VOLCANOES
Earth Science Fall 2012
Vocabulary:
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aa – jagged chunks of lava formed by rapid
cooling on the surface of the lava flow
Vocabulary:
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Caldera- large basin-shaped depression formed
when an explosion destroys the upper part of a
volcanic cone or when the magma chamber below a
volcano is emptied during an eruption
Vocabulary:
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Cinder cone- steep sloped deposit of solid
fragments ejected from a volcano
Vocabulary:
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Composite volcano- (strato volcano) steep sloped
volcanic deposit with alternating layers of
hardened lava flows and pyroclastic materials
Vocabulary:
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Crater- funnel-shaped pit at the top of a volcanic
cone; bowl shaped depression on the surface of a
planetary body
Vocabulary:
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Felsic lava- silica rich lava
Vocabulary:
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Fissure- crack in a rock surface through which lava
flows
Vocabulary:
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Hot spot- area of volcanism within a lithospheric
plate
Vocabulary:
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Lapilli- pyroclastic particles between 2mm & 64
mm in diamter
Lava- magma that reaches the earth’s surface
Mafic lava- dark colored rich in magnesium and
iron
Magma- liquid rock produced deep within the earth
Pahoehoe- solidified mafic lava with a wrinkled
surface
Vocabulary:
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Pillow lava- lava that flows out of fissures in the
ocean floor and cools rapidly in round shapes
Pyroclastic material- the rock fragments ejected
from a volcano
Sheild volcano- has a cone of hardened lava that
forms a broad base and slopes gently
Stratovolcano- also called composite volcano; steep
sloped volcanic deposit with alternating layers of
hardened lava flows and pyroclastic material
Vocabulary:
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Vent- opening through which the molten rock flows
onto the surface
Volcanic ash- pyroclastic particles between 0.25mm
and 2 mm in diameter
Volcanic block- the largest possible pyroclastic
material that is formed from solid rock blasted from
a volcanic fissure
Volcanic bomb- large, spindle-shaped clot of lava
thrown out of a volcano
Vocabulary:
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Volcanic dust- pyroclastic particles that are less than
2 mm in diameter
Volcanism-any activity that includes the movement
of magma toward or onto the earth’s surface
Volcano- lava and pyroclastic material built up on
the earth’s surface around a vent
Volcanoes & Plate Tectonics

Scientists have no direct way to measure
temperatures within the earth
 Analysis
of seismic waves & hear flow near the earth’s
surface & computer modeling allow scientists to
estimate those temperatures

Areas of solid rock melt to form magma
 Forms
when mantle material flows to areas of lower
pressure faster than it can lower its temperature by
hear flow or when fluids such as water are added to
the mantle material
Volcanoes & Plate Tectonics

Volcanism
 Any
activity that includes the movement of magma
toward or onto the surface of the earth
 Pockets of magma form due to melting of some of the
surrounding rock
 More
rock melts, magma pockets get large
Volcanoes & Plate Tectonics
most magma forms at plate boundaries, where one
plate is subducted beneath another plate
 Magma that erupts onto the Earth’s surface is lava
 The opening through which magma flows to the surface
is called a vent
 The structured formed by the vent and the material that
builds up on the earth’s surface around the vent is
called a volcano

Volcanoes & Plate Tectonics

Major Volcanic Zones
 Most
active volcanoes occur in zones near both
convergent and divergent boundaries of lithospheric
plates
 Subduction zones
 Deep
trench forms on the ocean floor where the continental
crust is being subducted
 The plate of continental crust buckles and folds, forming a
line of mountains along the edge of the continent
Volcanoes & Plate Tectonics
 Magma
erupts to the surface at the trench, forming volcanic
mountains near the edge of the continent
 Major zone of active volcanoes is the Pacific Ring of Fire
 If 2 oceanic plates collide one plate is subducted, forming a
deep trench
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After time a string of volcanic islands forms, called an island arc
Island arcs start as small islands and grow to large islands
 Ex: Aleutian Islands- small islands
 Ex: Japan – large islands
Volcanoes & Plate Tectonics
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Mid-Ocean Ridges
 The
largest amount of magma comes to the surface at
mid-ocean ridges
 Major zone of volcanic activity
 Upwelling magma adds material to the mid-ocean
ridge & creates new lithosphere along the rift
 Can
also form underwater volcanoes
 Iceland
is located on top of the mid-ocean ridge, it is
separated by a fissure
Volcanoes & Plate Tectonics
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Hot spots
 Areas
of volcanism within the interior of the lithosphere
 Appear to remain stationary, but actually drift slowly;
as a result the volcano actually is carried away
 When
the volcano is carried it away, it is moved away from
the hot spot and the activity of the volcano comes to an end
 Hawaiian
Islands are an example of a chain of
volcanic islands formed over a hot spot
Volcanic Eruptions
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Geologists have concluded that there are two
general types of lava:
 Mafic
lava- dark colored when hard and is rich in
magnesium and iron
 Forms
much of the ocean crust
 Felsic
lava- contains lots of silica, less magnesium and
iron, has a lighter color when hardened
 Common
in continental crust
Volcanic Eruptions
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When mafic lava cools quickly, the surface of the
lava flow forms a crust
 The
flow continues move and wrinkles up forming
pahoehoe, which means ropey in Hawaiian
 If the crust is deformed rapidly or grows too thick to
form wrinkles, it breaks into jagged chunks to form aa
 Sharp
blocky texture of the volcanic rock
Volcanic Eruptions
 Lava
cools so rapidly that the outside hardens leaving
a liquid core in the middle, called a lava tube
 Lava can also flow out of fissures on the ocean floor, it
cools rapidly separating into rounded blobs called
pillow lava
Volcanic Eruptions

Kind of Eruptions
 The
composition of the lava that reaches the surface
largely determines the force with which a volcano will
erupt.
 Ocean volcanoes, those that erupt on the ocean floor
and on oceanic islands, usually are produced by mafic
lava
 Mafic
lava is very hot and thin and flows easy
 Because gases escape quickly these eruptions are very quiet
Volcanic Eruptions
 Felsic
lavas of continental volcanoes tend to be cooler
and thicker
 Contain
large amounts of trapped gases and when the vent
or fissure opens up, the dissolved gases within the lava boil
out explosively, sending molten and solid particles shooting
into the air
Volcanic Eruptions
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Volcanic Rock Fragments
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Felsic lava explodes through pyroclastic material into the air
Rock fragments ejected from a volcano
 Some forms when cooling magma breaks into fragments, other
forms when a spray of lava cools and solidifies
 Volcanic ash and dust generally land close the volcano
 Lapilli are larger than ash and dust and generally land near the
volcano
 Volcanic bombs are formed when large clots of lava harden in the
air
 Volcanic blocks can be a large as a house
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Volcanic Eruptions
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Volcanic Features
 Piles
of volcanic material are volcanic cones
 Types of volcanoes
 Volcanic
cones that are broad at the base and have gently
sloping sides are called shield volcanoes
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Covers a wide area & generally results from lava eruptions
Slowly build up
 Ex: Hawaiian islands
Volcanic Eruptions
 Cinder
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cones- cause by explosive eruptions
Made of solid fragments ejected from the volcano
Usually have very steep slopes
Rarely more than a few hundred meters tall
 Composite
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volcanoes (stratovolcanoes)
Are formed by both quiet and explosive eruptions
Cone usually formed from lava flows
Explosive eruptions deposit lots of material around the vent
Cone is made of alternating layers of lava and pyroclastic
material
Ex: Mount Fuji , Mount Raineer, Mount Hood
Volcanic Eruptions
 Craters
& Calderas
 Craters
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Formed when material is blown out of the volcano by explosions
Usually gradually becomes wider
A small cone can form within a crater
 Caldera
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When the magma chamber below a volcano is emptied, the cone
may collapse, leaving a large, basin-shaped depression
Can also be caused by eruptions that discharge large amounts of
magma
Volcanic Eruptions
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Predicting Volcanic Eruptions
 Scientist
use measuring devices to detect geologic
events that may signal the beginning of an eruption
 Small
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earthquakes
Result from growing pressure on the surrounding rocks as magma
works it way upward
Temperature changes within the rock
 Slight
bulging of the surface of the volcano
 Change in composition of the gases given off by the volcano
 Study
previous eruptions
Extraterrestrial Volcanism
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Many of the planets and moons of the solar system
were volcanically active at one time.
The Moon
 Near
side is covered with basaltic lava flows
 Craters on the moon’s lunar surface come from
meteorite bombardment
 Maria – large impacts early in the moon’s history
created deep basins that later flooded with lava
 We don’t know how magma was formed on the moon
Extraterrestrial Volcanism
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Mars
 There
are numerous volcanoes and volcanic features on
the surface of Mars
 Largest volcano on Mars is Olympus Mons.
 Rise
nearly 28 km above the surface
 600 km across
 Caldera is 70 km across
 We
don’t know if the Martian volcanoes are still active
Extraterrestrial Volcanism

Io
 Moon
of Jupiter
 1st planetary body other than Earth where there have
been active volcanoes found
 Probably the most volcanically active body in the solar
system
 The material that erupts from these volcanoes is a
bright yellow-red
 Scietists
think that it is primarily sulfur and sulfur dioxide
Review

Temperature and pressure generally keep the rocks
in the athenosphere below melting point.
 Pink
True
 Yellow False
Review
Solid rock located deep in the mantle is called
magma.
Pink- True
Yellow- False
Review
Magma may sometimes break through to the earth’s
surface.
Pink- true
Yellow- false
Review
Magma is able to rise upward through the earth’s
crust because it is less dense than the surrounding
rocks.
Pink- true
Yellow- false