The following translation is based on a Jeju Special Self-Governing Province publication. It describes the geological features of the publically accessible sections of Manjang Cave that is open to the public. To the best of our knowledge, at present there is no such information available in English. — Ed.
Tumulus
Near Manjang Cave’s ticket office is a tumulus, a convex geological feature. These are formed when internal pressure increases due to the solidification of the front side of flowing lava or an increase in the amount of lava flowing inside the lava tube. A flat lava plateau connected to a tumulus is called a “bille” in Jeju dialect. A bille is formed by pahoehoe lava (which has a smooth, undulating, or ropy surface), and where billes form, there are likely to be lava caves as well.
Ropy structure
Ropy lava are mostly found on the surface of pahoehoe lava and are frequently seen on the floor of lava caves. They are crooked structures formed when the surface of cooled lava flows and bends as it gets forced along by a lava flow. Ropy lava can be seen outside Manjang Cave near the ticket office and near the lava stone columns inside the cave.
Entrance of the cave
The entrance of Manjang Cave was formed when the ceiling collapsed. Many rockfalls are evident near the entrance.
Flowline
Near the stairs of Manjang’s entrance are flowlines. These are striped lines that form when the level of lava flowing along the wall decreases or a later lava flow marks the inside of the cave.
Lava lining
On the wall inside Manjang Cave, linings with more than two or three layers can be seen, and collapsed linings are also frequently seen. Generally, they are several centimeters thick or even several tens of centimeters thick. The lining shows that lava existed before the caves were formed, and on the walls, partially melted wall structures are evident.
Surface structure of the cave floor
Manjang Cave has two types of floor; one is a relatively flat surface, and the other is made up of irregular blocks or overlapping basalt layers.
Lava stalactites
A lava stalactite is a cave structure that is formed when the ceiling partially melts due to rising heat as lava flows into the cave. Lava stalactites are frequently found in the low, narrow parts of a lava cave and look tooth-shaped, elongated tube-shaped, or irregularly shaped. When a lava stalactite drips onto the ground, a lava stalagmite may be formed. Sometimes, lava drops fall like the wax from a candle.
Lava flowstone
A lava flowstone is formed when lava flowing along the wall solidifies as the wall or the ceiling melts. Lava flowing along the wall forms various shapes depending on differences in temperature.
Cupolas
Inside Manjang Cave, there are spots where the ceiling is higher and concave. This kind of ceiling is called a cupola. There are two theories about the formation of cupolas. One is that lava melts the ceiling rocks when the lava level rises during a backup in flow from the narrowing walls. The other theory is that pressure inside the lava increases so that it pushes up the malleable ceiling.
Lava raft
A lava raft refers to when rockfalls, along with lava flowing on the ground, solidify in certain places. Rockfalls are formed when a ceiling or wall collapses while lava flows inside the cave. Hill-shaped lava rafts (made when rockfalls are completely surrounded by lava) are sometimes called lava balls.
Rockfalls and quartzite
Rockfalls are frequently seen inside most lava caves. They are formed when rocks collapse along the columnar joints, which are made when the volume of rock expands and then shrinks as lava cools. Rockfalls are also formed when an unstable ceiling or inner layers collapse. Manjang Cave’s rockfalls all consist of basalt. This basalt includes two to three kinds of rocks and minerals that are not usually found in basalt. These rock pieces are 1 to 5 centimeters in size, and they are a kind of quartzite, which is sandstone transformed due to heat.
Lava bench
A lava bench refers to a shelf-shaped formation that is formed when lava flowing inside the cave sticks on the wall or when the cave floor is peeled as the lava flows. Lava benches are subdivided into lava balconies and lava tables.
Turtle rock
Turtle rock is a well known Manjang Cave structure because it resembles Jeju Island. It is considered a lava raft, but its edge is located in the same place as a lava shelf, and its column is melted. Hence, it is possible that Turtle rock is not only a lava raft from rockfalls, but perhaps left from a lava shelf that partly melted.
Arch-shaped lava ceiling
Most ceilings of lava caves are arch-shaped. They form when the lava caves first form. If lava later flows to the cave again, it sometimes melts the cave ceiling. Arches are very stable formations, making cave ceilings resistant to collapse.
Scratches
Scratches on the wall indicate that the wall was not completely solidified when lava flowed inside the cave. Moreover, scratches also show the direction of flow and the angle variation. Scratches are especially visible on the walls of the upper level of the first part of Manjang Cave.
Lava toes
A lava toe is formed when lava flowing inside the upper level of Manjang Cave poured through a hole to the lower level of the cave. This lava consists of several lobes — each look like the toe of an elephant. Meanwhile, on the cave wall, lava flowstones, which are formed as the wall melts due to the heat of the lava that made the lava toe, can be seen well.
Lava column
The lava column is a well known part of Manjang Cave. It refers to a 7.8-meter column-shaped lava structure that formed when lava dripped from a hole in the ceiling. Lava then flowed for 100 meters on both sides of the column to form lava toes. <Jeju Weekly>
<Koh Yu Kyung contributor@jejuweekly.comⓒ Jeju Weekly All rights reserved>
