The new shape of plumes under the Hawaiian Islands?

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A recent scientific paper has re-kindled a long-running debate over the presence, shape, and depth of mantle plumes (sometimes called hot spots) under the Hawaiian Islands. Under basic plate tectonics theory, mantle plumes are persistent upwellings of molten rock or heat originating at the outer core of the earth.

The movement of tectonic plates over these molten upwellings produces chains of volcanic islands, such as the Hawaiian Islands and Emperor Seamounts farther north. Other notable hotspots fuel eruptions in the Canary Islands, in the Galapagos Islands, and at Yellowstone volcano.

Mantle plumes are primarily studied using earthquake seismograms of waves that travel through the plume structure. Two main techniques are used: the first identifies changes in the speed of waves as it travels through the plume (this method is analogous to a CT scan). The second technique uses the reflection of seismic waves to map boundaries within the plume. Such boundaries may be from mineral phase changes or changes in the amount of melt within the plume. Any earthquake greater than magnitude 6.0 can be well-recorded on seismometers around the world and may be used to study mantle plumes if the earthquake and seismometer are in the correct configuration to record a seismic wave that intersects the plume. Through time, more earthquakes occur in different places and are recorded on a progressively denser world seismograph network, providing a richer and more robust dataset from which to study earth structure.

With an improved dataset comes an improved understanding of the mantle plume under the Hawaiian Islands. The traditional view of the mantle plume under Hawai‘i is a vertical cylinder beneath the island that slows seismic waves that pass through it. These studies are confined to using seismometers on the Hawaiian Islands and a limited number of stations on the ocean floor around the islands. This configuration allows for good resolution in the upper mantle, where scientists have found a mantle plume to be present. Because of the limited geographic coverage of stations, results are also geographically limited to the area around the islands and have questionable resolution in the middle and lower mantle (>500 km or >300 mi depth).

In the newest study, the authors use seismic wave reflections off of well-known mantle layers using processing techniques borrowed from the oil industry. Since this technique uses waves reflected off of the underside of the layers of interest, the dataset is much richer, because earthquakes occurring on one side of the Pacific bounce off the layer of interest, and are recorded on the other side of the Pacific where seismometers are much more numerous and more broadly distributed.

The new results reveal significant variations in two layers at average depths of 520 km (255 mi) and 660 km (410 mi) over 1,000 km (600 mi) to the west of the island of Hawai‘i, leading the authors to hypothesize that the variations are due to the presence of a mantle plume west of the active volcanoes of Hawai‘i. The observed layer variations are outside the region of previous studies. Significant variations in the depths of these layers were not observed under Hawai‘i, and, thus, the authors suggest that the mantle plume at 660 km and likely deeper (410 mi and likely deeper) is shifted to the west of the Hawaiian Island chain. This plume then must transfer its molten material laterally to the Hawaiian Islands, where the plume is well resolved at shallower depths (<500 km or <300 mi), however the authors do not speculate or provide any evidence for how this might occur.

So does this mean that our traditional view of a mantle plume under Hawai‘i is wrong? Well, kind of, and that's okay. This study and subsequent debate illustrates all that is wonderful in peer-reviewed science. Observations are made and interpretations derived through the information that is available at that time. As new observations come along, it is important to reconsider the interpretations in light of the new data. New models of the mantle plume under Hawai‘i will now need to consider all of the observations together and be consistent with each observation. Through this process, we gradually converge to a fuller understanding of processes that we cannot directly observe.

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Volcano Activity Update

A small lava lake was present deep within the Halema‘uma‘u Overlook vent during the past week. Two deflation/inflation cycles were recorded at the summit this week. The level of the lake fell during the deflation and rose again slightly during the inflation, exposing a lava cascade within the vent when the lake level was low. Several periods of rise/fall cycles occurred when the lake level was high. Volcanic gas emissions remain elevated, resulting in relatively high concentrations of sulfur dioxide downwind.

Lava also erupted continuously within Pu‘u ‘Ō‘ō over the past week, feeding a lava lake perched above the crater floor. Small overflows topped the rim of the perched pond early in the week, but dropped below the rim and remained there the rest of the week. Two vents on the west and southwest crater floor were also active over the weekend but ceased erupting on the afternoon of June 13. Small overflows continued to pour over the edges of the perched lava lake. No lava is erupting outside the crater.

Two earthquakes beneath Hawai‘i Island were reported felt this past week. A magnitude-3.9 earthquake occurred at 10:21 p.m. on Monday, June 13, 2011, HST, and was located 7 km (4 mi) northeast of Ka‘ena Point at a depth of 9 km (5 mi). A magnitude-1.9 earthquake occurred at 8:06 p.m. on Wednesday, June 15, and was located 7 km (4 mi) northwest of Pāhala at a depth of 8 km (5 mi).