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Class Announcements
Term Break extra credit
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time.
Sign up today to attend a
Papago Park extra credit
field trip (worth 20 points).
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Wed., April 16: 10-11AM or 4-5 PM
Thurs., April 17: 10-11AM or 2-3 PM
Today’s Lecture: Chapter 9
Volcanic activity
Class presentations:
Mexican Volcanoes - Shawn Brown
Dissolved gases, silica content and
explosiveness of eruptions
Silicic volcanoes
Intermediate volcanoes
Materials extruded
extruded during
during an
an eruption
eruption
2. Materials
 lava
 gases
 pyroclastics
Magmas contain dissolved gases, held in by pressure!
Gases (volatiles)
1-6% by weight,
Mostly water vapor
Magma rises to surface
& pressure rapidly drops
gases expand
causing lava and
volcanic rock to
explode
Fig. 09.08
Stephen Marshak
Nature of volcanic eruptions
Importance of gas in eruptions
Expanding gas provides the force to fragment
and violently expel molten rock & ash.
Materialsextruded
extrudedduring
duringan
aneruption
eruption
2. Materials
 lava
 gas
 pyroclastics
Expanding gases pulverize rock, forming ash.
Violent escape hurls ash, blocks of rock
and blobs of magma into the air above the
volcano
Pyroclastic materials
Silica-rich magmas produce
explosive eruptions.
Violent volcanic eruptions produce:
 rock fragments
 finely fragmented ash
These accumulate
to form:
 molten bombs
 large angular blocks
pyroclastic
volcanic
rocks
Three basic general volcano types:
a. Shield volcanoes
b. Cinder cones
c. Composite cones
 Composed of
pyroclastics
 Small, steep sided
cones
Sometimes have
associated flows
Cerro Negro
Nicaragua
San Francisco Volcanic Field, AZ
Sunset crater, AZ
Types of Volcanoes
a. Shield volcanoes
b. Cinder cones
c. Composite cones
 Symmetrical form
 Intermediate size
Alternating lava flows
& pyroclastic deposits
Mt. Fuji, Japan
Composite
volcanoes
erupt
Mt. Mayon, Philippines
pyroclastics
and lava in
~ equal
Proportions.
Intermediate and silicic volcanism
Most found above subduction zones.
Subduction zones
Andes
Mt. Shasta, CA
Mt. Fujiyama, Japan
Stratospheric haze
Fig. 09.05a
Rising column
Falling
lapilli
Collapsing column
W. W. Norton
Nuée ardente
Features of Explosive
Volcanic Eruptions
Hot ash clouds are denser than air
and may collapse and rush down
volcanic slopes at high speeds forming
a nuee ardente (fiery cloud) or ash-flow.
yroclastic flows
Fig. 09.01a
Stephen Marshak
Fig. 09.01b
Stephen Marshak
Body cast of
Pompeii victim.
A mold was
formed
when hot ash
congealed
around body,
burning it away to
leave a mold.
Materialsextruded
extrudedduring
duringan
aneruption
eruption
2. Materials
 lava
 gas
 pyroclastics
If hot ash is erupted onto snow, or a lake,
or if once deposited, ash becomes
saturated by rain, volcanic mudflows
may form. These are highly fluid mixtures
of water, ash and rock that travel downslope
at very high speed, burying whatever is in
their path.
Volcanic mudflows or “lahars”
Fig. 09.07
U.S. Geological Survey
Mount Saint Helens Mudflow
Volcanic hazards
map for
Mount Rainier WA