Subject: Lavas Erupted at Kilauea Today

What are the most typical rocks currently being erupted from Kilauea's East Rift Zone? What is the general compositional category of the eruptive materials at Kilauea?

    Kilauea is a basaltic sheild volcano, erupting a type of basalt known as tholeiite. This type of lava is the dominant extrusive during the sheild building (the main stage) of hawaiian volcanism and is the dominant basalt type erupted on Earth. Mid-Ocean Ridge basalts formed at oceanic spreading centers are greater than 95% tholeiitic in composition. Another rarer basalt type, alkali basalt, is also erupted in Hawaii, but usually in the pre- and post sheild stages of the volcano's life. Some alkalic compositions can erupt during the sheild stage, but at Kilauea they are volumetrically minor (a few % at most). Alkali basalts have relatively higher concentrations of the elements Na and K and lower concentrations of Si relative to tholeiites, believed to be due to differences in the extent of melting in the mantle for the two (alkali basalts are smaller-degree melts).
    Although Kilauea's present activity is all along the East Rift Zone, Kilauea has had historical summit eruptions. Both locales erupt almost exclusively tholeiite.
    A second term you sometimes hear applied to Kilauean and other Hawaiian lavas is "Ocean Island Basalt", which for the same concentrations of the major constituent elements has higher (or "enriched") concentrations of some trace elements (such as Th, U, Ba, Pb, the Rare Earth Elements, etc..) than "depleted" tholeiites such as Mid-Ocean Ridge basalts. This difference is due to compositional variations existing in the mantle source of these lavas, where as the difference between alkali and tholeiite basalt has more to do with the conditions of melting.

Dr. Ken Rubin, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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