By YU FUJINAMI/ Staff Writer
September 9, 2020 at 07:30 JST
A remote volcanic island that has been growing in size could be reduced to nothing by the same forces of nature that created it.
Nishinoshima, an islet in the Pacific Ocean located 1,000 kilometers south of central Tokyo, is part of the Ogasawara island chain under the metropolitan government’s jurisdiction. It has been expanding since volcanic activity resumed in 2013.
Experts have noticed a recent change in the composition of volcanic ash spewing from the island.
The volcano’s magma is likely rising from greater depths beneath the island, and future developments could involve a collapse of the crater peak in the middle of the island, leading to a subsidence of the entire island, according to an expert.
The island, whose existence has been known from old times, erupted for the first time on record in 1973.
Following a lull of about four decades, volcanic activity resumed in 2013. The isle has since been erupting on and off, with the overall activity level decreasing.
However, the ongoing spell of activity that started late last year has been intense.
The average daily lava output has hovered around three to four times the corresponding figure from the 2013-2015 period. In late June, lava output peaked at 4.62 million cubic meters, or more than 20 times the levels from the previous period, according to estimates by the University of Tokyo’s Earthquake Research Institute (ERI) based on data from the Himawari-8 meteorological satellite.
The huge volumes of lava being spewed out have increased the landmass of the island by 40 percent from 2.89 square km in May 2019 to 4.1 square km on Aug. 14 this year, according to the Meteorological Research Institute on the basis of satellite images.
Previously only several hundreds of meters wide on each side, the island now measures more than 2 km across.
A shift has also been observed in the mode of its eruption.
Nishinoshima mainly emitted only lava until June, but it began to discharge huge volumes of volcanic ash in late July. The entire island ended up covered in brownish ash several meters thick.
Plants and seabirds were beginning to settle on the islet during the lulls of eruptions. But the plants and the seafowls’ nests were likely buried under the ash.
ERI’s analysis of volcanic ash collected aboard a vessel in July by the Japan Meteorological Agency showed its content of silicon dioxide (silica) had dropped from 60 percent or so to about 55 percent.
The analysis also showed magnesium, calcium and other ingredients had grown in content and resulted in higher magma densities.
Pressure and density in the Earth’s crust increase at greater depths. Magma, the source of volcanic ash, is also in equilibrium with the density of the surrounding medium. Therefore, magma of higher densities, which has been spewing out lately, likely has a deeper origin.
“It seemed as if the activity were winding down gradually after 2013, but underground, deeper magma had likely been rising gradually,” said Fukashi Maeno, an associate professor of volcanic geology with ERI.
Yoshihiko Tamura, a principal researcher of volcanology with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), said the shift in Nishinoshima’s activity mode evokes the process of an eruption more than 20,000 years ago at Sumisujima island (Smith Rocks), located between central Tokyo and Nishinoshima.
Magma gushing out of Sumisujima had a shallow origin during the initial phase of the eruption, but the magma gradually began to rise from greater depths.
As more magma of high temperatures and high pressures rises from deeper beneath a volcano, the magma chamber expands by melting the surrounding Earth’s crust.
When the magma undergoes a sharp drop in pressure, it comes to a sort of “boil,” causing an explosive eruption. A magma chamber emptied out by an intense eruption may no longer be able to withstand the weight of the island itself, causing the volcano body to subside over a broad expanse.
That is how a caldera measuring about 10 km across was likely formed on the seabed around Sumisujima island, leaving only part of the rock jutting above the sea surface.
In a similar development, a crater caved in on Miyakejima island, part of the Izu island chain, in 2000, creating a small caldera measuring 1.6 km across.
A JAMSTEC study has shown the crust thickness around Nishinoshima differs from the areas around Sumisujima and Miyakejima. So it is not clear if Nishinoshima will follow the same development as the two islands.
But the entire Nishinoshima island could cave in and sink beneath the ocean in the event of a caldera eruption, Tamura said.
“I cannot say if Nishinoshima will continue to grow in size or succumb to a caldera eruption,” he said. “Anyway, a collapse of the island body could trigger a tsunami, so it is essential to monitor the future course of the activity from the viewpoint of disaster management and disaster reduction.”
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