VOLCANISM
THE ROCK CYCLE
I. Introduction From: Roman god of fire, Vulcan A. What is a volcano? A conical mountain formed around a vent where lava, pyroclastic materials, and gases are erupted.
I. Introduction B. Volcanic activity: Active Dormant Extinct
B. Volcanic Activity 1. Active volcanoes activity in the last few centuries Ex: Vesuvius, 79 A.D. (50 times in 2000 yr) Ex: Mt. St. Helens (1980)
B. Volcanic Activity 2. Dormant volcanoes quiet for the last hundred to thousands of years, but still have potential to erupt. Mt. Rainier
B. Volcanic Activity 3. Extinct volcanoes No eruption in historical times No signs of erupting again
I. Introduction C. Volcano Distribution Most volcanoes occur in one of three areas: Circum-Pacific (i.e. The Ring of Fire) 60% Mediterranean 20% Spreading centers 10 15%
Volcano Distribution
II. Volcanic materials Three types of material expelled from volcanoes
Volcanic materials A. Lava ( the liquid ) Molten rock Si (silica) affects viscosity
II. Volcanic materials A. Lava 1) Pahoehoe lava Basaltic lava Low viscosity Cools moderately slowly Ropelike appearance
II. Volcanic materials A. Lava 2) Aa lava (pronounced aa-aa) Basaltic lava Higher viscosity Solidifies while flowing Angular pieces
II. Volcanic materials A. Lava 3) Pillow lavas Lava extruded underwater Cools and contracts Spherical masses Ocean floor
II. Volcanic materials A. Lava ( the liquid ) B. Ash and pyroclastic material ( the solid ) Airborne material ejected by a volcano Classified based on size:
B. Ash and pyroclastic material ( the solid ) * Volcanic ash Fine ash - <0.06mm Coarse ash 0.06mm to 2mm Composition = rock, mineral, and volcanic glass
B. Ash and pyroclastic material ( the solid ) * Cinders 2 mm and 64 mm Composition - same as ash Hazardous when falling
B. Ash and pyroclastic material ( the solid ) C) Bombs Larger than 64mm Molten rock solidifies in the air Shapes vary
II. Volcanic materials C. Volcanic gases ( the gases ) Volatiles H 2 S Hydrogen sulfide H 2 O Water vapor SO 2 Sulfides CO 2 Carbon dioxide N 2 Nitrogen HCl Hydrochloric Acid These Gases have been shown to contribute to extinction events on Earth
II. Volcanic materials A) Determines violence of an eruption High gas = violent eruptions Violent eruptions = felsic magmas High viscosity magma traps gas Expansion is prevented, pressure builds
III. Volcanic Landforms An erupting volcano will produce a number of distinct landforms including: A. Volcanic cones B. Flood basalts C. Calderas
A. Volcanic cones 1) Shield volcanoes Multiple layers of basaltic lava Shallow sides due to magma s low viscosity Gentle eruptions
A. Volcanic cones Mauna Loa volcano, Hawaii
A. Volcanic cones 2)Cinder cones Smallest volcanic cone Layered ash and cinders Short, narrow cone, Steep sides Violent eruptions Ex: El Paricutin
A. Volcanic cones Lassen National Monument, CA
A. Volcanic cones 3) Composite or stratovolcanoes Layered ash, lava, and mud Intermediate to felsic lava Steep sides, due to lava s high viscosity
A. Volcanic cones Mt. St. Helens, WA
III. Volcanic Landforms B) Flood basalts Large outpourings of basaltic lava Multiple, quiet eruptions Lava plateau
B) Flood basalts A portion of the Columbia Flood Basalts in WA
III. Volcanic Landforms C) Calderas Large depressions (> 1km) from violent eruptions Ugashik Caldera, AK
C) Calderas Two methods of formation: Method 1: Volcano rapidly empties its magma chamber, and support is lost
C) Calderas Method 1 (cont.): Overlying material collapses into magma chamber Caldera forms
C) Calderas Ex: Crater Lake, OR
C) Calderas Two methods of formation: Method 2: Volcano blows its top, leaving behind a void Inside the cone.
C) Calderas Two methods of formation: Method 2: Volcano blows its top, leaving behind a void Inside the cone.
IV. Volcanic hazards A) Lahars (hot mud flows)
IV. Volcanic hazards Lahars Sources of water Melting ice caps Excess rainfall
IV. Volcanic hazards B) Nuee Ardentes (Glowing Ash flows) Clouds of dense gas and debris French for glowing cloud High speeds and high temperatures
IV. Volcanic hazards How does a Nuee Ardente form? Volcano erupts Hot debris rises Gravity takes over
IV. Volcanic hazards How does a Nuee Ardente form? Debris descends rapidly (200 mph) Flows down mountain slopes Travel up to 80 miles
IV. Volcanic hazards C) Tsunamis Wave generated by volcanic explosion Japanese for harbor wave
IV. Volcanic hazards Tsunamis are extremely hazardous Travel vast distances Strike with short notice Krakatoa (1883) - 36,000 people died
IV. Volcanic hazards D) Lava flows Least dangerous Lava flows slowly (<10 mph) Dangerous to property