Download Volcanoes

Volcanoes
Earth’s Structure
Where Volcanoes Occur
• Volcanoes occur most frequently at plate
boundaries.
• Some volcanoes, like those that form the
Hawaiian Islands, occur in the interior of
plates at areas called hot spots.
• The greatest number of volcanoes occur
on the ocean floor along spreading ridges
spreading ridges.
• Over 80% of those on land occur at edges
of continents, or subduction zones, where
one plate dives, or subducts, under another
plate.
Why Volcanoes Occur
• Temperatures in the mantle are hot
enough to melt rock into magma.
• Less dense than the solid rock
around it, magma rises and some of
it collects in magma chambers.
• As the magma rises, pressure
decreases allowing trapped gases to
expand and propel the magma
through openings in the Earth’s
surface causing an eruption.
• •Erupted magma is called lava.
How Volcanoes Erupt
• Eruptions are described as explosive or
effusive (loosely flowing).
• How explosive an eruption is depends on
the magma’s chemical composition and
gas content, which affect the magma’s
stickiness, or viscosity.
• Magma’s low in silica are low in viscosity (mafic)
• Magma’s high in silica are high in viscosity
(felsic)
• •If magma is fluid, gases can escape
rapidly and lava flows; if magma is viscous
the gases can not escape and pressure
builds inside the magma until the gases
escape, sometimes violently.
Magma vs. Lava Magma
vs. Lava
• Magma is molten rock beneath
the surface.
• Lava is erupted magma. There
are 2 types of lava:
• A a (ah ah) is largely solidified
rock that gets pushed forward.
• – Pahoehoe (pahhoyhoy) is
flowing “liquid” with a ropy,
billowy surface.
Types of Lava Flows
• Lava flows are superheated streams
of molten rock that flow at 1 –50
mph.
• Pyroclastic flows are avalanches of
hot ash, rock fragments, and gases
that flow at speeds greater than 100
mph.
• Landslides are avalanches of rock,
snow and ice on slopes of volcanoes
(loosened and tumbling due to
seismic activity).
• Lahars(mud flows) are a mixture of
volcanic ash and water (like wet
concrete)
Volcano Terms
• A vent is an opening through
which eruptions take place.
• A crater is a basin like
depression over a vent, at the
summit of a volcano
• A caldera is a depression larger
than the original crater (>1km.
Diameter) that forms when the
summit is blown off, or when
the volcano collapses into the
Types of Volcanoes
• Repeated volcanic eruptions
build volcanic mountains of
three basic types three basic
types, or shapes, depending on
the composition of the
materials deposited by the
eruption.
• Shield volcanoes
• Stratovolcanoes
• Cinder cones
Shield Volcanoes
• Shield volcanoes are broad gently
sloping volcanic mountains slowly
formed by layer over layer of
solidified lava.
• Shield volcanoes are formed by
effusive eruptions of fluid lava.
• These can become very large as the
low viscosity lava spreads widely
and thickly.
• Examples:Kilauea, Hawaii and Mt.
Etna, Italy
Stratovolcanoes
(Composite)
• • Stratovolcanoes formed from both
explosive and effusive eruptions.
• •Layers of tephra alternating with
layers of viscous lava flows create
steep sided,often symetrical cones
that can be very large.
• •Formed over long spans of time as
periods of 100,000+ yrs. separate
periods of a few years of intense
activity.
• Examples: Aconcagua, Andes
(22,825’) and Mt. St. Helens
Cinder Cone Volcanoes
• • Cinder cones are the smallest
volcanoes (< 500’), formed by
explosive eruptions of formed by
explosive eruptions of lava.
• Blown violently into the air, the
erupting lava breaks apart into
fragments called cinders that fall
and accumulate around the vent.
• • Cinder cones are temporary
geologic features as they are easily
eroded. They have short life spans
as gas causing violent eruptions is
quickly depleted.
• Example: Paricutin, Mexico
Monitoring and Predicting
Eruptions
• Volcanic activity is monitored
using several observations:
• Land deformation: Rock structures bend
outward as gas build up.
• Gas Emissions: Higher release of Carbon
Dioxide and Sulfur Dioxide gases.
• Tremors- As magma moves under the
volcano and gas pressure builds, mild
earthquakes will occur in the area.
– Rock Fall: Due to movement of land, loose
rocks will begin to slide.