Holy Smokes!
A Volcano Primer
What we will learn
1. Plates move and cause major events.
2. The effects of volcanic eruptions on
earth.
3. Location of volcanic regions and why they
are found at those places.
4. What hot spots are and where
they are formed.
What is a volcano?
A volcano is a weak spot
in the crust where
magma comes out.
Magma is molten mixture
of rock-forming
substances, gases, and
water from the mantle.
When magma reaches
earth’s surface, it is
called lava.
When lava cools, it forms
rock.
Volcanoes and plate boundaries
Ring of Fire- A major volcanic belt that is
formed by many volcanoes that rim the
Pacific Ocean.
Volcanic belts form along the boundaries of
Earth’s plates.
Most volcanoes form along
diverging plate boundaries such
as mid-ocean ridges and along
converging plate boundaries
where subduction takes place.
Ex: Mount Etna formed near Eurasian and
The Ring of Fire
Diverging Boundaries
Volcanoes form along mid-ocean ridges
where plates diverge.
Ridges are long underwater mtn. ranges
that have a rift valley down the center.
Lava pours out of cracks in ocean floor,
building new mtns.
Volcanoes also form along
diverging plate boundaries on
land.
Ex: There are several large volcanoes along the
Great Rift Valley in East Africa.
Converging Boundaries
Many volcanoes form near converging plate
boundaries where oceanic plates return to
the mantle.
Volcanoes may form where 2 oceanic plates
collide or where an oceanic plate collides
with a continental plate.
Where 2 oceanic plates collide,
subduction takes place and the
older, denser plate sinks beneath
a deep ocean trench into the
mantle.
Converging boundaries (cont)
Some of the rock above the subducting
plate melts and forms magma.
The magma is less dense than the rock
around it, so it rises toward the surface, &
eventually breaks through the ocean floor,
and creates a volcano.
Quick Check- Do NOT look at
your notes or book.
1. What’s the difference in magma and lava?
2. Why does magma rise when it is inside
earth?
3. What is the ring of fire?
4. Why do volcanoes form at
divergent plate boundaries?
Island Arc
An island arc is a string of
volcanic islands.
The curve of an island arc is the
same curve as the trench
beneath it. Ex; Japan, New Zealand, Indonesia,
Philippines, Aleutians, and Caribbean islands.
Volcanoes also occur where an oceanic plate
is subducted beneath a continental plate.
This type of collision produced volcanoes of
the Andes Mtns in South America and the
volcanoes of the Pacific NW in
the USA.
Hot Spots
A Hot Spot is an area where material from
deep within the mantle rises and then
melts, forming magma.
A volcano forms above a hot spot when
magma erupts through the crust and
reaches the surface.
Some hot spots lie in the middle
of plates far from any plate
boundaries.
A hot spot in the ocean floor can
gradually form a series of volcaic
mtns.
Examples of hot spots
Hawaiian islands formed one by one over
millions of years as the Pacific plate
drifted over a hot spot.
Hot spots can also form under the
continents. Ex: Yellowstone
National Park in Wyoming marks a
hot spot under the North
American plate.
Compare Hot Spot to Island
Arc-What’s the difference?
Hot Spots
Summarizer
Write a ½ page summary answering these questions.
1. What is a volcano?
2. Where are volcanoes formed?
3. How did the volcanoes in the Andes Mtns form?
4. What is a hot spot and how does it
form?
5. Why are volcanoes are located in
the ring of fire?
3 Basic Volcano shapes
Shape
tells1.
usCinder Cone
something
about2.Shield
how the
volcano was
3.Composite
Cone
formed.
1. Cinder Cone Volcano
Formed by Tephra (volcanic
cinders, bit of solidified lava,
and bits of rocks) thrown into
the air during an eruption &
then fall back down around
the vent (volcanic opening)
forming a steep-sided loosely
packed volcano.
Example
Cinder Cone Volcano
Parícutin Volcano in
Mexico is a great
example of a cinder
cone volcano.
Paracútin
On February 20, 1943, a Mexican
farmer noticed that a hole in his
cornfield that had been there for
as long as he could remember was
giving off smoke.
Throughout the night, hot glowing
cinders were thrown high into the
air.
In just a few days, a cinder cone
several hundred meters high
covered his cornfield.
2. Shield Volcano
Quiet eruptions of
basaltic lava spread
out in flat layers.
The buildup of
these layers forms
a broad volcano
with gently sloping
sides.
Shield Volcano
Mauna Kea volcano
picture from www.usgs.gov
The shapes of these
volcanoes reflect the
fact that they are
constructed of lava
flows that erupted
non-explosively.
It is usually many
times larger than a
composite cone or
cinder cone volcano.
Example of Shield Volcano
Mauna Loa
Mt. Kilauea
Probably one of the
world’s most active
volcanoes.
The eruption of Kilauea
Volcano that began in
1983 continues at the
cinder-and-spatter cone
of Pu`u `O`o
Mt. Kilauea
Picture from http://www.britannica.com/eb/art89176/Hawaiis-Kilauea-is-an-active-volcano
3. Composite Volcano
This type of volcano is
composed of explosively
erupted pyroclastic
materials layered with
lava flows and volcanic
debris.
An explosive period can
release gas and ash,
forming a tephra layer.
Then, the eruption can
switch to a quieter period,
erupting lava over the top
of the tephra layer.
Example of
Composite Volcano
Mount St. Helen’s
Erupted in 1980
57 fatalities
Over 7000 big game animals
perished
4 billion board feet of timber
(enough to build about 300,000
two-bedroom homes) destroyed
Destroyed 27 bridges, nearly 200
homes. Blast and lahars destroyed
more than 185 miles of highways
and roads and 15 miles of railways.
Example of
Composite Volcano
Mount Pinatubo
Erupted in 1991
Killed 847 people
184 people injured
10, 000 home destroyed
and another 5,000 were
damaged.
The ash cloud took one
year to spread around the
globe, reducing global
temperatures. This
resulted in
Floods in 1993 along the
Mississippi River
Drought in Africa in 1993
The US had its 3rd
wettest & coldest winter
on record.
Example of
Composite Volcano
Krakatau
One of the most violent
eruptions in recent
times occurred on an
island in the Sunda
Straits near Indonesia
in August of 1883.
Krakatau, a volcano
on the island,
erupted with such
force that the island
disappeared.
Example of Composite Volcano
Krakatau
Killed 36,000 people most
were killed by a giant
tsunami
Destroyed 160 villages
Fine ashes from the
eruption were carried by
upper level winds as far
away as New York City
Volcanic dust lowered
global temperatures for
five years, this caused
Example of Composite Volcano
Krakatau
Unusual sunsets
and weather
patterns for
three years
Evidence:
William Ashcroft’s
paintings
The Scream
What determines how
explosive an eruption is?
Amount of trapped gases
Amount of Water Vapor
Composition of Magma
1. Trapped Gases
Gases such as water vapor
and carbon dioxide are
trapped in magma by the
pressure of the surrounding
magma and rock.
Gas escapes easily from
some magma during quiet
eruptions.
However, gas that builds
up to high pressures
eventually causes
explosive eruptions.
2. Amount of Water Vapor
The magma at some convergent
plate 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.
3. Magma Composition
The composition (what
it is made of)
determines how
explosive a volcano will
be.
Magma Composition
Basaltic
Magma made of basalt
is very fluid, allowing
trapped gases to
escape easily.
Volcanoes with
basaltic lava have
quiet, non-explosive
eruptions.
Magma Composition
Basaltic
•If
the same (palava
Asrock
pahoehoe
structures
flows
at
a lower
HOY-hoy)
lavatubes,
cools,
shaped like
temperature,
a
stiff,
underwater
pillow
it
forms
a
ropelike
balloons,
or
slowly moving aa (AHlava
pillows.
structure
.
ah)
lavaformations
forms.
Volcanoes with
basaltic lava produce:
Magma Composition
Basaltic
Examples
You can find volcanoes
with basaltic lava
Hawaiian Volcanoes,
including
Kilauea
Mona Loa
Iceland
Heimaey
Hekla
Magma Composition
Granitic Lava
Silica-rich, or granitic (also
called rhyolite) magma
produces explosive
eruptions.
Silica-rich granitic magma
is thick, and gas gets
trapped inside, causing
pressure to build up.
Magma Composition
Granitic Lava
“If it had erupted in a populated
area…"The magnitude of the
eruption can perhaps be best
realized if one could imagine a
similar outburst centered in New
York City. All of Greater New York
would be buried under from ten to
fifteen feet of ash; Philadelphia
would be covered by a foot of gray
ash and would be in total darkness
for sixty hours; Washington and
Buffalo would receive a quarter of
an inch of ash, with a shorter period
of darkness. The sound of the
explosion would be heard in Atlanta
and St. Louis, and the fumes noticed
as far away as Denver, San Antonio,
and Jamaica."
Some examples of granitic
volcanoes are
Yellowstone Caldera
It is a super voclano!
Katmai, Alaska
Last eruted in 1912.
Magma Composition
Andesitic Lava
Volcanoes with
andesitic magmas have
violent, explosive
eruptions.
It often forms at
convergent plate
boundaries where one
plate slides under the
other.
Magma Composition
Andesitic Lava
The word andesitic comes
from the Andes, which
are mountains located
along the western edge of
South America, where
andesite rock is common.
Many of the volcanoes
encircling the Pacific
Ocean also are made of
andesite.
Magma Composition
Andesitic Lava
Mount Pelee, Martinique
Famous for the May 8, 1902
eruption which killed 29,000 people
and destroyed the city of St.
Pierre. This is the largest number
of causalities for a volcanic
eruption this century.
Mayon, Phillipines
It is the most active volcano in the
Philippines. Since 1616, Mayon has
erupted 47 times.
It’s 1814 eruption killed 1,600.
Volcanoes in a nutshell
Three volcano types
Shape
Cinder
Violent eruptions
Composite
Violent & quiet eruptions
Shield
Quiet Eruptions
Volcanoes in a nutshell
Three factors affect how violent an
eruption will be
Water Vapor
More water vapor = bigger explosion
Trapped Gasses
How easily the gas escapes determines
how violent the explosion
Easy escape = quiet eruption
Difficult to escape = explosive eruption
Magma Type
Basaltic Magma = quiet explosion
Granitic/Andesitic = violent eruptions
Resources used
Volcano World
http://volcano.und.edu/
US Geologic Survey
http://www.usgs.gov/
Glencoe Earth Materials &
Processes