The shape of Hawaiian volcanoes tends to be long and linear, a striking contrast to the cone-like shapes we normally associate with volcanoes. The only other volcanoes in our solar system with even remotely similar shapes are those found along the mid-ocean ridge systems. Mid-ocean ridge volcanoes are elongated along the direction of spreading. This makes sense as the tearing apart of oceanic crust provides the pathway fractures for melt to reach the surface. Thus mid-ocean ridge volcanoes grow over these fractures. This NOAA image of the mid-ocean ridge off Washington and Oregon shows an elongated ridge with a summit caldera. The ridge has distinct rift zones with lots of little cones and domes scattered along it's length, not unlike Kilauea and Mauna Loa volcanoes. It's pretty clear that tectonics and faulting have a strong influence on mid-ocean ridge volcanoes.
Hawaiian volcanoes also have elongated zones called rift zones. This sonar image of Loihi made by the Monterey Bay Aquarium Research Institute clearly shows the elongated nature of Hawaiian volcanoes. The summit area at the left of the image is defined by a series of collapse pits, one of which formed in 1996!
Both the summit and the rift axes of Hawaiian volcanoes stand high because they are the predominant source of eruptions, as we discussed in the beginning of the Eruption section. But what does that really mean? Well one thing is that it must be easier for the magma to get to the surface along these zones. Clearly the summit and rift zones appear to be zones of weakness in the outer shell of the volcano. The tectonic reason for the elongation and weakness on mid-ocean ridges is pretty obvious, but the elongation of Hawaiian volcanoes is due to a combination of processes.
We have established that Hawaiian volcanoes have a central, high-level magma chamber that indicates a centralized supply system for the volcanoes. If there were no other influences, we would expect that Hawaiian volcanoes would have a circular form (like Olympus Mons on Mars) and a radial pattern of weaknesses. However, Hawaiian volcanoes form a chain due to the combined influences of a fixed hotspot and the movement of the Pacific plate which carries the volcanoes off the hotspot. So, when a new volcano forms it does so in the shadow of a much larger older volcano that is in the process of moving off the hotspot. Notice how the shapes of the earlier volcanoes have a profound effect on the shapes of the younger volcanoes in the next figure (subaerial rift zones in red, submarine in blue). In addition to the shapes of the earlier volcanoes, it is also probable that weaknesses in the underlying seafloor may control the initial shape of the volcano.
Though we can see that the presence and form of older volcanoes cause Hawaiian volcanoes to attain a linear shape, we are still left with the fundamental question of what produces the linear weaknesses in the volcanoes. One idea proposed by Richard Fiske in the 1960's was simply that the weight of the volcanoes caused them to spread laterally along their longest axes. This spreading created zones of weakness that the magma could exploit to get to the surface. These planes of weakness are extremely important in providing pathways for the magma, otherwise it would have to break through over 3 kilometers of solid basalt (which requires a minimum pressure of 1000 times atmospheric pressure simply to overcome the weight of the rock, not to mention its strength).
There are some interesting effects of bending these linear volcanoes as well. In the following JPL radar image and USGS map of Mauna Loa's vent systems, notice the lava flows and vents on the northwest side of the summit. These vents are crudely radial to the summit area and are a consequence of bending the volcano. Imagine bending a long 2x4 board. As you bend the board the outer edge must stretch and eventually will crack as it gets longer. In contrast, the inner edge is highly compressed and doesn't crack. The radial vents on Mauna Loa occur as the volcano grew against Hualalai and Mauna Kea and became progressively more bent with time.
During the 1960's Don Swanson and others at the Hawaiian Volcano Observatory became interested in exactly how the volcanoes behaved during and between eruptions. In order to get the best idea of the long term behaviour of the volcano, they went back to old surveys of the island and began collecting new survey data. They simply compared the old survey points to the new and found that they did not match, demonstrating that the volcano had been moving in historic time. Previous workers had suggested that the large scarps (or palis) on the south side of Kilauea were simple landslide scarps related to failure of the steep submarine slope of the volcano.
In contrast, the idea that the entire volcano moved southward suggested that the very large earthquakes of 1823 and 1868 were caused by a sudden sliding of the volcanoes on the ocean crust rather than simple slope failure. Following each dike intrusion the rift zones appeared to compress like a large spring. In fact, there was so much compression in the rift zones that Swanson and others actually suggested that Kilauea may be due for another large earthquake a couple of years prior to the 1975 magnitude 7.2 Kalapana earthquake.
This sequence of events led Swanson and his coworkers to propose that the forcible intrusion of dikes into the rift zones pushes the southern flank of the volcano seaward. Dikes that propagated from the summit out through the rift zones compress the zone until sufficient pressure builds up. This model requires that even though the dikes appear to follow the gravitationally weakened planes of the volcano, they are intruded under great pressure and not passively. Most of the dikes observed during this study were relatively shallow (less than 3 km) and did propagate from the summit into the rift zones. Deeper intrusions, up to 5-6 km, were also believed to play a part in the movement of the flank. While the flank was considered active, the large zone of faults south of the summit area (the Koae fault zone) was considered to be a passive reaction to the seaward offset of the East Rift Zone from the summit.
Eruptions on the SW rift zone of Kilauea are much less frequent than those on the East Rift Zone. Each time a dike is emplaced in the rift zone, the overlying rock is spread apart slightly. Large systems of cracks form on the surface above dikes on both rift zones. However, because eruptions are much more frequent on the East Rift, many of these fractures have become buried. In contrast, large crack systems are the most prominent feature of the SW rift zone (though the lack of vegetation also enhances the visibility of these cracks). The low lava supply to the SW Rift Zone is probably a consequence of being jammed between Mauna Loa and Loihi. It is very interesting to note that squeezing the rift zone has the effect of producing cracks on the surface. Lava that moves in the upper SW Rift Zone emanates in very shallow dikes from the area around Halemaumau. Magma that supplies eruptions on the lower SW RIft Zone moves directly south of the summit and then follows the Koae fault zone at 3-4 km depth to the SW rift.
Since the 1975 earthquake, the south flank of Kilauea has been moving much more freely. In fact, since the Puu Oo eruption began in 1983, the south flank has been moving seaward about 10 cm a year. There has also been little or no compression of the east rift zone, which is not surprising since the only major dike intrusion happened at the beginning of this eruption. So without anything to push the volcano, why is it moving seaward? This is a pretty good question that a number of people have pondered. Seismic evidence indicates that the sliding is happening either right on the interface between the volcano and the old ocean crust or along several parallel zones up within the volcano. Since the 1975 earthquake the south flank appears to have been sliding fairly smoothly (with the exception of the 1989 magnitude 6 Royal Gardens earthquake) and continuously. Prior to 1975, the south flank appears to have been locked up, allowing stress to build up prior to the 75 Kalapana earthquake.
During the late 1980's, Paul Delaney, Dick Fiske, and their HVO colleagues proposed that a deep magma body underlies the East Rift Zone and pressurization of this body was pushing the flank southward. While this model is similar to the earlier model of forcible dike injections, it differs in that the top of this magma body was thought to be at 4-5 km, whereas this was close to the base of the postulated magma zone in the shallow dike model. The principal evidence for the deep magma body came from the style of deformation of the lower East Rift Zone in the vicinity of Pahoa and the fact that the flank was moving during the late 1980's and early 1990's without any forcible intrusions of magma. In the Pahoa area the rift zone was observed to be progressively sagging with time over a wide area. The very broad nature of this sag was indicative of a very deep source; a shallow source would have produced a very narrow downdrop.
They also pointed out that while most dikes in the East Rift Zone did propagate from the summit area and move downrift, the 2 recent eruptions in the lower East Rift Zone (1955 and 1960) differed significantly in behavior. In contrast to eruptions initiated in the summit, both of these eruptions began in the lower East Rift zone with no apparent initial connection to the summit. Generally after a few days, the magma would aseismically (no earthquakes) migrate back uprift to the summit and cause a major deflation (draining the uppermost part of the magma chamber). This led Delaney to propose the existence of a deep (>4-5 km) magma body that not only allowed magma to move aseismically from the summit to the rift, but also provided the driving force for moving the rift seaward. This idea is appealing because it places most of the magma down deep in the volcano (from 3-4 km to the base of the volcano at 7-8 km) instead of up high in the volcano like the shallow dike model. However, it still requires a lot of driving pressure in the magma body to push the flank. In addition, seismic evidence, as we recall, shows a very dense body at 6-7 km beneath the summit and gravity anomalies show that the rifts also have very dense roots.
About this same time, Dave Clague was finding the first of the little grains of glass on the seafloor around Kilauea with the really high MgO contents (14-15%). As we all recall (don't we?) he inferred from these that the interior of Kilauea (and other Hawaiian shield volcanoes) must have a core of olivine cumulate (crystals that settled out of magma) that represents about 15-20% of the volcano.
This led Dave Clague and Roger Denlinger to propose that the deep seated magma body rested on a large, hot mobile core of olivine. Most of the olivine underlies the summit, with a decreasing proportion of olivine along the rift zones. This distribution of the olivine core mirrors the maximum movements along the south flank during the late 1980's and the 1990's very well. The maximum displacements almost directly to the south of the summit of Kilauea with decreasing displacements along the East Rift Zone. The Koae fault zone might know be considered the result of very active extension and spreading of the summit area rather than a completely passive structure. In addition, the rift zone moves more slowly and more sporadically due to the lower driving forces.
The cumulate model-deep intrusion produces a decidely different view of the eruptive mechanics of Kilauea than the shallow forcible dike model. The large pile of hot, mobile cumulate pushs the flank seaward and causes the volcano to extend and form weaknesses along its rift zones. A simple analogy would be pulling a deck of cards apart to produce a number of hairline separations. As the volcano extends, the rift zone weakens and magma exploits these zones by intrusion as sheet-like dikes and associated fissure eruptions. Each time the magma intrudes the cracks, it pushes the volcano back together, causing significant ground deformation. This model requires very little pressure to intrude dikes and creates a mechanism that is very similar to mid-ocean ridge volcanoes (which also have relatively small magma chambers floating on top of very large cumulate piles).
Finally, what is the deal with that strange bend in the East Rift Zone anyway? No other Hawaiian volcano appears to show this structure The bend is defined by the Chain of Craters and the locus of shallow dikes and fissures along the rift zone. Note how the fissures (and by inference, the dikes) remain parallel to the rest of the rift and cut across the bend. Swanson proposed that the curvature of the rift near the caldera resulted from the greater number of dikes near the summit (all dikes left the summit, but the number downrift diminishes as long dikes are much less frequent). This large number of dikes forced later dikes to the south, causing the rift to bend.
However, the direction of the shallow dikes appears to contradict this, perhaps lending support to the idea of dikes more passively filling fractures. Note that the dikes are roughly perpendicular to the direction of movement of the south flank, exactly the direction one would expect if they are related to stretching of the volcano. Because the volcano is growing and migrating southward, the zone of weakness also migrates in this direction. While dikes are free to follow the new path, the summit moves much more slowly as it has to migrate over the column of olivine cumulate. This migration can be seen as and old caldera rim north of Kilauea caldera, Kilauea caldera (the center when the caldera collapsed), and Halemaumau (the center during the 1800's. The bend may represent the "lag time" it takes for the summit to catch up with the rift.
It's interesting to see that even on the best studied volcano in the world there remain a number of fundamental questions as to how it works. There's still lots to learn and that's what makes science neat!
Extra Reading
Clague, D. A., and Denlinger, R. P., 1994, Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes: Bulletin of Volcanology, Volume 56, Number 6-7, p. 425-434.
Swanson, D. A., Duffield, W. A., and Fiske, R. S., 1976, Displacement of the South Flank of Kilauea volcano: The result of forceful intrusion of magma in the rift zones: US Geological Survey Professional Paper 963, 39 pages.
If you have comments or suggestions, email me at kenhon@hawaii.edu