Generational Differences in Volcanic Activity

Hawaiian Volcanoes and Relationship to a Deep-Mantle Plume

From the many topics that were given as options by the facilitator of this class, the author of this report has chosen to write about Hawaiian volcanoes and their relationship to the deep-mantle plumes that are nearby and close to them. The area that has come to be known as Hawaii exists within a hotbed of volcanic activity. Of course, this is true both above ground and below it as well. Volcanos play a huge part in how the earth has taken on its current shape as many islands and even many continents have been greatly impacted or formed by the activity of volcanoes. While the topic of this report makes it very clear that the deep-mantle plume and Hawaiian volcanoes are very heavily linked and this report will explore this in great detail using the suggested resources.

Analysis

The deep-mantle plume activity around the Hawaiian Islands is a study of contradictions in many ways. Just as one example, there is the assertion that there has been roughly a generation, about twenty years, of no major serious challenges or changes in the deep thermal mantle plume. Indeed, that is one of the major hypotheses that has been circling around among geologists and other volcanic experts. However, there are many features of the area that clearly do not conform to this theory and idea. One sterling example of this is the great "bend" that is present near the Mendocino fracture zone. This is the point where the Emperor seamount chain ends and the Hawaiian chain begins. This bend refers to the tectonic and other sub-surface masses that are running against each other. Apparently, the bend is not caused by that sort of relationship. Indeed, India and Asia have been colliding from a tectonic standpoint for many years but there is not an ostensible corresponding response when it comes to the fracture zones mentioned above. Whereas the normal driving forces of plate tectonics are caused by ridge push and slab-pull interactions, there seems to be something else going on with the edge of the Hawaiian zone (Fouldger, Anderson, Natland and Julian).

Also, the ostensible locus of active volcanism has not remained in a fixed place geographically. The zone itself is definable and locatable but the location of this zone relative to the earth itself seems to be ever-moving rather than affixed to one geographic area. Prior research has confirmed that the zone has shifted due south by approximately eight hundred meters and this was per research done nearly forty years ago. This has led to the conclusion that the Hawaiian and Emperor chains are actually distinct and separate from each other. Beyond that, there would seem to not be a defined and distinct plume head present in the Hawaiian zone. If it ever has, it has probably been sheared off due to the overall motions of the sub-surface of the area. Even if that were true, there has been no firm evidence collected that this is indeed the case (Fouldger, Anderson, Natland and Julian).

Some of the readings and observations gathered from the Hawaiian zone have been quite interesting. For example, the heat flow that is existing and moving along the Hawaiian bathymetric swell shows no significant amount of variation or change, at least in general. The one exception to this is found in the southern reaches of the Hawaiian zone and this phenomenon apparently cannot be classified as a thermal effect as alluded to earlier in this paragraph. Even more intriguing is the fact that there is no discernible thermal rejuvenation or thinning of the lithosphere as it passes areas with active volcanic activity. This runs directly counter to the commonly held beliefs and theories that pertain to the hypotheses and other works that relate to volcanic plumes. Another interesting metric is that the petrology of the Hawaiian lava flows suggest that the melt comes from a depth of about eighty to one hundred and twenty kilometers. If this is true, there is no petrological data known to exist that requires a deeper source of lava. This would put the depth at roughly near or above the base of the lithosphere (Fouldger, Anderson, Natland and Julian).

Even with the data above, other sources relating to the Hawaiian plume and area of volcanic activity proves that the area has been very busy and chaotic over time. Just one example is what happened on that island during the rule of Hawaii's first king, a man known as Kamehameha. There was an ongoing war at the time between the King and his rival, a man named Keoua. A lot of the fighting at the time led to a rather marked stalemate. However, things change quite quickly when the Kilauea Volcano in the area erupted. Apparently, there was a rather large eruption. It was large enough to remain the deadliest eruption in what currently makes up the United States in modern times. At least eighty people were killed and it is entirely possible that the number is north of one hundred. Regardless, there was a major eruption that led to searing ash and gas coming out of the ground. Quizzically, there has been very little similar activity in the two hundred-plus years since then. There was a much smaller series of explosions that occurred in 1924. Over a period of nearly three weeks, there was a good number of steam explosions that led to mud, debris and hot rocks weighing as much as eight tons being hurled into area. The very heavy rocks that were hurled traveled as far as two thirds of a mile, just to give an indication of just how powerful these plume- and volcanic-related explosions were. However, the last two to three centuries have been rather quite at the ground level when it comes to the Hawaiian volcanic range (Mastin et al.).

This is not to say that there have been no eruptions ... far from it. What is meant by the above is that eruptions, while common, are fairly minor and self-contained. It is to the point that eruptions of lava can be observed at a fairly close range with no concern for danger and safety. This has not always been the case. Even though scientific documentation and measurement is much better now than it was before the formation of the United States, it is known that Kiluea has actually been quite active over the centuries and the ones that preceded the late 1700's were particularly fully of activity. This can be proven by looking at the layers of ash that surround the volcano. As with many other facets of science when it comes to looking at terrain, the age of the ash can be ascertained using techniques like carbon dating and the like. In using this technique, it is clear that some very major eruptions occurred over the last three millennia. One happened only about 600 years ago and two more happened between two and three thousand years ago. This would place the latter two in the first millennia BC (Mastin et al.).

As for why the area is so quiet right now, this is mostly because of the location of the water table in the area. The reason this has changed in the past or could change in the future is the distance between the water table and the plume. While the margin is quite wide right now, it has come within three hundred feet (90 meters) in the fairly recent past. If those two areas were to meet, the circumstances could absolutely mimic what happened in the 1920's, the 1790's or during the other eruption-heavy times over the last three thousand years and beyond. For now, though, the area is relatively safe.