Volcanoes A Hot Topic.

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Presentation transcript:

Volcanoes A Hot Topic

What is a volcano? A mountain formed by lava and/or pyroclastic material

Nature of Volcanoes Explosive vs. Effusive (Erupts violently) (Quiet eruptions)

Nature of Eruption Controlled by Viscosity A material’s resistance to flow Factors controlling viscosity: Composition Temperature Dissolved gases

Factors Controlling Viscosity Composition ↑ SiO2; ↑ Viscosity ↓ SiO2; ↓ Viscosity Temperature ↑ temperature; ↓ Viscosity Dissolved Gases ↓ pressure; Gases come out of solution How easily gases escape determine eruptive style

Factors Controlling Viscosity Basalt Andesite Rhyolite Magma Type Basalt Andesite Rhyolite SiO2 50% 60% 70% Viscosity Low Intermediate High Gas content Explosiveness Nonexistent Explosive

What is Extruded? Lava Dissolved Gases Pyroclastic Material

Types of Lava Aa Pahoehoe Rough, jagged surface Higher viscosity Lower temperature Pahoehoe Smooth, ropy surface Lower viscosity Higher temperature

Dissolved Gases - Volatiles H2O steam CO2 Greenhouse gas SO2 “Rotten egg” smell HCl Acid rain

Pyroclastic Material Ground-hugging avalanche that rushes down the side of a volcano Material consists of: Hot ash Pumice Rock fragments Dust

Types of Volcanoes Shield Volcanoes Composite Cone (Stratavolcano) Largest, quiet eruption Composite Cone (Stratavolcano) Most explosive Cinder Cones Small, classic shape

Shield Volcanoes Broad, gently sloping - “Warrior’s Shield” Covers large area – lava tubes carry lava great distance Erupts mainly basalt

Shield Volcano Hawaiian Islands Mauna Loa, Hawaii

Composite (Stratovolcano) Alternating layers of lava and pyroclastic material Large, classic-shaped Associated with subduction zones Multiple eruptions

Composite Volcano Cascade Ranges Mount Lassen, California

Cinder Cones Cone built by cinders – lava fragments Explosive, but small One eruptive event

Cinder Cones California Cima Volcanic Field Amboy Crater

Deadly Volcanoes Lateral Blasts An explosion of rock, ash, and gas released at side of volcano May exceed speed of sound St. Pierre, 1902; 29,000 killed

Deadly Volcanoes Nuée Ardentes Pyroclastic flow of hot ash and gas Also known as “glowing avalanche” Mt. Pelee is in Martinique. A stratovolcano caused by the subduction of the North American Plate under the Carribean Plate Move at speeds up to 200 km/hr Montserrat, 1996

Deadly Volcanoes Lahars Volcanic mudflow Consist of water, rock, ash, and mud May travel >80 km distance at 35-60 km/hr Nevado del Ruiz, Colombia Causes: In 1985, 23,000 people died Heat melts snow and ice Rainfall eroding loose volcanic debris Landslides of water-saturated debris

Mount St. Helens, Washington After During before Lateral Blast

Mount St. Helens, Washington Lateral Blast Pyroclastic flow Lahar

Predicting Eruptions Seismicity Geomorphology Geophysical properties Ground shaking Geomorphology Changes in ground surface shape Geophysical properties Electrical conductivity, gravity anomaly Gas content ↑SO2

Mount Pinatubo, Philippines Successful Prediction Erupted in 1991 Prediction saved 1,000’s of lives Cloud circled entire equator Global temperatures decreased