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

2. Materials extruded during an eruption Materials extruded during an eruption u lava u gases u 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 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 u lava u gas u 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 Pyroclastic materials

Silica-rich magmas produce explosive eruptions. pyroclasticvolcanicrocks Violent volcanic eruptions produce: u rock fragments u finely fragmented ash u molten bombs u large angular blocks These accumulate to form:

a. Shield volcanoes b. Cinder cones c. Composite cones u Composed of pyroclastics u Small, steep sided cones uSometimes have associated flows Three basic general volcano types:

Cerro Negro Nicaragua

San Francisco Volcanic Field, AZ

Sunset crater, AZ

a. Shield volcanoes b. Cinder cones c. Composite cones u Symmetrical form u Intermediate size uAlternating 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 Most found above subduction zones.

Andes

Mt. Shasta, CA

Mt. Fujiyama, Japan

Fig a 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 a Stephen Marshak

Fig b 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 eruption Materials extruded during an eruption u lava u gas u 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”

Fig U.S. Geological Survey Mount Saint Helens Mudflow

Volcanic hazards map for Mount Rainier WA