• Dokdo in the East Sea
  • Educational material
  • Junior High School Version

Discovery Learning 1

Was Dokdo always this small?[Teacher Notes]

Underwater Topography near Ulleungdo and DokdoUlleung Plateau, Ulleungdo, Mt. An Yong-bok, Dokdo, Mt. Simheungtaek, Mt. Isabu
Submerged mountains over 1,000 meters in height are called underwater mountains.
Dokdo was formed approximately 4.6 million years ago when an underwater volcano erupted. Jejudo, Ulleungdo, and Dokdo are a few of the Korean islands that formed from underwater volcano eruptions. Dokdo was the first to form and is the smallest of the three. It is also the furthest from the mainland. Ulleungdo was formed about 2.5 million years after Dokdo, and Jejudo appeared around 3.4 million years after Dokdo.
When viewed underwater, Ulleungdo and Dokdo are part of a series of underwater mountain ranges and valleys. It is likely that these all formed from similar volcanic activity. The underwater mountain ranges to the east of Ulleungdo and Dokdo are older.

Seodo and Dongdo’s formationSeodo and Dongdo’s shapes were formed by wind and water erosion!
The islands were shaped by underwater volcanic activity!
※ ※ The height of the mountain has been exaggerated and enhanced by four times in relation to the width of the base.
Since Dokdo and Ulleungdo were created with similar volcanic activity, their geological composition is very similar. Japan’s Oki Islands have a completely different geological content than those found in Dokdo and Ulleungdo. In geographical terms, Ulleungdo and Dokdo are of the same formation, while Oki Islands are completely unrelated.
Dokdo features an underwater mountain approximately 2,200 meters in height, meaning that it is higher than Jejudo’s Mt. Halla. Dokdo was originally a volcanic mountain in which Seodo and Dongdo were unified. Over hundreds of thousands of years, wind and waves have slowly eroded away its original shape and today we have Seodo and Dongdo.
Discovery Learning 1
1. Explain how Ulleungdo and Dokdo are geologically similar.[Answer]
2. Describe how volcanic activity and erosion have shaped Dokdo.[Answer]

Teacher Notes
Dokdo is a volcanic island formed by hardened lava from the seabed some 2,000 meters below sea level. It began to form about 4.6 million years ago during the Pliocene period in the Cenozoic Era. Dokdo broke the surface of the water roughly 2.7 million years ago and began to take shape while being cooled by the ocean and the rain. The singular Dokdo began separating into two from around 2.5 million years ago as water eroded the island. It is believed to have taken its current shape some 2.1 million years ago as wind and water erosion continued to chip away at the island. Dokdo is the oldest of the volcanic islands of Korea. Ulleungdo and Jejudo formed about 2.5 million and 3.4 million years after Dokdo, respectively.
When viewed underwater, Ulleungdo and Dokdo are part of a series of underwater mountain ranges. It is likely that they were created by similar volcanic activity. Two underwater mountains roughly 15 kilometers and 55 kilometers to the east of Dokdo are Mt. Simheungtaek and Mt. Isabu, respectively. The underwater mountain ranges to the east of Ulleungdo and Dokdo are older. Dokdo features an underwater mountain approximately 2,200 meters in height, making it taller than Jejudo’s Mt. Halla. Dokdo’s Seodo and Dongdo are essentially mountain peaks that extend above the waterline.
An underwater mountain peak extending above the waterline is a rare occurrence. Erosion and sedimentation make it difficult for underwater mountains to retain their shape. Dokdo is a case in which the evolution of underwater mountains can be seen at a glance, and it is considered to be a geological monument.
Ulleungdo and Dokdo’s geological composition is very similar. Both Dokdo and Ulleungdo consist of trachyte volcanic rocks. When the composition of the radioactive isotopes is analyzed, it is difficult to distinguish between the two. They have unique characteristics that are very different from other Northeast Asia alkaline volcanic rocks formed during the Cenozoic Era. In contrast, the Oki Islands are from the Proterozoic Era (570 million to 2.5 billion years ago) and their geological composition is mainly gneiss. The gneiss found in the Oki Islands is similar to the continental crust but is fundamentally different from the volcanic rocks that compose Ulleungdo and Dokdo. Therefore, Ulleungdo and Dokdo are part of the same underwater mountain range and entirely different from the Oki Islands.
1. Both Ulleungdo and Dokdo were formed by similar volcanic activity, so they share a similar geological composition.
2. Dokdo was formed around 4.6 million years ago by underwater volcanic activity. Over time, the exposed island has been eroded away by wind and water to form the Seodo and Dongdo that we know today.
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