Loihi Volcano, the youngest volcano of the Hawaiian Island Chain, lies about 20 km off the south coast of the Big Island. Currently its summit is about 1000 meters beneath the surface of the sea, and it stand 3500 meters above the surrounding sea floor. We think that Loihi is an active volcano because of the distinctive volcanic earthquake swarms, similar to those found within Mauna Loa and Kilauea volcanoes. On these volcanoes such swarms are associated with the growth of dikes within the volcanic edifice. It is likely that these same processes give rise to the earthquake swarms on Loihi.
This map shows the Loihi's location south of the Big Island. Also shown are the rift zones trending from the summit areas of the five subareal volcanos comprising the island. Note that most have 2 rift zones, although Hualalai has a suggestion of a third, and Mauna Loa has a zone of radial fractures bisecting the rift zones on its north flank. Loihi, although its hard to see in this image, also has two rift zones trending roughly north and south. This will be more obvious in subsequent images. Click on the image to view at a large scale. In the enlarged image it is clear that Loihi does not lie on any obvious extension of Kilauea's Southwest Rift Zone.
This extraordinary image shows the bathymetry and onshore topography of the Kilauea's South Flank with Loihi visible just left of center along the bottom. This is a computer generated shaded relief map combining onshore data from the U.S. Geological Survey with bathymetry data obtained with the Sea Beam acoustic scanner. The same image is shown on the right with color coding with depth to more clearly reveal some geographic features. There is no vertical exageration in this image, and the submarine slopes of Kilauea are indeed much steeper than the subareal exposures. Recall from class that surface lavas are largely tube-fed and massive pahoehoe flows which brecciate into hyaloclastic material (black sand) as flows enter the ocean. Note the extension of Kilauea's East Rift Zone which extends roughly 60 km past Kumukahi where it dissapears beneath the waves. Also of particular interest is the change in slope roughness of the submarine flank south of land as compared to its extension adjacent to the submarine extension of the East Rift Zone. As you probably guessed, the reason for this is the venear of hyaloclastite derived from subareal flows entering the water. Such flows and their thermally brecciated products are of course lacking along the distal rift zone, although a small amount of such material is found their. Most of the surface past the Eastern tip of the Big Island (Kumukahi) is composed of pillow basalt, such as the example shown towards the end of this lesson. Occasionally lava rushes from the ground sufficiently rapidly as to form a submarine flow similar to pahoehoe flows on land.
This image shows a blowup of the bathymetry in the vacinity of Loihi. Loihi is the steep, banana-shaped seamount in the left lower part of the image. The clear scarp perpendicular to the shore is not completely understood, but some scientist think it may be the boundaries of a massive slump of the South Flank. These massive landslides are the subject of a later lesson. Others have suggested this feature to be a strike-slip fault, although there doesn't appear to be any current seismic expression of this feature either on land or in its offshore extension. The summit region is quite clear, as are both the North and South Rift Zones. The eastern flank is markedly steeper than the western. There is a complex summit caldera between the two rift zones, so that in general form Loihi is tectonically similar to other active volcanoes of the Hawaii Chain. This will be more clear in later closeups of the bathymetry data.
The image on the right shows a contours of bathymetry for the summit platform of Loihi. The summit crater differs from that of Kilauea and Mauna Loa in that it consists of two pits (northwest and southeast) seperated by a narrow septum trending northeast. Also unlike subareal volcanoes the summit caldera is rimmed by constructional cones particularly along its southwest margin. One of these, Pele's peak was until last July the highest point on Loihi. At the time of the seismic crisis discuss below it was inverted to become a deep pit known as Pele's pit. Presumably this occured during an intrusion or eruption from the lower flanks which drained magma from beneath this part of the summit platform. The beginnings of the North and South rift zones can be seen at the top and bottom of this map. There are some small pit craters along the proxymal parts of the rift zones, although they are fewer and somewhat smaller than their counterparts on Kilauea Volcano.
Some of the details of the morphology of the flanks of Loihi are shown on the bathymetry plot on the left. This and the remaining illustrations in this section were published in a paper in the Journal of Geophysical Research by Garcia and others. Tectonic details are sketched in the inset in the upper left part of the map. Again, the eastern flank is notably steeper than the western flank which appears to manifest two slump features separated by a broad buttress. It has been suggested that the buttress results from a previous trend of the South Rift, although no evidence other than the suggestions in the geomorphology support it. As with the submarine portions of Kilauea, mass wasting is the dominant force shaping Loihi. Much of the edifice presumably consists of pillow basalt stacked several kilometers high. The growth of Loihi then can be viewed as three steps forward and two steps back as the constructional gains are eroded by massive slumps of the flanks. To some extent this is what seems to have been involved in the 1996 seismic crisis.
The remaining images in this section consist of computer generated bathymetry maps with shaded topography. The data supporting these images was collected with the Gloria Multichannel acoustic scanner. In a real sense these can be thought of as pictures taken with sound instead of light. Obviously at the depth of Loihi no light penetrates and no human generated light safe to be around could illuminate the entire volcano. The images are provided in two sets, one as a grey scale relief map, and another with depth contoured in various colors. I have include both since each reveals topographic details in a slightly different way. My preference is the black and white images, which seem to be more detailed than the colored ones. This first set of images views Loihi from a point above its North Rift Zone. That is, the viewpoint is from the South shore of the Big Island looking south. The saddle between Loihi and Kilauea appears to be choked with landslide depris.
This next pair of images is similar to the first pair, but this time the view is looking shoreward from a point above the distal end of the South Rift Zone. The rift zone itself is a narrow, sharp ridge bounded by wasted topography on both sides. The western flank, on the left of the image, clearly reveals the two landslide amphitheaters and the buttress with a more gentle slope between. As mentioned before, the eastern flank on the right side of the image is much steeper. In this pair, the color images seem to reveal the structure of the western flank more clearly than the grey scale image.
The final image pair is from a view looking up the buttress on the western flank of Loihi. The deep amphitheaters resembling slump head scarps are clearly visible on both sides. In some ways, the geomorphology of Loihi is similar to the composite cones found along subduction zones bordering the Pacific Ocean, although the characteristics of the basaltic lava forming Loihi is much different than the andesitic magmas of the island arcs. So, how long will it be before the summit of Loihi appears above the surface of the sea. Estimates by the experts vary from a few tens of thousands of years to perhaps 100,000 years. I prefer these longer times for several reasons. Taking Mauna Loa as an example of a rapidly growing Hawaiian Volcano, it would appear that the most enthusiastic volcanos grow about 10 km (considering the downwarping of the lithosphere) during a lifetime of 500,000 to 1 million years. This yields a growth rate of about 500 m to 1 km per years averaged over the lifetime of a volcano. One presumes that the growth rate might be at a maximum during midlife, which for Loihi should be about now. Therefore, a reasonable guess for the emergence of Loihi might be from about 500,000 to 1,000,000 years. I wonder what will have happened to beach front realestate market by then.
Beginning on July 16, a swarm of earthquakes shook Loihi, producing more than 4000 locatable events over a two week period. The image on the left shows the number of earthquakes recorded during successive 6 hour periods. This and the remaining images in this section were selected from the Loihi Site maintained by The Hawaiian Center for Volcanology of the University of Hawaii at Manoa. The time sequence is similar to volcanic swarms on other active volcanoes, although the initial activity on July 16 has more the character of a mainshock - aftershock sequence than the more swarm-like activity that follows. In a mainshock sequence, the largest event tends to be one of the first few, with earlier quakes being identified as foreshocks. Swarms, by contrast, have their largest events occuring later in the sequence and often show a slow intensification of activity over a period of time, as is more characteristic of the activity beginning of July 19. The revised map of locations is shown on the right, with the earliest epicenters plotted in red. As shown, these earliest event appear to be assocated with the West Flank near the southern end of the northern most of the landslide amphitheaters discussed above. The depth of these events, about 7-8 km beneath the summit together with their more mainshock-like temporal characteristics suggests that they may have been tectonic events similar to the basal events of Kilauea's South Flank. These will be discussed in more detail in a later lesson. If this is so, then apparently the "plumbing system" was opened up permitting an intrusion of a dike from the summit into the South Rift giving rise to the more swarm-like later portions of the seismic sequence. I must caution that these speculations are my own and probably not generally shared by the volcanological community. They are, however, consistant with the observation that lava drained from beneath the summit platform as evidenced by the transformation of Pele's Peak into Pele's Pit as it is now named.
There is some evidence that there was some lava extruded onto the surface of Loihi Volcano, although so far no fresh samples have been obtained. The image on the right shows the results from a water temperature sensor whimsically called a "to-yow". It is towed behind a research vessel on a long teather, which is raised and lowered as it is pulled forward. The result is a pattern sampling the temperature over a vertical plane as depicted here. The black line segments are the path that the tow-yo followed along a profile trending southwest from Loihi's summit platform. What is revealed here is a warm plume (almost 1/4 degree celsius) above surrounding water. Admittedly this is a subtle temperature variation, but the distribution is suggestive of a source of heat at a depth of about 1.1km on the West Flank of Loihi Volcano. Furthermore, these subtle variations are consistant with down stream perturbations observed from thermal plumes along mid-ocean ridges after some some mixing with cold bottom layers in the ocean.
Should lava be extruded onto Loihi's surface it would likely have taken a form similar to the pillow basalt shown here. This is a sample actually collected from the Loihi's summit platform, although it is not suggested that it was erupted during the current crisis. As we discussed much earlier in the course, most of Loihi's bulk is made up of such rounded products of slow, submarine extrusion. During the immediate response to the seismic crisis, several very young looking samples were obtained near Loihi's summit. These are shown in a petrological graph on the right which graphs the percentage of alkalic component against silica content. This is the ordinary way of segregating alkalic lavas from tholeitic by rock chemistry. The blue samples are a variety of primarily dredge samples previously collected on Loihi's flanks. The three red sampls represent the recent collection. Note that these are well within the tholeitic field, which lends credibility to the notion that Loihi may have very recently or by concurrently emerging from the early alkalic phase into the main shield-building tholeitic phase. Obviously Loihi is an exciting place, and I think everyone should visit it at least once during their lifetime.
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