The image on the right is a computer generated digital elevation map of Kilauea and Mauna Loa volcanoes. Mokoweoweo can be seen on the left side of the image. The summit crater of Kilauea is located just below the center, and the East Rift Zone occupies the lower left quarter of the image. A red line shows the extent of fissuring occuring during the first few weeks of the Pu'u o'o eruptions (beginning January 3, 1983). The flanks of Mauna Kea are visible at the top of the image.
This is one of my favorite landscape views of the summit of Mauna Loa, taken with a low sun angle just after a light dusting of snow. Many features are visible under these conditions that are usually obscured by the drab colors of the lavas surrounding the summit. Many of the features of Mauna Loa are similar to those on Kilauea. Notably, there is a large, partially refilled central caldera (center back), with pit craters marking the proximal portions of two rift zones. Normal rifting is also a significant feature along Mauna Loa's rift zones, although this is not particularly evident in the photo. The view for this photo is taken looking uprift from the Southwest Rift Zone. Mauna Kea is visible on the horizon.
Lavas from Mauna Loa cover a smaller area of the Big Island than might be expected from its tremendous bulk. The reason is clear from the image on the right. Mauna Loa lavas are constrained by the two adjectent edifices of Mauna Kea on the North, and by Hualalai to the West. To the South, Mauna Loa lavas have been covered by flows from Kilauea's summit region. Because of this, lavas from Mauna Loa have been funneled throught the saddles between Mauna Kea and Hualalai and between Mauna Kea and Kilauea volcanoes. Note that this focusing has produced a concavity of the coast where the channeled lava enters the sea. Later we will see an example where blockage of flows produces a concavity South of the summit of Kilauea Volcano.
This map is from Volcanoes in the Sea, the text for this course. It is clear that short flows from the rift zones currently dominate activity on both Mauna Loa and Kilauea during the past two centuries. This has not always been the case, and apparently for both volcanoes rift zone eruptions produce less flow and coverage than stable, sustained eruptions from the summit area.
Also of note is the relative coverage areas of Mauna Loa lavas as compared to those erupted from Kilauea. This seems to refute the common notion that Kilauea is more active than Mauna Loa. In fact, given total volumes for the flows shown here, Mauna Loa has outpaced Kilauea during the past 150 years. However, as shown on the graph on the right, the eruption rates seem to have changed in mid-1950 so that today Kilauea is producing considerably more lava in an average year than Mauna Loa, and may soon pass Mauna Loa in total over that last couple of centuries. Part of the reason for this is that Kiluea has produced much of its lava during sustained eruptions lasting many years. Much of this lava simply enters the sea and does not contribute to increased areal coverage.
Much can be learned regarding the recent long term evolution of Mauna Loa volcano during the past few millenia. This map (from USGS Prof. Paper 1350, Chapter 18) shows lava ages over broad ranges or time during the past 4000 years. The lime green flows were erupted during the since 1843, and correspond roughly with the ages of those flows shown in the preceding flow map. As a side note, this rather strange color for a geological map was modified from the original, because the two most recent age ranges were almost indistinguishable when digitized in their published colors. The red group shows lavas erupted from about 1250 through 1843, about 6 centuries. Compared to the volumes in the last 150 years, it would seem that rift zone eruption rates have been much higher recently. Of course, one has to take into account that much of the older lava has been covered by younger flows. The reason for this is that during much of this time lava was infilling Mokuaweoweo after its formation sometime around the beginning of the 13th century A.D. The yellow lavas were erupted between 500 A.D. through 1250, keeping in mind that dates for many of these flows are less precise that would be liked. On thing is immediately obvious -- many of the flows in this time range seem to pour from both sides of Mokuaweoweo. Apparently during much of this time period, Mauna Loa was considerably higher than it is today. Lava was issuing from vents where now there is nothing but a large caldera. It has been suggested that Mokuaweoweo formed sometime around the boundary between the yellow and the red flows, or around 1250 A.D. It has also been suggested that the Panaewa flow forming the large coastal delta south of Hilo drained the summit and was responsible for the formation of the caldera at that time. The red lavas south of the summit caldera would seem to contradict this theory, although these lavas were early in the red time period, and small errors in dating could have led to this apparent contradiction. These flows are shown on the image on the right, and appear to issue from a vent that is inboard of the current caldera boundaries. Consequently, these boundaries must postdate the flows.
We have been discussing ages, so some mention should be made methodology of their determination. The primary method is based on carbon-14 dating of charcoal formed as forests were consumed by advancing lava flows. The finding of such carbon is an art itself. Too much heat, such as occurs for vegetation under thick, slowly cooling flows results in all of the material being turned to methane and carbon dioxide, both gases which are not retained and therefore not available for dating. Too cool, and the material is not completely carbonized and simply rots. Only near the edge, but not too near the edge can the right condition be found.
Only a fraction of flows can be dated by this method. For others no carbon has been found, so that other mechanisms must be used. Ordinary application of Steno's Laws (superposition) can be used to order some flows when they are adjacent and cover or are covered by dated flows. Still others can be dated by magnetic means using a chart such as the one shown on the right. Some thought is required to understand how this chart is used. As you may know from a previous course in Geology, the primary magnetic field of Earth varies slowly over time, a phenomenon known as secular variation. The curve shows variations in the declination and inclination of magnetic lines of force during the past three millenia. The current time is indicated as 0, and the curve is labeled in millenia before the present. Ticks are shown at approximately 100 year intervals. The curve is shown dashed where poorly determined.
On the Big Island, the declination varies about 10 degrees on both sides of true North. Similarly, the inclination, or the angle the magnetic field vectors plunges into the Earth at this latitude varies from about 10 degrees to as around 40 degrees. The magnetic field locked into a piece of basalt at the time it cooled sufficiently (the Curie temperature) is thus a snapshot of the field at that time. If the flow is also datable by carbon-14 methods, then it can be used to calibrate the curve shown here. If such dating is not available, then its magnetic field can be used to determine an estimate for its age. Note, that there are times when the curve crosses over itself. Flow ages near these points are ambibuous, and this ambiguity must be resolved by flow relationships and interlayering.
Hualalai is the third youngest of the volcanos making up the Big Island. It towers above above Kailua-Kona, and its steep slopes form the backdrop for the city. The most recent eruption of Hualalai Volcano occured in 1801 and covered an area shown on the map on the left. The character of this eruption was reported by John Young a form member of Captain Cook's crew and advisor to King Kamehameha I. Lava issued from two vents along the Northwest Rift Zone, with the upper being the first as is often the case on Mauna Loa's rift zones. Hualalai is in its Post-tholeitic, alkalic phase, and it is puzzling why the rift zones are still a preferred site of eruptive activity. Apparently its shallow magma chamber has essentially cooled, and the eruption seems to have come from a depth exceeding 10 km. The evidence for this is the unusual xenoliths (foreign rocks) that were piled near the vent. Many of these were dunites, which as you will recall are formed at the bottom of magma chambers. The fact that these rocks were brought to the surface seems to indicate that lava rose rapidly from below this depth. Also, the lava covering the xenoliths is frothy with large bubbles, seemly having a much high volatile content than is generally found in vesiculated, surface erupted lavas. The lava must of risen rapidly to the surface to carry such a large load of such dense xenoliths.
1801 Hualalai eruption Many people think of Hualalai as a dead or extinct volcano, although this is hardly the case. Over half of the surface lavas of Hualalai are younger than 3,000 years, and about 25 percent of the surface was covered by lava erupted later than 1000 A.D. The map on the right shows the age distribution of recent prehistorical flows on Hualalai volcano. Lighter shades represent younger lava, with the lightest less than 1000 years old, and the darkest shown greater than 10,000 years old. You will probably have to expand this view (click on image) to see the details of these flows. Again, as with the 1801 eruption, it seems that most flows erupt along the rift zone. Apparently Hualalai erupts every few centuries, the last time being in 1801. A swarm of large earthquakes beneath Hualalai in 1929 has been interpretted by many as an intrusion or dike that did not make it to the surface. Unfortunately, the availablility of seismic data at that time was not adequate to be definitive on this matter. Still, because of the steep slopes and fairly frequent eruptions, Hualalai poses a distinct threat to Kailua-Kona and agent communities.
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