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VOLCANISM:
All processes
associated with the
discharge of magma,
hot fluids, and gasses
Volcano- a mountain that forms when
layers of lava and volcanic ash erupt
and build up.
* most volcanoes on Earth are
dormant (not active)
*There are about 600 active
volcanoes on Earth.
Magma- molten (melted) rock that is
located beneath the Earth’s surface.
The more silica magma contains the
thicker it is.
Lava- magma that flows to the Earth’s
surface.
lava
magma
1) Divergent boundaries
* When 2 plates are moving apart, it creates
an opening or ridge.
* Magma flows into the ridge and is cooled
by sea water.
* pillow lava
* non-explosive
2) Convergent boundaries* Magma created in the subduction
zone is forced upward because the
HOT magma is less dense than
cooled rock of the surrounding
crust.
* example: Mount St. Helens in
Washington State
Formation of a volcano at subduction zone
TWO major belts
(volcanoes at convergent boundaries):
1) Circum-Pacific (Ring of Fire)
* Pacific plate
2) Mediterranean Belt:
* Eurasian, African, & Arabian plates
3) Hot Spots* Some areas of the mantle are
hotter than others. These areas
are called hot spots.
* These hot spots melt rock which is
forced upward to the Earth’s crust.
Hot Spots- OCEANIC
* As the plates move over the hot spot,
many volcanic islands may form.
* Example- Hawaiian Islands
A chain of island volcanoes can
form at a hot spot.
Hot Spots- CONTINENTAL
* Flood basalts form when lava flows out
of long cracks in the Earth’s crust called
fissures.
* This lava flow results in flat plains called
plateaus.
* Ex: Columbia Plateau in Washington &
Oregon
1)Shield volcano-
* quiet eruptions
* slow-moving, non-viscous lava
flows out in flat layers (low silica
content)
* creates a broad volcano with
gently sloping sides
2) Cinder Cone Volcano-
* explosive eruptions that shoot lava
high into the air (thick, viscous
lava that has high silica content)
* lava then falls back down and cools
* this creates a small, steep-sided
volcanic mountain (cone shape)
3) Composite volcano (stratovolcano)-
* both explosive and quiet eruptions
* First, an explosive period shooting
up lava. Then, a quiet period of
slow moving lava flows.
* large, steep-sided and symmetrical
built of layers of ash and lava
A Size Comparison of the Three
Types of Volcanoes
Figure 4.14
The type of eruption is determined by
the composition of magma.
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Temperature
Pressure
Presence of water
Amount of silica
AMOUNT OF SILICA
high silica= high viscosity
traps gasses
EXPLOSIVE!!
low silica= not viscous
easy flow
QUIET & NON-EXPLOSIVE!!
1) Basaltic
2) Andesitic
3) Rhyolitic
The Nature of Volcanic
Eruptions
• Factors affecting viscosity
– Lower silica content = lower viscosity or more
fluid-like behavior (e.g., mafic lava such as
basalt)
• Dissolved gases
– Gas content affects magma mobility
– Gases expand within a magma as it nears the
Earth’s surface due to decreasing pressure
– The violence of an eruption is related to how
easily gases escape from magma
The Nature of Volcanic
Eruptions
• Factors affecting viscosity
In summary
• Fluid basaltic lavas generally produce
quiet eruptions
• Highly viscous lavas (rhyolite or andesite)
produce more explosive eruptions
Ash cloud
Crater
Vent
Conduit
Magma Chamber
CRATER
Bowl-shaped depression formed around the vent, Less
than 1 km in diameter
VS.
CALDERA
Larger depression, can be up to 50 km in diameter,
often forms when top or sides of the volcano
collapses into the magma chamber
Terms to know:
Tephra
Pyroclastic flow
Lahar
Pyroclastic Materials
• “Fire fragments”; Tephra
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•
•
•
•
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Ash and dust – fine, glassy fragments
Pumice – porous rock from “frothy” lava
Lapilli – walnut-sized material
Cinders – pea-sized material
Blocks – hardened or cooled lava
Bombs – ejected as hot lava
A Volcanic Bomb
Figure 4.9 left
A Pyroclastic
Flow Moving
Rapidly Down
the Forested
Slopes of
Mt. Unzen
Figure 4.21 B
Pompeii
Aa: forms from cooler, thicker lava. Hardens
into a rock with a rough, jagged surface
A Typical Aa Flow
Figure 4.6 A
Pahoehoe: formed from thin, runny lava. Hardens
into a rock with a smooth, rope-like surface.
A Typical Pahoehoe Flow
Figure 4.6 B
Pillow lava: lava formed from eruptions under water
Pillow Lava
Figure 16.18
Active: a volcano that is erupting or shows signs that
it will erupt in the near future
Dormant: volcano that scientist expect to awake in
the future and become active
Extinct: volcano that is unlikely to erupt again