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: – Temperature – Composition – Dissolved gases

Factors Controlling Viscosity Composition – ↑ SiO 2 ; ↑ Viscosity – ↓ SiO 2 ; ↓ Viscosity Temperature – ↑ temperature; ↓ Viscosity Dissolved Gases – ↓ pressure; Gases come out of solution – How easily gases escape determine eruptive style

Factors Controlling Viscosity Magma TypeBasaltAndesiteRhyolite SiO 2 50%60%70% Viscosity LowIntermediateHigh Gas content LowIntermediateHigh Explosiveness NonexistentIntermediateExplosive Basalt Andesite Rhyolite

What is Extruded? Lava Dissolved Gases Pyroclastic Material

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

Dissolved Gases - Volatiles H 2 O CO 2 SO 2 HCl – steam – Greenhouse gas – “Rotten egg” smell – Acid rain

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

Volcano Anatomy Vent –Opening near surface Crater –Steep-walled depression Conduit –Narrow pipe Magma Chamber –Stored magma

Types of Volcanoes –Mauna Loa – the largest volcano in Hawaii –Compare Mauna Loa with Mt. Rainier –May be found on the flanks of shield and composite cones Shield Volcanoes Composite Cone (Stratovolcanoes) Cinder Cone

Shield Volcanoes Broad, gently sloping - “Warrior’s Shield” Covers large area Erupts mainly basalt in a relatively quiet eruption

Shield Volcano Hawaiian Islands Mauna Loa, Hawaii

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

Cinder Cones California Cima Volcanic Field Amboy Crater

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

Composite Volcano Cascade Ranges Mount Lassen, California

Deadly Volcanoes Nuée Ardentes Pyroclastic flow of hot ash and gas Also known as “glowing avalanche” Move at speeds up to 200 km/hr Montserrat, 1996

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

Mount St. Helens, Washington Lateral Blast before During After

Mount St. Helens, Washington Lateral Blast Pyroclastic flow Lahar

Predicting Eruptions Seismicity Geomorphology Gas content Geophysical properties – Ground shaking – Changes in ground surface shape – Electrical conductivity, gravity anomaly –↑SO 2

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