Volcanoes!
What is a Volcano?
Volcano: a mountain that
forms when layers of lava
& volcanic ash erupt &
build up.
Active Volcanoes spew
smoke, steam, ash,
cinders, & lava
(Tephra: rock fragments
thrown into air during a
volcanic eruption)
Currently about 550 active
volcanoes; most are
dormant on Earth
How do volcanoes form?
Deep inside Earth,
between the molten iron
core and the thin crust at
the surface, there is a solid
body of rock called the
mantle. When rock from
the mantle melts, moves to
the surface through the
crust, and releases pent-up
gases, volcanoes erupt.
But why does this solid
rock melt and come to the
surface?
From Magma to Lava
Magma being less dense than the
surrounding rock causes it to rise &
sometimes escape though areas in Earth’s
plates called vents(the opening where
magma reaches earth’s surface)
Magma that has reached the surface is
called lava.
What is Magma?
Magma: a mixture of molten rocks, minerals, &
gases.
Factors that affect magma formation:
1. Rock composition: different minerals melt at
different temps
2. Pressure: affects temp needed to melt rocks
3. Water Content: affects melting temp
4. Gas Content: more gas, more explosive eruptions
5. Silica Content: more silica, higher viscosity
6. Viscosity: the internal resistance of a substance to
flow
High silica content: high viscosity
Low silica content: low viscosity
Lava Types
Magma comes in different
"flavors," or compositions.
Each of these will produce
a different lava, from
fluid, fast-moving basalt
to slower, more viscous
andesite. Because rocks
are made up of collections
of minerals that melt at
different temperatures, the
makeup of the rock being
melted affects the magma
that results.
http://www.learner.org/exhib
its/volcanoes/movies/movies
3.html
Lava Types: Magma that reaches Earth's surface comes in
three essential compositions that produce the three common volcanic rock
types: basalt, andesite, and rhyolite.
Basaltic magma, the most common form of
magma, results from the partial melting of rock in
the zone of Earth's mantle called the
asthenosphere. Basalt has the lowest silica content
of the three common volcanic rocks, which makes
it the least viscous. It also has the lowest gas
content.
Fastest flowing lava (up to 16 km/hr)
Quiet eruptions
Usually formed from melted oceanic plate
Ex: Hawaiian Islands
Types of Basaltic Lava
Pahoehoe flow
Ropy pahoehoe
A'a flow
Lava Types: Magma that reaches Earth's surface comes in
three essential compositions that produce the three common volcanic rock
types: basalt, andesite, and rhyolite.
Andesite can arise in a variety of ways, but most is
produced by the partial melting of wet basalt. This
often occurs at colliding plate margins where
oceanic crust formed of basaltic magma is
subducted beneath another plate. Andesitic magma
can also be generated by the melting of continental
crust. Compared to basaltic magmas, andesitic
magmas are normally more viscous and contain
more dissolved gases.
Usually formed from interaction of oceanic plate +
continental plate
Ex: Mt. Saint Helens
Types of Andesitic Lava
Andesite flows on Lascar Volcano -Located in Chile, the Lascar volcano is
the most active stratovolcano in the
central Andes. Displaying here is a
large blocky lava flow.
Stubby Andesite Flow -- Thick,
short andesite flow on the flanks
of the Colima Volcano, Mexico.
Courtesy of J.C. Gavilanes,
Universidad de Colima.
Lava Types: Magma that reaches Earth's surface comes in
three essential compositions that produce the three common volcanic rock
types: basalt, andesite, and rhyolite.
Rhyolitic lava is rich in silica and therefore quite
viscous. It can arise from the partial melting of
continental crust. Because of its viscosity, which
results in the magma solidifying before it can
reach the surface, rhyolitic lava is not common.
Involved in explosive eruptions due to high viscosity &
gas content (think of shaking up a soda can & opening
it)
Ex: Yellowstone National Park
Types of Rhyolitic Lava
Obsidian flow -- The Long Valley Caldera, California, was created by crustal
collapse associated with an explosive eruption about 650,000 years ago.
Since that time, felsic eruptions of de-gassed magma have generated
viscous rhyolitic domes and short felsic flows. The example shown in the
foreground is Obsidian flow, which erupted about 600 years ago. The flow is
about 1.5 km wide across the photo. Wilson Butte, left of Obsidian flow, is a
felsic lava dome that erupted about 1400 years ago. Courtesy of S.R.
Bradley, USGS.
Why do volcanoes erupt in
different ways?
California's Mt. Shasta
Most volcanoes occur on
plate boundaries. Plate
boundaries are areas
where Earth's shifting
plates meet or split apart,
usually with violent
results.
Volcanoes can occur on
convergent or divergent
plate margins or over a
hotspot, a spot inside the
mantle that heats areas of
the plate above it.; hotter
than normal.
Volcanoes at Convergent
Boundaries
Along convergent margins, two
plates meet and one descends
beneath the other, a process
called subduction. As the
descending plate is forced
deeper into the mantle, parts of
it begin to melt and form
magma that rises to the surface,
often in explosive eruptions.
Convergent margins tend to
create large, classic, coneshaped volcanoes called
stratovolcanoes, such as Mt. St.
Helens in Washington State.
80% of all volcanoes
Ex: Ring of Fire
Mt. St. Helens before the 1980 eruption -- Mt.
St. Helens was considered by many to be one of
the most picturesque of the Cascade volcanoes.
This view of the northeastern flank of the
volcano is taken from Spirit Lake. Courtesy of
Jim Nieland, U.S. Forest Service
Mt. Saint Helens after eruption
Convergent Boundary Volcanoes
Convergent Boundary Volcanoes
The most volcanically active belt on Earth is known as the Ring of
Fire, a region of subduction zone volcanism surrounding the Pacific
Ocean.
Volcanoes at Divergent
Boundaries
Black smoker from the mid-Atlantic Ridge -- These
dark, billowing clouds of hot hydrothermal fluid are rich in
dissolved metals. As they rise into the cool ocean water,
they precipitate these metals along the sides of the vent,
thus creating a "chimney" that builds over time. The
fluids also provide nutrients to a variety of unusual plants
and animals that congregate around the vents in an area
of the ocean where life would generally not exist.
Courtesy of Peter Rona, NOAA.
At divergent margins, plates are
coming apart and hot rock
forces its way to the surface.
Many divergent plate margins
are under the oceans, creating
long undersea rift zones that fill
with lava. In some eruptions at
divergent margins, the
relatively calm, smooth flow of
lava creates volcanoes with
gently sloping sides, called
shield volcanoes.
Usually balsaltic lava flows so
quiet eruptions
Accounts for 15% of all
volcanoes
Ex: Mid Atlantic Ridge, Iceland
Divergent Boundary Volcanoes
Volcanoes at Hot Spots
Hotspots can also cause
shield volcanoes to form.
As plates move over
hotspots, volcanoes spring
up and die down in turn,
often creating an island
chain. The Hawaiian
Islands are the result of a
hotspot.
5% of all volcanoes
Provide important info
about plate movement
(plate moves while hot spot
is stationary)
http://www.learner.org/exhibit
s/volcanoes/movies/movies2.
html
Hot Spot Volcanoes
Types of Volcanoes
Although every volcano has a unique
eruptive history, most can be grouped
into three main types based largely on
their eruptive patterns and their general
forms.
Composite Volcanoes
Eruptions vary between quiet & explosive
Made from alternating layers (quiet & explosive)
Usually contain Andesitic Lava
Found mostly at convergent boundaries, above subduction zones
Ex: Mt Saint Helens
Cinder Cone Volcanoes
Formed from explosive eruptions that throw lava high into the air. This lava cools
quickly & forms different sizes of volcanic material collectively called tephra
(volcanic ash, cinders, “bomb” rocks,…)
Usually contain Rhyolitic Lava
Steep sides
Smallest overall size
Ex: Mount Shasta, Mt. Pele
Shield Volcanoes
Formed from quiet eruptions
Gently sloping sides with nearly circular bases
Usually contains Basaltic lava
Generally the largest
Ex: Hawaiian Islands (Mauna Loa is the world’s largest active volcano) & Iceland
Pyroclastic Flows!
What are Pyroclastic Flows?
http://volcanoes.usgs.gov/Mo
vs/anim_pf.mov
Pyroclastic flows are high-density
mixtures of hot, dry rock fragments
and hot gases that move away from
the vent that erupted them at high
speeds. They may result from the
explosive eruption of molten or
solid rock fragments, or both. They
may also result from the
nonexplosive eruption of lava when
parts of dome or a thick lava flow
collapses down a steep slope. Most
pyroclastic flows consist of two
parts: a basal flow of coarse
fragments that moves along the
ground, and a turbulent cloud of
ash that rises above the basal flow.
Ash may fall from this cloud over a
wide area downwind from the
pyroclastic flow.
What are Pyroclastic Flows?
Pyroclastic Flow: Highspeed avalanches of hot ash,
rock fragments, and gas
move down the sides of a
volcano during explosive
eruptions or when the steep
edge of a dome breaks apart
and collapses. These
pyroclastic flows, which
can reach 1500 degrees F
and move at 100-150 miles
per hour, are capable of
knocking down and burning
everything in their paths.
Remnant of a building in Francisco Leon that
was destroyed by pyroclastic flows and surges
during the eruption of El Chichon volcano in
southeastern Mexico between March 29 and
April 4, 1982. Francisco Leon was located
about 5 km SSE of the volcano. The
reinforcement rods in the concrete wall are
bent in the direction of flow (right to left).
The Inside of a Volcano!
Structure of a Volcano
A volcano constitutes a vent, a pipe, a
crater, and a cone.
The vent is an opening at the
Earth's surface.
The pipe/conduit is the main
passageway in the volcano in
which the magma rises through to
the surface during an eruption.
The crater is a bowl-shaped
depression at the top of the
volcano where volcanic materials
like, ash, lava, and other
pyroclastic materials are released.
A side vent is a vent on the side of
the volcano connected to the main
vent by a dike or fissure.
Solidified lava, ashes, and cinder form
the cone. Layers of lava, alternate with
layers of ash to build the steep sided
cone higher and higher.
Nice Side Vent!
This photograph is of a volcanic cone. The crater is
located at the top. The side vent is active and a lava
flow is running down the side of the cone. A fissure
is bringing the magma to the side vent.
Inside a Crater!
You are looking at the inside of a volcanic crater. The steep walls were produced be
many eruptions ejecting very liquid lava. This lava then lands on the crater walls
building them higher and higher. The lava in the main vent is extremely hot (probalbly
about 1800 degrees F.) The lava on top cools and hardens because the air that it is in
contact with is so much cooler than the lava. This hardened lava will then be dragged
back down under the surface and remelted. You probably noticed the same process if
you have ever heated soup on the stove. If you did not keep stirring the soup it formed
a "scum" on top.
Holy Caldera!
A large volcano crater is called a caldera. Caldera’s
are formed when the volcano “blows its top.”
Crater Lakes
Many craters eventually fill in with water and become
crater lakes, like this one; Crater Lake, Oregon.