Chapter 3: Volcanoes
Chapter 3.1
• Key Concepts:
–Where are Earth’s volcanoes
found?
–How do hot spot volcanoes
form?
Definitions:
Volcano: a weak spot in the Earth’s crust
where molten material (magma) comes to the
surface
 Magma: a molten mixture of rock forming
substances, gases and water from the earth’s
mantle.
Lava: when magma reaches the Earth’s
surface.
Volcanoes & Plate Boundaries
• 600 active volcanoes on land, many more
under the sea.
• Volcanoes occur in belts that extend
across continents and oceans.
• Ring of Fire - major volcanic belt formed
by the many volcanoes that rim the
Pacific Ocean.
Volcanic belts form along the
boundaries of Earth’s plates
• Volcanic belts form along the
boundaries of the Earth’s plates
• Diverging ( )and converging ()
occur.
• Results in fractures in the Earth’s crust
• Magma reaches the surface.
Most volcanoes occur at
• Divergent Boundaries
– Mid Ocean Ridge
• Convergent boundary
– Points of
subduction
ex. Mount Etna –
Eurasian and
African plates
Volcanoes on Divergent
Boundaries
• Under ocean: point of Mid ocean ridges
occurring ( long underwater mountain
ranges)
• Many have rift valleys down the center
• Lava pours out of rift valley – creates new
mountains.
• Land - Great Rift Valley in East Africa
Volcanoes on Convergent
Boundaries
• Volcanoes form when:
– Two oceanic plates collide
– Oceanic and continental plates collide
• In both situations, oceanic plates sink
beneath a trench.
• Rock above the plate melts to form
magma.
• Erupts to the surface as lava
Volcanoes on Convergent
Boundaries
Volcanoes on Convergent
Boundaries
• Magma – less dense than the surrounding
rock
– Magma rises to the surface and breaks
through the ocean floor creating volcanoes.
– String of islands formed: Island arc
– Examples: Japan, New Zealand, Indonesia,
the Philippines
Hot Spot Volcanoes
• Hot Spot – an area where material from
deep within the mantle rises and melts
forming magma
• Volcanoes form above a hot spot when
magma erupts through the crust and
reaches the surface.
Hot Spot Volcanoes
• Some hot spots lie in the middle of plates
far from plate boundaries.
• Others occur on or near plate boundaries.
Hot Spot Volcanoes
• A hot spot in the ocean floor can gradually
form a series of volcanic mountains
• Ex. The Hawaiian islands
Hot Spot Volcanoes
• A hot spot can also form under continents
• Ex. Yellowstone National Park, WY
3.2 PROPERTIES OF MAGMA
Vocabulary:
•
Element-a substance that cannot be broken
down into other substances
–
–
–
Compound-a substance made of two or more
elements that have been chemically combined
Physical Property-a characteristic of a substance
that can be observed or measured without
changing the composition of a substance
Chemical Property-any property that produces a
change in the composition of matter.
– Pahoehoe-fast moving, hot lava that has low
viscosity
– aa-higher viscosity, slower moving, cooler
lava that Pahoehoe
– Viscosity-the resistance of a liquid to flow
– Silica—major ingredient in magma made of
silicon and oxygen.
• Magma Composition
• Magma varies in composition and is
classified according to the amount of silica
it contains.
• The graphs show the average composition
of the two types of magma. Use the
graphs to answer the questions.
• What materials make up both types of magma?
• Silica, oxides, and other solids
• Which type of magma has more silica? About how much
silica does this type of magma contain? Rhyolite, 70%
• A third type of magma has a silica content that is halfway
between that of the other two types. About how much
silica does this type of magma contain? 60%
• What type of magma would have a higher viscosity?
Explain. The rhyolite forming would have higher viscosity
because it contains more silica
Volcanic Eruptions 3.3
• Objectives:
– Explain what happens when a volcano erupts.
– Describe 2 types of volcanic eruptions
– Identify stages of volcanic activity
• What comes out of volcanic explosions?
– Ash, lava, steam, rock, gases, dirt
Volcanic Eruptions 3.3
• Magma forms in the asthenosphere
– Convection currents in the mantle bring
magma towards the surface
• Magma tries to flow into any open crack
• When magma reaches a weak spot in the
crust, a volcano forms
• Lava is magma that has reached the
surface
Magma Reaches Earth’s Surface
• Volcanoes are systems of passage ways
through which magma moves.
• Inside a Volcano
– Magma collects below the volcano in the
magma chamber
– Magma flows upward through a tube (pipe)
that connects the magma chamber to the
surface.
Magma Reaches Earth’s Surface
• Inside a Volcano
– Gas and magma leave through vents
• Central vent is the opening at the top
• Side vent anywhere along the side of the volcano
– Lava pours out of the vents and creates a
lava flow
– A bowl shaped area called a crater may form
around the central vent
Magma Reaches Earth’s Surface
• A Volcanic Eruption
– Dissolved gases are trapped
in magma under extreme
pressure (CO2 in a soda bottle)
– as magma flows
, pressure
because
there is less rock on top of it.
– the dissolved gases expand and form bubbles
– When a volcano erupts, the force of the
expanding gas forces magma up the pipe until
it explodes out the vent
Content checkpoint… think/pair share…take
two minutes to answer these questions with a partner nearby…..
• What common everyday occurrence
can we relate to a volcanic eruption?
• What happens to the pressure in the
magma as it rises toward the surface?
Kinds of Volcanic Eruptions
• Volcanic eruptions can be quiet or
explosive.
• Eruptions depend on the properties of
magma
– Silica content
– Viscosity
Kinds of Volcanic Eruptions
• Quiet Eruptions
– Magma has low silica content = low viscosity
– Magma flows easier so gases bubble out
gently
– Lava can flow many kilometers from the vent
– Produce both pahoehoe and aa lava
– Example: Hawaiian Islands – Mount Kilauea
Kinds of Volcanic Eruptions
• Explosive Eruptions
– Magma has high silica content = high
viscosity
– Magma doesn’t always flow out of vent and so
it builds up (like a cork in a bottle)
– Trapped gases build up pressure until they
explode
– Magma is pushed out of the vent with
incredible force
Kinds of Volcanic Eruptions
• Explosive Eruptions
– Lava breaks into fragments that cool quickly
and harden into different sizes
• Ash – fine, dust sized particles of lava
• Cinders – pebble sized particles
• Bombs – baseball to car sized chunks of lava
– Pyroclastic flow: when an explosive eruption
hurls out gases, ash, cinders and bombs.
– Pumice forms when lava cools quick and
traps air bubbles inside
– Obsidian forms when lava cools quick leaving
the surface smooth and glass-like
Kinds of Volcanic Eruptions
• Volcano Hazards
– Quiet eruptions cause lava to flow far --burning and burying everything in its path
– Explosive eruptions can bury entire towns in
ash, cause landslides, avalanches, cause
damage from gases and cinders/bombs.
Content checkpoint…
think-pair-share take
.
two minutes to answer these questions with a partner nearby
• What is pyroclastic flow?
• How does volcanic ash cause damage?
• What is the main difference between a
quiet eruption and an explosive
eruption?
Stages of Volcanic Activity
• Life Cycle of a Volcano
– Scientists use historical records and monitor
volcanoes to determine what stage of activity
a volcano is in
– A volcano can be active, dormant or extinct
• An active volcano is erupting or showing signs of
near future eruptions
• A dormant (sleeping) volcano can awaken in the
future and become active
• An extinct (dead) volcano is unlikely to erupt ever
again
Stages of Volcanic Activity
• Monitoring Volcanoes
– Geologists use tiltmeters to detect slight
changes in surface elevation cause by magma
moving underground
– They monitor gases escaping from a volcano
– Increase in temperature might mean magma is
nearing the surface
– The changes detected may give a short
warning time BUT
• We cannot be certain the type of eruption or how
powerful it will be
Think and Discuss….
• Which is more likely to be dangerous – a
volcano that erupts frequently or a
volcano that has been inactive for a
hundred years? WHY?
3.4 Volcanic Landforms
• Objectives:
– List the landforms that lava and ash create
– Explain how magma that hardens beneath
earth’s crust creates landforms
– Identify other distinct features that occur in
volcanic areas
• Volcanic activity on and beneath the
surface has built up Earth’s land areas.
Landforms from Lava and Ash
• Volcanic eruptions create landforms made
of lava, ash and other materials
– Shield volcanoes
– Cinder cone volcanoes
– Composite volcanoes
– Lava plateaus
• Another landform results from the collapse
of a volcanic mountain
– caldera
• Shield volcanoes
–
–
–
–
Gently sloping mountains
Thin layers of low viscosity lava build up over time
Ex) the Hawaiian Islands
CSAV Hawaii: Pele's Hair (Volcanic Glass) - YouTube
Kilauea Lava Flows on May 6, 2012 - YouTube
Shield Volcano in Iceland
• Cinder cone volcanoes
– High viscosity lava produces ash, cinders and
bombs
– Those materials build up around the vent in a
steep cone shaped hill/mountain.
– Ex) Sunset Crater in Arizona
• Composite Volcanoes
– Form when volcanoes alternate between quiet
lava flows and explosive eruptions of ash,
cinders and bombs
– Tall, cone shaped mountain with alternating
layers of ash and lava
– Ex) Mt St. Helens, Washington and Mt. Fuji,
Japan
• Composite Volcanoes
• Form when volcanoes alternate between quiet lava flows
and explosive eruptions of ash, cinders and bombs
• Tall, cone shaped mountain with alternating layers of ash
and lava
• Ex) Mt St. Helens, Washington and Mt. Fuji, Japan
• Lava Plateaus
– Low viscosity lava flows out of several long
cracks and travels far before cooling
– After millions of years a high plateau forms
– Ex) Columbia Plateau in Washington, Oregon
and Idaho
• Calderas
– Huge hole left by the collapse of composite
volcanic mountains
– Filled with fallen pieces of the volcano, ash
and lava
– Enormous eruptions may empty the pipe and
the magma chamber
– there is only hollow space left that can’t
support the weight and it collapses on itself
– Overtime water from rain and snow may fill
the caldera
– Ex) Crater Lake, Oregon
• Soil from lava and ash
– People settle near volcanoes because of the
rich, fertile soil
– At first, after volcanic eruptions the land is
empty and barren
– Overtime the ash breaks down and provides:
• Potassium
• Phosphorus
• Other substances that plants need
Landforms from Magma
• Sometimes magma forces its way through
cracks in the crust but never reaches the
surface
• The magma cools and hardens within the
rock layers
• Overtime ice, wind, or rain erode away the
surrounding rock and expose the
hardened magma
• Volcanic Necks
– Forms when magma hardens in the pipe
– Overtime the outer rock is weathered away
– Looks like a giant tooth
– Ex) Ship Rock, New Mexico
• Dikes and Sills
– Dike: when magma forces its way across
multiple rock layers and hardens
– Sill: when magma squeezes between
horizontal layers of rock and hardens
– Ex) Palisades, NY & NJ
dike
sill
• Batholiths
– Large masses of rock that are formed when a
huge mass of magma cools inside the Earth’s
crust
– The outer layers of rock erode away leaving
the hardened magma exposed
– Ex) Sierra Nevada Batholith
• Dome Mountains
– Created by smaller bodies of magma
– The hardened magma uplifts the surrounding
rock and causes it to bend into a dome shape
– Ex) Black Hills, South Dakota
Geothermal Activity
• Geo = Earth, therme = heat
• Geothermal activity – when magma a
few km beneath the surface heats
underground water.
– Ex) hot springs and geysers found near past
and present volcanic activity
• Hot Springs
– Forms when underground water is heated by
magma or hot rock
– The heated water rises and collects in pools
• Geysers
– Fountains of water and steam that erupt from
the ground
– The heated water and steam build up
pressure until finally it erupts
– Ex) Old Faithful, Yellowstone National Park
• Geothermal Energy
– Heated water can provide an energy source
– Heated water can be used to power turbines
and create electricity
A panoramic view of
the Geysers
geothermal power
plant in Geysers,
Calif. The site,
located above Santa
Rosa, is the largest
geothermal
development in the
world.