Hot Spots

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Hot Spots
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Intraplate Volcanism
USGS
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Hawaii

• Best example of intraplate volcanism
• More lava is extruded here constantly than
  anywhere else on Earth!

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A Bigger Picture

• Looking at Hawaii, and volcanic seamounts nearby
  (underwater volcanic islands)

Current Hawaiian Islands
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What is a hot spot?

• Some mantle anomaly allows the oceanic or
  continental lithosphere to melt where it would
  not normally melt
• The anomaly (usually) stays stationary
• The plate moves over it

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The Hawaiian Hot Spot
University of North Dakota
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• Islands and seamounts get older as you move away
  from the hotspot

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Mantle Plumes

• The mantle anomaly thought to cause hot spots
• Numerous theories as to what plumes are, and
  whether they even exist

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Convection Model

• Plumes are areas of convective upwelling
• Same mechanism that drives plate motion causes
  mid-plate lithosphere to melt

USGS
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Problem with Convection Model

• Lavas at rift zones contain very little 3He
  (Helium-3)
• Hawaiian lavas contain lots of 3He
• 3He is not produced on Earth by any natural
  process
• Formed in Big Bang
• 3He was trapped in the core during the formation
  of the earth
• Plumes must have a deeper source than the same
  mantle convection that drives plate motion
• Or there needs to be some explanation for
  anomalous 3He in Hawaiian lavas

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Deep Mantle Plumes

• Plumes originate from or near the core-mantle
  boundary
• Represent the mechanism by which heat is lost
  from the core

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What would cause a deep mantle plume?

• Ultra-low velocity zones
• Areas of liquid/crystal mush at the core-mantle
  boundary
• Cover 12 of the globe, and 1/3 of all hotspots
  lie above them, including ALL the major ones
• Liquid/crystal mush conducts heat 100x better
  than solid mantle rock
• Sets up perhaps a deeper convection than that
  which drives plate tectonics

Solid plastically- deforming mantle
Liquid/Crystal Mush
Liquid Outer Core
Williams 1997
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Areas of ULVZ
Williams 1997
Yellow major hotspots Red ULVZ locations
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Two-layer convection?
Leeds University
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New Research

• Finding plumes
• Seismic techniques that detected
• Layers of Earth
• ULVZs
• Not sensitive enough to detect plumes
• Too small a feature?
• And/or plumes arent what we think they are
• Perhaps more than one type of plume