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

– ↑ 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

Magma Type Basalt Andesite Rhyolite

SiO2 50% 60% 70%

Viscosity Low Intermediate High

Gas content Low Intermediate High

Explosiveness Nonexistent Intermediate Explosive

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

• H2O

• CO2

• SO2

• 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

Types of Volcanoes

– Largest, quiet eruption

– Most explosive

– Small, classic shape

• Shield Volcanoes

• Composite Cone (Stratavolcano)

• Cinder Cones

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)

• Associated with subduction zones

• Large, classic-shaped

• Alternating layers of lava and pyroclastic material

• Multiple eruptions

Composite VolcanoCascade Ranges

Mount Lassen, California

Cinder Cones

• Cone built by cinders – lava fragments

• Explosive, but small

• One eruptive event

Cinder Cones California

Cima Volcanic FieldAmboy Crater

Deadly VolcanoesLateral 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 VolcanoesNuée Ardentes

• Pyroclastic flow of hot ash and gas

• Also known as “glowing avalanche”

• Move at speeds up to 200 km/hr

Montserrat, 1996

Deadly VolcanoesLahars

• 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 35-60 km/hr

Nevado del Ruiz, Colombia

In 1985, 23,000 people died

Mount St. Helens, Washington

• Lateral Blast

beforeDuringAfter

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

– ↑SO2

Mount Pinatubo, PhilippinesSuccessful Prediction

• Erupted in 1991

• Prediction saved 1,000’s of lives

• Cloud circled entire equator

• Global temperatures decreased