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Wed., April 16: 10-11AM or 4-5 PM

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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