Plate tectonics: Plate geometry 3

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Plate tectonics Plate geometry 3
Important This chapter follows mainly on chapter
2 and hilo.hawaii.edu/kenhon/GEOL205
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Hotspot tracks Global distribution
Volcanic chains and hotspot tracks
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Hotspot tracks A view on the Pacific
A closer look at the Pacific
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Hotspot tracks Hawaii
Linear increase of ages with distance along the
Hawaii-Emperor chain.
Gradual decrease in elevation with increasing
distance from the active volcano.
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Hotspot tracks Hawaii
The oldest seamounts are found at the northwest
end, poised to plunge beneath the Aleutian
volcanic arc, carried downward with the oceanic
lithosphere as it is consumed.
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Hotspot tracks Hawaii

• Note the abrupt bend about 44 millions years
  before the present, which indicates a major
  reorganization of plate motion at that time.
• While some think it was the collision of India
  with the Eurasian subcontinent, others suggest it
  was the beginning of spreading on the Antarctic
  Ridge south of Australia.

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Hotspot tracks Hawaii

• Another remarkable observation is that the
  eruption rate for Hawaiian volcanoes has remained
  quite constant over most of the 65 million years
  of preserved activity.
• This suggests that as volcanic material being
  erupted, new material is being supplied more or
  less continuously from below.

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Hotspot tracks Hawaii

• For the 10 million years following the bend,
  very little lava erupted. This is a bit of a bad
  situation for the previous inhabitants of the
  islands, since there is very little other dry
  land for thousands of kilometers. Indeed, almost
  all of the previous life must have been
  exterminated, so that the current flora and fauna
  must have arrived more recently.

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Hotspot tracks The plume model

• Morgans plume model (Morgan, 1971)
• Volcanic islands are produced by plumes rising
  through the mantle.
• The plumes come from the lower mantle - and are
  therefore fixed.
• Plume flow derives the plates.

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Hotspot tracks The plume model
The topographic swell
The sea floor surrounding the Hawaii chain of
islands is anomalously shallow, relative to
normal sea floor of the same age, over an area
about 1,200 km wide and 3,000 km long.
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Hotspot tracks The plume model
The topographic swell
Bathymetry of the North Atlantic. Iceland (shown
in the center) protrudes from the ocean basin
sitting on a large swell.
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Hotspot tracks The plume model
Seismic tomography
Seismic images suggest that some mantle plumes
originate at the lower mantle.
Figure from Montelli et al., 2004
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Hotspot tracks The plume model

• Distinct geochemical signature
• The content of incompatible elements is by 1 to
  2 orders of magnitude higher in Ocean Island
  basalt (OIB, e.g. Hawaii, EM-1 and HIMU) than it
  is in Mid-Oceanic Ridge Basalt (MORB).
• This implies different reservoirs for OIB and
  MORB.

Figure from Hofmann, 1997
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Hotspot tracks The plume model

• Distinct geochemical signature
• In general, Nd/Nd correlates negatively with
  Sr/Sr.
• MORB data are at the upper-left corner.
• The OIB are enriched in incompatible elements
  with respect to the MORB.

Incompatible rich
Incompatible rich
Figure from Hofmann, 1997
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Hotspot tracks The plume model

• Distinct geochemical signature
• The position of the OIB between MORB and
  continental crust suggests that OIB source may be
  the result of back mixing of continental material
  into the mantle.
• How different chemical reservoirs may still
  exist if the mantle is undergoing global mixing
  is yet an open question.

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Hotspot tracks The plume model
Association with flood basalt Morgan, in 1981,
pointed out that a number of hotspot tracks
originate in flood basalt provinces. He
explained that flood basalt was produced from a
plume head arriving at the base of the
lithosphere.
Flood basalt are the largest known volcanic
eruptions in the geologic record, and typically
comprise basalt of the order of 1 km thick over
an area up to 2000 km across.
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Hotspot tracks The plume model

• The association of the Deccan trap in India with
  the Reunion hotspot track.
• The flood basalt eruption is due to the arrival
  of the plume head, and the hotspot track is
  formed by the plume tail.

Figure from Dynamic Earth by G.F. Davies
Figure from White and McKenzie, 1989
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Hotspot tracks The plume model

• Summary of the arguments supporting the notion of
  a rigid plate moving atop of a deeply rooted
  mantle plume
• The straightness of the hotspot tracks and the
  linear increase of volcanic ages along the track.
• Topographic expression.
• The nearly constant eruption rate for Hawaiian
  volcanoes during the past 65 million years
  suggest that as volcanic material being erupted,
  new material is being supplied more or less
  continuously from below.
• Distinct chemistry for the OIB suggests deeper
  origin for the magma source.
• Seismic tomography.

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Hotspot tracks The fixity of hotspots
Paleo-magnetic data strongly suggests that all of
the lava solidified at 19.5 degrees north
latitude, precisely the latitude of the hotspot
today. At least with respect to latitude it would
seem that the Hawaiian hotspot has been nearly
fixed for at least the past 65 million years.
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Hotspot tracks The fixity of hotspots
That portions of island chains of similar age are
parallel to each other suggests that the hotspots
themselves remain mostly fixed with respect to
each other, otherwise the chains might be expect
to trend in different directions as the plumes
generating them moved independently.
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Hotspot tracks The fixity of hotspots
Parallel hotspot tracks within the Indian Ocean.
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Hotspot tracks The fixity of hotspots

• Summary of the geophysical arguments supporting
  the notion of fixity of hotspots
• Paleo-magnetic data indicate that the hotspot
  latitude has remained fixed during the past 65
  million years.
• Portions of island chains of similar age are
  parallel to each other suggests that the hotspots
  themselves remain mostly fixed with respect to
  each other.

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Hotspot tracks Absolute plate motion
Question In previous lectures we have discussed
the relative plate motion. Can we infer absolute
plate motion as well?

1. We have seen that the relative motion between
   plates and plumes may be inferred from the trend
   of hotspot tracks and the island ages.
2. Plumes are almost fixed.
3. From 1 and 2, it follows that hotspot tracks can
   be used to infer absolute plate motion.

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Hotspot tracks A plume next to a mid-ocean?
Difference in age between the volcanoes and the
underlying seafloor as a function of distance
along the island chain

• At present the age of the sea floor beneath the
  Big Island is roughly 95 millions years old.
• From the bend north along the Emperor chain the
  age difference steadily decreases until it is
  less than 10 million years for the oldest known
  volcanoes in the chain.
• If the trend is continued back to about 80
  million years, it would appear that the hotspot
  was building volcanoes on ocean floor of the same
  age.

Question how is that possible?
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Hotspot tracks A plume next to a mid-ocean?
Iceland is a modern example to a plume co-located
with a mid-oceanic ridge.
Iceland is the only place on Earth where an
active mid-oceanic ridge is exposed on land.
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Hotspot tracks Yellowstone
There is no reason why plumes be exclusively
under oceanic lithosphere and indeed several
plumes are found in continental areas too. The
Yellowstone is one such example
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Hotspot tracks Darfor-Levant volcanic array
(Garfunkel, 1992)
Hotspot track in Israel
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Hotspot on Mars Mt. Olympus

• Mars has no plate tectonics, so hotspot
  volcanism results in building huge volcanoes that
  dominate the surface of the planet.
• The moving plates on the Earth prevent any
  single volcano from sitting over the hotspot long
  enough to build such huge edifices.
• Earth's crust is also far too thin to support a
  volcano as massive as Olympus Mt..