Download Earthquakes and Volcanoes

Volcanoes
• A volcano is an
opening in the Earth
that erupts gases, ash
and lava.
• Kilauea is the world’s
most active volcano.
Lava
• When the magma reaches the surface, it
is called lava. The place in the Earth’s
surface through which magma and other
materials reach the surface is called a
volcano. In some places, lava can build
up to form a cone-shaped mountain.
• The opening from which lava erupts is the
vent. Volcanoes often have more than
one vent.
Formation of a Volcano
• The opening where
lava flows out is a
vent.
• The steep walled
depression around
the vent is a crater.
Volcanoes
• Volcanoes can be both destructive and
constructive forces.
Destructive Effects of Eruptions
• Lava destroys everything in its path. Ash
can cause lung damage.
• If ash and debris run down the side of the
mountain, this is called a pyroclastic flow.
Temperatures in this flow can be hot
enough to ignite wood.
• Sulfurous gases can mix with water vapor
and form acid rain that kills plants and
animals.
Constructive Effects of
Eruptions
• The soil formed from volcanoes is very
fertile and will eventually lead to abundant
plant growth.
• The sulfur in volcanic vents along the
ocean floor allow life to exist without
sunlight
• Volcanoes recycle earth’s minerals
Where do Volcanoes occur?
• Volcanoes form along
divergent plate
boundaries,
convergent
boundaries and hot
spots.
Divergent Plates
• Volcanoes often form
where plates are
moving apart, such as
the Surtsey in
Iceland.
• Iceland is located on
the Mid Atlantic
Ridge.
Convergent Plate Boundaries
• Soufriere Hills On the
Island of Monserrat is
an example of a
volcano that formed
when one plate slide
under another and
melted.
• The magma was
forced up and formed
a volcano.
Hot Spots
• Some volcanoes form
over hot spots in the
mantle.
• These are not on a
plate boundary but in
the center of a plate.
• The Hawaiian Islands
formed over a hot
spot in the Pacific
Plate.
Magma
Deep within the Earth, under tremendous
pressure and at great temperatures, rock
exists as a hot liquid called magma. This
molten rock is found in pockets called
magma chambers.
What Controls Eruptions?
• Eruptions are controlled by
• 1) the amount of trapped gas and the
water vapor
• 2) how much silica is present in the
magma
Trapped Gas
As magma reaches the surface, it is under
less pressure. Gas can escape. Gas
escapes easily during quiet eruptions.
• If gas build up to high pressure this will
cause an explosive eruption.
Amount of Water Vapor
• The magma at some convergent
boundaries contains a lot of water vapor.
• This is because oceanic plate material and
some of its water slide under other plate
material at some convergent plate
boundaries.
• The trapped water vapor in the magma
can cause explosive eruptions.
Viscosity of Magma
• Magma is divided into
two major types: silica
rich and silica poor
• The greater the silica
content, the more
viscous or thick the
magma.
Basaltic Magma
• Magma that is low in silica is called
basaltic magma. It is fluid and produces
quiet flows such as those at Kiluea.
• Basaltic lava flows under water produce
pillow lava formations.
Basaltic Lava
• Pahoehoe forms a
ropelike structure.
• If it flows at a lower
temperature it is stiff
and slow moving and
called aa.
Granitic Magma
• The second type of lava is light in color.
This lava, has high water content and high
silica content. Light-colored lava causes
explosive eruptions. Silica hardens in the
vents and form rocks. Steam and new
lava build up under the rocks. When the
pressure becomes great, a violent
explosion occurs. When this type of lava
cools on the surface it forms the igneous
rock, rhyolite, which resembles granite.
Andesitic Magma
• Andesitic magma is more rich in silica than
basaltic magma but it has less silica than
granitic magma.
• It forms at convergent boundaries where
one plate slides under the other.
• It is more explosive than basaltic magma.
• Krakatau was mainly andesitic in
composition. The word andesitic comes
from the Andes Mountains.
Gaseous Lava
• When lava that contains large amounts of
gases such as steam and carbon dioxide
hardens, it forms rocks with many holes in
them, due to the gas bubbles.
• Pumice and scoria are igneous rocks
formed from this type of lava.
Types of Volcanoes
• Different types of volcanic eruptions form
different types of volcanoes.
• Cinder cones
• Shield volcanoes
• Composite volcanoes
Cinder Cones
• Volcanoes made mostly of cinders and
other rock particles that have been blown
into the air are called cinder cones.
• Cinder cones form from explosive
eruptions. Because the material is loosely
arranged, the cones are not high. The
have a narrow base and steep sides such
as Paricutin in Mexico.
Tephra
• Explosive eruptions throw lava and rock
high into the air. Bits of rock or solidified
lava dropped from the air are called
tephra.
• The smallest particles are called volcanic
dust. (less than 0.25 mm)
• Volcanic Ash ( 0.25 -5mm) falls to the
Earth and forms small rocks.
• Volcanic bombs (a few cm to several
meters) are molten and harden as they
travel through the air.
Cinder Cone Volcano
Shield Volcanoes
• Volcanoes composed of quiet flows are
called shield volcanoes. Because it is
rummy, the lava flows over a large area.
After several eruptions, a dome-shaped
mountain is formed such as Mauna Loa
(4km over sea level) in the Hawaiian
Islands.
Shield Volcanoes
Flood Basalts
• Basaltic lava can also
flow onto Earth’s
surface through large
cracks called fissures.
• This type of eruption
forms flood basalts
• Columbia Plateau
was formed as the
lava spread out over
the ground.
Composite Volcanoes
• Volcanoes built up of alternating layers of
rock particles and lava are called
composite volcanoes. During the
formation of a composite volcano, a violent
eruption first occurs, hurling volcanic
bombs, cinder and ash out of the vent.
Then a quiet eruption, produces lava flow
that covers the rock particles. After
alternating eruptions, a cone-shaped
mountain forms such as Mount Vesuvius,
Mount Rainier or Sofriere HIlls.
Mount Vesuvius
Mount Vesuvius
• Images of victims in
eruption of Vesuvius
in 79 AD. Most died
as a result of
suffocation.
Composite Volcano
Krakatau
• One of the most
violent eruptions in
recent times occurred
on an island in the
Sunda Straits neat
Indonesia in 1883.
• 36,000 people were
killed.
• The island fell into the
magma chamber.
Crater
• There is often a funnel-shaped pit or
depression at the top of a volcanic cone.
This pit is called a crater. If the crater
becomes very large as a result of the
collapse of its walls, it is called a caldera.
A caldera may also form when the top of a
volcano explodes or collapses.
Volcanic Crater
Intrusive Igneous Rock
Features
• Most magma never reaches the surface.
• Much of it hardens in the earth.
• Sometimes the hardened magma
becomes exposed at the surface.
• These rock bodies are called intrusive
igneous rock features.
• Examples are batholiths, sill, dikes and
volcanic necks.
Batholiths
• The largest intrusive
igneous rock bodies
are called batholiths.
‘They form when
magma bodies that
are being forced
upward cool and
solidify before
reaching the surface.
Sills and Dikes
• Magma sometimes
squeezes into cracks
below the surface.
• Magma that cuts
across rock layers
and hardens is called
a dike.
• Magma that is forced
into a crack parallel to
the rock layers and
hardens is a sill.
Volcanic Neck
• When a volcano stops
erupting magma will
harden inside the
vent. Erosion wears
away the outside of
the volcano and the
solid igneous rock in
the vent remains.
• Ship Rock in New
Mexico was formed
this way.
Calderas
• Sometimes after an
eruption, the top of a
volcano can collapse.
This produces a large
depression called a
caldera.
• Crater Lake in
Oregon is a caldera
that filled with water
and is now a lake.
Volcanic Activity
• Volcanoes are rather unpredictable .
Some erupt regularly, others have not
erupted in modern history. Scientists
classify them as active, dormant or extinct.
Active Volcanoes
• An active volcano is one that erupts wither
continually or periodically such as Mount
Katmai in Alaska and Mount St. Helens in
the Cascade Range.
Dormant Volcano
• A volcano that has been known to erupt
within modern times but is now inactive is
classified as a dormant volcano. Mount
Rainier in Washington state are example
of dormant volcanoes in the United States.
Extinct Volcano
• A volcano not known to have erupted
within modern history is classified as an
extinct volcano. They have been worn
away almost to the level of their magma
chamber. Scientists can be wrong. Mount
St. Helens was considered to be dormant
but erupted after long periods of inactivity.
Volcano and Earthquake Zones
• Most major earthquakes and volcanic
eruptions occur in three zones of the
world. Scientists believe that there is a
great deal of movement and activity in the
Earth’s crust in these three zones.
Ring of Fire
• One major earthquake and volcano zone
extends nearly all the way around the
edge of the Pacific Ocean. This zone
goes through New Zealand, the
Philippines, Japan, Alaska and along the
western coasts of North and South
America. The San Andreas fault is part of
this zone.
Ring of Fire
Mediterranean Zone
• A second Major earthquake and volcano
zone is located near the Mediterranean
Zone and extends across Asia into India.
Many countries in the zone, including Italy,
Greece and Turkey, have violent
earthquakes. Many volcanic eruptions
also occur in this zone.
Mid-Atlantic Ridge Zone
• The third major earthquake and volcano
zone extends through Iceland and to the
middle of the Atlantic Ocean. There is
under the ocean a long range of volcanic
mountains called the Mid-Atlantic Ocean
Range. Scientists believe that the volcano
and earthquake activity are due to the
formation of new parts of the Earth’s crust
along the ridge. The volcanic island of
Iceland is part of this zone.
Earthquake Belts