Download Volcanoes 11.4 - Ramsey Public School District

Volcanoes
Where:
1. Almost all volcanoes are found @ Plate Boundaries.
Most of those volcanoes are found along the Pacific Rim
– a subduction zone called “The Ring of Fire”
2. Hot spots
Formation of Volcanoes
1. Magma forms: rock melts, forming liquid magma (melted
rock + trapped gas)
2. Magma rises through the crust, erupting at the surface.
Magma rises b/c it is less dense than rock (it’s hotter & has gas
in it).
3. Magma collects & melts more
rock …Pressure builds as more gas
is added.
4. Eruption: pressure gets too
high.
Melting Rock to Make Magma
1. Heat Melts Rock
Heat from mantle; Heat from friction of grinding plates; heat
from magma that intrudes into crust from other locations
2. Decrease Pressure on Rock
Decompression Melting: When pressure is reduced rock can
melt at lower temps. Pressure depends on depth. As hot rock
rises; it melts because there is less pressure.
* Rock deep in crust should melt; but is solid because of
pressure)
3. Add Water: “Wet” rock melts at lower temps.
Subduction drives water out of the subducting rock
(metamorphic change). “Dewatering the slab” lowers the
temperature of rock above.
Magma Composition and Characteristics
Granitic
Ocean Crust Melt
(Mafic Magmas)
Continental Crust
Melt
(Felsic Magmas)
Structure of Volcanoes
 Magma collects in magma chamber before eruption
 Magma exits through a
central vent or pipe to the
crater at the summit.
 Each eruption adds a layer to
the volcano
Eruption dates can be determined
by isotopic dating the rock layers
Structure of Volcanoes
Caldera : depression formed from collapsed
volcanoes (usually extinct); usually fill up
with water & b/cm lakes
Eruption Types:
Quiet = low silica/ high water/ high temp. /
Low viscosity/ low gas content
Eruption Types
Explosive Eruptions: high-silica / Low temps./
low water / high viscosity/ high gas content
“pyroclastic”
Mount St. Helens, WA
Eruption Types
Mount St. Helens, WA
Pre 1980
eruption
Today
Post 1980
eruption
Volcano Hazards
Least Hazard
Great Hazard
1. Lava flows (burns/
fire)
2. Ash (buries,
suffocates)
3. Pyroclastic flows
(gas, ash,
superheated rock
fragments
<bombs>)
4. Mudflows/Lahars
5. Acidification of
water
6. Climate
change/mass
extinction
Lahar: Volcanic landslide/mudslide
Eruptions and Eqs that accompany them trigger landslides;
Lava melts snow at the top of the volcano  Mudslide
Very Hazardous  wipe out villages/ fatalities
Volcano Types
1. Shield:
Broad / Flat, Gently sloping cone, (b/c
runny lava travels far before solidifying)
Quiet Eruptions:
Liquid lava (low viscosity / high water / low
silicates) fr. Single vent
Ex: Hawaiian Volcanoes
Volcano Types
2. Strato or Composite Volcano
Builds in layers of lava and ash & debris
Explosive “pyroclastic” eruptions (Hot gas, rock,
and ash)
High viscosity / high silicates /low water
Tall & Steep & Side Vents
Most Dangerous
Ex: Mt. St. Helens
Mt. Pinatubo
Volcano Types
3.
Cinder Cones
Simple, small, steep sided
Made from blobs of lava & ash ejected from a
single vent that fall back to the surface.
No repeated eruptions
Ex: California Volcanoes – Lassen Peak
Volcano Types
4. Under Water Volcanoes
Seamount – an underwater mt.
(volcano) that does not reach the
surface
Volcanoes - Most are located
near plate boundaries
“Ring of Fire” = Edge of Pacific
Ocean stretching from Alaska to
Japan to Indonesia, where most
of the world’s volcanoes are
located. (subduction zone)
Ring of Fire = Edge of the Pacific Plate. Most of the
worlds volcanoes are found here due to
SUBDUCTION of the pacific plate.
3 Ways Volcanoes Form
1. Subduction Zones (Convergent Plate Boundaries)
(Ring of Fire)
2. Rifting & Sea Floor Spreading (Divergent Plate
Boundaries)
(African volcanoes & Iceland’s volcanoes)
3. Hot Spots – can be in the middle of a plate
(Yellowstone, Hawaii)
– a region of active magma under a plate. This active
magma forces its way through weaknesses in the crust
to form volcanoes.
Hot Spots
The Hawaiian Islands were formed as
seamounts grew over hot spots in the
Pacific.
Hot Spots Around the World
Hot Spot Volcanoes
Hot Spot Volcanoes
 Hot spot stays in the same place while the plate moves above.
Other Extrusive Igneous Features
(not volcanoes)
Lava Plateau: Large amounts of easily
flowing lava erupting fr. cluster of long, thin
cracks in crust. Lava spreads out over
enormous area before solidifying.
Ex.: Columbia Platueau in Pacific NW (1
km thick / 200,000 square mi)
Basalt
Columbia Plateau in
Idaho =
Lava Plateau
Intrusive Igneous Features
Batholith: Large type of intrusive igneous
rock mass that can form the core of a
mountain range (Sierra Nevada’s in Calif.)
Sill: Magma squeezes through cracks in
rock layers and hardens / Paralles to rock
layers.
Dike: Igneous rock cuts across rock layers.
Volcanic Neck: magma hardens in
volcanic neck.
Half Dome at Yosemite
National Park (Sierra
Nevada Mts) = Forms
when batholith that was
originally under surface
reaches surface (rock on
top of erodes away or it is
pushed upwards)
Types of Volcanoes
http://vulcan.wr.usgs.gov/Glossary/VolcanoTypes/
volcano_types.html
http://vulcan.wr.usgs.gov/Photo/Pictograms/volcan
o_types.html
Volcano World – All things volcano
http://volcano.und.edu/
Smithsonian’s Weekly Volcano Report
http://www.volcano.si.edu/reports/usgs/