JEJU WEEKLY
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The hidden and remarkable geology of Manjang CaveTake a stroll in the closed section of Jeju's UNESCO World Heritage lava tube system
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승인 2011.05.30  01:21:09
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▲ 1. The entrance to Gimnyeong Cave, with well-developed lava flowlines. The scale of the cave is as large as that of Manjang Cave. 2. Lava flows are easy to see in Gimnyeong Cave. It was formed when the free flow of molten basalt rock was interrupted in smaller openings. 3. In the closed area of Manjang Cave is a very awe-inspiring sight. This lava bridge is left over from a collapse in the two chambered cave. Photos by Douglas MacDonald

Manjang Cave was first discovered with the light of a few torches and the protection of simple straw shoes carried and worn by an inquisitive teacher named Bu Jong Hyu and his young students of Gimnyeong Elementary School.

This curious team began their exploration of the lava tube at what is now referred to as entrance number one, near Gimnyeong MazeLand, in 1946. It wasn’t until 1947 that Bu and his team finally discovered entrance number three. It took many expeditions with a two-meter long rope before they could measure the full length of the lava tube.

At 5,296 meters, Manjang Cave is one of the longest lava tubes in the world. It is part of the 300,000-year-old Geomun Oreum lava tube system. Manjang Cave was formed 200,000 years ago, making it comparatively young worldwide.

There are in fact, two floors to the Manjang Cave. The upper level – which is too low for human visitors – is 2,120-meters long, while the lower floor – which is only partially open to the public – is 5,296-meters long, says Dr. Jeon Yong Mun of the World Natural Heritage Management Bureau.

 

   
▲ 4. Another entrance to Gimnyeong Cave. The upper layer is covered with thick vegetation that penetrates the rocks, thus limiting public access due to the danger of collapse. 5. An assortment of ropy lava along the floor. Inside the closed portion, ropy structures with a wide range of shapes and sizes are mostly undamaged. 6. In this part of Manjang Cave, Dr. Jeon explains how one can find well-preserved features including stones that fell from the ceiling into flowing lava. Photos by Douglas MacDonald

On May 17, I had the most auspicious opportunity to explore the closed area – to the left of entrance number two – of Manjang Cave with Dr. Jeon.

Manjang is home to a plethora of geological structures including lava stalactites and stalagmites, a lava column, flowstone, cave coral, lava rafts, lava bridges, shelves, ropy lava and grooved lava striations. However, the high volume of visitors to the public portion of the cave has expectedly affected the area, while the closed portion – or the off-limits portion of the lava tube – still remains a tumbling terrain of untouched volcanic phenomena.

Furthermore, the public portion of the tube has undergone a series of alterations from wall and roof reinforcements to light and walkways. The closed portion, however, is dark, dingy and rough. It is not necessarily an ideal visiting place for a sauntering tourist but is certainly a place for the curious adventurist like Bu and his team of young followers.

As I hiked through the closed portion, I was continuously bewildered by Bu and his team’s perseverance and strength. How did they traverse this landscape with just a torch and two straw shoes? Unlike Bu and his students, on May 17, we were well-equipped with hiking boots, flashlights, a first aid kit and helmets.

Also unlike the public portion, the closed portion is still very much alive – at least to the human eye. Everywhere I pointed my flashlight I could see evidence of a bubbling river of lava lapping and melting the walls of the tube as it rolled haphazardly along.

Where the public portion is relatively smooth and tubular, ridges jut out from the walls like jagged cliffs along both sides of the tube in the closed portion. As we walked along the notched ridges, we continuously stopped to observe and discuss.

One of the first things I noticed was the assortment of “ropy” along the floor and walls. At first sight, I thought it looked like wrinkled dinosaur skin, sometimes a wrinkled blanket, or even like several small, twisted ropes laying side by side.

During a lava flow, if the lava cools slowly from the top down, it forms a hard almost plastic malleable surface. While this crust sits on the surface, the lava under it continues to flow, pushing the crust, wrinkling it like a blanket, says Dr. Jeon. This is called “pahoehoe,” which is the Hawaiian word for ropy.

“It’s like the surface film of soup,” says Dr. Jeon.

Ropy differs depending on the strength of the lava flow – or the viscosity – the direction, and whether there is a large stone or pillar blocking the flow’s passage, says Dr. Jeon.

For example, we found a large boulder which had plummeted from the ceiling during the lava flow. Because it plopped down on top of cooled crust with flowing lava beneath, large wrinkles wrapped around the boulder, forming large tubes of ropy.

As we continued along, I began to notice a rainbow along the ridges of the tube exhibiting mostly red and a bit of white and blueish purple. This coloration is due to the oxidization of iron in the basalt rock, says Dr. Jeon.

Later what I mistook for ropy, were clumps of basalt in the shape of loaves of bread. These are called lava loafs, says Dr. Jeon. One in particular, was actually shaped like an elephant’s trunk.

Along the walls you can also find what appear to be white crystals. These are in fact bacteria, says Dr. Jeon. The bacteria began growing over 1,000 years ago he says.

What came to be the most astounding phenomena, however, were the lava bridges. Lava bridges are formed when a thick surface of the lava flow has cooled, followed by a later cooling of the inner or lower part, causing the inner or lower part to later collapse.

The first lava bridge was very low but still quite impressive. The second, double lava bridge, however, was what really made me gasp. The lower part of the bridge was, again very low; but the upper bridge was several feet high and leapt to the other side of the lava tube in a thin arc.

I realized then why this portion of the lava tube is protected from the public.

“Every time I visit here,” said Dr. Jeon, “I find something new.” This is due to thermal erosion, he explained.

The closed section of Manjang Cave is 800-meters deep. On May 17, we walked approximately 500 to 600 metres.

For more photos from this report, please go to https://www.facebook.com/media/set/?set=a.224805477529534.66749.174417479235001


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