Hot Spot
GEOL205 - Lecture Notes
In the last section we examined the Hawaiian-Emperor sea mount
chain which runs in two linear segments nearly 6000 km across
the sea floor.
We know that there has been a source of magma coming from deep
within the mantle which has been active for more than 65 my.
During this time enough lava has been produced to fill a box
100 km on a side.
Where did all this lava come from, since there is no great
depression marking the point it was generated?
What source of heat could remain active for tens of millions of
years?
Why has the rate of production been constant for millions of years?
Why did lava production dwindle dramatically for 10 million years
after the change in plate direction causing the bend?
Unfortunately we do not know the answer to these questions.
There are a number of models, though, that attempt to account
for what we observe along the chain.
All of the models proposed can be broadly grouped into four categories,
propogating fracture, convection, mechanical injection, and shear-heating.
None of these models is completely satisfying, although all account for
some observations better than others.
To be acceptable, a model must explain the uniform propogation of volcanism
along the chain, account for the apparent fixity of "hot spots" in
general, explain the variations in production rate such as that
following the great bend, as well as the details of the spacing between
individual vents.
The ultimate source of the material erupted must be found, since there is
no great hole centered on the Hawaiian "hot spot".
The lack of chemical variation in the basalts along the chain puts severe
constraints on possible models--models that change chemically with time,
as melting models usually do, are not acceptable.
The mystery is profound, and speculation is highly warranted!
Many of the ideas here follow the review by Clague and Dalrymple in U.S.
Geological Survey's Professional Paper #1350.
Propogating Fracture
The simplest models to understand, and probably the least likely to be
correct are the propogating fractures models.
In these models, external forces "rip" the lithosphere, and melt from
the upper part of the aesthenosphere well upward toward the surface.
The first such model was proposed in 1849 by Dana, suggesting that
the Hawaiian Islands were erupting through a world wide system of
"rents" associated with the slow cooling of the Earth.
The volcanoes on the Big Island were still active because the "rents"
were wider there.
Other scientists proposed similar models through the 1940's.
These models all suffer from the fact that they do not produce the
slow migration of volcanism along the chain at a constant velocity.
Indeed, such data was not available to these early geologists.
Beginning in the early 70's, scientists began to propose models involving
a propogating fracture, like a rip in a sheet used in a six team game of
tug-of-war.
The models vary in the source of the forces causing the lithosphere to tear.
In 1971 Green proposed that the fracture was a result of the lithosphere being
pushed over a stationary "bump" in the mantle.
It did not provide, however, an explanation as to how this "bump" could be
maintained for tens of millions of years, but it did explain the constant
velocity of the propogation of volcanism along the chain.
Two years later Menard expanded the model by relating the "bump" to
upwelling of material in the deeper mantle.
This explanation is something of a hybrid with the convection models
discussed below.
A fairly typical "propogating fracture" model by McDougal in 1971 is shown in the
sketch on the right.
As shown, the rise of mantle material into a propogating fracture causes
partial melting which continues until motion of the plate decapitates
the rising melt and the cycle begins anew.
It turns out that this particular model predicts propogation of the "hot spot"
that is not observed.
Walcot in 1976 proposed that the fracture was caused by the weight of the
volcanos themselves, and the once the process was started it would
continue indefinitely.
He did not mention, however, how things get started in the first place.
Many later workers attribute the forces tearing the lithosphere to stresses
in the oceanic lithosphere.
On such model was put forth by Jackson and Shaw in 1975 attributed these
stresses to boundary forces such as ridges and trenches or possibly to
flow in the deeper mantle.
Through out the 70's, Turcotte and Oxburgh suggested a source for the stress
as cooling of the lithosphere or membrane stresses assocated with moving
a rigid plate over a spherical earth (sketch C).
As objects cool they contract.
It was reasoned, that the resulting stresses should be parallel to the
associated rift
(sketch B).
But the Hawaiian chain is not perpendicular to the East Pacific Rise, but
Turcotte and Oxburgh pointed out that in a plastic material, fractures
occur at a slight angle (sketch A) to the applied stress, resolving this dilemna.
The problem with all of these models is how to account for the uniform rate
of volcanic propogation.
Convection Models
The convection models are those involving material rising in the mangle
below the active volcanism
Magma forms as a result of partial melting of the rising material as
it decompresses.
The reason for this partial melting is that the melting temperature of
rocks increases with pressure.
As mantle material rises, it eventually reaches a depth at which the
first melt appears from the melting of minerals with the lowest
melting temperatures constituting the rock.
It is important to note that rising mantle material undergoes very
little cooling as it rises because heat propogates very slowly by
conduction in rock.
As a result, the decrease in pressure dominates the partial melting of
rising mantle rock.
One of the first convection models was proposed by Wilson in 1963.
This model (shown on the left) held that melt arose from regions
of mantle convection that were relatively quiescent.
Over time the natural radioactivity present in most rocks could
increase the temperature sufficiently to begin partial melting.
The resulting derived liquid, basaltic melt, could then rise to the
surface, forming volcanoes as the overiding plate moved in a northwesterly
direction.
This model seems to suffer a number of problems, such as producing about
a million cubic kilometers of melt without leaving a hole in the sea
floor.
Another problem is that the hotspots move relative to ridges and trenches
while the underlying convective system would seem to be tied to
these features in some manner.
In the early 1970's, Morgan proposed the "plume" model, which is the
one most of us were taught at our mother's knees.
In this model proposes that thin plumes (about 150km across) arise from
very deep within the mantle, forming "hot spots", as well as supplying
the material that feeds the rift zones and tears continents apart.
Despite continued popularity, the model appears to suffer from some
difficulties.
It has been pointed out that mantle material rising through such a great
change in pressure would undergo substantial melting with a
chemistry much different than what appears to be the primitive
magma feeding "hot spot" volcanism.
There is also problems in fluid mechanics where convection in a homogeneous
fluid with properties like those of the mantle would not be expected to
produce such narrow "plumes".
The model does, however, predict the fixity of the various "hot spots" with
respect to each other.
An alternative to the fluid plume hypthesis proposed by Anderson in 1975
avoided some of the pitfalls encountered by Morgans model.
He proposed that the Earth had condensed inhomogeneously from the solar
nebula such that refractory minerals (those with a high melting temperature)
were concentrated near the center.
The minerals, which also tend to be radiogenic, melted as a result of
radiogenic heating and rose through the mantle leaving a trail
of refractory material in their wake.
The model suggests that the heating of the surrounding rock over time
generates the "hot spot" primitive magma.
There is some difficulty in understanding how the chemical trace generating
the heating can produce melt at a constant rate for tens of millions of
years.
The previous models have all involved mantle convection as a localized
plume.
A different approach was taking in 1975 in a series of papers by Richter
and Parsons, who considered the general pattern of convection in a
thin layer of viscous fluid such as the Earth's asthenosphere.
They found that when a shearing is applied, such as the motion of the
Earth's tectonic plates over the fluid region, convection occurs as a
series of parallel "rolls" aligned with the direction of plate motion.
These counter-rotating rolls cause alternating zones of compression
and extension in the tectonic plate.
The island chains, space at distance comparable to the depth of
convection, occur as a propating fracture in the extensional zone.
Unfortunately the model does not seem to predict the observed uniform
rates of propogation.
It does however, correctly predict the cessation of activity for millions
of years following the change in plate direction, a distinction that
no other proposed model seems to share.
Thermal Injection
Early on it was noted that the ocean is much shallower in the vicinity
of the Hawaiian "hot spot" than one would expect given its age.
Further, as volcanic islands are carried off the "hot spot", they
descend much more quickly than the aging of the sea floor would
suggest, a rate much more comparable to that for very young
ocean floor near a ridge.
This lead to a series of models inwhich the lithosphere was thinned and
"younged" beneath Hawaii as a result of heating.
The problem with early versions of such models was getting heat into
the tectonic plate fast enough to thin the lithosphere quickly enough.
Thermal conduction was nearly one hundred times too slow.
A mechanical "fix" to this problem was provided by models suggesting
a delamination of the lithosphere, with the lower portion descending
in the form of a kind of partial subduction zone.
This descending block of lithosphere should be seismically observable,
which it is not.
Also, the chemistry of the lavas produced at such a shallow depth is not
consistant with those erupted at the surface.
Shear-Melting
In 1973 Shaw published an intriguing model that was so unique as to warrant
a class by itself.
This model was a thermal-mechanical feed-back mechanism that produced
heat by the shearing of rock.
The heating mechanism is similar to that produced by rapidly flexing
a metal clothes hanger (not the ubiquitous, plastic variety).
In Shaw's model, shearing is concentrated in a particular part of the
asthenosphere, and thus produces more heat.
This heat, in turn, produces a melt which further weakens the surround
rock and rises to be the primitive source of Hawaiian lavas.
The weakening increases shear heating and the process continues.
One notable problem with this model is that it does not provide a means
of replacing removed material, which should result in a very large hole
in the ocean floor near Hawaii.
The model would also seem to be incapable of accounting for the similarity
in petrology (chemistry of the rocks) found throughout the Hawaiiand
and Emperor chains.
The model also lacks a clear starting mechanism, although one could
easily imagine an oceanic astroblem as the cause of the initial "bruise".
Subsequent revision of this model called for the anchoring of the "hot
spot" with a descending plume of residuum from the partial melting
producing the ascending magma.
One problem with this is that the partial melting of mantle rocks
should produce both a melt and residuu that are both less dense than the
parental material, and thus positively bouyant.
Ascent of Magma
Whatever the "hot spot" is, we must still address the mechanism by which a
partial melt works it's way towards the surface to become part of the
Hawaiian volcanic chain.
The earliest melt would appear as a thin film wetting the grain
boundaries.
As melting progresses, interstial melt is sqeezed into larger blobs,
that can begin to rise because they are less dense than the surrounding
rock.
In the asthenosphere, which is somewhat fluid and deformable, tadpole
shaped pods of magma known as diapirs rise like hot-air balloons to the
base of the rigid lithosphere.
Because the lithosphere is not nearly as deformable as the asthenosphere,
a different mechanism must be found to transport magma through
60 to 80 kilometers of brittle rock.
One such mechanism proposed by Hill combined a knowledge of tectonic
stresses with field observations of regions with similar stress
patterns at the surface of the Earth.
In this model, magma which has pooled at the base of the lithosphere
continues its upward ascent by filling cracks formed by bending tension
at the base of the lithosphere caused by volcanic loading.
The upward journey commenced as melt was squeezed through a network of
vertical cracks, with a lower crack closing as a higher crack received
its contents and expanded.
This mechanism would also seem to accord with the low frequency tremor
that comes periodically from intermediate depth in the mantle beneath
Hawaii.
During its passage through the asthenosphere and lithosphere the rising
primitive melt is undoubtedly contaminated by melting and mixing with
a significant amount of oceanic ridge basalt.
Eventually the melt is passed from crack to crack through the lithosphere
and nears the surface.
Its rise is halted about within a few kilometers of the surface because
of the decreasing density of the surrounding rocks.
At a depths of about 3 kilometers from the surface, the fractured and
vesiculated rocks of the edifice are approximately the same as the
rising melt, and magma pools in a magma chamber beneath the summit.
Pressure then rises within the summit magma chamber until it becomes
sufficent to fracture the shallow rocks capping the magma chamber and
magma erupts to the surface.
Sometimes, as we shall see later in the course, magma is forced into one
of two rift zones connected to the summit, and lava erupts further
downslope from one or the other of these.
Examination Questions
- Discuss various models for the Hawaiian "hot spot" siting
the advantages and disadvantages of each. What do you
think the "hot spot" is, and why?
- Describe 3 possible scenarios for how the hotspot might have formed and disucss the pro's and con's of these scenarios.
- Describe the evidence that the Hawaiian - Emperor Chain is a hotspot. How do we know that this spot has been at a fixed locality through time?
If you have comments or suggestions, email me at kenhon@hawaii.edu
Beginning in the early 70's, scientists began to propose models involving
a propogating fracture, like a rip in a sheet used in a six team game of
tug-of-war.
The models vary in the source of the forces causing the lithosphere to tear.
In 1971 Green proposed that the fracture was a result of the lithosphere being
pushed over a stationary "bump" in the mantle.
It did not provide, however, an explanation as to how this "bump" could be
maintained for tens of millions of years, but it did explain the constant
velocity of the propogation of volcanism along the chain.
Two years later Menard expanded the model by relating the "bump" to
upwelling of material in the deeper mantle.
This explanation is something of a hybrid with the convection models
discussed below.
