Earth Science
Volcanoes and Other Igneous Activity
Chapter 7 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition
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Mount St. Helens
Largest historic eruption in North America
Lowered peak by more than 400 m
Destroyed all trees in a 400 km2 area
Mudflows 29 km down Toutle River
Ejected 1 km3 ash more than 18 km into stratosphere
Mount St. Helens Versus Kilauea
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Mount St. Helens Versus Kilauea
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Kilauea
Quiet eruption of fluid basaltic lava
Occasional lava sprays
Eruption began in 1983 and has been ongoing for more than 20 years
Mount St. Helens Versus Kilauea
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Viscosity
Resistance to flow
How to decrease magma viscosity
Increase temperature
Decrease silica content
Rhyolitic magma (>70% Si) forms short, thick flows
Basaltic magma (~50% Si) is fluid
Gas content also dictates nature of eruption
Directly related to composition
Most common gas is water vapor
Quiescent Versus Explosive Eruptions
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The Nature of Volcanic Eruptions
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Triggered by addition of magma to near-surface magma chamber
Inflation and fracture of volcano summit
Fluid basaltic lava
Ongoing eruption of Kilauea since 1983
Quiescent Hawaiian-Type Eruptions
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Pressure decreases as magma rises
Dissolved gas forms expanding bubbles
Viscous magma expels fragmented lava and gas
Buoyant plumes of material (eruption columns)
Rapid ejection of magma
Reduces pressure in magma chamber
Causes further expansion and eruption
How Explosive Eruptions Are Triggered
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How Explosive Eruptions Are Triggered
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90% of lava is basaltic
Most is erupted on seafloor (submarine volcanism)
Flows in thin, broad sheets or ribbons
Flow rate ~10 to 300 m/hr
Up to 30 km/hr downhill
~9% is andesitic/intermediate
<1% is rhyolitic
Thick flows move imperceptibly slow
Don’t flow beyond a few km from vents
Lava Flows
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Two types of basaltic lava flows:
Aa
Rough, jagged blocks with sharp edges
Cooler, more viscous basaltic flows
Pahoehoe
Smooth, ropy surfaces
Hotter, less viscous basaltic flows
Lava Flows
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Lava Flows
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Lava tubes
Insulated pathways of a lava flow
Pillow lavas
Numerous tube-like structures stacked atop each other
Form on the ocean floor
Lava Flows
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Lava Flows
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Gases in magma are volatiles
Dissolved into magma because of confining pressure
~1–8% of total magma volume
Most abundant gases (in decreasing order): water vapor, carbon dioxide, sulfur dioxide, lesser amounts of hydrogen sulfide, carbon monoxide, and helium
Contribute to atmosphere
Significant quantities can alter global climate
Gases
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Pyroclastic materials or tephra
Particles erupted from a volcano (ash and dust)
Hot ash fuses to form welded tuff
Lapilli and cinders are the size of small beads to walnuts
Blocks are larger than 64 mm
Bombs are streamlined blocks ejected while still molten
Scoria is vesicular ejecta
Pumice is felsic equivalent
Pyroclastic Materials
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Materials Extruded During an Eruption
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Volcanic landforms are the result of many generations of volcanic activity
Eruptions start at a fissure
Flow is localized into a circular conduit
Conduit terminates at a vent
Successive eruptions form a volcanic cone
Anatomy of a Volcano
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Crater
Funnel-shaped depression at the summit
Form by erosion during, or collapse following, eruptions
Calderas are craters >1 km
Flank eruptions generate parasitic cones
Secondary vents that emit gas only are called fumaroles
Anatomy of a Volcano
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Broad domed structures built by accumulation of basaltic lava
Most begin as seamounts (submarine volcano)
e.g., Canary Islands, Hawaiian Islands, Galapagos, Easter Island, Newberry Volcano in Oregon
Shield Volcanoes
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9 km high
Low angle slopes
Well-developed caldera from collapse of magma chamber following eruption
Mauna Loa: Earth’s Largest Shield Volcano
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Kilauea is most active and studied volcano
50 eruptions since 1823
Most recent began in 1983
Magma chamber inflates and earthquake swarms indicate an impending eruption
Kilauea: Hawaii’s Most Active Volcano
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Cinder cones (scoria cones)
Symmetrical
Steep-sided
Loose accumulations of ejected scoria
Commonly pea- to walnut-sized fragments
Basaltic composition, reddish-brown color
Some produce lava flows
Craters are relatively large and deep
Cinder Cones
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Cinder cones form quickly
Many in less than one month
Generally in a single eruptive event
Small size (30–300 m)
Cinder Cones
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Cinder Cones
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Composite cones or stratovolcanoes
Located around the Ring of Fire
Large, symmetrical cones
Built by layers of cinder and ash alternating with lava flows
Primarily silica-rich andesitic magma
Associated with explosive eruptions and abundant pyroclastic material
Steep summit and gradually sloping flanks
Composite Volcanoes
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Composite Volcanoes
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70 volcanic eruptions expected each year
One large-volume eruption each decade
500 million people live near active volcanoes
Volcanic hazards include:
Pyroclastic flows
Lahars
Lava flows
Ash and volcanic gasses
Volcanic Hazards
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Volcanic Hazards
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Pyroclastic flow (nuee ardente)
Hot volcanic gas infused in incandescent ash and lava fragments
Gravity driven, can move up to 100 km/hr
Low-density cloud of hot gases and fine ash on top of layer of vesicular pyroclastic material
Caused by collapse of eruption columns
Pyroclastic Flow: A Deadly Force of Nature
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Volcanic Hazards
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Lahars
Fluid mudflows
Water-saturated volcanic debris move down steep volcanic slopes
Can occur on dormant/extinct volcanoes
Lahars: Mudflows on Active and Inactive Cones
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Tsunamis
Caused by collapse of volcano flanks into the ocean
Ash
Can damage buildings, living things, aircraft engines
Sulfur dioxide
Affects air quality and creates acid rain
Atmospheric cooling
Ash and aerosols reflect solar energy
Other Volcanic Hazards
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Volcanic Hazards
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Caldera
Steep-sided crater less than 1 km in diameter
Formed by summit collapse following draining of the magma chamber
Other Volcanic Landforms
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Fissure eruptions
Emit basaltic lavas from fissures (fractures)
Basalt plateaus
Flat, broad accumulations of basalt emitted from fissures
Flood basalts
Molten lava having flowed long distances within a basalt plateau
Other Volcanic Landforms
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Other Volcanic Landforms
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Volcanic necks (plugs)
Eroded volcanic cones expose the solidified magma inside the conduit
Other Volcanic Landforms
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Magma that crystallizes in Earth’s crust displacing host or country rock forms intrusions or plutons
Exposed by uplift and erosion
Classified according to shape
Tabular or massive
May cut across existing structures
Discordant
Or inject parallel to features
Concordant
Intrusive Igneous Activity
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Intrusive Igneous Activity
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Intrusive Igneous Activity
Tabular intrusive bodies
Magma is injected into a fracture or other zone of weakness
Dikes are discordant
Sills are concordant
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Columnar jointing occurs as a result of shrinkage fractures that develop when igneous rocks cool
Intrusive Igneous Activity
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Large intrusive bodies include:
Batholiths
Linear masses of felsic rocks hundreds of km long
Stocks
Surface exposure <100 km2
Laccoliths
Lift the sedimentary strata that they penetrate
Intrusive Igneous Activity
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Intrusive Igneous Activity
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Most volcanoes are found near:
Ring of Fire around the Pacific Ocean
Mid-ocean ridges
Few are randomly distributed
Plate Tectonics and Volcanism
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Plate Tectonics and Volcanism
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Intraplate volcanism
A mantle plume of hot material ascends to the surface
Plate Tectonics and Volcanic Activity
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