Volcano Vocabulary
VOLCANISM
VENT
CRATER
CALDERA
QUIET ERUPTION
EXPLOSIVE ERUPTION
PYROCLASTIC DEBRIS
CINDER CONE
SHIELD VOLCANO
COMPOSITE VOLCANO
STRATO VOLCANO
ACTIVE
DORMANT
EXTINCT
INTRUSION
DIKE
SILL
LACCOLITH
PLUTON
BATHOLITH
STOCK
ROCK CYCLE
REMELTED
BURIED
REMELTED
BURIED
HEAT
PRESSURE
ERODED
TRANSPORTED
DEPOSITED
ERODED, TRANSPORTED
DEPOSITED
HEAT
PRESSURE
I.
Heat within the Earth
A. For every kilometer we go beneath the surface the
temperature increases 30 degrees Celsius.
B. Sources of heat:
1. Radioactive decay- like a thermonuclear reactor is
partly responsible for heat within the Earth. Atoms are
splitting apart or decaying.
2. Original heat- Crust has insulated the heat trapped
within since the Earth's formation about 5 billion years
ago.
3. Friction- Tremendous heat created from the convection
currents within the Mantle.
II. Magma (molten rock beneath the surface that is mostly silica)
A. Formation
1. Molten rock within the Earth is called Magma. Molten
rock extruded onto the surface is called lava. Magma
forms at a depth of about 40 to 60 kilometers. Magma
chambers have a temperature of 650 to 1500 degrees
Celsius- or about 1,200 to 3,000 degrees Fahrenheit.
B. Types/ Chemical Composition of Magma
1. Basaltic- Also known as Pahoehoe, Mafic and Oceanic.
Contains less percentages of silica (SiO2) and more
Magnesium oxide (MgO). It has a very thin, runny
texture or low viscosity.
Temperature of Mafic lava ranges between 1000 and 1500
degrees Celsius. High relative density.
2. Granitic- Also known as aa, Felsic and Continental.
Contains more silica (SiO2) and Aluminum oxide (Al2O3).
It has a thick and pasty texture or high viscosity.
Temperature of Felsic lava is about 650 to 950 degrees
Celsius.
Low relative density.
Magma Type:
FELSIC
INTERMEDIATE
MAFIC
75% SiO<-------------<--------------<----------------<-----------------45% SiO2
increasing percentage of silica
<--------------<----------------<-------------------<--------------------increasing Na2O & K2O
---------------->---------------->-------------------->-------------------->
increasing CaO, FeO, & MgO
Common Rock: RHYOLITE
GRANITE
ANDESITE
BASALT
GABBRO
III. Anatomy of a Volcano
A. Common Features
1.
Magma Chamber- as magma rises to the surface it
forms a large pocket beneath the surface that holds
the magma.
AKA: Pluton, Batholith, Stock
2.
Pipe- is a narrow crack in the crust or bedrock
through which magma flows.
3.
Vent- central opening that allows the magma to flow
to surface. It is where magma leaves the pipe.
4.
Crater- the basin or bowl-like depression over the
vent at the summit of the cone.
IV. Kinds of Eruptions
A. Quiet Eruption
1. Typical of the Basaltic/Mafic/Oceanic lava flow.
A "fissure flow" describes lava running out onto the
surface in wide, thin sheets. Often these sheets form a
ropy surface called Pahoehoe- which means "satin-like".
These flows can lead to a topography called a Basalt
Plateau.
Mauna Loa (Hawaii) is a good example of a series of
quietly erupting lava.
B. Explosive Eruption
1. Typical of the Granitic/Felsic/Continental lava flow.
When lava of this type erupts, a great deal of gas and ash
erupt with it. The appearance of this kind of lava is sharp
and jagged which led to its name- aa. The particulate
matter that explodes from the volcano is called Pyroclastic
Debris.
a. Pyroclastic Debris (AKA: Tephra) is categorized by
size:
~Smallest is called Tuff or Dust
~Sand-sized particles are called Ash
~Pebble-sized particles are called Cinders
~Cobble to truck-sized debris called Volcanic Bombs.
V. Types of Volcanoes
A. Classification
Volcanic classification is determined by composition and
shape.
The Magma Chamber determines chemistry of magma/lava
and consequently the type of eruption as well. The material
ejected from the volcano and deposited around the vent
produces the shape.
B. Types
1. Cinder Cone- is a small, steep-sided volcano made
mostly of cinders & tuff (tephra), often with lava flows
intermixed.
The height is usually less than 600 feet (200 meters). The
Cinder Cone tends to erode quickly and may bleed from
the bottom or sides- called a "flank eruption". These
volcanoes do not often cause damage in that they are
small, intermittent explosions of Felsic lava.
Examples are Wizard Island (Crater Lake) and Paricutin
(1943, Mexico).
2. Shield Volcano- is usually wider than it is tall, much
like a shield. It is almost entirely made of mafic lava
(Pahoehoe).
The Shield has a 2 to 10 degree slope and most often found
near oceans or on oceanic crust. The lava flow itself can
cause significant damage though its eruptions are
tectonically quiet.
An example is Mauna Loa (Hawaii)
a. Sheet Volcano- is similar in structure and
composition to Shield Volcanoes. In this type of
structure, huge quantities of lava exude onto the
surface from fissures in the crust. These volcanoes
are very difficult to identify yet are very common.
Sheet Volcanoes are the largest volcanoes in area,
often covering thousands of square kilometers.
An example is the Columbia Plateau (Washington
& Oregon).
3. Composite or Strato Volcano- is a large, steep-sided
volcano made of alternating layers of lava flow and
pyroclastic debris. Its structure and behavior is highly
complex. Strato Volcanoes are considered to be extremely
dangerous due to their inconsistent behavior. These
volcanoes are extremely explosive at times, while at other
times the eruptions are quiet. The nature of the eruption is
determined by the feeder location.
Examples of Composite Volcanoes are Mount St. Helens
(Washington), Mount Vesuvius (Mediterranean), and
Mount Pinatubu (Philippines) which exploded in June of
1991.
a. The Glowing Cloud, or nuees ardentes, is the
main killer in explosive eruptions. The superheated
gas rushes down the cone in excess of 300 miles per
hour and often exceeds temperatures of 800 degrees
Celsius or 1,500 degrees Farenheit. On May 8, 1902
Mont Pele’e erupted on the Caribbean island of
Martinique. A nuees ardentes blew down the slope at
over 700 degrees Celsius killing 30,000 people in the
city of St. Pierre within only two minutes. Only two
people are known to have survived.
b. Caldera- a volcanic depression much larger than
the original crater. It is either an exploded volcano
or a collapsed volcano.
Examples are Krakatoa (Indonesia) Crater Lake
(Oregon), and Mount St. Helens after 1980
(Washington).
VI. Life Cycle of Volcanoes
1. Active Volcano- is a volcano that is erupting now, has
erupted in the recent past, or shows signs of erupting in the
near future.
2. Dormant Volcano- is a volcano that has not erupted
recently, but has during recorded history. It is expected to
erupt again in the future.
3. Extinct Volcano- is one that has not erupted in recorded
history. It is unlikely to erupt again.
VII. Volcanism Underground
A. Intrusions- are flows of magma that cool and harden before
they reach the surface. Intrusive igneous rocks have a large
grain, or crystal, size.
The deeper the intrusion, the larger the grain size
1. Volcanic Neck- is an intrusive structure composed of
cooled magma trapped in the throat or vent of a volcano.
2. Dike- is a tabular, typically vertical intrusion
(discordant).
Dikes formed at shallow depths can have small grained
texture, while those at greater depths may have a coarse
grained texture. Often dikes will be more resistant to
erosion and leave a wall-like protrusion on the surface.
An example is Ship Rock (New Mexico). There are ancient
dikes in Wisconsin.
3. Sill- is a relatively horizontal structure that is parallel
(cordant) to the layers of country rock. Magma squeezes
in between sedimentary rock and solidifies into a sill. Like
dikes, the grain texture of a sill is dependant upon depth.
4. Laccolith- is a close relative of the sill which often
leaves a bulge at the surface. It has a slight dome or
mushroom shape, with the middle of the structure being
thicker than the sides. These can form smaller dome
mountains.
5. Pluton- is an igneous body that has cooled and
crystallized at great depths. Plutons have irregular shapes
and often cause doming of bedrock at the surface.
a. Stock- is a small discordant (nearly vertical)
pluton with an outcrop area of less than 100 square
kilometers.
b. Batholith- is a large discordant pluton with an
outcrop area of more than 100 square kilometers.
Most batholiths are felsic in composition, being
mostly granite.
Written and compiled by
Peter Watts
Riverside Middle School
131 Hall Street
Watertown, WI 53094
414-262-1480
wattsp@watertown.k12.wi.us
http://rms.watertown.k12.wi.us/RMS_Staff/wattsp/wattsp.htm