine, and Fluorine gases
• The more gas, the more explosive the eruption!• However, gas content affects viscosity as well:
– High Gas = Low Viscosity– Low Gas = High Viscosity
Practice Questions
• What are 3 things that affect the viscosity of magma?– Silica content– Heat– Gas content
• How does each of these affect viscosity?
Viscosity Review
1) Temperature - temp. Viscosity2) Silica Content - silica Viscosity3) Dissolved Gas - gases Viscosity
• Deep below the surface of the earth, the pressure is great.
• Volcanic gases are dissolved in the liquid….
• If the magma has low viscosity, the gas bubbles can easily escape...
• If the magma has high viscosity, the gas bubbles are trapped in the thick, gooey liquid. And gas pressure builds up until….
Kaboom!An explosive eruption!
So low viscosity magmas that allow gas to escape produce non-explosive eruptions.
High viscosity magmas that trap gas produce explosive eruptions.
Types of Eruptions• Non-Explosive
Lava Flows – Low viscosityEx. Hawaii & Mid-Ocean Ridge
• ExplosiveHigh viscosity lava flows & high gas
contentPyroclastics – “fire fragments”Ex. Mount St. Helens, Krakatoa
Soda Bottle Analogy
• Dissolved gases under pressure inside bottle.
• Soda (lava) escapes violently as pressure drops!
Practice Questions• What type of eruption would
happen if the magma has a high silica content and a high gas content?– Explosive
• What type of eruption would happen if the magma has a low silica content and a low gas content? – Non explosive
Lava Flows• Dark colored = mafic lava (rich in
magnesium & iron, low in silica) – low viscosity– Also called basaltic lavas
Pahoehoe – smooth, rope-like lavaaa – clinkery, rough lava
• Light colored = felsic lava (rich in silica)– High viscosity
Pyroclastics“Fire Fragments”
• Ash- very fine glassy fragments• Welded tuff – fused glassy shards • Cinders – pea sized pyroclasts• Lapilli – walnut sized pyroclasts• Lava blocks – large hardened
pyroclasts• Lava bombs – incandescent lava that
cools as it flies through the air, football shaped
• Pumice – frothy material w/ air spaces
Volcanic Locations1) Divergent Plate Boundaries
Where two plates are pulling apart--Mid Ocean Ridge & Continental Rift
2) Subduction Zones Where an oceanic plate converges
with another oceanic plate or a continental plate--Pacific Ring Of Fire
3) Hot Spots Fixed source of magma in the center
of a plate-- Hawaii, Yellowstone
Shield Volcano• Mafic, basaltic lava• Rapid streams of low viscosity lava
flow easily form gentle slopes.• Very flat and low compared to
diameter• Each flow is only a few meters thick• Mauna Loa
2.5 miles above see level (13,679 ft)
6 miles above sea floor60 miles across at base
Cinder Cone• Small, steep cone shaped volcanoes• Usually very forceful release of gas• Little to no lava flow• Magma and rock are flung from
volcano• Pea size pyroclastics (cinders) pile up
and form distinctive steep sided cone
Composite Cone
• Alternate layers of lava and pyroclastics build up
• Viscous, gas charged andesitic lava
• Violent eruptions• http://www.pbs.org/wnet/savag
eearth/animations/volcanoes/index.html
Fissure Eruptions• A fissure is a
fracture or crack in rock along which there is an obvious separation
• Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
Practice Questions• Arrange the types of volcanoes from least
explosive to most explosive. (in general)Shield, cinder cone, composite/strato
• Which volcanoes have lava as part of the eruption? shield, fissure eruptions & composite/strato
• Which volcanoes have pyroclastics as part of the eruption? cinder cone & composite/strato
Volcanic Hazards• Lava and Pyroclastics•Lahars – volcanic mudflows
created by ash and water•Nuée Ardente – incandescent debris w/ hot gases that moves like an avalanche
Glowing cloud
20000 F
120 mph
•Toxic Gases – CO, CO2, Sulfur, HCl
Lake Nyos - Cameroon
•Steam Explosion
Phreatic eruption
Seawater is heated and explodes
Krakatau100 million tons of TNT
Future Eruptions
• Decade Volcanoes– 16 volcanoes that have been designated as
Decade Volcanoes by the International Association of Volcanology and Chemistry of the Earth's Interior b/c of their eruptive history and proximity to major population centers.
Volcano Pictures
• Mount Etna, Italy
•Pacific Northwest – Rainier and Mt. St. Helens
•Hekla, Iceland
•Olympus Mons, Mars