Volcano Watch — Clues to Mauna Loa's plumbing system

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Recent work sheds light on Mauna Loa's magmatic plumbing. The U.S. Geological Survey has embarked on scientific investigations of the plumbing system of Mauna Loa in cooperation with researchers from University of Washington and University of North Carolina.

These researchers are looking at xenoliths. The derivation of the word xenoliths comes from "xeno" meaning foreign and "lith" meaning rock. Any rock material carried by lava but not genetically related to it can be called a xenolith.

Geologic mapping has discovered several deposits of crystal-rich rocks from deep inside the volcano that were brought to the surface by volcanic eruptions. These rocks did not crystallize from the magma that carries them and therefore are xenoliths. The xenoliths are comprised of the minerals olivine (a green mineral), plagioclase (a white mineral), pyroxene (a black mineral), and combinations of each.

The texture and mineral compositions of the xenoliths provide clues about the chemical and physical environment in which they formed. In general, the compositions of the minerals record changes in the magma chemistry with time, and textural information mirrors changes in the physical environment.

One area that contains xenolith-bearing flows that were studied is near Na`alehu. These flows contain xenoliths made of olivine, plagioclase, and pyroxene. Detailed chemical and microscopic analysis of the samples gives us insight into the complex nature of Mauna Loa's plumbing system. After studying the xenoliths, it was decided that they could be subdivided into two main groups.

Neither group of xenoliths possesses affinities indicative of mantle (deep earth) origin. The first group is comprised predominantly of plagioclase-bearing xenoliths that reflect a complex history of growth and development. This group of xenoliths crystallized within the southwest rift zone that was infrequently recharged with magma. The other group, dominated by the minerals olivine and pyroxene, suggests a deeper depth of formation. These xenoliths formed at the base of the volcano adjacent to the oceanic crust, at depths approaching 12 to 16 km (7-10 miles). Their limited compositional range suggests an isolated storage zone that mixed with the host magma just prior to being erupted.

Another region in which xenoliths have been studied is around Moku`aweoweo, the summit caldera of Mauna Loa. Here the xenoliths are part of the debris that was ejected during one or more explosive eruptions. The xenoliths are primarily plagioclase-rich.

These plagioclase-bearing xenoliths all seem to share a common history. The textural and chemical data are comparable to some samples acquired during the drilling of Kilauea Iki lava lake. Within some of the xenoliths are veins of volcanic glass. These veins are very silica-rich and have chemical compositions that are unusual for Hawaiian lava. They represent the very last vestiges of melt in a magma body that is solidifying. This study implies that the xenoliths at Moku`aweoweo were formed from magma that was stored in the shallow subsurface and remained isolated until an explosive eruption occurred. This finding is consistent with other lines of data that suggest Mauna Loa has a shallow magma reservoir beneath Moku`aweoweo.

The examination of these xenoliths by HVO scientists and university colleagues improves our knowledge of the plumbing system of Mauna Loa. Studies like these help further our understanding of this volcano and other volcanoes.

Note regarding a recent article..."There's no Pahoehoe town, just Ho`okena" By Kent Warshauer, Sugar Mill Spy.

It is our intention to provide factual information when writing Volcano Watch. In addition, we include place names and locations to make the articles familiar to our audience. Specifically, to the question of whether there was a Pahoehoe town in 1950, authors Ruy Finch and Gordon Macdonald wrote in a USGS publication "Hawaiian Volcanoes During 1950" pg. 55, first paragraph, and note the destruction of "several homes, a filling station, and the Ho`okena Post Office at the village of Pahoehoe."

Who is correct? I think the jury is still out, and the Sugar Mill Spy may be full of bagasse.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Molten lava is flowing near the end of the Chain of Craters road, and the National Park Service is allowing visitors to get up close to the action. The Mother's Day flow keep on providing spectacular lava cascades down Pulama pali and Paliuli. The ocean entry is adding new land to the northeast end of the Wilipe`a bench and also along a portion of the seaward edge of the older bench.

Two earthquakes were reported felt during the week ending on September 12. Residents from Waimea to Volcano felt an earthquake at 11:00 p.m. on Tuesday, September 10. The magnitude-2.9 earthquake was located 7 km (4.2 mi) southeast of the summit of Mauna Kea at a depth of 26.7 km (16 mi). Residents of Maui at Haiku and Wailuku felt an earthquake at 9:45 p.m. on September 11. The magnitude-3.4 earthquake was located 24 km (15 mi) south of Hana at a depth of 30.8 km (18.5 mi).