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Volcanoes
Chapter 6
By
6C
Vulcan, Roman God of Fire

The word volcano comes
from name of the roman
god of fire, Vulcan.
What is a Volcano?
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A volcano is a place on the
surface where molten rock,
gases and pyroclastic debris
erupt through weak spots in
the earth's crust.
Volcanoes vary quite a bit in
their structure - some are
cracks in the earth's crust
where lava erupts, and some
are domes, shields, or
mountain-like structures with
a crater at the summit.
Magma or Lava?
Magma is a molten mixture
of rock forming substances
(liquid rock), gases, and
water vapor.
 When it reaches the surface
it is called lava.
 It forms igneous rock when it
cools.
 Reshapes the surface by
adding to land, creating
mountains and islands.

Location of Volcanoes
Most occur on diverging plate boundaries or in
subduction zones at converging boundaries.
 About 600 active volcanoes on land, and more under
the ocean.

Mid-ocean Ridge
Underwater rift valleys
where new crust is being
created from lava
pouring out of cracks in
the ocean floor.
 Only in a few places like
Iceland and the Azores
do they rise above the
ocean surface.
 Divergent Boundaries
 Worlds longest mountain
range
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World Wide Mountain Range
Convergent Boundaries
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Subduction can cause a series of volcanoes parallel to the plate
boundary.
This creates mountain ranges like the Andes and volcanoes in
the Pacific Northwest.
At sea they create island arcs like Japan.
Hot Spot Volcanoes
Can occur in the middle
of plates.
 The plate moves over a
stationary hot spot.
 A chain of islands is
formed over millions of
years as the plate drifts
over the hot spot.

Yellowstone Hot Spot
Magma and the Earth’s Surface
Magma rises because it
is less dense than the
surrounding solid
material.
 It will rise to the surface
unless it is trapped
beneath layers of rock.

Chapter 9
Section 1 Volcanic Eruptions
Magma and Vents
Click below to watch the Visual Concept.
Visual Concept
A Volcano Erupts
When a volcano erupts,
the dissolved gases
(carbon dioxide and
water vapor mainly) rush
to the surface, carrying
the magma with it,
 At the surface the
magma becomes lava.

Exploring a Volcano
Magma Chamber
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A large underground
pocket formed by the
rising magma.
Pipe
Side Vent
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Sometimes lava will exit
from cracks in the side of
a volcano.
Pipe
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Narrow, almost vertical
crack in the crust that
carries the magma to the
surface.
Vent

Point on surface where
the magma leaves the
pipe.
Crater
Bowl shaped area that
forms around the vent.
 Crater Lake in Oregan
was formed when Mount
Mazama blew its top
some 8,000 years ago.

Chapter 9
Section 2 Effects of Volcanic Eruptions
Craters
Click below to watch the Visual Concept.
Visual Concept
Lava Flow

The river of lava that
gravity causes to flow
down the side of a
volcano
Viscosity
Is the resistance a liquid
has to flowing.
 Honey has a high
viscosity as it pours very
slowly.
 Water has a low viscosity
as it pours quickly.

Characteristics of Magma
The viscosity of magma
depends on the amount
of silicates, amount of
dissolved gas, and the
temperature.
 More silicates, higher
viscosity.
 Higher the temperature,
the lower the viscosity.
 More gas, less viscosity.

Quiet Eruptions
Type of eruption is
determined by the
amount of gas and
the viscosity of the
magma.
 Low pressure gas
and low viscosity
magma will ooze
out, forming shield
volcanoes.

Explosive Eruptions
Thick magma can plug
the volcano pipe like a
cork.
 The pressure can build
until it suddenly
explodes.
 The higher the gas
content the larger the
explosion.
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Volcanic Material
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Rock fragments thrown into
the air are called tephra.
They are classified by size.
Dust is the smallest, followed
by ash.
Larger pieces are called
lapilli, which means “little
stones.” Also called
cinders.
Bombs/blocks: The largest
tephra thrown from a volcano
can be the size of a car or
small building. Bombs cool
on the way down.
Pyroclastic Flows
Very deadly cloud of ash,
gas, dust, and other
tephra that rushes down
the sides of a volcano at
speeds of nearly 200km/hr.
 Temperature can exceed
700°C.

Active Volcanoes
One that is currently erupting or
may erupt in the very near future.
 Sakurajima is one of the most
active volcanoes on Earth. Since
764 AD eleven eruptions have
caused fatalities. The largest
historic eruption was in 1471-1476
and caused an unknown number of
deaths, probably from pyroclastic
flows. An eruption in 1779 caused
153 deaths from tsunami and
tephra. Evacuation of the island in
1914 prevent a larger death toll
from that eruption. The current
eruptive activity began in 1955.

Mount Vesuvius
Only volcano on
European mainland to
have erupted in last 100
years.
 Considered to be most
dangerous volcano as 3
million people live nearby
(Naples) and because of
its explosive nature.
 Last eruption in 1944.

79 AD
Vesuvius is best
know for the eruption
in 79 CE that buried
the towns of
Herculaneum and
Pompeii.
 Killed between
10,000 and 25,000
people.
 Pliny the Younger
was the first to
describe pyroclastic
flow.

Pompeii Today
Krakatoa
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The best-known eruption of
Krakatoa culminated in a
series of massive explosions
on August 26–27, 1883,
which was among the most
violent volcanic events in
recorded history.
Exploded with 4 times the
power of the largest H-bomb.
Over 36,000 died (possibly
over 100,000) from
pyroclastic flow, ash, and
tsunami.
The Year Without Summer
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Volcanic ash caused global
temperatures to fall by over
1.2 degrees Celsius . Acted as a
“sun filter.”
Weather patterns were chaotic for
years and temperatures did not
return to normal until 1888.
In the US 1816 was the “Year
without Summer,” as frost in May
and snow in June destroyed the
crops in New England.
Caused famine and death from
disease throughout the world.
Volcanic Eruptions and Climate Change

During a large-scale
volcanic eruption,
enormous amounts of
volcanic ash and gases
are ejected into the
upper atmosphere.

As volcanic ash and
gases spread throughout
the atmosphere, they
can block enough
sunlight to cause global
temperature to drop.
Chapter 9
Section 2 Effects of Volcanic Eruptions
Effects of Volcanoes on Earth
Click below to watch the Visual Concept.
Visual Concept
Dormant Volcanoes
Sleeping volcano.
Expected to erupt again
in the future.
 Can be thousands of
years between eruptions.
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Extinct Volcanoes

A dead volcano, not
considered likely to ever
erupt again.
Shield Volcanoes
Built by repeated lava
flows from quiet
eruptions.
 Builds up a gently
sloping volcanic
mountain.
 Very large.

Cinder Cone Volcanoes
Erupt explosively,
forming, the cinders pile
up around the vent.
 This forms steep, cone
shaped volcanoes.
 Tend to be very small.

Composite Volcanoes
Also called
stratovolcanoes
 Has alternating
layers of ash and
lava.
 Can be quiet or
explosive

Volcanoes Compared
Calderas
Sometimes a massive explosion can empty the main
vent and magma chamber.
 A hollow shell is left.

Thera
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The island was the site of one of the
largest volcanic eruptions of the last
several thousand years when it
erupted cataclysmically about 3,500
years ago.
The eruption left a large caldera
surrounded by volcanic ash
deposits hundreds of feet deep, and
its effects may have indirectly led to
the collapse of the Minoan
civilization on the island of Crete,
110 km (70 mi) to the south.
One popular theory holds that the
Thera eruption is the source of the
legend of Atlantis.
Soils from Lava and Ash
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Initially barren, the ash, lava,
and cinders will eventually
break down into soil.
Volcanic soils can be among
the richest in the world.
Rich soil means fertile soil.
Volcanic soil is fertile as it is
full of minerals brought up
from deep within the Earth.
That’s why people are willing
to live near volcanoes.
Landforms from Magma
When magma cools it
hardens to solid rock.
 Igneous rock is usually
fairly hard, so when the
softer rock and soil
around it erodes, these
landforms are exposed
to view.

Volcanic Necks
When the magma in the
volcano pipe hardens it
forms a volcanic neck.
 It looks like a giant tooth
sticking out of the ground
when the softer rock
around it erodes away.

Dikes
Magma that forces itself
across rock layers is
called a dike.
 Tend to be vertical

Sills
Magma that squeezes
between layers of rock
before hardening forms
sills.
 Usually horizontal

Batholiths
Very large rock masses
that form the core of
some mountain ranges.
 Occurs when a large
amount of magma cools
below the surface.
 Stone Mountain in
Georgia

Dome Mountains
Occur when rising
magma is blocked by
horizontal layers of rock.
 It pushes the layers up
into a dome shape.
 Eventually the rock
layers erode away,
exposing the igneous
rock of the mountain.

Super Volcanoes