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Class AnnouncementsClass AnnouncementsTerm Break extra credit option due Friday at class time.
Sign up today to attend a Papago Park extra credit field trip (worth 20 points).
Options:
Wed., April 16: 10-11AM or 4-5 PM
Thurs., April 17: 10-11AM or 2-3 PM
Today’s Lecture:Today’s Lecture: Chapter 9 Chapter 9Volcanic activityVolcanic activity
Class presentations:
Mexican Volcanoes - Shawn Brown
Dissolved gases, silica content and explosiveness of eruptions
Silicic volcanoes
Intermediate volcanoes
2. Materials extruded during an eruptionMaterials extruded during an eruption lava gases pyroclastics
Magmas contain dissolved gases, held in by pressure!
Magma rises to surface
& pressure rapidly drops
gases expand causing lava and volcanic rock to explode
Gases (volatiles) 1-6% by weight, Mostly water vapor
Fig. 09.08
Stephen Marshak
Importance of gas in eruptions
Expanding gas provides the force to fragment
and violently expel molten rock & ash.
Nature of volcanic eruptionsNature of volcanic eruptions
2. Materials extruded during an eruptionMaterials extruded during an eruption 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
PyroclasticPyroclastic materials
Silica-rich magmas produce explosive eruptions.
pyroclastic pyroclastic
volcanic volcanic
rocksrocks
Violent volcanic eruptions produce:
rock fragments
finely fragmented ash
molten bombs
large angular blocks
These accumulate
to form:
a. Shield volcanoes a. Shield volcanoes
b. Cinder conesb. Cinder cones
c. Composite conesc. Composite cones
Composed of pyroclastics Small, steep sided conesSometimes have associated flows
Three basic general volcano types:
Cerro Negro
Nicaragua
San Francisco Volcanic Field, AZ
Sunset crater, AZ
a. Shield volcanoes a. Shield volcanoes
b. Cinder conesb. Cinder cones
c. Composite conesc. Composite cones Symmetrical form Intermediate sizeAlternating lava flows & pyroclastic deposits
Types of Volcanoes
Mt. Fuji, Japan
Mt. Mayon, Philippines
Composite
volcanoes
erupt
pyroclastics
and lava in
~ equal
Proportions.
Subduction zones
Intermediate and silicic volcanism Intermediate and silicic volcanism
Most found above subduction zones.
Andes
Mt. Shasta, CA
Mt. Fujiyama, Japan
Fig. 09.05a
W. W. Norton
Stratospheric haze
Rising column
Collapsing column
Nuée ardente
Falling lapilli
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.
Pyroclastic 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.
2. Materials extruded during an eruptionMaterials extruded during an eruption 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”Volcanic mudflows or “lahars”
Fig. 09.07
U.S. Geological Survey
Mount Saint Helens Mudflow
Volcanic hazards
map for
Mount Rainier WA