Subject: Oahu Volcanoes
We are a 4th grade class and we are in search of some answers to questions that we have posed in class. Our school, Hahaione Elementary, is located in Hawaii Kai and is located near Koko Crater. We learned that it is part of the large Koolau Shield, but we aren't sure if Koko Crater itself is classified as part of a shield volcano or a separate type. We know that Diamond Head is a tuff cone but we are unsure about Koko Crater. Could you help us out on this question. Also, if you could list other noteworthy volcanoes on Oahu along with its classification (i.e. Punch- bowl) we would really appreciate it.
Thank you for your question. I certainly can understand
the source of some confusion on this classification scheme! Actually there
are many more than three types of volcanoes, although the three you name
are probably about the most common types. Shield volcanoes typically are
very large- Mauna Loa is a good example, as are other Hawaiian volcanoes
active and inactive. However when they get deeply eroded, as the Koolau and
Waianae shield on Oahu have, their gentle 'shield' profiles are difficult
to see.
Composite volcanoes are the type typically found above subduction zones
near deep sea trenches at continental margins. Good examples are Mt St
Helens and other volcanoes along the west coasts of the Americas and the
Western Pacific, like the Mariannas, Tonga, Aleutians, etc..
Cinder cones are typically much smaller than these two main types and
have much simpler eruption histories. Whereas shield and composite
volcanoes are ususally active for at least several million years and are
made of many, many individual eruptions and eruptive episodes, cinder cones
usually form in a single eruptive episode lasting only a few weeks or
possibly months. So grouping these three types together is like grouping a
basketball and a volleyball together with a pea! They are all spherical,
but ....
On Oahu, almost all of the so-called 'rejuvinated' volcanics (called the
Honolulu series) are cinder cones. This includes the ones you mentioned and
any others with that general shape and size. In other cases, eruptions of
the Honolulu series produced isolated single flows or little mounds of
flows, examples are the the flow in Manoa valley that can be seen in the
old quarry on the UH campus and the Kaimuki "mound" near the Kaimuki fire
station respectively.
Dr. Rodey Batiza, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822
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