Dynamic Earth
Class 13
21 February 2006
Volcanic Imagination(Chapter 4, continued)
Exploring the Earth’s Interior
How do we know about the Earth’s Interior?
• Meteorites
• Direct observation
• High-pressure experimental petrology
• Earthquake waves (seismology)
Layers of the Earth
• Crust
• Rigid upper mantle (+crust = lithosphere)
• Asthenosphere
• Upper mantle
• Lower mantle
• Outer core
• Inner core
Earth’s internal heat•Original heat
•Subsequent radioactive decay
•Conduction
•Convection
Crust as an Elastic Sheet
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic uplift
Return to isostatic equilibrium
The less dense crust “floats” on the less buoyant, denser mantle
MohorovicicDiscontinuity
(Moho)
Convection in Earth’s 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
Convection in Earth’s Mantle
• Assumptions– Solid mantle behaves fluidly over time
– Mantle and core do not mix
– Heat generated from within the Earth
Temperature vs. Depth
Convection as a Possible Mechanism for Plate Tectonics
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
Basics of Tomography
Tomography of the Mantle
At 2770 km
Tomography at the Base of the Mantle
Tomography Beneath Active Volcanoes
Tomography Beneath Active Volcanoes
Volcanic Activity on Earth
• Spreading centers (ridges)
• Island arc
• Hotspots
Hotspots
• Areas with volcanic activity NOT explained by plate tectonics
• Mantle beneath may be hot, wet, or chemically different
• Commonly active for long time
Global Hotspots
http://www.hvo.usgs.gov
Flood basalts
Hotspot tracks
Oceanic plateaus
Karoo / Etendeka Flood Basalts
Columbia River Flood Basalts
Linear Volcanic Chains
Hawaiian Islands - Emperor Seamounts
QuickTime™ and aTIFF (LZW) decompressor
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The trail of the Hawaiian Hot Spot goes all the way to the far northwest Pacific
In addition to the Hawaiian Hot Spot track, there are several others in the Pacific
Age Progressions Along Volcano Chains
Hawaiian Islands
Oldest
Youngest
Hotspot Observations
• Volcanic activity, NOT explained by plate tectonics
• Active for long time
• Age varies (youngest in opposite direction of plate motion)
Hot spots are regions in the Asthenosphere that are hotterthan their surroundings
Molten magma rises to the surface to form volcanoes, similar to what happens at
Spreading Centers
Hot spot volcanoes are relatively small, isolated
features.
Hot spots are believed to be fixedrelative to the mantle.
BUT – this is controversial!
When a plate moves over afixed Hot Spot,
a linear chain of volcanoes is formed.
Oceanic crust
Lithosphericmantle
6-9 km
10-60 km
H o t
C o ld
200-400 km
Oceanic flood basalts
Hotspot Origins - Mantle Plumes
Mantle plumes and eruption sizes
Convection in the Mantle
Convection and Mantle Plumes
Why Linear Chains of Volcanoes?
Model of Mantle Plumes
A mantle plume rising beneath a slow-moving plate or continent will “puddle” beneath the lithosphere
Model of Mantle Plumes
When eruptions begin, they are voluminous, causing oceanic plateaus
and flood basalt provinces
Model of Mantle Plumes
Model of Mantle Plume
Mantle Plume Shape Unknown
Model of Mantle Plumes
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Models of Mantle Plumes
Wolfe et al., Nature, [1997]
At 2770 km
Tomography at the Base of the Tomography at the Base of the MantleMantle
Instability Causes Mantle Plumes
How Can Plate Tectonics and Mantle Plumes Work Together?
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)
Volcanic Eruptions and the Atmosphere
Life on Earth is Difficult!
• Earthquakes
• Floods
• Climate changes (draught, ice ages)
• Other weather hazards (tornadoes, cyclones)
• Volcanic eruptions
• Meteor / asteroid impacts
Meteor / Asteroid Impacts
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Meteor / Asteroid Impacts
Environmental Catastrophes and Hotspots
End Cretaceous (65 Ma)
Eruption of Deccan Traps (Reunion hotspot)
Extinction Percentages and Hotspots
Extinction is Forever
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Dinosaurs - ~65 Ma
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Giant Ground Sloth ~10 Ka
Trilobites~ 300 Ma
ThursdayVideo:
Death of the Dinosaurs