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VOLCANOES
MICHELE HOLLER
MAIDEN HIGH SCHOOL
MOUNT SAINT HELENS
• Erupted May 18, 1980 to
create the largest
volcanic eruption in North
America
• The blast blew the entire
north flank of the volcano
leaving a gaping hole
• It ejected nearly a cubic
kilometer of ash and rock
debris
FACTORS AFFECTING
ERUPTIONS
• Magma composition, magma
temperature and the amount of
dissolved gases determine
whether an eruption is violent
or quiet
VISCOSITY AND MAGMA’S
TEMPERATURE
• A substance’s resistance to
flow. (viscosity)
• The cooler magma is, the more
its viscosity increases. The
warmer the easier it flows
• Is directly related to the
composition. The more silica
that is found in the magma, the
more viscosity that magma has
DISSOLVED GASES
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During explosive eruptions, the
gases trapped in the magma
provide the force to eject molten
rock from the vent, which is an
opening to the surface
Most gases are water vapor and
carbon dioxide
The reduced pressure near the
surface allows the dissolved
gases to be released suddenly
Basaltic magmas allow the gases
to escape easily and have
relatively quiet eruptions
Highly viscous magmas, slow the
upward movement and the gas
bubbles increase in size until they
eject molten rock from the volcano
VOLCANIC MATERIAL
Pahoehoe flow
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Evidence of a a
flow
LAVA FLOWS
basaltic lavas are very fluid. When basaltic lava hardens, it forms a
relatively smooth skin as the still-molten subsurface lava continues to move,
this is called pahoehoe. It resembles twisted braids of rope.
Basaltic lava can also have a a flow. It has a rough jagged surface with
dangerously sharp spiny edges
GASES
• The gaseous portion is only about 1-6% of the total weight
• Composition is important. They have contributed greatly to the
gases that make up the atmosphere
• Hawaiian eruptions have a gas makeup of 70% water vapor, 15%
carbon dioxide, 5% sulfur, 5% nitrogen and lesser amount of
chlorine, argon and hydrogen
Water vapor
Carbon
dioxide
PYROCLASTIC MATERIALS
• Name given to particles
produced by eruption. They
range in size from very fine
dust to pieces that weigh
several tons
• Lapilli- range from small beads
to walnuts (2-64 mm). Also
called cinders
• Blocks- anything larger than 64
mm and are made of hardened
lava and bombs (which are
pieces of semi-molten lava
hurled in the air. Form
elongated rocks)
ANATOMY OF A VOLCANO
• Volcanic activity begins
when there is a crack or
fissure that develops in
the crust
• The gas rich magma
moves up the fissure
through a circular pipe
that ends at the vent
• Repeated eruptions
eventually build a volcano
• Located at the summit of
the volcano is a steep
walled depression called
SHIELD VOLCANOES
• Produced by the
accumulation of fluid
basaltic magma
• Have the shape of a
broad, slightly domed
structure
• Most have grown up from
the ocean floor to form
islands
• Examples include the
Hawaiian Islands and
Iceland
CINDER CONE VOLCANO
• Formed from ejected lava
fragments the size of cinders
• Product of relatively gas-rich
basaltic magma
• Have steep sided slopes that
are maintained by loose
pyroclastic materials as they
come to rest
• After the eruption, which can
last days or weeks, the pipe
solidifies
• Have a short life span, so they
are typically small
• An example is: Mount Etna
COMPOSITE VOLCANO
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Most beautiful and potentially
dangerous volcanoes
Most located in the Pacific Ring of
Fire which includes the Andes and
the Cascades (Mount St Helens,
Mount Rainier)
Stretches from the Aleutian
Islands to Japan
Nearly symmetrical with layers of
both lava and pyroclastic materials
Magma has andesitic composition
Have a high silica content and
very explosive eruptions
Composite volcano in New Zealand
DANGERS FROM COMPOSITE
VOLCANOES
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Pyroclastic flows are the most
dangerous. They consist of hot
gases, glowing ash, and larger
rock fragments
Pyroclastic flows can race down at
speeds of 200km per hour
Can blast sides of the mountain
out
Can also generate mudflows that
are called lahars. These occur
when the volcanic debris becomes
saturated with water and rapidly
moves down slope
lahars can be triggered by heavy
rain
OTHER VOLCANIC LANDFORMS
CALDERAS
• Large depression in a
volcano
• Form:
1. After a collapse of the
top of a composite
volcano after an
explosive eruption
2. Top of a shield volcano
collapses
• Example is Crater Lake in
Oregon
OTHER VOLCANIC LANDFORMS
CONTINUED
NECKS AND PIPES
• Most volcanoes are fed
through conduits called
pipes
• Cinder cone volcanoes
erode easily and leave
behind the hardened pipe
that is now called a
volcanic neck
• Best known pipes are the
diamond bearing pipes in
South Africa
OTHER VOLCANIC LANDFORMS
CONT.
LAVA PLATEAUS
• Greatest volume of
volcanic material is
extruded from fissures
• These extrusions cover a
large area and form
plateaus
• Example is the Columbia
Plateau
PLUTONS
• The structures that result
from the cooling and
hardening of magma
• They all form beneath the
surface and can only be
studied after uplift and
erosion
• Are generally classified
according to their size,
shape and relationship to
the surrounding rock
layers
SILLS AND LACCOLITHS
• Form when magma
intrudes close to the
surface
SILL
• Formed when magma is
injected along
sedimentary bedding
surfaces, parallel to the
bedding planes
• Sills only form at shallow
depths where pressure is
low
sill
laccoliths
• Occur close to the
surface
• Magma that
generates laccoliths is
more viscous
• Collects as a lens
shaped mass that
pushes the overlying
strata upward
DIKES
• Formed when magma is
injected into fractures that
cut across preexisting
rock
• Are sheet like structures
that are a few meters
thick and extend a few
kilometers
• Form when magma from
a magma chamber
invades the surrounding
rock
BATHOLITHS
• Largest intrusive
igneous body
• Must have a surface
exposure of greater
than 110 square
kilometers
• Smaller plutons called
stocks are parts of a
batholith
Yosemite National
Park
ORIGIN OF MAGMA
• Originates when essentially
solid rock in the crust and
upper mantle partially melts.
• Heat plays a major role
• The change in temperature
with depth is called geothermal
gradient
• Heat can be generated at
subduction zones with the
friction that is generated
• Crustal rocks are heated
during subduction
ORIGIN OF MAGMA CONT.
• Pressure also plays a role
• Pressure also increases with
depth
• Melting occurs at higher
temperatures because of the
greater confining pressure
• Reducing the confining
pressure reduces the rock’s
melting point. This is called
decompression melting
• Decompression melting has
generated magma beneath
Hawaii
ORIGIN OF MAGMA CONT.
• THE ROLE OF WATER
• Water causes rocks to
melt at lower
temperatures
• Wet rock has a much
lower melting point than
dry rock
CONVERGENT PLATE
BOUNDARIES
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Plate tectonics and volcanism
have one connection and that is
plate motion
At convergent boundaries,
oceanic crust is pushed into the
mantle
The magma that is formed, slowly
migrates upward forming
volcanoes like Mount Saint Helens
Ocean-ocean-have a chain of
volcanoes on the floor called
volcanic island arcs
Ocean-continental-produces a
continental volcanic arc. Magma
changed composition due to the
high silica content of the
continental crust
DIVERGENT PLATE
BOUNDARIES
• Most magma is produced
along the oceanic ridges
• As rock rises the
confining pressure
decreases and
decompression melting
occurs
• The new magma is less
dense and rises to the
surface forming rift
valleys
INTRAPLATE IGNEOUS ACTIVITY
• Intraplate volcanism occurs
within a plate
• Occurs where a mass of hotter
than normal mantle material
called a mantle plume, rises to
the surface and creates hot
spots
• Causes decompression
melting of basaltic magma
• Mantle plumes also have
created lava plateaus