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Stage 1 is
sometimes referred to in more detail as the pre-shield alkalic stage.
The only example we have of such volcanism is at Lo`ihi ,
a newly growing submarine volcano that lies southeast of the Island
of Hawai`i. It is unknown whether east Maui or other volcanoes of
the chain must go through a pre-shield alkalic stage. The lava flows of
stage 1, if present, was subsequently buried by products of succeeding
eruptive stages.
Stage
2 is the shield-building stage. Over 95 percent of a Hawaiian volcano's
volume is emplaced during shield building, during a period that may
span about 600,000 years. The Earth's crust, unaccustomed to the load
of the volcano, subsides greatly during this stage--as much as 3 mm per
year using current subsidence rates from the Island of Hawai`i as
a guide. Early eruptions are entirely underwater, but the rate of upbuilding exceeds the rate of subsidence. The volcano grows to
reach the ocean surface and becomes an island about midway through
its shield-building years, after about 300,000 years.
At
East Maui volcano, we see the final lava flows of the shield-building
stage in exposures along the north shore of the island from Honomanu
Stream eastward to Nahiku. For convenience of discussion, geologists call these flows the Honomanu Basalt, naming the
sequence for a site where the lava flows are exceptionally
well exposed. To imagine what the shield looked like, we must
consider the shape of Mauna Loa and Kilauea on the Island of
Hawai`i. That's because East Maui's Honomanu shield is obscured
by younger rocks.
The
third volcanic stage is the capping or post-shield alkalic stage.
East Maui entered its capping stage about 900,000 years ago. As
the cross-sectional figure suggests, this stage produces lava
flows that mantle much of the preexisting surface. But the rocks
form only a small part of the total volume of the island, about
one percent. Clearly the rate of volcanism diminishes greatly in
the post-shield alkalic stage.
At East Maui, strata in the main part of this stage have been grouped into the Kula Volcanics, named for the upcountry town. In most places, lava flows of the Kula Volcanics extend from the coast to the summit area of Haleakala, where they are well exposed in the walls of Haleakala Crater. The ages obtained from Kula volcanic rocks indicate they span the period from 950,000 to 150,000 years ago. Other volcanoes elsewhere in the Hawaiian Islands that are currently in the post-shield alkalic stage are Hualalai and Mauna Kea on the Island of Hawai`i.
Newly determined isotopic ages show that East Maui persists in the postshield stage as its eruptive vigor wanes. Strata in the youngest part of the postshield stage have been named the Hana Volcanics, after the town. Representative products include the young cinder cones and lava flows that blanket the floor of Haleakala Crater. Equally young lava flows and cinder cones continue southwest and east along the major rift zones of the volcano. The east rift zone extends into the ocean at the village of Hana, ending a short distance eastward.
East Maui was
once thought to have already entered the fourth volcanic stage, the rejuvenated or
renewed volcanism stage. Lengthy periods of erosion may precede or be interspersed with
eruptions of the renewed volcanism stage. Recent eruptive products from Ko`olau
volcano on the island of O`ahu are classic examples of rejuvenated-stage
volcanism.
