Dynamic Earth

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Dynamic Earth

• Class 13
• 21 February 2006

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Volcanic Imagination(Chapter 4,
continued)Exploring the Earths Interior
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How do we know about the Earths Interior?

• Meteorites
• Direct observation
• High-pressure experimental petrology
• Earthquake waves (seismology)

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Layers of the Earth

• Crust
• Rigid upper mantle (crust lithosphere)
• Asthenosphere
• Upper mantle
• Lower mantle
• Outer core
• Inner core

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Earths internal heat

• Original heat
• Subsequent radioactive decay
• Conduction
• Convection

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Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
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The less dense crust floats on the less
buoyant, denser mantle
Mohorovicic Discontinuity (Moho)
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Convection in Earths Mantle

• Convection happens when
• Temperature gradient exists
• Heat is directional
• Conduction operates slowly
• Surface area to depth ratio is low
• Viscosity of material not extremely high

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Convection in Earths Mantle

• Assumptions
• Solid mantle behaves fluidly over time
• Mantle and core do not mix
• Heat generated from within the Earth

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Temperature vs. Depth
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Convection as a Possible Mechanism for Plate
Tectonics
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Mantle Tomography

• Uses numerous seismic data
• Uses small changes in speed of seismic waves
• Faster wave motion may correspond to denser or
  colder regions
• Slower wave motion may correspond to buoyant or
  warmer regions

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Basics of Tomography
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Tomography of the Mantle
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Tomography at the Base of the Mantle
At 2770 km
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Tomography Beneath Active Volcanoes
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Tomography Beneath Active Volcanoes
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Volcanic Activity on Earth

• Spreading centers (ridges)
• Island arc
• Hotspots

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Hotspots

• Areas with volcanic activity NOT explained by
  plate tectonics
• Mantle beneath may be hot, wet, or chemically
  different
• Commonly active for long time

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Global Hotspots
http//www.hvo.usgs.gov
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Hotspot tracks
Flood basalts
Oceanic plateaus
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Karoo / Etendeka Flood Basalts
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Columbia River Flood Basalts
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Linear Volcanic Chains
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Hawaiian Islands - Emperor Seamounts
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The trail of the Hawaiian Hot Spot goes all the
way to the far northwest Pacific
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In addition to the Hawaiian Hot Spot track,
there are several others in the Pacific
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Age Progressions Along Volcano Chains
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Hawaiian Islands
Oldest
Youngest
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Hotspot Observations

• Volcanic activity, NOT explained by plate
  tectonics
• Active for long time
• Age varies (youngest in opposite direction of
  plate motion)

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Hot spots are regions in the Asthenosphere that
are hotterthan their surroundings
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Molten magma rises to the surface to form
volcanoes, similar to what happens at Spreading
Centers
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Hot spot volcanoes are relatively small, isolated
features.
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Hot spots are believed to be fixedrelative to
the mantle. BUT this is controversial!
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When a plate moves over afixed Hot Spot, a
linear chain of volcanoes is formed.
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Hotspot Origins - Mantle Plumes
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Mantle plumes and eruption sizes
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Convection in the Mantle
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Convection and Mantle Plumes
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Why Linear Chains of Volcanoes?
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Model of Mantle Plumes

• A mantle plume rising beneath a slow-moving plate
  or continent will puddle beneath the lithosphere

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Model of Mantle Plumes

• When eruptions begin, they are voluminous,
  causing oceanic plateaus and flood basalt
  provinces

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Model of Mantle Plumes
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Model of Mantle Plume
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Mantle Plume Shape Unknown
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Model of Mantle Plumes
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Models of Mantle Plumes
Wolfe et al., Nature, 1997
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Tomography at the Base of the Mantle
At 2770 km
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Instability Causes Mantle Plumes
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How Can Plate Tectonics and Mantle Plumes Work
Together?
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Why Are Hotspots Important

• Associated with Large Volcanic Eruptions
• May inject gas and particles into air
• May re-landscape large areas
• May decrease habitable areas
• May make life difficult for some plants and
  animals (and cause mass extinction)

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Volcanic Eruptions and the Atmosphere
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Life on Earth is Difficult!

• Earthquakes
• Floods
• Climate changes (draught, ice ages)
• Other weather hazards (tornadoes, cyclones)
• Volcanic eruptions
• Meteor / asteroid impacts

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Meteor / Asteroid Impacts
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Meteor / Asteroid Impacts
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Environmental Catastrophes and Hotspots
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Extinction Percentages and Hotspots
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Extinction is Forever
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Thursday

• Video
• Death of the Dinosaurs