Hawaiian-Emperor Seamount Chain

The Hawaiian Island chain is one of the largest and most striking features on the surface of our planet, yet it is not related to any of the major types of plate boundaries. Since most volcanoes on the Earth's surface are related to plate boundaries, the Hawaiian Islands represent a profound enigma and a reason for geologists to spend copius amounts of time in Hawaii. The Hawaiian Island chain consists not only of the main Hawaiian islands and adjacent French Frigate shoals, but are also connected to the Emperor Seamounts, a large, linear submarine range that runs northward to the Aleutian subduction zone where it disappears. This extensive line of volcanoes represents anomalous lava production and by inference a zone of excess heat in the underlying mantle. In the early 1960's the term "hotspot" was coined for regions like Hawaii where anomalous heat could be recognized, though the origin of such regions still remains quite puzzling. In this lecture we'll explore the structure of the Hawaiian-Emperor Chain and in the next we'll talk about possible theories of how hotspots evolve.

Note the difference between Olympus Mons, a hot spot on Mars, and the Hawaiian Island Chain and Emperor Seamounts on Earth. Mars has no plate tectonics, so hotspot volcanism results in building huge volcanoes that dominate the surface of the planet. The Hawaiian Island chain and related Emperor Seamounts are conspicuous features, but don't stand out nearly as well. The image of Earth uses submarine topography in combination with a satellite photo to see the seamounts beneath the ocean, otherwise only a few of the largest islands would be visible from space. The moving plates on the Earth prevent any single volcano from sitting over the hotspot long enough to build such huge edifices. Instead long linear chains of islands form as the Pacific plate moves over the Hawaiian Hotspot. And as we will learn in this section, the Earth's crust is also far too thin to support a volcano as massive as Olympus Mons.



The Hawaiian-Emperor chain shown in the figure on the left is the most famous and well-studied of the dozens of "hot spots" that dot our planet. The youngest islands lie to the southeast, with the Big Island being the newest. As we shall see, the islands and seamounts become progressively older towards the northwest, bending sharply toward the north about halfway along its length. The oldest seamounts are found at the northwest end, poised to plunge beneath the Aleutian volcanic arc, carried downward with the oceanic lithosphere as it is consumed. The oldest volcanoes yet to be consumed are just over 65 million years old, erupting just about the time that the last dinosaur sank to its knees (or whatever) and died. There may have been older volcanoes that have now been subducted--we have no way of knowing, since the geologic record has been erased. 65 million years later, and 6000 kilometers to the southeast, the "hot spot" continues to pump lava to the surface, currently building the Big Island.



By contrast with the dinosaurs, that prowled the Earth for nearly 200 million years, man is a relatively new addition to the biosphere. Indeed, modern man arrived while Haleakela was just reaching it prime, and since then the plates have moved less than a few hundred kilometers over the Hawaiian "hot spot".

Volcanic island chains run like smoke on the wind and are generated as fresh lava from each "hot spot" builds massive mountains of basalt on the sea floor, through 5 kilometers of water, and then several more kilometers above the sea. In this lesson we investigate what can be learned from these linear mountain chains on the bottom of the sea. The image on the right shows some of the other "hot spots" scattered about the floor of the Pacific Ocean. It is intriguing, that portions of island chains of similar age are parallel to each other. This suggest that the "hot spots" themselves remain mostly fixed with respect to each other, otherwise the chains might be expect to be curvilinear, or trend in different directions as the "hot spots" generating them moved independantly.

How do we know the ages of the underwater volcanoes making up the Hawaiian and Emperor Island seamount chains? The answer is radiogenic dating. The basalt that makes up the seamounts contain minerals with a relatively generous amount of the element potassium. This is especially true for the last lavas to be erupted as the seamount moves off the "hot spot". Some of the pottasium consists of K40, an unstable isotope that spontaneously decays to the inert gas Argon (Ar40). Since argon is chemically inert, it is not generally found bound into minerals making up igneous rocks. As a rock ages, from the time it solidifies, argon atoms slowly build up, trapped in the crystal lattice, while the amount of unstable potasium slowly decreases at a very uniform, and predicitable rate. By carefully counting the numbers of each the age of a rock may be determined. On the average it takes about 1.3 billion years for a single atom of K40 to decay by capturing an electron into its nucleus, changing a neutron into a proton. Another way of looking at it is that a pile of 1.3 billion atoms of K40 would produce one such transition in an average year.



Needless to say, counting potassium and argon atoms requires some rather fancy equipment. The figure on the left shows the range of ages for specific volcanoes plotted as a function of distance along the island chain. Clearly the ages increase in a fairly uniform manner, but each volcano also has a range of ages, since individual edifices remain active for several hundred thousand years. We can't sample the oldest lavas from each seamount, because it is locked deep within many kilometers of younger basalt. Therefore, we must make due with the ages of the last lavas to be erupted. This is less than ideal, since there is considerable variation in the lifetimes of individual volcanic seamounts.

The incredibly striking linear progression in age along the chain is shown by the figure on the right. The best straight line through all of the data (ironing out the bend, of course) gives a tectonic plate velocity over the "hot spot" of about 8.6 centimeters per year, much slower than hair or fingernails grow. Actually, I think maybe my hair grows more slowly than plates move, but that is a rare exception! Two lines fit the data somewhat better, as will always be the case. The two line solution suggests that recently tectonic motion has slowed somewhat with respect to rates prevalent in the Pacific more than 30 million years ago. One could just as easily, or perhaps more easily, fit a slowly varying curve to the data. This curve would indicated a small but significant slowing of plate velocity over the past 70 million years, a result that seems reasonable as the heat driving plate motion is slowly lost to space.

The plates show a remarkable and abrupt bend about 44 millions years before the present. There has been much speculation regarding the cause for this bend. Geophysicists generally agree that the bend originated with an abrupt change in plate motion. Prior to 44 million years ago the plates were moving in a much more northerly direction. There is considerable less agreement, however, on just what caused this bend. Many scientists believe it was the collision of India with the Eurasian subcontinent, and event that has raised the Himalayan Mountains, that did the dirty deed. Other feel that it was the beginning of spreading on the Antartic Ridge south of Australia that was the culprit. Whatever the cause, it is clear that there was a massive reorganization of plate motion nearly 50 million years ago.

Perhaps it is even more amazing that in the past 65 million years there has been only one such bend. Even more remarkable is the observation that the straight portions of the chain are straight. As we shall see below, the configuration of the plate boundaries in the Pacific have changed dramatically during the lifetime of the Hawaiian hotspot. If, as many geophysicists believe, subduction drives tectonics, then how on earth can the straight parts be so straight and move at constant velocities for tens of millions of years? The answer to these questions remains a mystery!

One other remarkable consistancy remains to be discussed. The eruption rate for Hawaiian volcanoes has remained quite constant over most of the 65 million years of preserved activity. This is shown by the figure on the left, where cumulated volume is plotted against distance along the chain. For each distance, the volume plotted is the total amount of lava erupted previosly by all older volcanoes. Over the last 65 million years, about 1 million cubic kilometers of lava has been produced. This is enough lava to fill a box 100 kilometers on a side. The width of the Big Island is roughly 100 kilometers to give you some idea of how much material this represents. If all this lava were somehow removed from the vicinity of the Hawaiian "hot spot", there should be a great hole in the bottom of the ocean nearly 100 km in depth. Since there is no such hole, there must be some mechanism to replace the material lost and erupted to the surface. Essentially this proves that there must be some kind of convection or fluid motion in the rocks that make up the Earth's upper mantle.

It should be noted that for the 10 million years following the bend, very little lava erupted. Dave Clague, former Scientist in Charge of the USGS Hawaiian Volcano Observatory, suggestss that during this time there were no Hawaiian Islands at all above the surface of the sea. This is a bit of a bad situation for the previous inhabitants of the islands, since there is very little other dry land for thousands of kilometers. Indeed, almost all of the previous life must have been exterminated, so that the current flora and fauna must have arrived more recently. Although we don't have a clue as to why this cessation occured immediately following the reorganization of plate motion, its existence would seem to provide some powerful constraints on the models proposed for the "hot spot" discussed in the next lecture.

Looking Back in Time



At present the age of the sea floor beneath the Big Island is roughly 95 millions years old. This means, that it was created at a mid-oceanic ridge 95 million years ago before being rafted to the central Pacific Ocean. This is shown in the figure on the left, where the difference in age between the volcanos and the underlying seafloor is plotted with distance along the island chain. Curiously, this age difference does not change as far back in time as the bend, demonstrating that the active volcanoes were being built on sea floor somewhat more than 90 million years old for the past 44 million years. After the bend, however, the picture changes. From the bend north along the Emperor chain the age difference steadily decreases until it is less than 10 million years for the oldest known volcanos in the chain. This means that these volcanos were being erupted on sea floor that was much younger than is the case today. If the trend is continued back to about 80 million years, it would appear that the "hot spot" was building volcanoes on ocean floor of the same age. How can this be? The answer, clearly, is that roughly 80 million years ago the Hawaiian "hot spot" was collocated with an oceanic ridge, much as the Iceland "hot spot" is today. But which ridge was it near. Current ocean floor beneath Hawaii was constructed at the Mid-Pacific rise just off the west coast of the Americas. To answer these questions we must reconstruct the configuration of the Pacific ocean floor going back nearly 100 million years, a daunting task indeed.

The situation during the past few million years, an eyeblink for a geologist, is shown in the figure on the left. The plate is moving in a northwesterly direction, leaving a trail of distinct volcanos on the sea floor. The H marks the current "hot spot" position. Similarly, the Yellowstone "hot spot" is shown by a large block labelled with the letter Y.

The image on the right shows the situation about 43 million years ago, just after the platemotion reorganized into the present configuration. Note the Emperor chain trending toward the North. There is also a small piece of ridge off the Pacific Northwest known as the Kula Ridge. This ridge has since been subducted beneath Alaska, but then it was actively making oceanic lithosphere.

The next image on the left shows the situation about 56 million years ago. The oldest volcanos of the chain that still exist today were then only about 10 million years old. The Kula Ridge can now be seen to be much closer to the "hot spot", and consequently the age of the sea floor at the "hot spot" is much younger as discussed previously.

The trend continues with the image on the right. Again, the Kula ridge is quite close to the Hawaiian "hot spot", and the ocean floor beneath the growing volcanoes is very young. This image shows the configuration when the oldest volcanoes that still exist today were just being formed. Remember, this is also the time of the extinction of the dinosaurs.

Going back still further, roughly to 80 million years before the present, the Kula Ridge lies south of the "hot spot", and will shortly be passing it as it moves north. The idea that ridges and trenches move around so dramatically might seem strange to you. Somehow one gets the idea in high school or introductory geology courses that these things are relatively fixed with respect to each other. Unlike their cousins, the "hot spots", this is clearly not the case.

Finally, at 100 million years before the present, shown on the image on the right, the configuration of plates in the Pacific is almost totally unrecognizable compared to contemporary tectonics. To reiterate an earlier point, and considering the extreme changes in plate geometry over the lifetime of the Hawaiian "hot spot", the amazing linearity and constancy of motion of the plate as evidenced by linear, progressive aging of the volcanoes along the chain is truely incredible and mysterious!

There are a few additional points worth making involving the notion of the fixity of the Hawaiian "hot spot". These involve the Earth's magnetic field. As you probably know, the Earth's magnetic field is dipolar, quite similar to that surrounding a simple bar magnet. As shown in the figure, the magnetic field lines intersect the Earth's surface at an angle everywhere but at the equator, where the field lines are perfectly horizontal. At the North and South poles, the field lines are vertical, and magnetic compasses are totally useless.

As lava cools, magnetic minerals align with the magnetic field, essentially preserving a record of its orientation at the time. The important component is the angle from the horizontal, known as the magnetic inclination. At a given latitude, the magnetic inclination is always the same. In effect, known the inclination of the magnetic field of a lava sample tells you the latitude at which it cooled and solidified. The magnetic inclination data for rocks cored from submerged seamounts along the island chain is shown on the diagram on the right. Although there is considerable scatter in the data, it is strongly suggested that all of the lava solidified at 19.5 degrees north latitude, precisely the latitude of the "hot spot" today. At least with respect to latitude it would seem that the Hawaiian "hot spot" has been fixed for at least the past 65 million years.

Geodynamics



The masses of the great volcanoes raised above the "hot spot" places a tremendous load on the plate, depressing it by up to 5 kilometers. This bending changes the bathymetry (depth of the ocean) in the central Pacific, and as we shall see leads to a class of earthquakes that can effect much of the State of Hawaii. The figure on the left shows the contours of depth plotted over the topography of the Hawaiian Islands.

Main eruptive vents are shown as stars. The Hawaiian Ridge is the range of volcanic seamounts itself. It fills the deepest part of the trough caused by the down bowing of the lithosphere, which is roughly 80 kilometers thick. The remainder of the depression adjacent to the islands is known as the Hawaiian Deep. As the central part of the trough is bent downward under the weight of the volcanoes, material in the asthenosphere is pushed to the side. This, together with flexure of the plates produces a rise paralleling the island on both sides known as the Hawaiian Rise. Centered on the Hawaiian "hot spot", and extending nearly 1000 km in all direction is a general shallowing of the sea known as the Hawaiian Swell. The broad, gentle upbowing of the lithosphere is probably assocated with a similarly broad upwelling of material in the mantle. This also would seem to hold some important clues as to the nature of the "hot spot".

The bowing of the crust in the vicinity of the active end of the chain is shown in cross section on the figure on the right. The top figure shows the section along the chain. As the plate moves to the left, a "bow wave" is produced ahead of the growing island chain as the plate begins to bend downward under the approaching load. The two lower sketches show cross sections perpendicular to the trend of the chain. The sketch to the left shows the almost complete bending on a cross section through Mauna Loa. The one on the right shows the immature bending at about the position of Loihi, southeast of the Big Island. The relationship between the trough, the Hawaiian Ridge and the Hawaiian Deep are clearly shown in this illustration.

How do scientists study the effects of volcanic loading on the sea floor over geologic time. It turns out that some of the answers can be pieced together from the effects glacial episodes have had on the Hawaiian Islands. Although it is true there were glaciers on Mauna Kea and Mauna Loa just a few thousand years ago, these are not the effects to which I am referring. At the onset of an "ice age", water from the oceans is locked onto land in the form of huge sheets of ice. As the glaciers form, sea level drops world wide at a rate of a few millimeters each year. Coincidentally, the islands themselves, because of the loading of the lithosphere, are sinking at about the same rate. The result is that coral reefs surrounding the islands remain in the photonic growth zone much longer and become quite massive. At the end of an ice age, the volcanoes continue to sink as sea level rises, exterminating the coral. The result is a series of coral rings at increasing depths surrounding the islands. These reefs have maintained a continual record of tectonic deformation during the past million years or so. In addition, the existence of coral far north of where it could ever have grown is another argument for the fixity of the Hawaiian "hot spot", as least as far as latitude is concerned.

The slow sinking of the island is clearly shown by the sea level data plotted on the figure on the right. As the island sinks, tide guages fixed to the islands are dragged down with them. The result is that if the island is sinking, the sea level appears to rise. For the Big Island this rate, as shown in the figure, is roughly 3 millimeters per years, or 30 centimeters per century. By contrast, tide guage data from Oahu, which has ceased growing and is therefore stable, shows very little evidence for continued subsidence. The slight upward trend is probably due to slow expansion of the sea as a result of global warming as well as the continuing melting of the world's glaciers. The shallowing of the ocean as the plate arives at the "hot spot" followed by a slow increase in depth subsequently is shown in the figure on the left. Even as the effects of loading dwindle, however, the islands continue to sink very slowly as the lithosphere upon which they are constructed slowly ages, and becomes more dense. The depth of the ocean is strongly correlated to the age of the underlying seafloor, a remarkable fact. As the seamounts are carried further from the "hot spot", they are dragged down by the aging sea floor until coral reefs and atoll are found more than a kilometer beneath the surface of the sea--a depth much to great for light-loving coral to grow.

The bending and sinking of the island chains help to partially clear up an interesting paradox. The volume estimates of the Hawaiian-Emperor Chain derived from measuring all of the seamounts are about 1 million cubic kilometers. However, estimates of lava production over the hotspot are greater than 0.1 cubic kilometers per year. Discounting some of the low production zones in the chain, there should be a volume more like 5 million cubic kilometers of lava produced during this time. The fact that about half of the volcanoes mass lies beneath the level of the ocean crust would roughly double the measure volumes to about 2 million cubic kilometers. Still there is an annoying discrepancy of about an equal volume. One idea is that the recent production of the hotspot has increased to levels not previously attained. Another idea is that landslides may have significantly reduced the volumes of the older islands and seamounts, making volume estimates prone to great errors. More on this story as it develops....





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