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
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.
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.
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.
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.
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.
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.
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