Transcript iceland volcano

Tectonics III: Hot-spots and mantle plumes
Hotspot tracks: Global distribution
Location of hot-spots and hot-spot tracks:
Figures from Turcotte and Schubert
Hotspot tracks: A view on the Pacific
A closer look at the Pacific:
Hotspot tracks: Hawaii
Linear increase of ages with
distance along the HawaiiEmperor chain.
Hotspot tracks: Hawaii
• Gradual decrease in elevation
with increasing distance from
the active volcano.
• 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.
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.
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.
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. Almost all of the
previous life must have been
exterminated, so that the current
flora and fauna must have arrived
more recently.
Hotspot tracks: The plume model
Morgan’s 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 drives the plates.
Hotspot tracks: The plume model
The topographic swell:
Hawaii
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.
Figure from Ribe, 2004.
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.
Images produced by Richard Allen
Hotspot tracks: The plume model
Seismic tomography:
Seismic images suggest the Hawaiian plume
originates at the lower mantle.
Figure from McNutt and Caress (2007),
reproduced from Lei and Zhao, 2006
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
Hotspot tracks: The plume model
Distinct geochemical signature:
Incompatible rich
• In general, Nd/Nd correlates
negatively with Sr/Sr.
• MORB data are at the upperleft corner.
• The OIB are enriched in
incompatible elements with
respect to the MORB.
Incompatible rich
Figure from Hofmann, 1997
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.
Figure from Hofmann, 1997
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.
Figure from
Richards, Duncan
and Courtillot,
Science,1989
* 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.
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
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 suggests that as volcanic material is 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.
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.
Info. Box: magnetic inclination and paleo-latitude:
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.
Hotspot tracks: The fixity of hotspots
Parallel hotspot tracks also within
the Indian Ocean.
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 suggesting
that the hotspots themselves remain mostly fixed with respect to each other.
Hotspot tracks: Absolute plate motion
Question: In previous lectures we have discussed the relative plate motion.
Can we infer absolute plate motions 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.
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 30 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.
Fig. from Keller et al., Nature, 2000
Question: how is that possible?
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.
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:
Hotspot tracks: Darfor-Levant volcanic array (Garfunkel, 1992)
Hotspot track in Israel:
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.