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 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 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 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 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 Materialsextruded extrudedduring duringan aneruption eruption Materials ◆ 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” Fig 09.07 U.S Geological Survey Mount Saint Helens Mudflow Volcanic hazards map for Mount Rainier WA