It occurs during the later stages of volcanic activity on oceanic islands such as Hawaii,[2] which happens to be when the alkali metals are most present.[3]
Hawaiite is an
aphanitic (fine-grained)
volcanic rock produced by rapid cooling of
lava moderately poor in
silica and enriched in
alkali metal oxides (
potassium oxide plus
sodium oxide). It is often impractical to determine the
mineral composition of such a fine-grained rock, and so hawaiite is defined chemically. Under the
TAS classification, hawaiite is sodic
trachybasalt, with a silica content close to 49 wt%, a total alkali metal oxide content close to 6%, and Na2O wt% > K2O wt% + 2. This places hawaiite in the S1 field of the TAS diagram.[5][6]
Hawaiite is not a recognized rock type in the
QAPF classification of
igneous rock, which is based on the relative proportions of
quartz,
alkali feldspar, and
plagioclase in the mineral composition. However, hawaiite is composed mostly of
andesine (plagioclase feldspar with an
albite content of 50% to 70%) and
pyroxene with smaller amounts of
olivine.[1] This would fall into the
andesite/basalt field of the QAPF diagram.[7]
Occurrence
Hawaiite is erupted in the late stages of ocean island volcanism, forming part of the
alkaline magma series characteristic of such eruptions. It is preceded by silica-poor
ankaramite and followed by intermediate-silica
mugearite as the
magma evolves by crystallization in the underlying
magma chamber. These rocks form an alkalic cap over the older rocks of the island.[8] Hawaiite can occur at earlier stages in the evolution of some volcanoes in other tectonic settings, for example during the middle stage of volcanic activity in the Kekuknai volcanic massif (in
Kamchatka, Russia) that formed in a volcanic
back-arc basin.[9]
Other settings in which hawaiite and other alkaline volcanic rocks are found include regions of continental extension, such as the
Basin and Range Province of western North America[10] and the
Red Sea Rift.[11]
References
^
abMacDonald, Gordon A.; Abbott, Agatin T.; Peterson, Frank L. (1983). Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii press. pp. 127–129, 150–152.
ISBN9780824808327.
^Hazlett, R.W.; Hyndman, D.W. (1996). Roadside Geology of Hawaii. Missoula: Mountain Press. p. 13.
ISBN0-87842-344-3.
^Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 140–143.
ISBN978-0-521-88006-0.
^Vaniman, D. T.; Crowe, B. M.; Gladney, E. S. (1982). "Petrology and geochemistry of hawaiite lavas from Crater Flat, Nevada". Contributions to Mineralogy and Petrology. 80 (4): 341–357.
Bibcode:
1982CoMP...80..341V.
doi:
10.1007/BF00378007.
S2CID128583787.
^Camp, Victor E.; Roobol, M. John; Hooper, Peter R. (1 March 1991). "The Arabian continental alkali basalt province: Part II. Evolution of Harrats Khaybar, Ithnayn, and Kura, Kingdom of Saudi Arabia". GSA Bulletin. 103 (3): 363–391.
Bibcode:
1991GSAB..103..363C.
doi:
10.1130/0016-7606(1991)103<0363:TACABP>2.3.CO;2.