Introduction to Volcanoes
The landforms created when
magma escapes Earth’s
interior.
I. Factors controlling types
of volcanic eruptions
A. Viscosity: resistance to
flow
1. Temperature
2. Magma composition
directly related to silica
content - - - SiO2 chains
even before crystallization
begins
Introduction to Volcanoes
I. Factors controlling types of
Volcanic Eruptions
B. Gases: 1-9% of magma
Volatiles: H2O, CO2 readily gas
at Earth’s surface.
Dissolved underground due to
pressure, as magmas rises
pressure decreases and gas
unmixes from the magma.
Gas less dense than liquid
magma so it rises pushing
the magma in front of it
Introduction to Volcanoes
I. Factors controlling types of
Volcanic Eruptions
B. Gases: 1-9% of magma
Low viscosity magmas: the
volatiles can escape more
easily & don’t accumulate
If rock above/blocking magma, gas
pressure builds up behind and finally
overlying rock shatters (throat clearing)
High viscosity magmas:
volatiles cannot escape
easily, so gas pressure
builds up within the magma
and when it reaches the
surface it explodes
explosively (soda pop
example)
Introduction to Volcanoes
II. Types of Volcanic Eruptions
1. Effusive (non-explosive): lava
flows out fairly easily
a. primarily basaltic
b. low viscosity
c. low gas content
d. more frequent non-explosive
events.
e. Lava destructive to property,
but rarely kills people.
f. mid-ocean ridges (oceanic
divergent plate boundaries),
oceanic hot spots
g. Examples: Hawaii & Laki,
Iceland
Introduction to Volcanoes
II. Types of Volcanic Eruptions
2. Explosive / Pyroclastic
a. Primarily andesitic and some
rhyolite.
b. High viscosity(rhyolite is often
too viscous to even get out of
the ground)
c. High gas content
d. Erupt less frequently, decades
to centuries—the longer the
interval, the larger the
eruption
e. Very dangerous to people and
property
f. Over subduction zones,
continental hot spots,
continental rifting zones
g. examples: Cascades, Andes,
Aleutian Islands
Introduction to Volcanoes
III. Volcanic Deposits
A.
Lava: effusive deposits
1. Basaltic Lava: most likely to
erupt because low viscosity
8 cm to several meters
thick
up to 20 mi/hr on slopes—
usually 30-1000 feet/hr
travel up to 90 miles or
more before cooling
Introduction to Volcanoes
III. Volcanic Deposits
A.
Lava: effusive deposits
1. Types of Basaltic Lava
a. pahoehoe: ropy (in Polynesian
dialect) lava
Surface cools to form and elastic
skin which gets folded into
ropes
b. ‘a’a: rough jagged basalt flows
- - - more viscous than
pahoehoe.
Resembles an advancing mass of
lava rubble, travels 5-50 m/hr
Introduction to Volcanoes
Introduction to Volcanoes
Introduction to Volcanoes
Introduction to Volcanoes
III. Volcanic Deposits
A. Lava: effusive deposits
Non-explosive
1. Basalt
2. Andesitic Lava
B. Pyrolcastic deposits
Explosive deposits
1. Andesitic
2. Rhyolitic
Viscous: it tends to cool
underground or explode
violently.
Introduction to Volcanoes
III. Volcanic Deposits
2. Andesitic Lava
Can flow like basalt (effusive)
or erupt explosively.
More viscous, flows do not
travel as far and generally
thicker flows.
Introduction to Volcanoes
III. Volcanic Deposits
3. Rhyolitic Lava: (rare)
Rhyolite magma is so viscous it
tends to cool underground or
explode violently.
If it has already been degassed—
the magma left over after an
eruption (at the bottom of a
magma chamber), it may
ooze out, but doesn’t get far
Forms domes -- Mt. St. Helens
Basaltic flow
Andesitic flow
Rhyolitic dome
Rhyolitic spire
Introduction to Volcanoes
Introduction to Volcanoes
B. Pyroclastic Deposits: air
borne deposits from explosive
volcanic eruptions. (Greek
Pyro = fire, klastos =
fragments)
1. Consists of volcanic blocks, not
from magma or lava flows
Pieces of the volcano that were
ripped out during the explosion,
up to the size of houses!
Paricutin, Mexico, During its brief nineyear cinder cone soon after its birth in
1943 in a Mexican cornfield. Photograph:
U.S. Geological Survey (K. Segerstrom).
Throat clearing to remove all the
stuff that was in the way.
When a volcano erupts, rapidly
expanding gases carry small
fragments of magma into the
air, like champagne
Introduction to Volcanoes
B. Pyroclastic Deposits: air
borne deposits from explosive
volcanic eruptions. (Greek
Pyro = fire, klastos =
fragments)
1. Consists of volcanic blocks, not
from magma or lava flows
Pieces of the volcano that were
ripped out during the explosion,
up to the size of houses!
Paricutin, Mexico, During its brief nineyear cinder cone soon after its birth in
1943 in a Mexican cornfield. Photograph:
U.S. Geological Survey (K. Segerstrom).
Throat clearing to remove all the
stuff that was in the way.
Often solidified magma from past
eruptions
Introduction to Volcanoes
B. Pyroclastic Deposits: air borne
deposits from explosive volcanic
eruptions. (Greek Pyro = fire,
klastos = fragments)
1. Consists of volcanic blocks and ejecta
2. Pieces of the volcano that were ripped
out during the explosion, up to the size
of houses!
Throat clearing to remove all the stuff
that was in the way.
Often solidified magma from past
eruption
When a volcano erupts, rapidly
expanding gases carry small fragments
of magma into the air, like champagne
Introduction to Volcanoes
Soufriere, St. Vincent, W.
Indies. April 22, 1979
6 km eruption cloud rising into
the early morning sky.
The cloud eventually rose to an
altitude of 15 km.
Photograph: U.S. Geological
Survey (F.C. Whitemore).
Introduction to Volcanoes
B. Pyroclastic Deposits : air borne
deposits from explosive volcanic
eruptions.
2. Tephra—pyroclastic material that
cools and solidifies in the air
a. Volcanic dust particles: 1/1000 mm
in diameter (cake flour) & can stay
in atmosphere for up to two years
after an eruption.
b. Volcanic ash: <2mm diameter, fine
sand to rice size, remains in the
atm for hours to days
c. Lapilli: (cinders), peas to walnuts in
size (2-64 mm). Italian for “little
stones”
d. Volcanic Bombs: (>64 mm), form
from sizable blobs of lava erupt
and solidify in mid-air
Introduction to Volcanoes
~1630 BC eruption of Santorini.
Size of tephra and thickness of layers varies as a function of distance
from a vent
Introduction to Volcanoes
Changes of tephra from volcanic source
Introduction to Volcanoes
Tephra layers
Ash
Lapilli
Bombs
Relative volcano size
Shield and Composite volcano
Relative volcano size
Shield, composite, and cinder cone
Relative volcano size