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					VOLCANOES
   A few volcano basics…
• Active – a volcano that has erupted
  recently (geologically speaking)

• Dormant – (sleeping) has erupted
  within the past few thousand years –
  will likely erupt again

• Extinct – hasn’t erupted in 10’s of
  thousands of years; unlikely to erupt
  again
Mt. Rainier -
active
Mount Baker -
dormant
Shiprock Peak –
extinct volcano
magma: molten rock inside
       the Earth


  lava: magma that has
 emerged on the Earth’s
         surface
er   lava




       magma
 hot spot: area of
  volcanic activity
   independent of
lithospheric plates

    ex. Hawaii
Different types of
volcanoes are categorized
based on the viscosity of
the magma.
viscosity: a fluid’s resistance to
flow

high viscosity = high resistance
to flow

ex. honey is more viscous than
water
• Low viscosity magmas allow
  gases to escape easily (they
  are hotter!)
• In higher viscosity lavas, gas
  pressures build up and erupt
  explosively
Viscosity is determined by
    silica content and
       temperature.
 The higher the silica
content, the higher the
       viscosity.

The hotter the magma,
 the less viscous it is.
TYPES OF VOLCANOES

  1. Shield
  2. Composite
  3. Cinder cones
Shield Volcanoes
- largest volcanoes (tallest when
measured from the ocean floor)
- shaped like a gentle arch or
shield
-basaltic lavas (low silica
content)
- usually non-explosive
eruptions
- mainly lava flows
- found in Hawaii (Mt.
Kilauea) and Iceland
lava tubes: caves left behind
       after lava flows
•
Composite Volcanoes
- coned shaped; sometimes
called stratovolcanoes
- lava contains mostly
andesite or rhyolite
(higher silica content)
- more viscous
-combination of explosive
activity (pyroclastic) and lava
flows
- responsible for most deaths
- ex. Mount Saint Helens
Mt. Pinatubo
Mt. Fuji
Mt. Vesuvius
Mount Saint Helens, Washington State
Mt. Saint Helens, after eruption
Mount Pinatubo, 1994
Mount Fuji, Japan
Mount Vesuvius / Ruins of Pompei
Cinder Cones

- smaller volcanoes
- form quickly
-are active for a few years
and then usually go dormant
Wizard Island
      ex. Paracutin, Mexico

- 1943, hole in ground
- 2 weeks prior, small tremors
- 1st day grew 10 m
- By 5th day it was 100 m tall
- After 2 yrs, it was 400 m tall
- went quiet after 9 years
    Ring of Fire
• Area of the world where
  volcanoes are common
• Here is what a volcano on
  the ring of fire looks
  like:
   Features of Volcanoes
crater - depression found at
the top of a volcano; formed by
the explosion of the upper
portion of the cone
hot springs: water is heated
by the hot rock and reaches
      Earth’s surface
geyser: heating and
circulation of water forms
a pattern (RARE!)
ex. Old Faithful
 caldera: a giant crater that can
 be more than 12.5 miles in
 diameter
- formed by rare, very violent
eruptions
- none have occurred in recorded
history (they are 1000 times more
violent than Mt. St. Helens!)
- Yellowstone caldera was formed
600,000 years ago!
Crater Lake, OR
Long Valley, CA
Aniakchak, AK
   Volcanic Hazards



Primary Effects
 Lava flows: eruption of
magma at Earth’s surface

Lava flows vary based on the
 composition of the magma.
We are specifically going to
 look at basaltic lava flows.
pahoehoe: fast moving; low
viscosity

- smooth ropy texture when
it hardens
- move at a rate of up to 20
mph
aa: slow moving; higher
viscosity

- blocky texture when
hardens
- move at rates of a few
meters per day
-found further away from
volcano
Pyroclastic Activity:
explosive volcanism where
tephra is physically blown
into the atmosphere

tephra: any material that
is blown out of a volcano
(mostly ash)
ash fall: huge quantities of
rock, glass and gas are blown
high into the air
  - kills vegetation
  - contaminate water
  - structural damage
  - jet engine “flame out”
  - respiratory irritation
lateral blast: explosions of
gas and ash from the side of
the volcano; destroys part of
the mountain
ex. Mt. St. Helens
Ash flow (nuee ardentes)

- move very quickly
- destroys everything in its
path
      Poisonous Gases
- CO2, carbon monoxide,
sulfur dioxide, and hydrogen
sulfide
- can cause acid rain;
contaminates water supply
and vegetation
   Secondary Effects
lahar: debris flows and
mudflows

-volcanic debris becomes
saturated with water
-ex. Mt. Pinatubo, Philippines
Buildings buried after lahars at Pinatubo
Aerial View, Pinatubo Aftermath
  Predicting Volcanoes

1. Monitoring of Seismic
Activity- often the
earliest sign
2. Thermal monitoring -
measuring ground
temperatures
3. Topographic Monitoring -
mountain may tilt and swell
before an eruption

4. Volcanic Gas Emissions -
changes in composition of
gases may indicates rising of
magma
5. Geologic History -
geologists map and date
rocks around the area

				
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posted:9/29/2012
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