Volcanoes
by Jim Alton
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What are volcanoes?
Today we will look at:
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Where they occur
“Continental Drift” & “Plate Tectonics”
Types of volcanic eruptions
Historical eruptions
Volcano environments
• Most active volcanoes are located along, or near, the
margins of the continents.
• More than half en circle the Pacific Ocean.
• The area in red, below, is called the "Ring of Fire."
Plate-tectonics theory
• Alfred Wegener first proposed “Continental Drift” Theory in
early 1900’s. This theory was widely criticized.
• Discovery of mid-ocean ridges gave birth to “Seafloor
Spread” theory.
• The demonstration below shows the drift in reverse,
ending with the super-continent “Pangea”.
What drives plate
tectonics ?
• One theory is that convection
within the Earth's mantle
pushes the plates, in much the
same way that air heated by
your body rises upward and is
deflected sideways when it
reaches the ceiling.
• Another theory is that gravity is
pulling the older, colder, and
thus heavier ocean floor with
more force than the newer,
lighter seafloor.
Divergent boundaries (Seafloor Spread)
• As the two sides of the mountain move away from each
other, magma wells up from the Earth's interior.
• It then solidifies into rock as it is cooled by the sea,
creating new ocean floor.
Convergent boundaries
• New crust is continually being pushed away from
divergent boundaries, increasing the Earth's surface.
• The ocean crust subducts, or slides under, other pieces
of Earth's crust.
Collisional boundaries
• When two land masses meet, neither will slide under the
other.
• They crumple and fold. Some pieces of land are thrust
over or under other pieces. The result is a mountain
range.
Transform boundaries
• Two plates move against each other, building up
tension, then releasing the tension in a sudden and often
violent jerk.
• This sudden jerk creates an earthquake.
Hot Spots
• The Hawaiian volcanoes were formed by a hot spot that
remained fixed as the pacific plate moved to the north
west.
Island Arcs
• One plate sinks into a
trench. A continuous
series of earthquakes
is created. The plate
then starts to heat up
at a depth of about 75
miles. certain
magmas are melted
and rise toward the
surface, forming
volcanoes.
• If the upper plate is
oceanic, the
volcanoes pile up until
they poke through the
surface of the ocean
and form an elegant
arc.
Types of Eruptions
Principal Types of Volcanoes
• Cinder cone volcanoes
• Composite volcanoes
• Shield volcanoes
Cinder cones
• are the simplest type of
volcano
• are built from particles and
blobs of congealed lava
ejected from a single vent
• gas-charged lava is blown
violently into the air, it breaks
into small fragments that
solidify and fall as cinders
around the vent to form a
circular or oval cone.
• have a bowl-shaped crater at
the summit and rarely rise
more than a thousand feet or
so above their surroundings
Composite volcanoes
(stratovolcanoes)
• are typically steep-sided,
symmetrical cones of large
dimension built of alternating
layers of lava flows, volcanic
ash, cinders, blocks, and
bombs and may rise as much
as 8,000 feet above their
bases
• Most have a crater at the
summit which contains a
central vent or a clustered
group of vents
Shield volcanoes
• are built almost entirely of fluid
lava flows
• Lavas also commonly erupt
from vents along fractures (rift
zones) that develop on the
flanks of the cone
Strombolian-type
eruption
• huge clots of
molten lava burst
from the summit
crater to form
luminous arcs
through the sky
"Vulcanian"-type
eruption
• a dense cloud of ashladen gas explodes
from the crater and
rises high above the
peak
• Steaming ash forms a
whitish cloud near the
upper level of the
cone
"Hawaiian" eruptions
• may occur along
fissures or
fractures that
serve as linear
vents
• may occur at a
central vent
"Phreatic" (or steam-blast)
eruptions
• driven by explosive expanding
steam resulting from cold ground
or surface water coming into
contact with hot rock or magma
• they only blast out fragments of
pre-existing solid rock from the
volcanic conduit
• generally weak, but can be quite
violent in some cases
– (Mt St. Helens)
“Plinian" eruptians
• Most powerful
• Involve the explosive ejection of
relatively viscous lava
• Fast-moving deadly pyroclastic
flows are common
Largest eruptions
Power of a Volcano
• The following slides demonstrate the power of one of
the better known American volcanoes.
• Mount St. Helens erupted on May 18, 1980.
• 57 victims were claimed in the eruption.
• 4 victims were in the restricted zone.
• The remaining 53 victims were in areas believed to be
safe.
• Some victims were as far as 13 miles away.
Mount St. Helens - 1980
Before and After – Obscurity Lake
Before and After – marker 393
Krakatau
• Krakatau erupted in 1883, in one of the
largest eruptions in recent time
• The explosions were heard on Rodriguez
Island, 4653 km distant
• Ash fell on Singapore 840 km to the N,
Cocos (Keeling) Island 1155 km to the
SW, and ships as far as 6076 km WNW.
Darkness covered the Sunda Straits from
11 a.m. onthe 27th until dawn the next
day.
• Giant waves reached heights of 40 m
above sea level, devastating everything in
their path and hurling ashore coral blocks
weighing as much as 600 tons
• At least 36,417 people were killed, most
by the giant sea waves, and 165 coastal
villages were destroyed
• When the eruption ended only 1/3 of Krakatau,
formerly 5x9 km, remained above sea level, and
new islands of steaming pumice and ash lay to
the north where the sea had been 36 m deep.
Mount Tambora
• The 1815 eruption of
Tambora was the
largest eruption in
historic time.
• About 150 cubic kilometers of ash were erupted
(about 150 times more than the 1980 eruption of
Mount St. Helens).
• About 10,000 direct deaths were caused by
bomb impacts, tephra fall, and pyroclastic flows
• An estimated 82,000 were killed indirectly by the
eruption by starvation, disease, and hunger.
• Talcous dust was carried high into the
stratosphere where it began to float around
the earth.
• Due to this immense cloud of dust a part of the
incoming sunlight was bounced back into space.
“The Year Without a
Summer”
• The 1815 eruption of Tambora caused the
"Year without a Summer."
• Daily minimum temperatures were
abnormally low in the northern hemisphere
from late spring to early autumn.
• Famine was widespread because of crop
failures.
So, Are you feeling lucky?
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Quiz Index
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Plate Boundary Quiz
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Volcano Quiz
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Plate Boundary Quiz
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Convergent Boundaries
Divergent Boundaries
Collisional Boundaries
Transform Boundaries
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Volcano Quiz
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Largest Eruption in Prehistoric Time?
Largest Eruption in Modern Time?
Smallest volcano “type”?
Least explosive volcano “type”?
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Which is an example of a transform boundary?
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Coast of California
West Coast of South America
Himalaya Mountains
Iceland
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Which is an example of a divergent boundary?
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Coast of California
West Coast of South America
Himalaya Mountains
Iceland
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Which is an example of a convergent boundary?
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Coast of California
West Coast of South America
Himalaya Mountains
Iceland
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Which is an example of a collisional boundary?
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Coast of California
West Coast of South America
Himalaya Mountains
Iceland
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What was the largest known eruption in prehistoric
time?
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Krakatoa
Yellowstone
Mt. St. Helens
Vesuvius
Mt. Pinatubo
Tambora
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What was the largest eruption in modern (historic)
time?
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Krakatoa
Yellowstone
Mt. St. Helens
Vesuvius
Mt. Pinatubo
Tambora
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Which of the below is the smallest type of
volcano?
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Cinder cone
Shield
Strombolian
Composite
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Which of the below is the Least explosive Volcano
“Type”?
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Cinder cone
Shield
Strombolian
Composite
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