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 shield volcano, erupting a type of basalt
known as tholeiite. This type of lava is the dominant extrusive
during the shield 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 shield 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
Return to the Ask-An-Earth-Scientist © page