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NATURE VOL. 230 MARCH
5 1971 |
Convection Plumes in
the
Lower Mantle |
The concept of crustal plate motion over mantle hotspots
has been advanced1 to explain the origin
of the Hawaiian and other island chains and the origin
of the Walvis, Iceland-Farroe (sic) and other
aseismic ridges. More recently the pattern of the
aseismic ridges has been used in formulating continental
reconstructions2. I have shown3
that the Hawaiian-Emperor, Tuamotu-Line and Austral-Gilbert-Marshall
island chains can be generated by the motion of a
rigid Pacific plate rotating over three fixed hotspots.
The motion deduced for the Pacific plate agrees with
the palaeomagnetic studies of seamounts4.
It has also been found that the relative plate motions
deduced from fault strikes and spreading rates agree
with the concept of rigid plates moving over fixed
hotspots. Fig. 1 shows the absolute motion of the
plates over the mantle, a synthesis which satisfies
the relative motion data and quite accurately predicts
the trends of the island chains and aseismic ridges
away from hotspots.
I now propose that these hotspots are manifestations
of convection in the lower mantle which provides the
motive force for continental drift. In my model there
are about twenty deep mantle plumes bringing heat
and relatively primordial material up to the asthenosphere
and horizontal currents in the asthenosphere flow
radially away from each of these plumes. The points
of upwelling will have unique petrological and kinematic
properties but I assume that there are no corresponding
unique points of downwelling, the return flow being
uniformly distributed throughout the mantle. Elsasser
has argued privately that highly unstable fluids would
yield a thunderhead pattern of flow rather than the
roll or convection cell pattern calculated from linear
viscous equations. The currents in the asthenosphere
spreading radially away from each upwelling will produce
stresses on the bottoms of the lithospheric plates
which, together with the stresses generated by the
plate to plate interactions at rises, faults and trenches,
will determine the direction in which each plate moves.
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Fig.1 The arrows show
the direction and speed of the plates over the
mantle; the heavier arrows show the plate motion
at hotspots. This synthesis was based on relative
plate motion data (fault strikes and spreading
rates) and predicts the directions of the aseismic
ridges/island chains emanating from the hotspots. |
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Evidently the interactions
between plates are important in determining the net
force on a plate, for the existing rises, faults and
trenches have a self-perpetuating tendency. The plates
are apparently quite tough and resistant to major changes
because rise crests do not commonly die out and jump
to new locations and points of deep upwelling do not
always coincide with ridge crests. (For example, the
Galapagos and Reunion upwellings are near triple junctions
in the Pacific and Indian Oceans. Asthenosphere motion
radially away from these hotspots would help to drive
the plates from the triple junctions but there is considerable
displacement between the “pipes to the deep mantle”
and the lines of weakness in the lithosphere would enable
the plates to move apart.) Also, a large isolated hotspot
such as Hawaii can exist without splitting a plate in
two. I believe it is possible to construct a simple
dynamic model of plate motion by making assumptions
about the magnitude of the flow away from each hotspot
and assumptions about the stress/strain rate relations
at rises, faults and trenches. Such a model has many
possibilities to account for past plate motions; hotspots
may come and go and plate migration may radically change
the plate to plate interactions. But the hotspots would
leave visible markers of their past activity on the
seafloor and on continents.
This model is compatible with the observation
that there is a difference between oceanic island and
oceanic ridge basalts5,6. It suggests a definite
chain of events to form the island type basalt found
on Hawaii and parts of Iceland. Relatively primordial
material from deep in the mantle rises adiabatically
up to asthenosphere depths. This partially fractionates
into a liquid and solid residual, the liquid rising
through vents to form the tholeiitic part of the island.
The latter alkaline “cap rocks” would be
generated in the lithosphere vent after plate motion
had displaced the vent from the “pipe to the deep
mantle”. In contrast, the ridge basalts would
come entirely from the asthenosphere, passively rising
to fill the void created as plates are pulled apart
by the stresses acting on them. The differences in potassium
and in rare earth pattern for island type and ridge
type basalts may be explained by this model. Moreover,
the 2 billion year “holding age” advocated
by Gast7 to explain lead isotope data of
Gough, Tristan da Cunha, St Helena and Ascension Islands
may reflect how long the material was stored in the
lower mantle without change prior to the hotspot activity.
My claim that the hotspots provide
the driving force for plate motions is based on the
following observations to be discussed below. (1) Almost
all of the hotspots are near rise crests and there is
a hotspot near each of the ridge triple junctions, agreeing
with the notion that asthenosphere currents are pushing
the plates away from the rises. (2) There is evidence
that hotspots become active before continents split
apart. (3) The gravity pattern and regionally high topography
around each hotspot suggest that more than just surface
volcanism is involved at each hotspot. (4) Neither rises
nor trenches seem capable of driving the plates.
The symmetric magnetic pattern and
the “mid-ocean” position of the rises indicate
that the rises are passive. If two plates are pulled
apart, they split along some line of weakness and in
response asthenosphere rises to fill the void. With
further pulling of the plates, the laws of heat conduction
and the temperature dependence of strength dictate that
future cracks appear down the centre of the previous
“dike” injection. If the two plates are
displaced equally in opposite directions or if only
one plate is moved and the other held fixed, perfect
symmetry of the magnetic pattern will be generated.
The axis of the ridge must be free to migrate (as shown
by the near closure of rises around Africa and Antarctica).
If the “dikes” on the ridge axis are required
to push the plates apart, it is not clear how the symmetric
character of the rises could be maintained. The best
argument against the sinking lithospheric plates providing
the main motive force is that small trench-bounded plates
such as the Cocos plate do not move faster than the
large Pacific plate8. Also, the slow compressive
systems would not appear to have the ability to pull
other plates away from other units. The pull of the
sinking plate is needed to explain the gravity minimum
and topographic deep locally associated with the trench
system9, but I do not wish to invoke this
pull as the principal tectonic stress. This leaves sub-lithospheric
currents in the mantle and the question now is: are
these currents great rolls (mirrors of the rise and
trench systems), or are they localized upwellings (that
is, hotspots)?
A recent world gravity map10
computed for spherical harmonics up to order 16 shows
isolated gravity highs over Iceland, Hawaii, and most
of the other hotspots. Such gravity highs are symptomatic
of rising currents in the mantle. Even if the gravity
measurements are inaccurate (different authors have
very different gravity maps), the fact remains that
the hotspots are associated with abnormally shallow
parts of the oceans. For example, note the depth of
the million square kilometres surrounding the Iceland,
Juan de Fua, (sic) Galapagos, and Prince Edward
hotspots. The magnitude of the gravity and topographic
effect should measure the size of the mantle flow at
each hotspot.
There is evidence of continental expression
of hotspot activity in the lands bordering the Atlantic:
the Jurassic volcanics in Patagonia (formed by the present
day Bouvet Island plume), the ring dike complex of South-west
Africa and flood basalts in the Parana Basin (Tristan
da Cunha plume), the White Mountain Magma series in
New Hampshire (the same hotspot that made the New England
Seamount Chain (Azores plume?), the Skaegaard and the
Scottish Tertiary Volcanic Province (Iceland plume)
and perhaps others. I claim this line-up of hotspots
produced currents in the asthenosphere which caused
the continental break-up leading to the formation of
the Atlantic. Likewise the Deccan Traps (Reunion plume)
were symptomatic of the forthcoming Indian Ocean rifting.
A search should be made for such continental activity,
particularly in east Africa and the western United States
(the Snake River basalts?) as an explanation for the
rift features found there. There is a paucity of continental
hotspots in Fig. 1; perhaps this is a bias due to continental
complexity versus oceanic simplicity, but the model
presented here predicts that most hotspots will be near
a spreading rise.
I thank Kenneth Deffeyes and Fred Vine
for their contributions to the ideas in this letter.
This work was partially supported by the US National
Science Foundation and the Office of Naval Research. |
W. J. Morgan |
Department of Geological and Geophysical
Sciences,
Princeton University,
Princeton, New Jersey
Received December 21, 1970
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258, 145 (1965).
2 Dietz, R. S., and Holden, J. C., J. Geophys. Res.,
75, 4939 (1970).
3 Morgan, W. J., Hess Memorial Volume (edit. by Shagam,
R.), Mem. Geol. Soc. Amer. (in the press).
4 Francheteau, J., Harrison, C. G. A., Sclater, J. G.,
and Richards, M. L., J. Geophys. Res., 75,
2035 (1970).
5 Engel, A. E. J., Engel, C. G., and Havens, R. G.,
Geol. Soc. Amer. Bull., 76,
719 (1965).
6 Gast, P. W., Geochim. Cosmochim. Acta, 32,
1057 (1968).
7 Oversby, V. M., and Gast, P. W., J. Geophys. Res.,
75, 2097 (1970).
8 McKenzie, D., Geophys. J. 18,
1 (1969).
9 Morgan, W. J., J. Geophys. Res., 70,
6189 (1965).
10 Kaula, W. M., Science, 169,
982 (1970). |
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