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Class Announcements. Term 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: Chapter 9 Volcanic activity.

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Class Announcements

Term 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: Chapter 9

Volcanic activity

Class presentations:

Mexican Volcanoes - Shawn Brown

Dissolved gases, silica content and explosiveness of eruptions

Silicic volcanoes

Intermediate volcanoes


  • lava

  • gases

  • pyroclastics

Materials extruded during an eruption

2. Materials extruded during an eruption

Magmas contain dissolved gases, held in by pressure!

1-6% by weight,

Mostly water vapor

Gases (volatiles)

gases expand causing lava and volcanic rock to explode

Magma rises to surface

& pressure rapidly drops


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.


  • lava

  • gas

  • pyroclastics

Materials extruded during an eruption

2. Materials extruded during an eruption

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

  • molten bombs

  • large angular blocks

These accumulate

to form:

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

pyroclastics

and lava in

~ equal

Proportions.

Mt. Mayon, Philippines


Intermediate and silicic volcanism

Most found above subduction zones.

Subduction zones


Andes


Mt. Shasta, CA


Mt. Fujiyama, Japan


Fig. 09.05a

Stratospheric haze

Rising column

W. W. Norton

Collapsing column

Falling lapilli

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.

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.


  • lava

  • gas

  • pyroclastics

Materials extruded during an eruption

2. Materials extruded during an eruption

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


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