Lecture 18 more volcanoes

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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


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.