On the southeastern section of the Big Island of Hawaii, there is an unusually long and
deep crack which runs across a swath of the Kilauea volcano.
Known as "The Great Crack", it measures 8 miles or 13 kilometer long, is up to 66 feet
deep and up to 50 feet wide.
Since this feature is located on the southwestern section of the Kilauea volcano, does the crack
directly indicate that a section of the island is breaking away?
No, although this feature does have a volcanic origin.
For context, the Kilauea volcano in Hawaii is one of the world's most active volcanoes
which as of January 24th of 2023 is erupting right now.
While many of its eruptions have occurred at or near its central caldera, Kilauea also
has two so-called rift zones which are are weak points on the volcano from which lava
can erupt at more distant locations away from a volcano's summit via one or more points
along a chain of activity.
There is the east rift zone which stretches all the way to the ocean and was last the
site of a volcanic eruption in 2018, while to the southwest is the southwestern rift
zone.
While the southwestern rift zone is also covered in numerous geologically recent lava flows,
during the past 50 years the east rift zone has been dominant as the southwest rift zone
last produced an eruption in 1974.
While The Great Crack is the largest crack on the Kilauea volcano, marking part of its
southwest rift zone, it is not the only lengthy crack.
All across both rift zones you can see other smaller cracks, which exist due to the same
reason.
Per a direct quote from a U.S. Geological Survery webpage, "Similar, though shorter,
wide cracks occur on the east rift zone.
One, between Napau Crater and Pu`u `O`o, has existed for many decades, as did another east
of Pu`u `O`o, now filled in with new lava flows.
Thus, the Great Crack is not unique; it is simply longer than the others."
The reason why the Great Crack exists could best be described as due to a combination
of magmatic intrusions and the weight of erupted materials emplaced in close proximity to the
eventual crack.
In other words, whenever a shield volcano develops, even if magmatic intrusions due
to dikes and sills are randomly emplaced around the volcano in all directions, the random
pattern will eventually create an area where a higher proportion of intrusions occur compared
to the rest of the volcano.
When magma intrudes into the crust, it does not simply melt all of the overlying rock
creating a net zero situation but rather adds to the mass of existing rock as more material
is forced into the same space.
As a result, dikes and sills of magma uplift and deform the ground above them.
These dominant areas thus spread the crust above them which can form cracks due to the
strain it places on overlying rock.
This spreading then causes additional magmatic intrusions when they occur to often use these
rift zones as a path of least resistance, which causes more crustal spreading and creates
a positive feedback loop.
While the age of the Great Crack is unclear, it could be anywhere in my opinion from a
few hundred to 5,000 years in age.
During the last 250 years, the crack itself has been the source of two eruptions, one
between 1790 and 1815 and the other during 1823.
In the next few hundred years, this crack and the area in close proximity to it will
likely be the source of additional eruptions, although it could be decades or centuries
before one occurs there.
While the Great Crack is an area of crustal spreading, it's existence alone will not cause
a large area to dramatically collapse into the ocean.
How do we know this?
If a large area was part of a weakened area which would eventually collapse, you would
expect large magnitude earthquakes to move the entire area, even if the movement is small
being only a few inches.
However, per a direct quote from a U.S. Geological Survey webpage, "...there was no growth of
the Great Crack during the largest historic earthquake (magnitude 7.9) in 1868.
Measurements spanning the magnitude-7.2 south flank earthquake in 1975 show no change across
the area.
Thus, the two largest earthquakes in historic time have not affected the Great Crack."
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