The Japanese island of Iwo-Jima is well known as it was the site of an infamous world war
2 battle.
However, many people are not aware that this entire island is an active volcano.
This island has experienced an extreme amount of uplift, as a very large volume magma chamber
has pushed towards the surface.
In fact, the beaches where US marines first landed are now 17 meters above sea level.
This volcano last erupted in 2016, and could soon be on its way to a major caldera forming
eruption.
This video will discuss this fascinating volcano, and mention what its future hazards are.
Iwo Jima is located 1,250 kilometers south of Tokyo in the Pacific Ocean.
It may seem strange, but Iwo Jima is not a typical volcanic island.
Although there are several volcanic centers on its surface, these have not erupted a sufficient
volume of material to build the overall island.
Rather, much of Iwo Jima is what geologists call a resurgent dome.
Although it isn't easily visible, there is a 10 kilometer wide caldera on the ocean floor
surrounding the volcanic complex, resulting from a massive ancient eruption.
During this eruption, what existed of Iwo Jima collapsed downwards, thus filling a portion
of the magma chamber.
Over the years since this collapse, magma has pushed the remnants of the volcano upwards.
As an example, a shoreline visited by Captain Cook's crew in 1779 is now 40 meters above
sea level.
With this being said, we do not know exactly when this ancient caldera collapse occurred.
All that we do know is that it occurred before 850 BC, when a different major eruption occurred.
This eruption in 850 BC destroyed a forested volcanic complex on the modern north of the
island, called Motoyama.
For the next several hundred years, the entire island remained underwater, but was slowly
pushed upwards.
Eventually, the eruptive center of the volcano changed during this period.
This new eruptive center was located to the south of the previously mentioned complex,
and began building a volcanic cone out of an unusual type of rock called trachyandesite.
Eventually, this cone, called Suribachi, breached the surface.
Suribachi is well known, as it was the site of a famous world war 2 era photograph where
marines raised the US flag.
By 1543, the older eruptive center to the north had risen above the surface of the ocean,
thus forming two relatively close islands.
Eventually, further uplift and explosive eruptions caused the area between these two islands
to merge into the modern Iwo Jima island.
Since 1888, at least 23 small scale explosive eruptions have occurred on various sections
of the island.
These eruptions occur at random, and occur due to the interaction of a shallow magma
chamber with groundwater.
The heat from the magma chamber causes the groundwater to flash to steam, thus creating
an explosion.
Although scientists have not documented all of the recent phreatic eruptions on Iwo Jima,
spotting their aftermath on the island is relatively easy.
These eruptions create an explosion crater dozens of meters wide, and generate small
pyroclastic flows.
After the eruption is over, a highly altered area of rock remains, often with a mixture
of brown, white, and yellow coloration.
Some craters even contain active fumaroles.
Luckily, scientists were able to snap two photographs of one of these phreatic eruptions
while it occurred.
The eruption in question took place in 2001, and formed a 47 meter wide explosion crater.
While looking at satellite overlay of the island, I was able to spot a recent undocumented
eruption, which formed two new 30 meter wide craters sometime between 2014 and 2017.
I have left the coordinates of this recent eruption on screen.
In summary, Iwo Jima is a highly dangerous volcanic complex which erupts about once every
5 years.
These small eruptions are potentially deadly, as they occur anywhere on the island.
With this being said, I have created a risk map of where they are most likely to occur.
However, a much greater hazard remains.
I am relatively certain that if you were to visit Iwo Jima 1,000 years from now, you would
find no island, but rather a shallow submarine caldera.
I am not the only volcanologist with a similar perspective.
On a list of volcanoes most likely to produce the next VEI 7 sized eruption, Iwo Jima was
universally placed as number one.
When the next mega eruption occurs, it will likely be preceded by a very large series
of submarine eruptions around the volcano, along with an energetic earthquake swarm with
thousands of earthquakes.
Although it likely will be some time before this next mega eruption, when it does occur,
it will generate a large tsunami which will impact both Japan and China.
I hope that you enjoyed this video!
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