People are often taught that the Hawaiian Island Chain which is formed by a mantle hotspot
has the following ages, with islands getting progressively older to the west, matching
the direction the Pacific Tectonic Plate has moved during that timespan.
Yet, this isn't the full story.
As, what if I were to tell you that the Big Island with its 4 active volcanoes Kilauea,
Mauna Loa, Hualalai and Mauna Kea, in addition to the island of Maui with its Haleakala volcano
were not the only Hawaiian islands to in recent geologic timescales produce volcanic eruptions.
As, for example, Hanauma Bay on the island of Oahu formed in an explosive volcanic eruption
only 30,000 years ago.
Or, the basaltic rock around the Spouting Horn Blowhole on the island of Kauai which
is only 150,000 years old.
Or, on Molokai, we have a peninsula which jets out from the northern section of the
island with a prominent vent and small ravine where lava tubes once existed, having all
formed in a brief timespan around 350,000 years ago.
What these three sets of volcanic rock on separate islands have in common is that they
all represent an interesting phenomenon often witnessed in island based shield volcanoes,
that is a phase of some shield volcano's lifespans, referred to as rejuvenated volcanism.
This is best shown via a graphic I have created which shows the timespan when each of the
main 8 Hawaiian Islands was in an active phase with occasional eruptions.
Note how the 5 oldest islands all have produced long pauses in eruptive activity lasting from
150,000 to several million years in length before being followed by an onset of more
volcanic activity.
I have now color coded all of the rejuvenated volcanism in orange.
If you are wondering why Maui has orange surrounded by two reds, that is because the orange represents
rejuvenated volcanism from the now extinct West Maui volcano while the surrounding red
represents eruptions from the still active Haleakala volcano.
Thus, here are the areas rejuvenation stage volcanics covered on each island.
After building a vast shield volcano between 5.7 and 4.7 million years ago, the island
of Ni'ihau produced a series of pahoehoe dominated lava flows on its western half between 2.3
million and 400,000 years ago.
These formed a fairly flat section on the island.
The most recent vents erupted on the northern section of the island, with a tuff cone forming
400,000 years ago.
Kauai's rejuvenation stage volcanism occurred between 3.85 million and 150,000 years ago,
building low lying vents some of which are still quite visible today on the southeast
section of the island.
The island of Oahu was constructed via three separate shield volcanoes, but approximately
1.3 million years ago after the Koolau volcano had not erupted for 200,000 years, it collapsed
into the ocean in a massive landslide.
Removing 1/3 of the island, this seemingly sped up the eventual rejuvenation stage volcanism,
allowing it to occur beginning 900,000 years ago and continuing to 30,000 years ago.
This activity most famously constructed a series of explosively formed tuff cones and
maars, including the diamond head tuff cone, in addition to a series of lava flows which
were generally centered slightly offshore or near the shoreline on the eastern third
of the island.
After Moloka'i ended its main phase of eruptions 1.3 million years ago, a brief series of rejuvenated
eruptions occurred around 350,000 years ago, forming a 3.7 square mile peninsula and building
a slightly more than 330 foot high vent.
Kaho'olawe's brief rejuvenated eruptions 950,000 years ago produced a series of very small
volume lava flows on the eastern edge of the island.
While the Haleakala volcano was erupting, the West Maui volcano went on to produce a
series of cinder cones around its eastern and southern edge between 610,000 and 440,000
years ago.
As to why rejuvenation stage volcanism seemingly occurs at some but not all shield volcanoes,
it might relate to their size.
I hypoethesize the following.
The Hawaiian Islands and their crust naturally float on the mantle.
The thicker the crust is, the more it sinks downwards, hence natural subsidence on Hawaii's
Big Island.
On the other hand, islands which have experienced a significant amount of erosion now have less
mass in a concentrated point than they did before, causing islands such as Oahu to rise.
This uplift causes decompression melting to occur in the lithosphere, causing small batches
of magma to intrude into the crust via weak points, before occasionally erupting onto
the surface in rejuvenation stage eruptions.
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