Volcanoes
ORIGINAL SOURCE:
SITES.TENAFLY.K12.NJ.US/~PLOBOSCO/
Volcanoes and Earth’s Moving Plates
 Objectives:
 To describe how volcanoes can affect people.
 To discuss the conditions that cause volcanoes.
 Identify the relationship between volcanoes and
Earth’s moving plates.
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.
Types of Volcanoes
 Objectives:
 To discuss how explosiveness of a volcano is related
to the silica and water content.
 To discuss the three types of volcanoes.
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 (p-hoi-hoi)
forms a smooth, billowy,
ropelike surface.
 Aa, flows at a lower
temperature, is stiff and
slow moving.
https://encryptedtbn1.gstatic.com/images?q=tbn:ANd9GcQxYSoNsSaKu47pJNuJkI
E49xVUxFS-fXrKwh50nQnXPV0OrUB7
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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.
https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSlJYtYiNswbpjWkiOrzUzpySSw-UEbaw4L9RyuDZeFbCokLFXEA
3 Types of Volcanoes
Different types of volcanic eruptions form different
types of volcanoes.
 Cinder cones
 Shield volcanoes
 Composite volcanoes
Cinder Cones
 They are steep sided cones of basaltic fragments and
are smaller and simpler than composite volcanoes.
 Streaming gases carry liquid lava blobs into the
atmosphere that fall back to earth around a single
vent to form the cone.
 The volcano forms when ash, cinders pile up around
the vent to form
a circular or oval cone.
 Most common kind of volcano
http://www.k12.hi.us/~kapunaha/student_projects/volc_blowout/cinder_cone_volcano.htm
https://encryptedtbn3.gstatic.com/images?q=tbn:ANd9GcSeCsTPToq3GDWqNxDijDfx1gIDwDm1RpArCYsmE17
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Tephra
 Explosive eruptions throw lava and rock high into the air.
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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.
Cinder (5mm-a few cm)
Volcanic bombs (a few cm to several meters) are molten
and harden as they travel through the air.
Paricutin Volcano in Mexico
http://4.bp.blogspot.com/-x6k8MObhuIg/T0vDJmMSxI/AAAAAAAAGv8/myMqukEGkwk/s1600/paricutin.jpg
Shield Volcanoes
 Volcanoes composed of quiet flows are called shield
volcanoes.
 Because it is runny, 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.
Mauna Loa, Hawaii – Earth’s Largest Volcano
http://hvo.wr.usgs.gov/maunaloa/4303062_M.jpg
http://hvo.wr.usgs.gov/maunaloa/
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 (Stratovolcano)
 Most deadly
 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.
 Lower slope is gentle, but they rise steeply toward
the summit
Composite Volcano (Stratovolcano)
Mount Vesuvius (near Naples, Italy)
http://www.history.com/topics/pompeii/videos
Mount St Helens
http://dsc.discovery.com/tvshows/discoverypresents/videos/understandingvolcanoes-mt-saint-helens.htm
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.
 http://www.history.com/s
hows/how-the-earth-wasmade/videos/the-krakatoavolcano#the-krakatoavolcano
Igneous Rock Features
 Objectives
 To describe intrusive rock features.
 To explain how a volcanic neck and a caldera form.
Predicting Volcanoes
There are four indicators of volcanic activity:
 Rising magma (increase in electric current)
 Seismic activity (tremors)
 Ground deformation
 Changes in gases (carbon dioxide and sulfur dioxide)
Instruments used to Detect Volcanic Activity
 Tiltmeters are sensors that use laser beams to help create
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map of the physical changes in the earth’s surface that
rising magma causes.
Scientists also measure the levels of gases escaping, such as
carbon dioxide and sulfur dioxide.
Gravimeters measure the electrical currents given off by
magma. An increase indicates a rise in the level of magma
The most recent development uses satellites to detect heat
from the gases released by volcanoes.
The satellite Landsat uses infrared sensors.
Limits of Seismography in
Detecting Volcanic Activity
 Seismographs cannot determine magma height,
depth or how fast the magma is rising.
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.
Yosemite National Park
 The granite
domes of
Yosemite
National Park are
exposed
batholiths.
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 whether
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 is an example of a dormant volcano in the
United States.
 Scientists can be wrong. Mount St. Helens was
considered to be dormant but erupted after long
periods of inactivity.
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.
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.
 Under the ocean, there is 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
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