Volcanic

Mount ST. Helens, Washington

Largest historic eruption in North America (1980).

• Erupted a cubic kilometer of ash and rock debris

• Associated ash, pyroclastic flows, and mudflows

• Claimed 59 lives

Kilauea, Hawai’i

Began in 1983 and is the most recent of at least 50 eruptive phases

• Non-explosive eruptions of fluids lavas

• Destroyed 180 homes and a national park visitor center

The Nature of Volcanic Eruptions: Magma

Magma is the source material for volcanic eruptions

• Composition: Basaltic and Granitic

• Basaltic: rich in dark silicates

• Granitic: rich in light silicates

The Nature of Volcanic Eruptions: Where magma is generated?

• Basaltic: results from partial melting of upper mantle

• Granitic: results from melting of continental crust

Effusive versus explosives eruptions are controlled by…

Viscosity and Gas content

Viscosity: the resistance to flow

• Higher Silica content = higher viscosity

• Higher Viscosity are more explosive

Gas content: amount of dissolved gasses present in magma

• Higher viscosity have more tapped gasses = more explosive

High Gas content / High Silica

Conical Composite

• Ex: MT.ST.Helens

• made of two things (ash and lava)

High Gas Content / Low Silica

Cinder Cone

• Ex: Amboy

Low Gas Content / High Silica

Dome

• Ex: MT.ST.Helens

Low Gas Content / Low Silica

Shield Volcano

Ex: Hawai’i

Eruptions of highly viscous lavas may produce explosives clouds of hot ash and gases called…..

Eruption Columns

Materials Extruded During an Eruption: Lava Flows - Basaltic lavas are much more….

Fluid!

What are the 3 types of lava

• Pahoehoe lava

• Aa lava

• Pillow Lava

Pahoehoe Lava

resembles a twisted or ropey texture

Aa Lava

rough, jagged blocky texture

Pillow Lava

occurring along divergent plate boundaries on the ocean floor or other underwater eruptions

Materials Extruded during an Eruption: Lava Flows - Andesitic/Rhyolitic lavas are viscous….

Black Lavas (rough, travel short distances)

Materials Extruded During an Eruption: Dissolved Gasses

• one to six percent of a magma by weight

• Mainly water vapor and carbon dioxide

Materials Extruded During of Eruption: Pyroclastic Materials (“Fire Gragments”) what are they?

• Pumice

• Ash and Dust

• Lapilli

• Cinders

• Blocks

• Bombs

Pumice

Porous rocks from “frothy” lava

Ash and Dust

fine, glassy fragments

Lapilli

walnut-sized materia

Cinders

pea-sized material

Blocks

hardened or cooled lava

Bombs

ejected as hot lava

• molten lava

Anatomy of a Volcano: General Features

opening at the summit of a volcano

• Crater

• Caldera

• Vent

Crater

steep-walled depression at the summit, generally less than 1 kilometer in diameter

Caldera

a summit depression typically greater than 1 kiometer in a diameter, sometimes produced by collapse following a massive eruption

Vent

a general name for any opening in a volcano that materials come out of, such as a gas vent (fumarole), lava vent, etc.

All craters are……….But not all vents are……..

all craters are VENTS but not all vents are CRATERS

Shield Volcanos

Features:

• Largest Volcanoes

• Broad, slightly dome-shaped

• Basaltic Lava

• Mild eruptions of large volumes of lava

• Most begin on the ocean floor (called seamounts)

Cinder Cones

Features:

• Built from ejedted basaltic lava (mainly cinder-sized) fragments, forming a steep slope

• Small in size and frequently occur in groups

Composite Volcanoes

Features:

• Large, classic-shaped volcano (thousands of feet high and several miles at base)

• composed of interbedded lava flows and layers of pyroclastic debris

• violent eruptions

• Most are located adjacent to the the pacific ocean (e.g. Fujiyama, MT.ST. Helens)

Volcanic Hazards

• Pyroclastic flow

• Lahar

• Tsunamis

• Volcanic Ash and Aviation

• Volcanic Gas and Respiratory Health

• Effects on Climate

Pyroclastic Flow

a fast-moving, ground-hugging avalanche of hot gas, ash, and volcanic rock (tephra) that rushes down a volcano's flanks during explosive eruptions

• A flow of hot gas, ash, rock or any combination of the three

• Driven by gravity down the slopes of a volcano at speeds over 200 kilometers per hour

• can be over 1100 degress celsius (2000 degrees Fahrenheit)

• Destroyed the ancient cities of Pompeii and Herculaneum as well as many others

Lahar

• Mudflow from interaction with ash/rock and water

• Follow stream valleys

• Can result from melting of summit glaciers or snowpack

• a violent, fast-moving mixture of water, volcanic ash, and debris that flows down volcano slopes like wet concrete, reaching speeds over

Tsunamis

triggered from collapse of volcanic landforms in the ocean

Volcanic Ash and Aviation

clouds of volcanic ash damage planes

Volcanic Gas and Respiratory Health

poisonous gases can be inhaled

Effects on Climate

dust and gas can significally alter global temperature

Other Volcanic Landforms: Calderas

Calderas are steep-walled depressions that are generally larger than 1 kilometer

They form from either…

• Collapse of an empty magma chamber

• collapse of the top of a shield volcano from subterranean drainage

• Collapse of a large area from discharge of large volumes of pumice and ash

Other volcanic landforms: Fissure eruptions and basalt plateaus

Fluid Basaltic lava extruded from crustal fractures called fissures

• Example: Columbia River Plateau

Other volcanic landforms: Volcanic Necks

Volcanic necks are resistant vents left standing after erosion has removed the volcanic cone

• Ex: Ship Rock, New Mexico

Intrusive Igneous Activity

• Intrusions (plutons)

• Tabular Intrusive Bodies

• Massive Intrusive Bodies

Intrusive Igneous Activity: Intrusions (plutons)

magma rises through crust

Intrusive Igneous Activity: Tabular Intrusive Bodies

magma forcibly injected into a fracture zone or bedding plane

• Dikes

• Sills

Dikes

Cuts across bedding surfaces

Sills

travel along bedding planes

Intrusive Igneous Activity: Massive Intrusive Bodies

large amounts of intruded material

• Batholiths

• Stacks

• Laccoliths

Batholiths

• Largest intrusive bodies

• occur as long, linear features, often under mountain

Stacks

small batholith

Laccoliths

• Forcibly injected between strata

• Can cause strata on top to rise

Partial Melting and the origin of magma: Earths Crust and Mantle are composed primarily of…..

Solid, not molten rock

Partial Melting and origin of Magma: Partial Melting

incomplete melting of rocks that produces most magma

• generating magma through decompression

• adding water causes melting of lower temperatures

Plate Tectonics and Volcanism: Global distribution of igneous activity is…….

Not random!

Plate Tectonics and Volcanism: Most Volcanoes are located…..

within or near plate margins

Plate Tectonics and Volcanism: Divergent Boundaries…..

produce greatest volume of lava

Plate Tectonics and Volcanism: Convergent boundaries result…..

in more explosive volcanism

Plate Tectonics and Volcanism: Interplate Volcanism

• activity within a tectonic plate

• Associated with plumes of heat in the mantle

• Forms localized volcanic regions in the overriding plate called a hot spot

• Produces basaltic magma sources in oceanic crust (e.g., Hawai’i and Iceland)

• Produces granitic magma sources in continental crust (e.g. Yellowstone park)

How many volcanoes are there?

550 volcanoes