Volcanoes within Hawaii's main islands which have erupted in recent geologic timescales
are typically associated with the Big Island.
There, the Mauna Kea, Hualalai, Mauna Loa, and Kilauea volcanoes have all produced at
least one volcanic eruption in the last 5,000 years.
Yet, Hawaii's Big Island is not the only recently volcanically active region in the state.
Moving to the northwest on the island of Maui is another volcano which has erupted since
the area was settled.
What I am referring to is the highly scenic and tall shield volcano known as Haleakala,
whose summit is dotted with numerous cinder cones and young lava flows.
And, this might come as a surprise to some people, but in ranking the number of volcanic
eruptions each active or potentially active volcano in Hawaii which is above sea level
has produced in the last 1,000 years, Hakeakala does not rank last out of the 5 volcanoes.
Rather, it ranks 3rd as it has produced 10 volcanic eruptions since 1,000 CE.
This means it has produced more eruptions in the last 1,000 years than the Mauna Kea
and Hualalai volcanoes, but less than the Mauna Loa and Kilauea volcanoes.
Much of this recent volcanism has occurred along two rift zones, one which goes towards
the east and another which trends to the southwest.
Hakeakala's most recent eruption occurred around 1540 CE, depositing the lava flows
shown on screen.
However, this same southwest rift zone contains a more distant feature which is not on the
island of Maui proper.
3 miles offshore of Maui's coastline is a ring shaped structure which is popular among
divers and is volcanic in origin.
Known as Molokini, this structure formed more than 100,000 years ago in an unusual submarine
explosive eruption and will luckily never erupt again.
Although submarine volcanic eruptions are uncommon in the context of Hawaii, they have
occurred in historical times and been witnessed.
For example, in 1877 a submarine volcanic eruption of the Mauna Loa volcano was witnessed
by numerous individuals on the volcano's western flank.
However, this eruption completely remained underwater as if it had continued, it might
have formed a feature similar to Molokini.
Molokini rises a little more than 600 feet from the ocean floor, to a height of approximately
150 feet above sea level.
It has a currently crescent shape much like a waxing gibbous moon, but this only describes
its surface above sea level.
As, if you outline an approximately 1,736 foot wide circle you can see the original
rim that Molokini once had.
Although Molokini might look like the exposed rim of a cinder cone it is instead a different
type of volcanic vent known as a tuff cone.
While tuff cones can form in a wide variety of circumstances, they are a hallmark of volcanic
eruptions regardless of source lava composition which originate at shallow depths in a lake,
sea, or ocean.
For example, approximately 14,000 years ago, a vent which would later become known as Tabernacle
Hill erupted at shallow depths of the now nonexistent Lake Bonneville.
This feature which is now exposed provides a good indication of what Molokini once looked
like.
Or, if you want to see a tuff cone in the context of a vent which erupted at shallow
depths in an ocean, I present the once central portion of the Hunga Tonga Hunga Ha'apai volcano
as it appeared during 2020.
Molokini originated approximately 230,000 years ago, after a strong series of earthquakes
occurred on the southwestern flank of the Haleakala volcano.
These earthquakes over a timespan of perhaps 48 hours subsequently progressed further downslope
until they reached submarine depths.
Then, lava finally erupted onto the ocean floor in an explosive manner, sending a plume
of steam and ash rising to the surface, and ejecting fragments of volcanic rock in a wide
area.
As the vent continued to build from the ocean floor, its eruption continued to get more
and more explosive.
Surtseyan style eruptive activity then occurred, which has a distinguishable white and gray
coloration in its plume.
After several days to weeks, a distinguishable ring shape island formed.
The reason why this style of eruption forms tuff cones in the shape of a ring is because
although the most abundant accumulation of ashfall occurs right adjacent to an erupting
vent, semicontinuous explosions will carve out the central portion of the circle, forming
a ring.
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