Transcript Have plumes been detected seismologically?

Have plumes been
detected seismologically?
Maeve O’Shea
University of Durham
October 2004
Introduction
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Historically, volcanic activity has been explained by the Plate
Tectonic theory.
Hotspots do not fit this model.
In 1971, Morgan produced the Plume theory
→ proposed that beneath hotspots are narrow plumes which originate from
CMB.
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Model been refined over past 3 decades.
Popular, little opposition.
Recent debate as to whether plumes actually exist.
One of most popular ways to investigate the presence of plumes is
to track seismic waves through the mantle.
Seismological Methods
A number of different techniques have been used to detect plumes,
each with limitations:
1. Seismic tomography
(i) Teleseismic
(ii) Whole-mantle
(iii) Surface-wave
2. Receiver functions
3. Multiple ScS
4. Plume waves
Recognising a “plume”
What seismological signals do we look for when searching for plumes
in the mantle?
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Large areas of low velocity
Shape: bulbous head with thin stem.
Must be careful when interpreting low velocity zones as plume
structures because a number of factors affect the speed of waves in
the mantle:
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Temperature
Pressure
Rock composition
Melting
Anelasticity
Anisotropy.
Case studies of three
hotspot locations
Evidence for the presence of deep mantle plumes
Hawaii:
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Low velocity zone detected, but ~1000km SE of Hawaii- explained by
“mantle wind” (Russell et al., 1998).
However, location of anomaly disputed (Bréger and Romanowicz, 1998).
Regional imaging can be difficult due to remote location.
Different method of receiver functions used: thinned transition zone
detected (Li et al. 1999).
Global tomographic imaging of the Hawaiian hotspot (Zhao, 2001):
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Deep mantle plume theory is
supported by new method of
global tomography: finitefrequency (Montelli et al.,
2004).
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Alternative theory proposed:
Tear in the crust (Foulger).
Other deep plumes:
Ascension, Azores, Canary, Easter, Samoa
and Tahiti (Montelli et al., 2004)
Evidence for the presence of shallow plumes
Iceland:
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First imaged as a plume
originating from CMB
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(Bijwaard & Spakman, 1999):
Foulger et al. (2000) started
seriously questioning the
belief that all plumes
originated from lower mantle
Used teleseismic images
 Found problems with past
models of the Iceland plume.
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Ritsema et al. (1999)- whole
mantle tomography as
evidence for NO deep plume.
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More recently, others
have come to a similar
conclusion that Iceland
originated from the
upper mantle (Allen et
al., 2002; Montelli et al.,
2004).
Other shallow plumes:
Bowie, Eastern Australia, Eifel,
Etna, Cocos-Keeling, Galapagos
and Juan de fuca/Cobb
(Montelli et al. 2004).
Absent Plumes
Yellowstone:
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First thought to be a typical
deep plume hotspot.
Scientists have since realised
that it did not fit deep-mantle
plume model:
Teleseismic tomography: shallow
plume, extends only as far as 200
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km depth
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Receiver functions: do not show
any thinning of transition zones
(Christiansen et al., 2002)
Whole-mantle tomography: no
evidence for lower mantle
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influence (Ritsema et al., 1999).
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Montelli et al. (2004)- no
evidence for a substantial
plume underneath Yellowstone.
Conclusion
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Low velocity anomalies been detected under many hotspots.
However, still dispute over whether to call them plumes.
Difficult to accept since not all hotspots fit the deep-mantle
plume model.
Limitations in imaging Earth’s interior, therefore new data and
more effective analysis techniques are needed.
Combine seismic evidence with geochemical observations in
order to get a clearer picture of what exactly is happening
underneath hotspots.
The current research project on Hawaii will provide greater insight
into the debate of whether plumes really do exist.
Additional material
Methods:
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Receiver functions