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The Seogwipo Formation, Jeju's foundationThough everyone equates Mt. Halla with the island, in reality it's just the tip of the iceberg
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¡ã Above the red dotted line, 400,000 year-old rocks (lava bed). Below the red dotted line, 1 million year-old rocks of the Seogwipo Formation. Photo courtesy Jeju Special Self-Governing Province

At the beginning of October, 2010, the Global Geoparks Network announced that Jeju Island would become its 20th member, making it Korea’s only province to be assigned Geopark status. In the categories of geology, archaeology, biology and cultural interest, nine sites on the island were inspected and later certified. This article briefly introduces the fourth site: the Seogwipo Formation — Ed.


The Seogwipo Formation

- A 100-meter thick layer of rock underlying the entire island
- Over 1.2 million years old
- Only 1.5 kilometers are exposed as 40-meter-tall cliffs on the south-central shoreline of Seohong-dong
- Found at the entrance of Cheonjiyeon Waterfall
- A UNESCO-mandated environmental designation: Geopark status as certified by the Global Geoparks Network (2010)
- Korean Natural Monument No. 195 (1968)
- Of strong scientific interest since the 1930s, with distinct sedimentation layers, a fossil record and information that shows past changes in sea levels and climate change
- A mostly impermeable layer (called an aquiclude) prevents groundwater from escaping, thus giving Jeju Island a precious natural resource: pure spring water (yongcheonsu)


How it formed
¡ã Photo courtesy Jeju Special Self-Governing Province

Given the underlying and large-scale importance of the Seogwipo Formation, we should take a look back at how the entire island formed. According to provincial government information, Jeju is an island created during five volcanic stages and by more than 110 confirmed eruptions. Mt. Halla might appear to be the source of the entire island, but it is more like an ice cream cone on top of a layer cake.

Before Jeju formed, there was just the continental shelf floor of the southeastern Yellow Sea. Drifting mud and sand were slowly deposited under the relatively shallow sea (estimated at 100 meters in this area.) This “Unconsolidated Formation,” also known as the “U Formation,” was a layer that pre-dated any island volcanism.

The first eruption stage of the island was about 1.2 million years ago when a combination of basal basalt and sedimentary layers formed over the U Formation. This layer is the 100-meter-thick Seogwipo Formation.

The second eruption stage — the one that formed the island, occurred between 1.2 million and 700,000 years ago. Still under the surface of the ocean, lava pushed through the Seogwipo layer and spread out gradually to form a large lava plateau. More eruptions pushed up through sea level to build the foundations of various landmarks such as Mt. San-bang, Byuldo Peak and Dansan.

Jeju’s coastal lowlands formed during the third eruption stage, dating around 300,000 years ago. This eruption stage is marked by low viscosity lava, meaning it flowed easily and widely, like water rather than molasses. Geologists estimate Mt. Halla was less than 1,000 meters-high at the time due to the ease with which the lava moved away from the crater.

It was not until the fourth stage, between 300,000 and 100,000 years ago, that Mt. Halla grew to its present height and overall shape. The viscosity of the lava during this period was high, and therefore the lava accumulated around the crater mouth rather than moving down to the sea.

The fifth and last eruption stage brought with it the present-day landmarks of oreum (parasitic volcanic cones) and an eruption to the northeast of Mt. Halla that resulted in its present-day distinctive shape at Baekrokdam.


The geology

Looking underneath Jeju Island requires a combination of indirect (radar) and direct (borehole) observations. Since the 1960s, thousands of groundwater boreholes have been strategically drilled and their cores studied.

“That rock is the base of Jeju Island,” said Prof. Yoon Seok Hoon, of Jeju National University. “It is very important that it is an impermeable layer for ground-water.”

The first studies of the Seogwipo Formation’s stratigraphy, sediment layers and depositional environment were conducted by Prof. Yoon and together with Prof. Sohn Young Kwan, of Gyeongsang National University, they reinterpreted and published their findings in the American journal Geology, last year.

“Actually the Seogwipo Formation consists of two very different kinds of rocks. The first is volcanic ash... the so-called Tuff, and the other is molten sedimentary units including many fossils,” Prof. Yoon said. “Skeletal banks were formed during active storm periods where small fossils and organisms were dumped in the coastal area by the [churning] sea.”

Prof. Yoon said that apart from providing data from careful readings of oxygen isotopes and the study of the micro-fossils present in sedimentary layers to determine past ocean levels, conditions, and temperatures, the Seogwipo Formation is a source of fossil data that cannot be found on mainland Korea. It is also the only place on the island where one can view 1 million year old rocks.


The fossil layers

According to the Jeju Special Self-Governing Province Geopark application dossier, there are a variety of fossils present in the Seogwipo Formation, and Prof. Yoon confirmed that they extend throughout the layer, not just the coastal outcrop. Among the specimens visible or detectable are fossilized sponges, coral, mollusks, barnacles, and even whale bones, among many others.

According to stratigraphy done by Prof. Yoon and others, there are 10 fossil-bearing layers at the exposed cliff wall and four fossil-free layers.

Prof. Yoon added that there are no paleontologists on Jeju. “Most all of the work [studying the Seogwipo Formation fossils] was done by people outside Jeju.”

Unfortunately, the average tourist will not get a good sense of the fossil record on a casual viewing of the Seogwipo Formation, but it’s good to know the fossils are protected and being studied.

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Todd ThackerÀÇ ´Ù¸¥±â»ç º¸±â  
¨Ï Jeju Weekly 2009 (http://www.jejuweekly.com)
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