Lecture 18 more volcanoes

Today’s Lecture: Chapter 9Volcanic activity Class presentations: Mexican Volcanoes - Shawn Brown Dissolved gases, silica content and explosiveness of eruptions Silicic volcanoes Interm

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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

2 Materials extruded during an eruption Materials extruded during an eruption

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 eruptions

2 Materials extruded during an eruption Materials extruded during an eruption

Silica-rich magmas produce

explosive eruptions.

pyroclastic volcanic rocks

Violent volcanic eruptions produce:

a Shield volcanoes

b Cinder cones

c Composite cones

 Composed of pyroclastics

 Small, steep sided cones

Sometimes have associated flowsThree basic general volcano types:

Cerro Negro

Nicaragua

San Francisco Volcanic Field, AZ

Sunset crater, AZ

a Shield volcanoes

b Cinder cones

 Intermediate sizeTypes of Volcanoes

Mt Fuji, Japan

Mt Mayon, Philippines

Compositevolcanoeserupt pyroclastics and lava in

~ equalProportions

Intermediate and silicic volcanism

Most found above subduction zones.

Andes

Fig 09.05a

W W Norton

Stratospheric haze

Rising column Collapsing column Nuée ardente

Falling

lapilli

Features of ExplosiveVolcanic 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

Fig 09.01a

Stephen Marshak

Fig 09.01b

Stephen Marshak

Body cast of Pompeii victim

A mold was formed when hot ash congealedaround body, burning it away toleave a mold

2 Materials extruded during an eruption Materials extruded during an eruption

 gas

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

Fig 09.07

U.S Geological Survey

Mount Saint Helens Mudflow

Volcanic hazards

map for Mount Rainier WA