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‘There could be volcanic activity, but we don’t know’Jeju younger than previously thought, highlights dearth of knowledge about Mt. Hallasan
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승인 2014.09.28  18:47:22
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▲ Tavurvur in Papua New Guinea belches ash and pumice into the twilight. Scenes such as this could have been witnessed on Jeju as recently as 1,000 years ago. Photo courtesy Taro Taylor

A July report from the Korea Institute of Geoscience and Mineral Resources (KIGAM) surprised volcanologists by dating Jeju’s most recent volcanic eruption to 5,000 years ago. Media outlets gleefully reported Mt. Hallasan was not dead, or dormant, but alive.

The research team, led by Jin-yeong Lee, radiocarbon dated carbonised wood (charcoal) below the basalt layer at Sangchang-ri, Seogwipo City, to 5,000 years old. This was 2,000 years more recent than the 7,000-year-old eruption at Mt. Songaksan, thought to be Jeju’s last volcanic activity.

Scientists had speculated that the basalt layer at Sangchang-ri was formed 35,000 years ago, yet the carbonised wood was below the basalt, making the rock at least as young as the ancient trees. Sangchang-ri was thus confirmed as the site of the most recent volcanic activity in South Korea. (The title of most active volcano on the peninsula goes to Mt. Baekdusan in North Korea, which last erupted in 1903.)

The findings were picked up by media outlets and headlines stated that Mt. Hallasan was “alive” and not dead, a fact already known as the earlier Songaksan activity was already within the same Smithsonian Institute’s Global Volcanism Program (GVP) 10,000 year timeframe for active volcanoes. Nevertheless, the Science Daily headline, “Jeju Island, Korea is a live volcano,” typified the reaction.

The Weekly wanted to get a little deeper into the discussion and arranged a meeting with Koh Gi Won, a local geologist at the Jeju Water Resources Research Center, who recently lead-authored ‘Volcanism in Jeju Island,” published in the Journal of the Geological Society of Korea. Koh was also a consultant on the KIGAM research at Sangchang-ri.

Koh’s research stretches back over a decade to a 2001 collaboration with Oregon State University, and it proposes a new genetic model for Jeju volcanism. Challenging the classification of Jeju’s “oreum” as parasitic volcanoes, the paper suggests that as independent eruptions, they are actually “monogenetic volcanoes”.

His research also seems to confirm the younger classification of Jeju volcanic activity after dating over 1,000 rock samples from over 100 exploratory boreholes around the island. The argon-argon dating technique, a kind of radiometric dating, indicates that Jeju’s rocks are between 1 million and 5,000 years old.

However, Koh cautions that rock-dating is fraught with uncertainty as base layers are extremely difficult to date and sensitive to contamination; thus, to reach uncontaminated samples, the team of geologists drilled 100 meters with a bore 25~30 c.m. in diameter. If carbonised wood is extracted, as is usual in sedimentary layers, particles showing signs of rotting and oxidation must be removed to ensure there is no misreading under analysis.

▲ A cross-section of the sediment deposits at Sangchang-ri. Photo courtesy KIGAM

Tantalizing clues

While scientific evidence of volcanism on Jeju Island is proving difficult to confirm, one piece of historic evidence suggests that the island was active much more recently than 5,000 years ago. The “Dongguk Yeoji Seungram,” a Joseon Dynasty geography textbook (multiple volumes published between 1481 and 1530) includes this seemingly eye-witness account.

“In June 1002 CE, a mountain arose in the middle of the sea. There were four giant holes at the top of the mountain, out of which red liquid flowed and soared, and thick smoke plumed for five days. All the red liquid hardened and became stone like roof tiles.”

The account has long puzzled geologists, particularly as few clues were given as to the location of the eruption. The most fancied site had been Biyangdo, an island off of Jeju’s west coast, followed by the peaks of Songaksan and Dansan, both in the southwest of the province.

Investigations at Biyangdo, however, showed the island to be 27,000 years old, with no signs of hydrovolcanic activity. (Hydrovolcanism, present when hot lava interacts with water, is characterized by extensive rock fragmentation and the formation of tuffs such as at Seongsan, Suwolbong and Songaksan.) Reflecting on the continuing mystery, Koh said: “I personally think that it could still be under water.”

Does this bring Hallasan’s activity even closer to the present, making the island active? Koh is cautious, saying that despite the findings outlined, he prefers to refer to Jeju as dormant as we still lack clear evidence as to where the reported eruption took place.

This lack of certainty clearly troubles Koh, who would like to see more investment from local and central government. Whereas in Hawaii the United States Geological Survey monitors all volcanic and seismic activity, Korean scientists at KIGAM know next to nothing about Hallasan. Without reliable data any prediction would be speculative; all dormant volcanoes are potentially re-eruptable, says Koh.

“There could be some volcanic activity there, but we don’t know. We need more geophysical monitoring data... especially deep geophysical monitoring. We don’t even know how deep the magma is,” he said. “We have the technology but no investment.”

ⓒ Jeju Weekly 2009 (http://www.jejuweekly.com)
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