The Hawaii Hot Spot (Cold Spot) Page
An hypothesis on the formation and periodicity of the Hawaii-Emperor volcanic chain*
In 1971, Geologist, Jason Morgan proposed the mantle plume or hot spot theory for the origin of Hawaii. In simple terms it said that the Hawaiian and Emperor Island chains were formed when continental drift moved the oceanic crust and lithosphere over a fixed mantle hot spot. The cartoons at the right illustrate.
|When Morgan made his
proposal, "he opened with, 'consider plumes rising
from the core mantle boundary.' Before uttering the next
sentence, of what may have been a rehearsed opening
gambit, Xavier Le Pichon blurted out loudly from the
front row, 'Why?' Somewhat flustered Jason said, 'Well,
just consider it . . .'" Molnar, P., From Plate
Tectonics to Continental Tectonics, in Plate Tectonics,
N. Oreskes, ed, 2001. Though Le Pichon reportedly
persisted, the question of "why" was never
answered . . . perhaps it can't be. The notion of hot
spots has evolved considerably since 1971--so much so
that some would argue that there is no underlying
principle at all.
In the mean time, a considerable body of data contrary to the notion of mantle plumes has accumulated. A good place to find out about this debate is http://www.mantleplumes.org. Some of the significant problems for the Hawaii example are listed by Foulger and Anderson in their web paper The Emperor and Hawaiian Volcanic Chains: How well do they fit the plume hypothesis? http://www.mantleplumes.org/Hawaii.html . Another author concluded in a paper on mantleplumes.org that "hotspots like Hawaii are initially formed by tectonic (i.e., shallow) processes, although the mechanisms for their longevity remain unknown." Norton (2006) http://www.mantleplumes.org/Hawaii2.html
So, are hot spots or mantle plumes real? The jury is still out. It seems that a new hypothesis might be in order. And a good place to start might be Hawaii. That is the purpose of this page.
A Starting Point--Styles of Convection
Of course there could be much debate about what constitutes first principles for this problem. I think it is safe to say that Jason Morgan didn't start with first principles. He made no such pretense. The notion of plumes from the core mantle boundary assumes that heating is at the mantle-core boundary. In fact, the mantle and the core are heated internally by radioactive decay. There has been some work on what convection would look like where heating is internal and cooling at an upper boundary--the earth's surface. The result looks like Morgan's assumption tipped upside down. Here are two screen captures from movies in Moresi and Lenardic, Three-dimensional mantle convection with continental crust: first-generation numerical simulations, http://www.esscc.uq.edu.au/~muhlhaus/WWW-Solidmech/Publications/Moresi/Moresi+Lenardic.EI98/HTML/Moresi+Lenardic.EI98.paper.html
|The plumes are downward at
least as much as they are upward. This dripping is most
often referred to as a Rayleigh-Taylor instability. There
are may references to it in the literature and on the web
in the context of mantle convection. Look here. The point
to take away from this work is that, in general, downward
convection is likely to be in drip-like forms. This is
not really much like the uniform slabs that one usually
associates with subduction. Of course, Hawaii is not
subduction of a plate--but even subduction on plates in Moresi
and Lenardic's modeling evolves to drip-like form at
greater depth. Drips have significance.
There are a unstable elements in the mantle and perhaps at the crust mantle boundary. For example, the diagram at the right shows that at the base of continental crust, an eclogitic root is denser than underlying upper mantle.
I am not sure if similar instabilities exist in oceanic crust--I am still looking--but they might. There are incipient instabilities within the mantle itself. An endothermic phase change at 660km could result in thermal sinking--or dripping--should the boundary be depressed. While I have not found numeric models documenting the notion, it seems possible that there could be cascading drips. A drip originating at an instability at the crust mantle boundary could push down to the 660km phase-change boundary where it would be perpetuated by the exothermic change. '
from Don Anderson, Large Igneous Provinces, Delamination, and Fertile
Mantle, Elements, Dec 2005, http://www.mantleplumes.org/WebDocuments/Anderson_Elements.pdf
|What are the
If instabilities exist, then a downward push can trigger Rayleigh-Taylor instability and a drip. Depending on the instabilities in the mantle, the drip may be a cascade of drips moving from layer to layer.
The idea of driving mantle events from the top down is discussed in Don Anderson's article in Science. A figure from that article is shown at the left. In my opinion, the top-down concept portends a revolution in many areas of geology on the surface of the earth--not just hot spots.
from: D.L. Anderson, Top-Down Tectonics, Science, V. 293, P. 2016. http://www.mantleplumes.org/WebDocuments/TopDown.pdf
ways to flow. (Top) A fluid layer cooled from
above or from the side, or heated from within, develops
narrow cold downwellings that cool the interior. The
downwellings are terminated by phase changes, density
increases due to composition, or high viscosity. There
are no active or hot upwellings. This resembles the upper
mantle. (isolated chemical layer Middle) A high
viscosity or cooled from above develops large cool
downwellings. This mimics the mid-mantle (1000 to 2000
km). (Bottom) A deep, dense, high-viscosity layer
with low thermal expansion overlying a hot region
develops large, sluggish upwellings. This mimics the deep
where's the push?
To the right is a cross section showing the crust mantle boundary below Oahu. The isostatic disturbance caused by the volcanic islands and their roots would be expected to cause a depression in the crust-lithosphere boundary and perhaps the lithosphere-asthenosphere boundary. Could the isostatic push of an accumulated volcanic pile cause a drip? I think it is possible.
from: Emily Van Ark, Jian Lin, Time variation in igneous volume flux of the Hawaii-Emperor hot spot seamount chain, JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B11401, (2004) http://quake.usgs.gov/research/deformation/modeling/papers/2004/Van_Ark_Lin_JGR_2004.pdf
|What are the consequences of a drip?|
|The drip is in essence a point of subduction. Under the right circumstances, it will draw oceanic crust into the mantle. That could cause volcanism. Once the drip has been consumed, the mantle will rebound and the decompression could cause volcanism. Here is an example of how it might happen in continental crust. (Again, sorry about the lack of an oceanic example.)||Figure 2. Schematic model for loss
of the lithosphere by a Rayleigh-Taylor instability. As
the instability falls, asthenosphere is sucked into the
resulting lithospheric dome, and may melt adiabatically.
A convection cell forms in the lithospheric dome due to
horizontal temperature gradients, and may also cause
adiabatic melting. As the instability falls, if it is
hydrous, it may dewater, triggering melting of the mantle
or of itself.ũ
from: Linda T. Elkins-Tanton, Continental magmatism caused by lithospheric gravitational instability, http://www.mantleplumes.org/LithGravInstab.html
(Figure at right modified per personal communication with L.T.E-T. Updated figure below.)
|Drips in the
So far, in the context of the Hawaiian "hot spot," the drip theory doesn't explain the movement or periodicity of the spot--the thing that got Jason Morgan thinking in the first place. The explanation for this can be found by looking at the time it takes for a drip to form and rebound. The diagram to the left is taken from time-dependent surface topography in a coupled crustmantle convection model, Russell N. Pysklywec and M. Hosein Shahnas, July 12, 2003 15:21 Geophysical Journal International http://www.physics.utoronto.ca/~shahnas/Publication/gji1987[268-278].pdf . It shows a numeric simulation for the topographic impact of a downward plume--a drip--for a thin weak continental crust situation. (That was as close to oceanic crust as I could find.)
As can be seen the impacts exist on a significant time scale that allows for the possibility that the crust and lithosphere may have moved away from the drip and thus the magma may appear to be stationary with respect to some off-plate reference.
From: Time-dependent surface topography in a coupled crustmantle convection model, Russell N. Pysklywec and M. Hosein Shahnas, July 12, 2003 15:21 Geophysical Journal International http://www.physics.utoronto.ca/~shahnas/Publication/gji1987[268-278].pdf
|Add the Plate Motion
To better describe plate motion, I refer to Carlo Doglioni & Marco Cuffaro, The hotspot reference frame and the westward drift of the lithosphere, http://www.mantleplumes.org/Hotspots.html. Doglioni and Cuffaro argue that plate motions are indicative of a shallow origin for the Hawaiian volcanic source. Their argument appears consistent with the hypothesis of this page, but I cannot say whether it is right or wrong. Figure 3 from this paper illustrates relative motion of the lithosphere, asthenosphere and mantle.
Figure 3 The Hawaiian volcanic track indicates that there is decoupling between the magma source and the lithosphere, which is moving relatively toward the WNW. If the source is below the asthenosphere (e.g., in the sub-asthenospheric mantle, option 1), the track records the entire shear between lithosphere and mantle. In the case of an asthenospheric source for the Hawaiian hotspot (option 2), the volcanic track does not record the entire shear between the lithosphere and sub-asthenospheric mantle, since part of it operates below the source (deep missing shear). Moreover the larger decoupling implies larger shear heating, which could be responsible for the scattered, punctiform Pacific intraplate magmatism .
|(As an aside, Doglioni and Cuffaro's rationalization of all continental movement to the westwith apparently larger vectors near the equatoris consistent with a lunar tidal force as a significant factor in plate tectonics. So far as I have been able to discern, the Moon gets too little credit as a driving force. See http://www.mantleplumes.org/WebDocuments/DoglioniGSABull2005.pdf )|
Drip, Move, Melt, Volcano, Push, Drip, Move, Melt, etc. etc.
So, to put it all together, once a Rayleigh-Taylor drip starts, it moves down through the lithosphere to the asthenosphere. It comes up again in the form of volcanism in a different place--because the surface has moved! Once it comes up, the volcanic accumulation in a new place causes a new drip and repeats the cycle.
With the help of Linda Elkins-Tanton's cartoons--or modified versions, one can get an idea how the self perpetuating chain continues.
How did it start? I don't really consider that here. (Meteorite impact?)
Does it work? Does it fit geologic and geophysical evidence?
Send your comments to Eric Twelker, firstname.lastname@example.org
Your comments here:
A reader writes: I don't entirely agree that your hypothesis is feasible . . . . One problem is that the melts produced in Hawaii are inconsistent with melts produced by wet flux from sinking lithospheric material, as is required to produce the chaining of volcanoes. Hawaiian melts are relatively dry and petrologically consistent with temperatures far higher than those found under normal oceanic lithosphere. A further problem is that oceanic lithosphere is unlikely to have sufficient volatiles to trigger melting in the asthenosphere by devolatilizing during its fall. Perhaps you can rebut these thoughts.
of all, I will have to admit to being a neophyte at this,
but at first glance there are a few things that could
overcome the problems mentioned. First, as is noted in
Dr. Elkins-Tanton's paper, the circulation caused by the
drip will increase the potential for adiabatic melting
that could result in dry melts. Second, the Hawaiian
lavas may have benefited from small volatile
introductions. See The Minimum Potential Temperature
of the Hawaiian Mantle is About 1420ēC, G.H.
Gudfinnsson & D.C. Presnall, http://www.mantleplumes.org/Abstracts/GudfinnssonAGU2002.html. A third aspect that has been suggested is the
possibility of asthenospheric shear heating. This diagram
to the left from The
hotspot reference frame and the westward drift of the
lithosphere Carlo Doglioni & Marco Cuffaro http://www.mantleplumes.org/Hotspots.html illustrates:
Having two or more of these factors in the same location could affect melting temperatures and melt characteristics as well. The details of how this would happen are not clear to me now.
More to follow when I have had more time to think about this. ET
|Figure 4. If locally the viscosity of the asthenosphere is higher than normal, the shear stress and shear heating are also higher, providing an increase in asthenospheric temperature. Variations in lithosphere velocity (100 or 200 mm yr-1) and local increases of the viscosity (4 x 1019 or 1020 Pa s) can determine different excess temperatures ranging between 12ēK and 120ēK. The highest excess temperatures could result in extra melting and possibly uprising intraplate magmatism (after Doglioni, et al. http://www.mantleplumes.org/Penrose/BookChapterPDFs/Doglioni_Accepted.pdf )|
Comment on this idea vis a vis the ideas of Carlo Doglioni and the Universitā La Sapienza di Roma group.
On reflection, this hypothesis closely relates to the ideas expressed by Carlo Doglioni and his colleagues cited above. Because the drip hypothesized here is traveling partly in the lithosphere and partly in the asthenosphere, the movement of the apparent hotspot will be significantly slower than the motion of the lithosphere relative to the mantle. Thus measurements of plate velocity based only on the hotspot track would be significantly too slow. Dr. Doglioni and his group have recently suggested that faster plate velocities may in fact be possible and marshaled some supporting evidence. http://www.mantleplumes.org/WebDocuments/DoglioniGSABull2005.pdf
If indeed the faster lithosphere speeds are a reality, then the implications are revolutionary. As they point out in their work, a decollement--or something similar--between the lithosphere and the asthenosphere is required. They suggest an astronomical driving force for the lithosphere. This is at odds with the notion of mantle convection as the driving force of plate tectonics. It also requires rethinking of the mechanisms of subduction and spreading--and of course the mechanism of heat dissipation as well. Some of that work has been started, but if it is the revolution that it appears to be, this will take time. (C. Doglioni, M. Cuffaro and E. Carminati, What moves slabs? Bollettino di Geofisica Teorica ed Applicata Vol.47,n.3,pp.227-247;September 2006 http://tetide.geo.uniroma1.it/sciterra/sezioni/doglioni/Publ_download/WhatMovesSlabs.pdf )
The notion of a decollement and decoupling has implications that feed back to the hypothesis expressed on this page. If the lithosphere and asthenosphere are separated by a low viscosity layer, then they have the potential to move independently--or somewhat independently. The mantle may make turns that are independent of the plates or it may move at an angle to plate motion. The result could be hotspot tracks that diverge from plate motion. Tracks that cross spreading centers would be conceptually possible too. Because we know so little about motion below the asthenosphere, this may be an exercise in speculation--but then that is what much of this topic is made of anyway. ET
Re: Don Anderson's New Theory of the Earth . . .
While I haven't been able to get Don to comment on this page, he did mention in his new book that "Construction and erosion of volcanoes change the local stress field and can generate self-perpetuating volcanic chains; the load of one triggers the next." Hmm.
* I expect this page to be updated as this idea is developed, data added, and concepts refined--or perhaps deleted if it is completely falsified. I encourage researchers to follow up on the ideas presented here. I have no intention of publishing this in the scientific literature, so any published exposition would be welcomed. A citation or acknowledgment to this work would be appreciated.
First posted: October 26, 2006
Last update: October 1, 2007