Volcanoes
Volcanism – all of the processes associated with the discharge of magma, hot water, and steam from within the Earth.
Magma – a mixture of molten rock, mineral crystals, and gases that is found down inside of the Earth.
Lava – the term given to magma that reaches the Earth’s surface.
Tephra (pyroclastics) – Debris that is ejected from a volcano (dust, ash, volcanic rocks, etc.) This is the stuff that
when combined with superheated gases and traveling at a high speed is known as a pyroclastic cloud.
Most volcanoes on land are a result of the subduction of oceanic crust under continental crust.
Continental Arc
Explosive Eruptions
Why? As the oceanic plate is
subducted it takes some water with
it. This water is heated and turned
into a gas (water vapor) that
expands and rises up under
pressure causing a violent eruption.
Ex.: In the Cascade Range,
Mt Saint Helens erupted violently in May
of 1980 (It is formed from the subduction
of the Juan De Fuca Plate under the North
American Plate)
Major Zones of Volcanic Activity
Circum-Pacific Belt (Ring of Fire) – a major
zone of volcanic activity that is found circling
the Pacific Ocean. The volcanoes making up
the Andes Mtns., the Cascade Range, the
Aleutian islands, Kuril islands, Japanese
islands, and the Mariana islands are all found in
this zone.
Mediterranean Belt – a smaller zone of volcanic activity that is found
between the plate boundaries of Africa, Eurasia, and the Arabian plates.
Mediterranean Belt
Examples of volcanoes found in the
Mediterranean Belt:
 Mt. Vesuvius
 Mt. Etna
 Mt. Stomboli
Volcanism at divergent boundaries
Rift Valley
When the lithospheric plates move apart magma rises to fill in
the gap that is created.
The volcanism that occurs here is typically non-explosive with
large amounts of lava spilling out.
2/3 of Earth’s volcanism occurs underwater along ocean ridges.
This lava quickly cools when it comes in contact with water and
takes the form of giant pillow-like shapes. It is appropriately
called pillow
lava.
Click this link to view pillow lava being made:
Pillow Lava Video
Hot Spot Activity
Hot Spot – an unusually hot region in the Earth’s upper mantle just under the Earth’s lithosphere.
It is a place where high-temperature plumes of magma are able to burn their way
through the crust and on to the surface producing volcanoes. Hot spots remain
stationary while the crust above continuously moves.
The Hawaiian Islands are a prime example of islands formed from hot spot activity under Pacific
plate oceanic crust. Also related to this hot spot activity are the Midway Islands located to the
Northwest of the Hawaiian Islands. These islands were at one time also over the same hot spot long
before the Hawaiian Islands were even formed.
Hot Spot Animation
On the Big Island of Hawaii, the world’s most active volcano is found: Kilauea, it has been erupting
constantly since 1983.
Hot Spot Animation
Because the Pacific Plate continues to move and the hot spot remains stationary, eventually Kilauea
will be carried off of the hot spot and it will become inactive. A new volcano is already forming on
the ocean floor to the southwest of the Big Island of Hawaii. It may one day grow large enough to
rise above the water and form a new Hawaiian island.
By mapping out the positions of the islands as well as underwater volcanoes (called seamounts or
flat–topped seamounts that are called guyots), we can determine the rate and direction of plate
movement for the past 80 million years.
Hot spot activity can also occur under continental crust. When this occurs this can lead to the
formation of Flood Basalts.
Flood Basalts – thick lava flows on the continental crust that form when lava comes out of long
cracks in the Earth’s crust called fissures. These eruptions can last for hundreds
or even thousands of years and result in thick deposits of hardened lava that can
form into raised up flat plains called plateaus.
Click this link to see:
Flood Basalt Animation
Structure of a Volcano
Magma Chamber – the reservoir of
molten rock under a volcano. It is
connected to the interior of the Earth.
Volcanic Neck (conduit) – the
passage way for the molten rock to
travel towards the vent at the top.
Vent – the opening through which the
molten rock comes out of. The
hardened lava that hardens around this
opening creates the volcanic cone.
Volcanic neck (conduit)
Side Vent – an opening found on the
side of a volcanic cone that is formed
from a lateral extension of the
volcanic neck.
Crater – a relatively small sized
bowl-shaped depression that
surrounds the vent and is typically
found at the top of the volcano. They
are usually less than ½ mile across.
Calderas
Caldera – A much larger basin-shaped depression (up to 30 miles across) at the top of a volcano.
They often form after the magma chamber beneath a volcano empties from a major
eruption or a decrease in pressure (the magma gets sucked back into the earth).
This animation illustrates how a caldera forms: (click this link)
http://glencoe.com/sec/science/earthscience/2007/concept_motion/animated_art/CalderaFormaiton18_8.avi
One of the best examples in the world of a caldera forming is in the Cascade Range in the Pacific
Northwest. The name of the volcano is Mt. Mazama. It formed a huge caldera that filled in with
water creating Crater Lake. The volcano had periods of volcanic activity following the caldera
formation that resulted in an a volcanic island forming in Crater Lake, its name is Wizard Island.
6 miles across
Crater Lake is the deepest lake in the United States. At 1,932 feet deep, this
beautiful lake has no inlet or outlet and is known for its intense blue color.
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Crater Lake is the deepest lake in the United States
The second deepest lake in the Western Hemisphere
The seventh deepest lake in the world
Maximum lake depth: 1932 feet
Maximum lake width: 6 miles
Precipitation, yearly average: 66 inches
This is what keeps Crater
Snowfall, yearly average: 44 feet
Lake filled up with water.
The lake was formed after the collapse of an ancient volcano,
posthumously named Mount Mazama. This volcano violently erupted
approximately 7,700 years ago. That eruption was 42 times as
powerful as the 1980 eruption of Mt. St. Helens. The basin or caldera
was formed after the top 5,000 feet of the volcano collapsed.
Subsequent lava flows sealed the bottom, allowing the caldera to fill
with approximately 4.6 trillion gallons of water from rainfall and snow
melt, to create the seventh deepest lake in the world at 1,932 feet.
The clean, clear, cold lakewater contained no fish until they were
introduced by humans from 1888 to 1941. Today, rainbow trout and
kokanee salmon still survive in Crater Lake.
Why is Crater Lake so blue?
One of the most dramatic features of Crater Lake is its blue color; in fact early
explorers called the lake “Deep Blue.” When sunlight penetrates into the lake, the
red through the green portions of the light are preferentially absorbed by water and
suspended particles. The blue light, which penetrates more deeply, is eventually
scattered by water molecules and returns to the lake surface and our eyes.
Concentrations of nutrients such as nitrogen and phosphorus are low in the lake,
which explains why growth of microscopic plants (called algae) is low. The
blueness of water is greatest when densities of particles and algae are low.
3 Types of Volcanic Cones
Shield Cone – a volcano that forms when thin and runny lava flows out of the Earth and spreads out
over a wide area. It typically has non-explosive eruptions, a very broad base, and is
composed of layers of solidified lava. It is the largest of the 3 types of volcanoes.
Ex.: Kilauea in Hawaii (the world’s most active volcano)
Olympus Mons on Mars (the largest volcano in our solar system – 3 times taller than Mt. Everest)
Cinder Cone – a volcano that forms when tephra (volcanic rock fragments) is thrown out of the
earth and piles up near the vent. It typically has explosive eruptions, steep slopes,
and is relatively small. It is the smallest of the 3 types of volcanoes.
Ex.: Paricutín in Mexico. (It was born in Feb. 20, 1943 in a corn field and grew to 300 feet in 5 days)
Composite Cone (stratovolcano) – a volcano that is made up of alternating layers of
tephra and lava flows.
It will experience both explosive and non-explosive eruptions.
Ex.: Most of the volcanoes of the Cascade Range:
Mt. Rainier and Mt. Saint Helens