In mid 2018, the Kilauea volcano had been continuously erupting for the past 35 years.
This primarily occurred via a lava lake in the center of its summit caldera, but on occasion
produced more distant flank eruptions which flowed into the ocean.
Then, on April 30th, something unexpected occurred at the volcano.
The level of its caldera began rapidly dropping, indicating that magma was now being funneled
to a distant flank vent.
Earthquakes were subsequently tracked to the east of the caldera, and continued onwards
in that direction.
48 hours later, this magma began pooling underneath the Leilani Estates subdivision, causing the
ground to uplift and causing large cracks to open in the ground.
After the area was evacuated, a series of 23 fissures opened, ejecting molten rock several
hundred feet into the air.
These vents formed an almost uniform northeast trending line, representing the underlying
dike of magma.
While 22 of these vents erupted basaltic lava in a relatively nonexplosive manner, a single
fissure known as fissure 17 erupted something completely different.
Instead of erupting fast moving basalt lava, it instead explosively erupted andesite lava.
Andesite is a type of volcanic rock which is typically seen at stratovolcanoes, as since
it has a higher viscosity it moves more slowly, thus creating steep slopes.
Kilauea on the other hand is a shield volcano, where its low viscosity basalt lava moves
quickly, creating a broad and gentle slope.
This begs the question, why did this fissure erupt a type of viscous volcanic rock which
had never before been witnessed at Kilauea?
This video will answer this question, as the answer explains how certain volcanoes evolve
over time.
Dike intrusions underneath volcanoes are quite common, and 9 times out of 10 they do not
result in an eruption.
When an eruption does not occur, the magma remains at depth where it takes a significant
amount of time to cool.
This could take anywhere from several years to hundreds of thousands of years, depending
on the initial temperature, depth, and volume of molten rock.
It has been known for some time that the area adjacent to Leilani Estates contains several
such at depth dike intrusions, as in 2005 a body of dacite magma was encountered at
a depth of 2,488 meters or 8,162 feet while drilling a hole for a geothermal well.
Further analysis of this magma, although inconclusive, suggested that it originated from an intrusion
of magma in 1955.
So, how did the composition of molten rock change from basalt to dacite in merely 50
years?
The answer is the same reason a body of basalt changed to andesite which later erupted in
2018; fractional crystallization.
Bodies of magma are not universally composed of one substance, but rather contain numerous
minerals in them which vary depending on the temperature of the overall mass of molten
rock.
In the case of basaltic composition magma, primary minerals which form in solidified
lava include olivine, pyroxene, and plagioclase feldspar.
Each of these minerals have different crystallization temperatures; in other words, they begin to
solidify at differing temperatures.
As the overall body of magma cools, one of the first minerals to crystallize is olivine,
which often crystallizes at between 1200 to 1300 degrees Celsius.
Once the overall magma body drops below this temperature and the olivine has crystallized,
the magma becomes more silica rich as the olivine contained less silica than the average
figure within the magma body.
By weight, olivine contains between 29.5 and 42.7% silica, while in this case the magma
body contains 51% silica.
As a result, the average percentage by weight of silica increases as the magma cools, causing
the magma's composition to change from basalt to basaltic andesite and then andesite.
This process then repeats with other minerals, allowing for basaltic composition melts to
theoretically transition all the way to rhyolite.
This process is what occurred to a body of magma underlying Leilani Estates at fissure
17.
Once the 2018 eruption occurred, moving fresh magma interacted with it, triggering the material
to erupt onto the surface.
What was left behind by this specific vent was an unusually viscous blocky andesite lava
flow.
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