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Geomorphic and Structural Characteristics of the Dokdo Area

HOME > Marine Science Reports > Geomorphic and Structural Characteristics of the Dokdo Area
A bird’s-eye view of Dokdo tells us that it is not more than 170 meters in height and 1,000 meters in width.
Below the sea, however, lies a huge monolith of an underwater volcanic edifice that is over 2,000 meters high, with an upper platform that has a diameter of 12 kilometers and a base with a diameter reaching 30 kilometers(Figure 1).
Figure 1. Dokdo volcanic edifice
Figure 1. Dokdo volcanic edifice
The upper platform of this gigantic Dokdo volcanic edifice extends as a flat wave-cut platform to a depth of about 200 meters. The diameter of this upper platform is a huge 12 kilometers. The area of this upper platform is eight times the size of Yeouido. The rock composition and structure is almost homogeneous throughout the whole upper platform, which seems to have been formed through the erosive actions of high wave-energy storms. Dokdo seems to be part of the crater rim of this volcanic edifice and is located on the southern part of this upper platform.
Waters between Dongdo and Seodo are only 10 meters in depth, and are populated with numerous jagged rocks that may or may not be exposed above water. These rocks are found most plentifully in the southwest seabed closest to Seodo. Dokdo is a volcanic island that was formed through volcanic activity during the late Tertiary and early Quaternary periods of the Cenozoic Era that consists mainly of neutral or basic low-viscosity trachyte, agglomerate and tuff. Rocks with similar composition take up most of the seabed in shallow waters near Dokdo in the depth range of 10 to 20 meters. 400 meters to the northwest of Seodo, a wave-cut platform that is 450 meters long and 200 meters wide extends to the north. This area has particularly well-developed wave-cut platforms along with numerous sea stacks and sea caves. The surface of the seabed near Seodo is much more irregular and has exposed and protruded rocks in many more places than in the seabed near Dongdo. The water channel connecting Dongdo and Seodo shows a shallow depth ranging from 1 meter to 10.
Research by KIOST has shown that the Dokdo caldera is about 2.5 kilometers wide along its major axis and 1.5 kilometers along the minor axis. Dokdo seems to be a part of this caldera, and is located at the southern most point of the crater. At its highest point above sea level, the Dokdo volcanic edifice reaches 168 meters. In the depth range of 90 to 175 meters, a very mild incline tapers off into a rather flat upper platform with a diameter of 12 kilometers, which then plunges deep into the sea in a cliff-like fashion from 200 to 2,000 meters in depth. In waters around Dokdo that are 90 to 130 meters deep, there are numerous platforms and cliffs formed by the erosive action of waves, all of which seem to have been formed along the coast of Dokdo by sea-level changes that occurred in the Quaternary period(Figure 2).
Figure 2. 3-dimensional imaging of the underwater geomorphology of the northeast seabed of Dokdo
Figure 2. 3-dimensional imaging of the underwater geomorphology of the northeast seabed of Dokdo
There are two more volcanic edifices similar in size with Dokdo that reside to the east of the Dokdo volcanic edifice. One is called the ‘Sim Heung-taek Sea Mount’ and the other is called the ‘Isabu Sea Mount’. A series of volcanic edifices therefore stretches from Ulleungdo to Dokdo, Sim Heung-taek Sea Mount and Isabu Sea Mount(Figure 3). The tops of these edifices are increasingly more eroded and more immersed, telling us that the eastern edifices were mostly likely formed far before the western ones. It is estimated that Ulleungdo was formed by volcanic activity 2.5 million years ago, Dokdo 4.6 million years ago, and the other two edifices perhaps several million years before that.
Figure 3. Distribution of underwater mountains near Dokdo
Figure 3. Distribution of underwater mountains near Dokdo
Overlapped images of underwater geomorphology and the seabed show us that a complex system of base rock is widely prevalent up until depths contours of 80 meters, and that lines extended from these base rocks tend to overlap with the ridge topography(Figure 4).
Figure 4. Seabed image map for the upper platform of the Dokdo volcanic edifice
Figure 4. Seabed image map for the upper platform of the Dokdo volcanic edifice
These underwater base rocks seem to be remainders of the crater rim that was formed at the beginning of volcanic action that formed the Dokdo edifice and that has since been washed away by waves and has collapsed with time. On the northeast and northwest parts of the seabed on top of the Dokdo upper platform, seabed imagery shows us the shape of small craters, and these are thought to have been created by later eruptions after the formation of the Dokdo volcanic edifice.
As for magnetic patterns, a high-anomaly field is located to the northwest of the research area, while complex anomaly profiles have been detected right around Dokdo. Smaller magnetic anomaly fields have also been observed quite clearly(Figure 5).
Figure 5. Magnetic anomaly map for the upper platform of the Dokdo volcanic edifice
Figure 5. Magnetic anomaly map for the upper platform of the Dokdo volcanic edifice
The analytic signals show extensions to the northeast and northwest from the land areas of Dokdo. These anomaly fields are consistent in location with the distribution of rocks that appear as remains of the crater rim defined by seabed imagery(Figure 6).
Figure 6. Analytic signal map for the upper platform of the Dokdo volcanic edifice
Figure 6. Analytic signal map for the upper platform of the Dokdo volcanic edifice