A towering 500 foot tall gray and glowing lava flow creeps forwards at an incredibly
slow rate of up to 1 foot an hour.
Ash up to 2 feet thick litters the ground, only being broken by occasional fragments
of pumice and dark black obsidian.
Numerous loud and repeating explosions can be heard, creating large shockwaves and creating
ground hugging plumes of 900 degrees Fahrenheit gas and rock known as pyroclastic flows which
raced downslope at a speed of 100 miles per hour.
As you struggle to walk on the finely grained material as it is much like sand, some lightning
is generated due to the rubbing of ash together in the air.
The sequence of events I just described might sound like a possible eruption sequence of
the Mount Saint Helens volcano.
Yet, the eruption which was just described did not originate in the continental United
States or even Alaska, but rather to the possible surprise of many, Hawaii.
On the Big Island of Hawaii, it is largely taken for granted that volcanic eruptions
there are not very explosive due to the highly fluid basaltic lava 4 separate volcanoes can
erupt.
This material which cools as a black color is known as basalt, and can move at a speed
of up to 19 miles per hour.
Because of this, basalt lava does not pool above an erupting vent, so gas does not get
trapped underneath it, allowing the volatile material to escape in an effusive manner as
it degasses at a regular rate.
Yet, under a specific set of circumstances, two volcanoes could theoretically produce
a highly explosive eruption more typical of a stratovolcano, with one specific explosive
eruption having occurred 103,000 years ago.
This is the story of the highly unusual Pu'u Wa'awa'a vent on the northeastern flank of
the Hualalai volcano.
Zooming in on the vent which bears this name, you might notice some unusual features, including
toes of lava and ash and a breached summit crater to the southeast.
As, unlike the medium sized cinder cones seen around the summit region of Maune Kea, Pu'u
Wa'awa'a is not a cinder cone.
Rather, like the central cone of Oregon's Newberry volcano, Pu'u Wa'awa'a is classified
as a pumice cone.
As the name implies, pumice cones are largely made out of pumice but also small amounts
of scoria and some obsidian, which overall form in highly explosive eruptions around
a central vent.
Although other variables exist that control how explosive a volcanic eruption is such
as gas, generally speaking, the more silica rich a lava type is, the more explosive the
eruption it is associated with will be.
Kilauea during the last 100 years has almost exclusively erupted basalt, which has more
than 45 but less than 52% by weight silica.
On the other hand, the lava which Pu'u Wa'awa'a erupted had between 57.5% and 69% silica which
was also far more alkali rich, being known as the rock type trachyte.
The trachyte lava produced during this ancient eruption breached the 570 foot tall pumice
cone it created to the side, going on to form blocky lava flows which were more than 100
feet in height.
This trachyte lava flowed up to a distance of 8 miles away, forming a steep blocky ridge
which is now the site of the Makani Golf Club.
The lava flow took approximately 7 years to cover a total of 9.4 square miles, totaling
4.6 cubic kilometers of material.
This was more than the combined lava flows Kilauea produced during its lengthy 1983 to
2018 eruption.
The reason this trachyte lava was able to be produced in the first place relates to
the stage of its lifespan that Hualalai is at.
Shield volcanoes in the Hawaiian Islands undergo long periods of activity which largely determine
the nature of eruptions they produce and their topography.
Hualalai entered what is known as a "post-shield" stage around 105,000 years ago which continues
to the present.
During a post shield stage infrequent eruptions eventually fill in the caldera which once
existed, many cinder cones form, and eruptions become slightly more explosive.
These eruptions erupt slightly more silica rich rock with basalt erupting alongside trachybasalt,
basaltic trachyandesite, trachyandesite, and trachyte also erupting.
This occurs largely through long term leaching of silica rich crust into the magma chamber
and fractional crystallization, which over time allows basalt to progress to trachybasalt
and eventually trachyte.
Both Mauna Kea and Hualalai are in their post shield stages, but only Hualalai albeit once
has produced highly viscous trachyte lava.
The explosive eruption which created the Pu'u Wa'awa'a pumice cone and the associated lava
flow likely was comparable in explosiveness to the 1914 to 1917 eruption of Lassen Peak
in California, being rated on the volcanic explosivity index as a 3.
Although the potential does exist for Hualalai to produce another alkali rich viscous lava
in its post shield stage, the fact that only one such event has occurred in the last 110,000
years suggests that the odds of this repeating are incredibly low in the next few thousand
years.
Hualalai will most likely erupt again in the next 1,000 years, but its eruptions will involve
either effusive or moderately explosive erupted alkali basalt.
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