Brimstone over Iwo Jima!

Joe Rosenthals photograph from 23 February 1945 is one of the most iconic images ever taken. If the camera would be pointed at the same spot today at the same elevation we would only see rock since the place have been magmatically uplifted 17 meters.

68 years ago US troops stormed the beaches of Iwo Jima’s Mount Suribachi, the site where they landed is now a full 17 meters above the ocean surface.

I have often written about caldera forming eruption being cyclic, and that as the magmatic system starts to evolve after the cataclysmic event it starts with small eruptions ever so slowly getting larger as the magma chamber grows in size with each magmatic influx. This is though only true for some volcanoes like Grimsvötn or Krakatau. There is a class of volcanoes that behaves radically different.

These volcanoes are the resurgent domes and Iwo Jima is one of them. Iwo Jima is not one of the easiest volcanoes to write about since there is not a whole lot of research that have been done on it, at least not a lot that has been released to the public. It has after all been a military facility for 75 years, and during those years it was even for a time a nuclear strategic air bombardment base.

Resurgent domes form as a magma is filling under the rubble of the cataclysmic caldera forming event. The rubble first get soldered together as magma oozes up through old feeder channels, as the magma filled the area between the rubble it was in the case of Iwo Jima quenched and in the end a hard but flexible layer 2 km thick had formed.

By then the magma started to accumulate under the current Island of Iwo Jima slowly pushing it up from the bottom of the caldera floor until it breached the surface. When exactly that happened we do not know, but if the rate of uplift have been constant it happened roughly 680 years ago. After that the Island grew into today’s height of roughly 170 meters.

Iwo Jima Is situated inside a nine kilometer wide caldera, at 21 kilometers it takes up about one third of the original size before the caldera forming event. Iwo Jima is surprisingly benign for being such a massively inflating volcano. There are a number of reasons for this.

The first reason is that even though inflation is changing between 10 centimeters to 80 centimeters per year it is fairly constant. And by that the roof of the magma chamber has time to move without cracking. If you bend rock over a long time it is surprisingly bendable, especially since the rocks over the chamber are rather hot making them very ductile. If the rate of magmatic influx was lower the roof would get less heat and thusly becoming more brittle, and during a sudden magmatic intrusion it would crack. If the rate of influx was higher it would in the end just melt through the roof.

Classical phreatic detonations photographed at Iwo Jima.

The roof is though not perfect, there are a crack in it so that gasses can pass out from the volcano, and it is from there the name comes, Sulphuric Island. This degassing lowers the volatility of the magma giving additional time for the swelling before a larger eruption would occur. There is just not enough force for a lot of magma to punch through.

Now most say that Iwo Jimo erupts quite often, but in reality it erupts less frequently then commonly believed. Instead most of the eruptions are phreatic detonations, which even though they are spectacular, are not true eruptions since no new lava is ejected out of a vent. Instead it is just rock being blasted as water comes into contact with a highly warmed piece of rock (or even magma) at depth causing a hydrothermal explosion.

Inevitably though the roof will burst the same as any overinflated balloon would do, when that will happen is anyone’s guess. In theory you could try to measure the quality of the roof over the magma chamber to decide how durable it would be according to a number of criteria. Sadly enough it is impractical onto the boarder of impossible since it is not a homogenous piece of rock.

And that is the problem with your average resurgent dome. It will go about pushing up the bottom of the old caldera floor, having some minor activity like Iwo Jima with phreatic detonations, some small eruption with a little lava, active fumaroles and hydrothermal fields. And it will be fairly gaseous, all right then, stinky is the word. And in all that benign activity it is hard to remember that the island is doomed to one day blow up and we will not know if it will happen tomorrow, or thousands of years into the future. The most likely though is that Iwo Jima will not survive for that much longer, probably not more than a few hundred years, even before it has grown to the same size as the old volcano. And the reason for that is that it has a very shallow magma chamber. If we compare it to other known resurgent domes the chamber is just too shallow to be able to take the strain for much longer.

CARL