Prepared by Mark R. Noll

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Prepared by Mark R. Noll SUNY College at Brockport
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Fig. 22.2. Volcanism associated with Plumes
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Hotspots

• Long, vertical columns of hot magma
• First evidenced in Hawaii
• Shield volcanoes not associated with other
  tectonic activity
• Age of islands get progressively older
• Similar trends seen in other linear island chains

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Evidence of Mantle Plumes

• Evidence is indirect
• Local zones of high heat flow
• Hotspots do not drift with plate movement
• Geochemistry of basalt is distinct
• Deep mantle source
• Oceanic islands associated with swells
• Seismic studies

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Hotspot Characteristics

• Distribution is linear
• Produces submarine volcanoes
• Some become islands
• Lithosphere moves over mantle plume
• One volcano becomes dormant, a new one develops
• Magma generation is in the lower mantle
• At least 700 km, maybe at mantle base

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Fig. 22.5. Formation of island chain
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Evolution of Mantle Plumes

• Plumes are a form of convection
• Less dense material at base of mantle
• Less dense material begins to rise
• Diapirs
• Starting plume enlarges
• Large bulbous head grows
• Narrow tail feeds material upward

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Fig. 22.6. Plume evolution and geometry
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Evolution of Mantle Plumes

• Rising plume swells lithosphere
• Plume rises and spreads beneath lithosphere
• Reduced pressure allows magma generation
• Rifting provides conduits for magma
• Most of plume head cools
• Tail may continue to feed new material

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Fig. 22.8. A starting plume
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Making Magma

• Magma is generated by decompression melting
• Lower pressure allows material to partially melt
• Similar process at midocean ridges
• Occurs at 100 km deep
• Less dense magma continues to rise
• Source of material controls geochemistry

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Fig. 22.9. Decompression melting
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Making Magma

• Source of magma appears to be mantle material
  contaminated with ancient oceanic crust and
  sediments
• Cold oceanic crust is metamorphosed during
  subduction
• Resulting material is very dense
• Dense material sinks to base of mantle

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Composition of different basalts
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Mantle Plumes - Oceanic

• Starting plumes generate flood basalts
• Broad oceanic plateaus
• Extremely large volcanic event
• Oceanic crust increased in thickness by up to 5x
• No large shield volcanoes
• Magnetic stripes hidden
• Eruption rate similar to all of ridge system

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Mantle Plumes - Oceanic

• Tail plumes create island chains
• Large shield volcanoes produced over plume tails
• Quiet flows of basaltic lava
• Collapse caldera forms at summit
• Vertical tectonic processes from high heat flow
  and weight of volcano

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Hawaiian Plume

• Best example of still-rising tail plume
• Hawaii is active portion of chain of islands
• Remaining islands are extinct volcanoes
• Most are now below sea level
• Hawaii has 2 active volcanoes
• Mauna Loa Kilauea

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Fig. 22.1. Hawaiian Island chain
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Hawaiian Earthquakes

• Earthquakes are relatively small and infrequent
• Most are shallow associated with magma movement
  or slumping
• Usually magnitude 4.5 or less
• Some quakes form in the mantle
• May be larger, up to 6.2 magnitude

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Hawaiian Volcanism

• Volcanism dominated by basalt
• Partial melting of mantle material
• Low water and volatiles content compared to
  subduction zone basalts
• Few andesites or rhyolites
• No continental crust component
• Eruptions commonly form along fissures

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Evolution of Seamounts Islands

• Grow by extrusion of lava at various points of
  their surface - mostly subaqueous
• Intrusive features also form
• Base subsides as volcano grows upward
• Submarine mass movements, mainly along faults
• Islands subject to subaerial erosion
• Subsidence occurs away from hotspot

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Fig. 22.16. Evolution of a volcanic island
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Mantle Plumes - Midocean Ridges

• Plumes may form coincident with midocean ridge
  system
• Iceland formed at intersection of ridge and
  hotspot tail plume
• Combination produced island
• Basalt geochemistry shows mixing
• Rhyolite forms as basalt partially melts
• Plume may have assisted in initial rifting

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Fig. 22.18. History of Iceland
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Mantle Plumes - Continents

• Plumes beneath continents create regional uplift
  and bimodal volcanism
• Lithosphere gently warps from rising plume
• Flood basalts erupted
• Rhyolite forms from melting of crust
• May initiate continental rifting

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Yellowstone Plume

• Yellowstone plume has evolved from head to tail
  stage
• Starting plume produced Columbia River flood
  basalts
• Uplift created rifting in Nevada
• NA has moved SW over plume
• Tail plume forms Yellowstone volcanics and geysers

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Fig. 22.21. Cenozoic features of NW U.S.
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Fig. 22.23b. Cross section of Yellowstone plume
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Continental Rifting

• Rifting may be initiated by mantle plumes
• Rising starting plume spreads out beneath
  continental lithosphere
• Buoyant plume domes lithosphere
• Extension may lead to rift development
• Etendeka and Parana basalt provinces

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Continental Rifting

• Plumes do not always cause rifting
• Major mantle plumes produce continental flood
  basalts
• Rifting occurs in an intraplate environment on a
  plate already in motion
• Siberian flood basalts - latest Paleozoic
• Lake Superior - Precambrian
• Yellowstone

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Plumes, Climate Extinction

• Mantle plumes may affect Earths climate and
  magnetic field
• Starting plumes create enormous amount of
  volcanism over short time period
• May change composition and circulation in ocean
  and atmosphere
• Large volumes of volcanic gases produced,
  including CO2

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Plumes, Climate Extinction

• Flood basalts may be correlated with climate
  change and extinction events
• Ontong-Java Plateau - Cretaceous warming
• Deccan Plateau - Cretaceous Tertiary boundary
• Siberian flood basalts - late Paleozoic
  extinctions

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Plumes, Climate Extinction

• Plume events may correlate with polarity
  reversals
• Large number of plume events correlates with
  decreased polarity changes during Cretaceous
• Plume events may remove heat from outer core
  area, slowing convection

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End of Chapter 22