Volcanoes and Seismic Hazards Volcanoes I What is

Volcanoes and Seismic Hazards

Volcanoes I. What is a volcano? • Opening in Earth’s crust through which molten rock, gases, and ash erupt. Volcano Formation: Crater – a basin-like rimmed structure at top or on flanks of a volcanic cone. Vent – opening at earth’s surface through which volcanic material is extruded. Magma Chamber – reservoir of magma in the shallow part of the lithosphere

II. Magma Formation: 3 conditions that allow magma to form Example of location this would occur Decrease in pressure lowers melting Rift valley at midtemperature of materials in ocean ridge asthenosphere Temperature increase can cause Asthenosphere at a materials to melt hot spot Increase in amount of water in asthenosphere can lower melting temperature of materials there Asthenosphere at subduction boundaries

What Controls the Type of Magma and Eruption Style? - The viscosity of the magma • What effects viscosity? - Temperature of magma – hotter, lower viscosity, more fluid - Composition of magma (amount of silica) – more silica, higher viscosity, more resistant to flow.

What Makes Magma/Lava Erupt? • Magma is a liquid, less dense than surrounding material. • Gases within magma rise to surface • Low viscosity lava: gases easily rise and are gently released • High viscosity lava: gases build up – explosive eruptions

III. Types of Magma Characteristic Basaltic Magma (Mafic) Andesitic Magma Rhyolitic Magma (felsic) Silica content ≤ 50% ≈ 60% ≈ 70% Gas content Least Intermediate Most Viscosity Least Intermediate Most Type of eruption Rarely explosive Sometimes explosive Usually explosive Melting temperature Highest Intermediate Lowest Location Rifts, oceanic hotspots Subduction boundaries Continental hotspots

Basaltic Magma: Kilauea

Andesitic Magma: Mt. St Helens

Rhyolitic Magma: Yellowstone Caldera (rim)

IV. Ejected Material: A. Lava: liquid molten rock 1. Pahoehoe – ropelike lava 2. Aa – crumbly lava 3. Pillow lava – thought to have formed under water

B. Solid Pyroclastic Material: 1. Ash – microscopic solids Cinders – pea-sized 2. Lapilli – walnut size 3. Blocks/Bombs – football size or bigger 4. Pyroclastic flow – avalanche of burning ash

V. Volcanic Landforms • Cinder Cone: steepsided, formed by explosive eruption of cinders • Small height, short lived • Sunset crater, Arizona

Composite Volcano (Stratovolcano) Steep-sided, built by lava flows and pyroclastic deposits (tephra) alternating layers, intermediate composition (andesitic), most dangerous Osorno volcano in the Chilean Andes

Shield Volcano Gentle slope, resembles a warriors shield, quiet eruptions of fluid lava flows (basaltic), largest landforms on earth Mauna Loa

Caldera Crater-shaped basin formed after top of a volcano collapses Crater Lake, Oregon

Match description to type of eruption and volcano • 1. Thin mafic lava flows, gentle slopes of hardened lava layers shield. • 2. Felsic thick lava flows, much pyroclastic debris/steep slope composite • 3. Small steep-sided, formed by explosive eruption of cinders cinder cone • 4. Mt. St. Helens composite volcano • 5. Mt. Pinatubo part of a chain of composite volcanoes • 6. Mt. Fuji stratovolcano or composite • 7. Kilauea shield • 8. Craters on the Moon lava flow field with cinder cones

Mt Ruapehu, New Zealand • A cone volcano – • Very active • Last erupted in 1995

Lahars Volcanic mudflow often formed when hot ash mixes with water from melted snowand ice or a crater lake.

Earthquake Destruction 1. Ground Shaking There are several controls to amount of damage any area may sustain: • Duration of shaking • Distance from epicenter • Type of bedrock material • Amount of slippage along faults

2. Building Collapse Earthquakes don’t kill people, buildings kill people.

3. Fire Photograph showing the great fire following the 1906 San Francisco Earthquake – magnitude 8. 1 -8. 2.

4. Landslides and Ground Subsidence Damage from the 1964 Alaskan Earthquake – a massive earthquake where over 200 acres of land slid toward the ocean. (9. 4)

Liquefaction- Stable ground turns into fluid not capable of supporting structures

5. Tsunami – “ Harbor Wave” • Most triggered by subduction-zone earthquake and earthquake induced landslides. • Wall of water is pushed up from the ocean floor • Can travel across ocean as a series of waves

6. Ring of Fire Earthquakes and volcanoes are not distributed randomly – they occur in specific regions- usually along zones. where plate boundaries meet.

7. Volcanic Hazards Lava ash is deadly Mudflows (Lahars) are deadly • Toxic gases • Pyroclastic bombs, • Cause acid rain • Landslides • Even though violent – often results in fertile soil (Ruapehu Lahar emergency) • •

1953 Tangiwai disaster

Mt. St, Helens Mudflow and Pyroclastic Blast

Seismic Risk Map (USA)

Geologic Hazards and Emergency Preparedness • Millions live in hazardous areas • Many have no choice • Many choose to live there • Risks and Benefits: volcanic ash creates fertile soil • tourists • geothermal energy Scientists try to predict hazards in advance - Monitoring escaping gas, - increased magma temperature, - animal behavior

Good planning reduces the effects of the hazard • Monitoring – warning • Emergency supplies • Families can organize • Local emergency services be prepared • Information available • Buildings and roads designed to cope.