Volcano Watch — Mirror, mirror, on the wall, who's the hottest of them all?

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Hawai‘i is the quintessential "hotspot." The geologic definition of a "hotspot" is that region of the Earth beneath an area that has experienced high levels of active volcanism for an extended period of time. The 5,800-km (3,200-mi) -long Hawaiian-Emperor Seamount chain is a testament to the longevity of the Hawaiian hotspot.

Hotspots are fed by mantle plumes, a rising column of magma from the interior of the earth. Besides Hawai‘i, other localities that owe their volcanism to a hotspot include Yellowstone, Iceland, the Galapagos, and the Reunion-Maldives islands.

Although it's difficult to establish the dimensions of the Hawai‘i hotspot definitively, we can deduce its size from the area containing active volcanoes. Hualālai, Mauna Loa, Kīlauea, and Lō‘ihi, a seamount southeast of Hawai‘i Island, have been active within the last few hundred years.

Lō‘ihi, the youngest volcano, must be near the leading edge of the hotspot. Correspondingly, the post-shield volcano Hualālai must be close to the trailing edge of the hotspot. The distance between the leading and trailing edges is approximately 90 km (56 mi) in a northwest to southeast direction.

What about the cross axis dimension? At the very least this dimension should include the summits of the active volcanoes and the dormant Mauna Kea. If we draw a line between the summits of Kohala, Mauna Kea and Kīlauea, parallel to the Pacific plate motion, and another line between Lō‘ihi, Mauna Loa, and Hualālai, the average distance between the two lines is approximately 30-35 km (18-22 mi). Therefore, an approximate minimum dimension for the hotspot is a 90 km by 35 km (56 mi by 22 mi) ellipse.

Coincidently, these two lines are referred to by geochemists as the Loa and Kea trends. Lava erupted from volcanoes along the Loa trend (Lō‘ihi, Mauna Loa, Hualālai and Kaho‘olawe, including other volcanoes up the island chain) is chemically different from lava erupted from volcanoes along the Kea trend (Kīlauea, Mauna Kea, Kohala, and Haleakalā).

Temperatures within the hotspot are not uniform and vary concentrically. The center represents the hottest portion of the hotspot or mantle plume. As you move outward, away from the center, the temperature gradually decreases toward the edge.

Where is the hottest part of the hotspot plume relative to the volcanoes on Hawai‘i Island? For this we rely on a geothermometer using the composition of lava and olivine, the common green mineral in Hawaiian lavas. Using this approach, Mauna Loa takes the prize, because its lavas are hotter than those of neighboring volcanoes. The composition of Mauna Loa lavas suggest that they have been produced by greater amounts of melting of the hotspot source than lavas from the Kea volcanoes, Kīlauea and Mauna Kea.

To further constrain the hotspot and the Loa and Kea trends, geochemists rely on elements such as the isotopes of lead (Pb), strontium (Sr), hafnium (Hf), neodymium (Nd), and helium (He). These elements and their isotopes provide clues to the structure and geometry of the hotspot or mantle plume.

Simple growth and evolution of Hawaiian volcanoes support a zoned hotspot. Most isotopes (Sr, Nd, Hf) support a concentric, thermally zoned mantle plume. However, lead isotopic data complicate the picture by inferring that the composition of the hotspot is asymmetrical. Helium data, which is more abundant on the Loa trend, also supports an asymmetrical model and suggests that the magma supply is deep and hot.

But all data agree on the answer to the question--"Mirror, mirror on the wall, who's the hottest of them all?"--and, for now, that answer is Mauna Loa.


Volcano Activity Update

Lava continues to enter the lava tube system and is carried downslope to Puhi-o-Kalaikini lava delta, near Kalapana, where it enters the ocean and creates a large steam plume. Lava breakouts from the tube fed small surface flows that were active near the end of Highway 130, just west of Kalapana, through much of the week.

At Kīlauea's summit, the circulating lava lake deep in the collapse pit within the floor of Halema‘uma‘u Crater has been visible via Webcam throughout the past week. The circulation pattern was interrupted sporadically by abrupt increases in the height of the lava surface. These periods of high lava level have been short-lived, lasting up to several hours, and each ended with a sudden drop of the lava surface back to its previous level. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

One earthquake beneath Hawai‘i Island was reported felt during the past week. A magnitude-2.3 earthquake occurred at 00:11 a.m. H.s.t on Sunday, October 17, 2010, and was located 15 km (10 miles) southeast of Hāwī at a depth of 27 km (17 miles).