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Volcanic Landforms
I. Effusive eruptions:
relatively quiet, non-explosive
mostly basaltic lava, flows
freely.
A. Central Vent Eruptions—
lava flows out (sometimes
fountaining) from one central
vent, then the lava solidifies in
approximately the same
volume all around.
Shield volcano: a low, broad,
cone-shaped structure - looks
like a warrior’s shield
Mauna Kea - 13,792 ft above sea level
Mauna Loa - 13,678 ft above sea level
Low angle slopes of 1-10
Both ~30,000 ft from their base
Composed primarily of basalt
lava flows
Largest volcanoes in volume
Volcanic Landforms
Shield Volcanco
Low angle slopes of 1-10
Composed primarily of
basalt lava flows
Volcanic Landforms
Effusive eruptions – Shield volcanoes
Volcanic Landforms
Effusive eruptions – Shield volcanoes
Volcanic Landforms
B. Fissure Eruptions on Land—
basalt may flow out of large cracks
in the ground (fissures)
Flood Basalts—large volume of very
fluid basaltic lava may gush out at
speeds of 25 miles per hour
Columbia River Basalts (CRB)
170,000 km3
about 17-14 Mya
Over 60 individual flows, covering
some areas in over 2 km of basalt!
One flow alone could pave I-90 from
Seattle to Boston 575 feet deep!)
Siberian Flood basalt 900,000 km3
about 245mya
Lava Plateaus—thick plateaus of
lava spreading over areas
thousands of kilometers
Fig. 7.18a
W. W. Norton
Volcanic Landforms
B. Fissure Eruptions on Land:
Flood Basalts
basalt may flow out of large cracks in
the ground (fissures)
Volcanic Landforms
B. Fissure Eruptions on Land:
Flood Basalts
Columbia River Basalt: basalt may
flow out of large cracks in the
ground (fissures)
Volcanic Landforms
II. Pyroclastic Eruptions: Explosive,
involve viscous, gas-rich magma.
The more gas-rich it is the higher the
tephra column; less gas results in
pyroclastic flows.
A. Cinder Cones: formed by gas-rich lava
of any composition (usually basaltic).
Built of tephra that is remarkably vesicular
(pumice to scoria)
Generally short lived eruptions - weeks to
a few years until the magma is degassed,
then it solidifies in the pipe and flows form
from the base
After they’re done, they never erupt again!
Paricutin, Mexico, cinder cone soon
after its birth in 1943 in a Mexican
cornfield.
Smallest volcanic features have large
craters with steep slopes of 30-40
Volcanic Landforms
B. Composite Cone or
Stratovolcano
Volcanoes on continents over
subduction zones
Built up by alternating layers
(lava and pyroclastic
deposits)
Steeper slopes 10-25
Izalco, El Salvador, December 1949.
Cascades, Andes, Aleutian
Islands
Small steam eruption and a view of the older
lava flow on the side of the cone in the
foreground.
Built over tens to hundreds of
thousands of years
Composite composite cone
2
1
Lava flow
Lava flow
3
Blast cloud
Pyroclastic
flow
Eruption on
flank of upbuilding
composite cone
4
Summit crater
Volcanic
neck
Layers of
lava flows
&
pyroclastic
s
Volcanic Landforms
1.Lava Dome
Degassed magma may
erupt in the crater and
harden there without
flowing anywhere
Produces a plug in the
volcanic vent which must
be blown away before
future eruptions can occur
Traps gases inside so
they build up pressure.
2. Calderas
Energetic eruption, blasts
out everything, then
collapses
Caldera
A large amount of
magma erupts
explosively to form
ash fall and ash
flow deposits,
partially emptying
the underlying
magma chamber.
There is essentially a big "hole" the
overlying rock collapses, leaving a
depression.
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Volcanic Landforms
1.Calderas
The following diagrams
show the formation of
Crater Lake during the
climactic eruption of
Mount Mazama.
Eruption deposits airfall
pumice and ash, blown by
winds to north and east.
Volcanic Landforms
1. Vent enlarges and eruption column collapses.
2. Pyroclastic flows deposit the Wineglass Welded Tuff on north and
east flanks of Mt. Mazama
Volcanic Landforms
Caldera has been partly
filled with pumice and ash
from the eruption with
blocks of rock from the
caldera walls
Weak, dying explosions
within the caldera deposit
ash on the caldera rim
Pyroclastic-flow deposits
develop fumaroles and
gradually cool.
Volcanic Landforms
Crater Lake today
Volcanic Landforms
C. Ash-flow eruptions
Eruption not from a cone, felsic
magma
1. Felsic magma pushes up into
the crust near the surface,
bulging the overlying rock.
2. Creates ring fractures over
the bulge.
3. May collapse, magma forced
into fractures and erupts
Forms large calderas
Largest and most devastating
eruptions in history
Volcanic Landforms
C. Ash-flow eruptions
Eruption not from a cone, felsic
magma
1. Felsic magma pushes up
into the crust near the
surface, bulging the
overlying rock.
2. Creates ring fractures over
the bulge.
3. May collapse, magma forced
into fractures and erupts
Forms large calderas
Largest and most devastating
eruptions in history
Volcanic Landforms
C. Ash-flow eruptions
Eruption not from a cone, felsic
magma
1. Felsic magma pushes up
into the crust near the
surface, bulging the
overlying rock.
2. Creates ring fractures over
the bulge.
3. May collapse, magma forced
into fractures and erupts
Forms large calderas
Largest and most devastating
eruptions in history
Volcanic Landforms
B. Composite Cone or
Stratovolcano
Mt. St Helens pyroclastic
eruption on the volcano
flank.
Lateral blast
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Volcanic Landforms
C. Ash-flow eruptions
Eruption not from a cone, felsic
magma
1. Felsic magma pushes up
into the crust near the
surface, bulging the
overlying rock.
2. Creates ring fractures over
the bulge.
3. May collapse, magma forced
into fractures and erupts
Forms large calderas
Largest and most devastating
eruptions in history
Volcanic Landforms
C. Ash-flow eruptions
Examples:
Toba, Indonesia
75,000 years ago
Caldera is 30 x 60 miles
Covered 10,000 square
miles!—1000 feet thick!
Yellowstone
3 major eruptions in last 2
million years
Approximately 1000 times
larger than Mt. St. Helens!
New felsic magma may be
pooling, thermal features are
heated by the magma
Volcanic Landforms
The major eruptions of the volcanic field were exceedingly voluminous, but their
products are only surficial expressions of the emplacement of a batholithic volume
of rhyolitic magma to high crustal levels in several episodes.
The total volume of magma erupted from the Yellowstone Plateau volcanic field
since 2.5 million years ago probably approaches 6,000 cubic kilometers.
Volcanic Landforms
C. Ash-flow eruptions
Examples:
Long Valley caldera near
Mammoth Lakes ski
resort in California, north
of Bishop, CA
Last erupturion 700,000
years ago
Over the past 20 years the
floor has risen 9 inches
Magma recently risen from 5
miles depth to 2 miles
Eruption very likely, but
timing not certain
Volcanic Landforms
C. Ash-flow eruptions
Examples:
Long Valley caldera near
Mammoth Lakes ski
resort in California, north
of Bishop, CA
Last erupturion 700,000
years ago
Over the past 20 years the
floor has risen 9 inches
Magma recently risen from 5
miles depth to 2 miles
Eruption very likely, but
timing not certain
Volcanic Landforms
Volcanic Landforms
Imagine the effects of a large
caldera forming eruption
Ash covering the US!
Clogs all air filters, engines 
no cars, no electricity, not
air travel
Abrades all moving parts
(ash + water is very heavy 
building collapses
Centimeters of ash on all crops
 crop failure and famine
Volcanic Landforms
Imagine the effects of a large
caldera forming eruption
Ash covering the US!
Clogs all air filters, engines 
no cars, no electricity, not air
travel
Abrades all moving parts
(ash + water is very heavy 
building collapses
Centimeters of ash on all crops
 crop failure and famine
Volcanic Landforms
Shield volcano (e.g. Hawaii)
9 km
150 km
Composite volcano (e.g. Vesuvius)
p.194-195c
3 km
15 km
original artwork by Gary Hincks
Cinder cone (e.g. Sunset crater)
0.3 km
1.5 km
Non-violent vs. explosive
eruptions
Basalt: flows onto the
surface
Andesite/Rhyolite:
explode, huge eruptive
clouds
Depends on the viscosity of
the lava- resistance of lava
to flow (water vs. molasses)
Viscosity is controlled by:
Santa Maria, Guatemala. Santa Maria had a
huge eruption in 1902, from a vent on the
other side of the cone as viewed from this
direction.
The 1902 eruption was not from the summit.
Starting in 1929, a lava dome began to grow
in the 1902 crater, and it is still active today. It
is named Santiaguito.
1) silica content
2) temperature
3) gas content
Basaltic lava
Basaltic lava
Silica content
As silica tetrahedra bond
together they make the magma
thicker, or more viscous.
Similar to slushy as ice bonds
start to form. Slushy is thicker
and more viscous than water.
Thus more silica = more viscous
Mafic magmas = <50% silica
Intermediate magmas >60-65%
(Andesite)
Sarigan volcano, Northern Mariana volcanoes.
It has not had any recorded eruptions but it is
very young. That is just a regular cloud over its
summit.
Felsic magma’s = >65%
(Ryolite).
Felsic (granitic) magmas = more
viscous based on silica content.
Temperature
The hotter a liquid is the less
viscous it is. Example:
heating honey or molasses
to make them flow more
readily. Breaks bonds.
Temperatures required to
melt the minerals in mafic
vs. felsic magmas.
Lascar Volcano, Chile, The most active
stratovolcano in the central Andes. Note the two
massive andesite flows exhibiting thick flow
margins tens of meters high and well-developed
lava levees. Courtesy of Peter Francis.
Minerals in mafic magmas
have higher melting
temperatures- therefore the
magmas must exist at higher
temperatures.
Minerals in felsic (granitic)
magmas have lower melting
temperatures- cooler, thus
more viscous.
Gas Content
Gases in magmas = mostly
water vapor, also SO2, H2S,
CO2, HCl, …
If the magma has a low
viscosity (e.g., basaltic
magmas) the gases can
escape easily.
Colima Volcano, Mexico. Thick, short
andesite flow on the flanks of Colima.
Courtesy of J.C. Gavilanes, Universidad de
Colima.
If they magma has a high
viscosity the gases are
trapped, they build up until
they explode (felsic
magmas)
Buoyancy
There are several factors that
are important in allowing magma
to move to the surface and erupt
as a volcano.
The first is buoyancy. Buoyancy
is the tendency for a less dense
substance to move up or float.
Pacaya, Guatemala is a volcanic complex
of two small strato-volcano cones and older
lava domes.
It has erupted over twenty-two times since
its birth in 1565 and nearly annually since
1965.
Generally, a liquid is less dense
than a solid of similar chemical
composition.
Buoyancy
Since magma is liquid rock,
surrounded by solid rock, the
magma will tend to move up
through the crust toward the
surface.
As a magma is moving toward
the surface, it is moving into
cooler areas of the crust
(geothermal gradient)
Pacaya, Guatemala. Eruptions are
generally characterized by explosions, but
recent eruptions have also produced lava
flows. Here, an ash eruption shortly after
the February 4, 1976, magnitude 7.5
earthquake.
Begins to cool down.
Buoyancy
What happens to magma when it
cools down?
It begins to crystallize. If the magma
gets to about 50% crystallized, it will
stop moving up. ( “crystal mush”).
All those crystals make the liquid
too sluggish to flow very easily, and
it simply stops moving.
Therefore, whether or not a magma
makes it to the surface is really a
race between how fast it moves up
and how fast it crystallize
The hotter a magma starts out, the
more likely it is to get to the surface
before it has reached that 50%
crystallization.
Eruption
Analogy to soda bottle:
When soda can is closed and
the soda is under pressure the
carbon dioxide (CO2) is
dissolved in the soda.
Open the bottle, release the
pressure, the carbon dioxide
comes out of solution and
bubbles out.
Gas expands and escapes.
Can happen slowly, controlled,
or violently.
Magmas generally have a
high gas content. Like soda,
the gas is dissolved within
the magma when the magma
is under pressure.
Pressure builds up until there
is an eruption, this releases
the pressure.
Again, gases expand and
escape.
Obsidian flow, Long Valley Caldera,
California
If gases escape easily and
gradually (non-viscous) it is a
non-violent eruption (basalt),
If gases can't escape easily
and gradually it results in a
violent eruption (felsic
magma).
Magmas generally have a
high gas content. Like soda,
the gas is dissolved within
the magma when the magma
is under pressure.
Gas bubbles and froth on
surface of the lava, similar to
bubbles on top of soda.
Obsidian flow, Long Valley Caldera,
California, was created by crustal
collapse associated with an explosive
eruption about 650,000 years ago.
Since that time, felsic eruptions of degassed magma have generated
viscous rhyolitic domes and short felsic
flows.
Produces distinctive texture
in the rock.
Types of explosive
volcanoes:
Composite or
stratovolcanoes
these types of eruptions,
which often alternate with
more effusive eruptions,
produce composite or
stratovolcanoes.
Very steep sided volcanoes
that are characterized by
interbedded or alternating
deposits that result from
explosive (pyroclastic rocks)
and effusive eruptions
(lavas).
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