Volcanoes

Prepare for Quiz Print Name: Bill Shields Lab Section: 12 TA: Ryan Signature: #&$*&(*&)*(&

1. Igneous rocks high in silica are dark in color. A. true B. false 2. Igneous rocks which cool slowly have coarse-grains, (large crystals). A. true B. false 3. Volcanic igneous rocks are fine-grained, (small crystals). A. true B. false 4. Extrusive igneous rocks are plutonic. A. true B. false 5. Mafic igneous rocks are low in iron and magnesium. A. true B. false

Increasing Fe and Mg Increasing silica (SiO 2 )

Mount Vesuvius

Pompeii

Types of Volcanoes *Based on type of eruption Nonexplosive – shield volcano Explosive – composite (strato) volcano Largely a function of- viscosity - dissolved gases

Types of Volcanoes

Shield Volcanoes produce Non-explosive eruptions

Shield Volcano: gentle slopes, layered lava flows

Shield Volcano: gentle slopes, layered lava flows

Fissure Flows

Basalt Flow

14-16 million years ago 3500m thick 300 separate flows Covers 164,000 sq. Km

Composite Volcanoes Produce explosive eruptions

Composite (Stratovolcano) Alternating layers of lava, ash, and pyroclastics

Composite (stratovalcano) steep-sided, alternating layers of lava, ash, and pyroclastics

Mt. St. Helens – before eruption Mt. St. Helens – after eruption

Volcanism at Divergent Margins Few actual volcanoes, lava erupts from fissures. Molten results from decompression melting. Mafic in chemical composition.

Divergent Plate Boundaries

Pillow Lava

Submarine eruption of basaltic lava

Volcanism at Convergent Margins Volcanoes form in a line parallel to the subduction zone. Molten material is generated by heating "wet" rocks that are being subducted.

Convergent Plate

Convergent plates of the Northwest

Volcanism at Convergent Margins Stratovolcanoes - steep-sided - alternating layers of lava, ash and pyroclastics - explosive (due to high magma viscosity and gas content Silicic magma chemistry

Mantle Plumes (Hot Spots) Molten material rising from deep within the Earth. Mafic magma chemistry. Can form shield volcanoes – broad and gently sloping sides, non-violent eruptions, built by repeated layers of lava flows (Hawaiian Islands).

Hawaiian Islands

Tephra - A mixture of hot gas and fragments of all sizes

Ash

2-64 mm Lappili

Block & Bombs

Pahoehoe Lavas

Lavas – A’a

Formation of craters and calderas

Resurgent Dome

Hazardous Volcanic Geologic Processes 1.Tephra Fall 2.Pyroclastic Flows 3.Pyroclactic Surges 4.Explosive Ejection of Ballistic Projectiles 5.Lateral Blasts 6.Lava Flows 7.Lahars

Tephra Fall Mixture of hot gas and Fragments less dense than air

Major Hazards of Tephra Fall 1.Impacts 2.Burial of structures 3.Suspension of abrasive aerosols

Pyroclastic Flows Avalanches of hot, dry, volcanic rock fragments and gases Denser than air O F Up to 200 mph Tends to channel into valleys

Causes of Pyroclastic Flow 1. High vertical eruption columns 2. Boil over 3. Disruption of hot dome

1902 Eruption - Mt. Pelee St. Pierre, West Indies

Pyroclastic Surges Avalanches of hot, dry, volcanic rock fragments and gases Less dense than air O F Up to 200 mph Not confined to channels

Pyroclastic Surge Hazards Incineration Destruction by high-velocity ash-laden winds Impacts by rock or flying material Exposure to noxious gases (SOx) (CO2)

Explosive Ejection of Ballistic Projectiles Not constrained by wind direction Not dependent on eruptions Large projectiles thrown 1/2 mile

BOMBSRibbon Rotational Bread crustFusiform

BLOCKS

Lava Flows Flow controlled by topography Destructive, but not life threatening Speed related to slope and viscosity

Pahoehoe Flow

Aa - lava

Pahoehoe

‘A’a Flow

Diversion Barriers

Lahars Rapid flow of sediment, rock, and water Channelized Twice as fast as water

Lahars

Predicting Eruptions Ground tilt and displacement Increases in surface temperature Monitoring earthquakes Changes in gas composition

Tragedy at Lake Nyos, Aug, 1986

Nyos Degassing