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Haleakala Eruption History

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Eruptive History of East Maui in a nutshell
The early history of volcanism on East Maui is buried beneath innumerable lava flows thousands of meters thick. But Hawaiian volcanoes follow an overall pattern of eruptive growth and decline. The accompanying diagrams track the growth of East Maui through time.


Stage 1
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 offshore of the Island of Hawaii. It is unknown whether east Maui or other volcanoes of the chain must go through a pre-shield alkalic stage. The lava of stage 1, if present, was subsequently buried by products of succeeding eruptive stages.


Stage 2
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 typically spans 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 Big Island of Hawaii as an 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, Earth scientists call these flows the Honomanu Basalt, naming the sequence for a site where the layers (strata) 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 Hawaii. That's because East Maui's Honomanu shield is obscured by younger rocks.










Stage 3
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 Hawaii.

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.


Stage 4
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.

Submergence
Subsequent stages, entirely nonvolcanic and not portrayed here, encompass the changes that bring the volcanic islands back to low eroded atolls and finally, when fully drowned, to subsea plateaus known as seamounts. For a glimpse of how the volcanoes of big islands become submerged to form numerous smaller islands, examine a bathymetric map of the State of Hawaii.





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