Transcript Geology of the Hawaiian Islands

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?
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Meteorites
Direct observation
High-pressure experimental petrology
Earthquake waves (seismology)
Layers of the Earth
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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
Mohorovicic
Discontinuity
(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
Tomography at the Base of the Mantle
At 2770 km
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
Hotspot tracks
Flood basalts
Oceanic
plateaus
Karoo / Etendeka Flood Basalts
Columbia River Flood Basalts
Linear Volcanic Chains
Hawaiian Islands - Emperor Seamounts
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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 hotter
than 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 fixed
relative to the mantle.
BUT – this is controversial!
When a plate moves over a
fixed Hot Spot,
a linear chain of volcanoes is formed.
Hotspot Origins - Mantle Plumes
Oceanic flood basalts
Oceanic crust
6-9 km
Lithospheric
mantle
10-60 km
200-400 km
H o t
C o ld
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
QuickTime™ and a
TIFF (LZW ) decompressor
are needed to see this picture.
Models of Mantle Plumes
Wolfe et al., Nature, [1997]
Tomography at the Base of the Mantle
At 2770 km
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!
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Earthquakes
Floods
Climate changes (draught, ice ages)
Other weather hazards (tornadoes,
cyclones)
• Volcanic eruptions
• Meteor / asteroid impacts
Meteor / Asteroid Impacts
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Meteor / Asteroid Impacts
Environmental Catastrophes and Hotspots
Eruption of Deccan Traps
(Reunion hotspot)
End Cretaceous (65 Ma)
Extinction Percentages and Hotspots
Extinction is Forever
Dinosaurs - ~65 Ma
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Trilobites
~ 300 Ma
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Giant Ground Sloth ~10 Ka
Thursday
Video:
Death of the Dinosaurs