After anywhere from 500,000 to 3 million years of inactivity,
some Hawaiian volcanoes become reactivated. This is after erosion
has greatly altered the original form of the volcano. The best
examples of this rejuvenation stage (sometimes called the post-erosional
stage) are found on the Ko'olau and East Maui volcanoes. Ko'olau
volcano makes up the eastern half of O'ahu. The
prominent landmarks
around the city of Honolulu (Diamond Head, Punchbowl crater, etc.,
collectively called the Honolulu volcanic series)
started erupting
some 1 million years after the last eruption of the tholeiite
lavas (Ko'olau apparently never went through a post-shield alkalic
stage). The other fine example is the East Maui
volcano, commonly
(but incorrectly) called Haleakala. The "crater" at
the summit (which is properly called Haleakala) actually
formed from the coalescence of two very large valleys that in
the 400,000 years after the cessation of the post-shield alkalic
stage, were able to eat out the heart of the volcano. After this
long eruptive repose the rejuvenation stage filled these valleys
with lava flows and cinder cones, providing the
spectacular scenery
found today.
The lava erupted in these rejuvenation-stage eruptions is highly
alkalic and geochemically indicates that it came from a great
depth (Chen &
Frey 1983);
a few of the deposits contain garnet-bearing xenoliths. These
are indicative of a rapid and violent journey from the zone of
magma generation. The volume contribution from these rejuvenation-stage
volcanics is <<1% of the total for a Hawaiian volcano, and
they form monogenetic fields - each vent only erupting once. Like
monogenetic fields elsewhere in the world the overall eruption
rate during this stage is very small
(Walker 1990).
It is not known why this stage of volcanism occurs. There have
been numerous explanations put forth, the most popular one today
suggests that the lithosphere rebounds upward after having been
depressed while directly over the hotspot. This rebounding is
because the lithosphere is no longer being thermally weakened
and because the overlying volcanoes are eroding. Depressurization
due to this uplift would then lead to melting and magma generation.
It is not clear that the lithosphere does indeed rebound in that
way, however. Perhaps batches of magma attempt to make it to the
surface all over under the Pacific plate, and only where the plate
has been fractured and weakened by hotspot volcano formation are
they able to make it to the surface. On the Ko'olau volcano, many
of the rejuvenation stage vents are found to lie along rifts that
are perpendicular to the trend of the old Ko'olau volcanic structure.
Many of the Honolulu volcanic series vents happened to erupt into
shallow seawater, and the eruptions were phreatomagmatic.
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