Lecture 2b: Hot spots

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Lecture 2b Hot spots

• Questions
• Why are there volcanoes in the middle of plates?
• How do such volcanoes grow and evolve?
• What is the connection between hotspots and flood
  basalts?
• Tools
• Plate tectonics, geochronology, igneous
  petrology, isotope geochemistry, etc.

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Hot spot chains
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Hot spots, flood basalts, LIPs
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• Chains of volcanoes in the middle of plates, if
  long-lived and with an age progression along the
  chain, are called hot spots.
• There are continual arguments over how many hot
  spots there are some are obvious, some are
  marginal (usually because it is hard to establish
  the age progression). The most common catalogue
  has 40, others go over 100.

• Hawaii and Iceland are biggest, by buoyancy flux
  and by volume of volcanism.

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Hot spots correlated with geoid?
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Mantle Plumes
The initiation of a new plume is thought to
involve a very large blob of hot material
arriving at the base of the lithosphere and hence
a large episode of excess volcanism. This is
supported by the association between many active
hotspots and older continental flood basalts or
oceanic plateaux (collectively, large igneous
provinces or LIPs).
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An experimental starting plume (in glucose syrup)
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Flood Basalts and hotspot tracks
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Flood Basalts and hotspot tracks
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Today
120 Ma ago
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Flood Basalts

• Flood basalts are big and erupt very quickly
• Siberian traps, 2 x 106 km3 within 1 Ma at 250
  Ma (Permian-Triassic)
• Deccan traps (India), 106 km3 within 1 Ma at 65
  Ma (K-T)
• Columbia River Basalts, 2 x 105 km3 within 1 Ma
  at 16 Ma.
• It may or not be coincidence that big flood
  basalt eruptions coincide with major extinction
  events in the fossil record!
• Flood basalt petrology and chemistry there is a
  general progression through
• small volume of early alkali basalt and olivine
  tholeiite with mantle isotope signatures and high
  3He/4He
• massive volume of quartz tholeiite with isotopic
  signature of subcontinental lithosphere
• small late eruptions with wide range of
  compositions and evidence of crustal components

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Flood Basalts

• We can think of this sequence as the result of
  emplacement of a big thermal anomaly at the base
  of the continental lithosphere.
• The first products are small degree,
  high-pressure melts of the plume itself, that
  escape quickly.
• Then heat flow, a slow process, raises the
  temperature of the cold non-convecting part of
  the mantle attached to the base of the continent
  until it melts over a wide area, in a process
  that is characterized by positive feedback
  between melting and heat flow, giving high magma
  flux for a short time.
• Finally, heat from the plume head reaches the
  crust itself, mostly by advection of magmas, and
  some crustal melts occur.
• Oceanic plateaux are basically similar to flood
  basalts, except they presumably occur when a
  plume head comes up under oceanic lithosphere.
• The biggest on earth is the Ontong-Java plateau,
  which is really two oceanic plateaux on top of
  each other, one 122 Ma and one 90 Ma. At that
  time the Pacific plate was hardly moving, and two
  plume-head like blobs came up the same conduit
  and hit the same area of lithosphere.

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Ocean Island Volcanoes
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• Mid-plate volcanoes in age-progressive chains are
  presumably the product of long-lived plume
  tails.
• They show a sequence driven by motion of new
  lithosphere over the plume (rather than arrival
  of new plume under lithosphere).
• At least at Hawaii, the lifecycle of one volcano
  is typically divided into four stages

• Preshield low flux of alkali basalt, erupted
  submarine, very pure plume component (high
  3He/4He, etc.)
• Shield-building stage very large flux and large
  volume of tholeiite, progressing towards an upper
  mantle/oceanic lithosphere affinity.
• Alkalic capping stage small flux of alkali
  basalts, no steady shallow magma chamber.
• Posterosional stage very small volume of
  extremely alkalic lavas that erupt 2 Ma after
  end of capping stage.

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Ocean Island Volcanoes

• In addition to characteristic chemistry, these
  stages generate characteristic morphology and
  structures
• The pre-shield stage, erupted underwater, is
  mostly a big mound of pillow basalts, relatively
  steep sided. At present, this stage is only known
  from Loihi seamount its role in other volcanoes
  is inferred.
• The main shield stage creates an edifice that
  emerges above sea-level.

• Subaerial tholeiite flows have low viscosity and
  long cooling times and can travel far down low
  slopes, allowing the volcano to assume the
  characteristic shield shape, perhaps 50 km in
  diameter for each 1 km above sea level at the
  summit.

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Ocean Islands Main Shield Stage

• A large summit caldera develops when the roof
  collapses into a shallow (lt1 km below summit)
  magma chamber. Most lavas ascend to this summit
  magma chamber and degas and differentiate there,
  even if they erupt down on the
• Rift zones that develop when gravitational
  stresses and push from intruding dikes break the
  edifice into three (or two if buttressed by an
  older volcano on one flank) sectors. The ongoing
  Puu-Oo eruption of Kilauea is on Kilaueas
  southeast rift zone.
• Occasionally, large sectors of a Hawaiian volcano
  will fail catastrophically and produce an
  enormous landslide, with the potential to drive
  km-high tsunamis. There are frequent earthquakes
  on shield volcano islands, sometimes on nearly
  horizontal faults.

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Ocean Islands Main Shield Stage
By way of advertisement, when you get to end of
our Ph.D. program, you will have the opportunity
to participate in project Pahoehoe and see a
shield volcano for yourself. Here is Caltech
undergrad Laura Elliott lava-dipping in 2004.
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Ocean Islands Hawaii
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• Topography and bathymetry show the shield shapes,
  summit calderas, rift zones, Loihi seamount, and
  sector-collapse landslide deposits.

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Ocean Islands Post-shield stage

• The alkalic lavas of the post-shield capping
  stage are smaller in volume and more viscous.
  They build a steeper mound on top of the shield
  (see Mauna Kea at present), with many near-summit
  cones, but no major caldera. These lavas
  frequently carry xenoliths from the oceanic crust
  and the cumulate pile inside the volcano,
  implying that they pond at the base of the crust
  and do not pause at any shallow magma chamber.

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Ocean Islands Post-erosional stage

• The post-erosional lavas are easily recognized as
  a series of cinder cones and explosive craters on
  top of a major unconformity and soil horizon from
  1-2 Ma of erosion and weathering. See Diamond
  Head on Oahu. These flows may carry mantle
  xenoliths, including garnet peridotites,
  indicating rapid ascent from mantle depths with
  no permanent magma chamber at any level.

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Oceanic basalts, conclusion

• In addition to the obvious morphological
  differences between mid-ocean ridges and ocean
  island volcanoes, there are important
  petrological differences relating to degree of
  melting, volatile content, and extent of melting.
• Moreover, there are essential, first-order
  differences in trace-element ratios and
  radiogenic isotope ratios.
• Broadly, MORB is from a depleted and degassed
  source, presumably the upper mantle
• OIB sources tend to be less depleted, nearly
  primitive, or even enriched relative to bulk
  earth and show evidence for a primordial noble
  gas component, hence they are thought to sample
  the lower mantle in some way.
• The existence of distinct isotopic reservoirs in
  the mantle constitutes the essential geochemical
  commentary on issues of whole-mantle vs. layered
  mantle convection, a subject on which
  geophysicists also have opinions.
• We will return to the global geochemical dynamics
  of the mantle as expressed through oceanic
  basalts at the very end of the course.

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