Download Volcanoes

Volcanoes
Windows Into the Earth
Important Vocabulary
•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
• 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. Lava
Both 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 is a fluids resistance to flow
• Viscosity can be affected by
many variables
Mineral composition
Temperature
Dissolved gases
Practice Question
• Rank the following substances from
high to low viscosity.
– Ketchup
– Vitamin Water
– Magma
– Maple syrup
1.
2.
3.
4.
Magma
Ketchup
Maple syrup
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 content
High 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.  Viscosity
2) Silica Content -  silica  Viscosity
3) 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 nonexplosive eruptions.
High viscosity
magmas that trap
gas produce
explosive eruptions.
Types of Eruptions
• Non-Explosive
Lava Flows – Low viscosity
Ex. Hawaii & Mid-Ocean Ridge
• Explosive
High viscosity lava flows & high gas
content
Pyroclastics – “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 lava
aa – 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 Locations
1) 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
Types of Volcanoes
• Shield Volcano
• Cinder cone
• Composite (stratovolcano) cone
• Fissure eruptions
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 floor
60 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/savageea
rth/animations/volcanoes/index.ht
ml
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
•Steam
Explosion
•Lahars
•Nuée
•
Lava
Ardente
–
and
volcanic
Pyroclastics
–
incandescent
mudflows
created
debris
by
•Toxic Gases – CO, CO2, Sulfur, HCl
ash
w/
hot
and
gases
water
that
moves
like
an
Phreatic eruption
Lake Nyos - Cameroon
avalanche
Seawater is heated and explodes
Glowing cloud
Krakatau100
million
tons
of
TNT
20000 F
 120 mph
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
Eyjafjallajökull, Iceland 2010
The End