Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Exploring Geology Chapter 6 Volcanoes and Volcanic Hazards

06.00.a Before 1980 eruption Mount St. Helens After 1980 eruption During eruption

Ash from eruption rose 25 km Ash spread across Washington, Idaho, and Montana Lava dome formed in crater after eruption Volcano constantly monitored by USGS Mt. St. Helens disintegrates in enormous landslide -hvgP2uY

Crater Vent where magma erupted Commonly a hill or mountain Can be erupted from fissure or depression, so not a hill Observe some characteristics of a volcano a Can erupt lava or ash

06.01.b Erosion of flow Fissure feeds a lava flow Hill was not over a vent (so not a volcano) Example of such a hill that is not a volcano Observe how a hill capped by volcanic rocks can form. Is it a volcano?

Observe how a hill capped by volcanic rocks can form Consider if this would be classified as a volcano EXPLANATION Lava erupts from vent or fissure Erosion around lava flow Fissure location marked by dike Eventually forms as a flat-topped mesa It is a hill, but not a volcano NOTES Photograph: Hopi Buttes, Arizona

06.01.c Shield Volcano Scoria Cone Types of Volcanoes: Part 1

Scoria cone also called cinder cone usually basaltic several hundred meters high Shield volcano gentle slopes usually basaltic with scoria and ash size ranges from a kilometer across to huge mountains NOTES Scoria cone: northern Arizona Shield volcano: Iceland

Paracutin eruption of

06.01.c Volcanic Dome Composite Volcano Types of Volcanoes II

Composite volcano Symmetrical mountains Interlayered lava flows, pyroclastic deposits, mudflows Mostly andesite, but also contains felsic and mafic rocks Volcanic dome dome-shaped constructed feature Solidified lava with volcanic ash and rock fragments Very viscous felsic or intermediate magma NOTES Composite volcano: Ecuador Dome: Mount St. Helens, Washington

06.02.a Observe these types of eruptions Dome Lava flow Lava fountain Eruption column

Observe these types of eruptions and discuss what you see with a classmate clow: magma erupts onto surface and flows away Lava fountain: gas propels molten pieces of lava into air Dome: viscous lava piles up around vent Eruption column: volcanic ash, pumice, rock fragments ejected into air Upper two photos are mafic and lower two are felsic (could bring in viscosity) NOTES Lava flow: Kilauea, Hawaii Lava fountain: Kilauea, Hawaii Dome: Mount St. Helens, Washington Eruption column: Augustine volcano, Alaska

06.02.b2 Dissolved gas held in magma by pressure What happens when you open a soda? Under less pressure, gas forms bubbles Released gas propels eruption and forms ash How Do Gases Affect Magma? Opening top releases dissolved gas held in by pressure

Magma, like soda, contains dissolved gases including water, carbon dioxides, and sulfur dioxide As magma rises, pressure decreases and gases come out of solution

06.02.c How Does Viscosity Affect Eruptions? More viscous: difficult to flow and traps gas Less viscous: flows easier and gas can escape

Felsic magmas contain high amounts of silica so more viscous Mafic magmas contain less amounts of silica so less viscous NOTES More viscous: Mount St. Helens, Washington Less viscous: Kilauea, Hawaii

06.03.a Scoria Cones and Basalt Flows: Rock Types Nonvesicular basalt Vesicular basalt Scoria

06.03.a Features of Lava Flows AA lava Lava tubes Pahoehoe lava

Lava tubes (Hawaii) surface of lava flow solidifies but interior still moving tube can drain, leaving behind cave AA lava (Hawaii) breaks apart into fragments as it flows Pahoehoe lava (Hawaii) surface forms small folds usually fed by a lava tube