#kitadaito

Eroded limestone

Terraces

These spectacular photos show Minamidaito and Kitadaito islands, found far to the south of Japan on the Philippine Sea Plate, were captured by cosmonaut Oleg Artemyev from the International Space Station. The islands are made of limestone, the remants of ancient coral reefs. The rings of plants on these islands are old sea terraces, a testament to the geology in this area. These coral reefs poked their holes above the ocean’s surface at least a few hundred thousand years ago. Once they appeared above the Pacific, waves crashing gainst the coastline began eroding the limestone. When sea level stays constant over a long enough period of time, this erosion will form a sea cliff; a boundary several meters high just above high tide line that marks the upper limit of erosion.

120,000 years ago, sea levels were a little bit higher than they are today because more ice melted then than at the end of the last ice age. Sea level stayed at this point for a few thousand years before continental glaciers began forming again, and during that time the ocean cut a sea cliff on this island. When the most recent ice age began, sea levels retreated by over a hundred meters, raising this island up out of the ocean. Then, 10,000 years ago, the last ice age ended and sea levels returned, but the island was not in the same place. The sea cliff from the last ice age had, on its own, moved upwards.  

These two islands are found to the east of a subduction zone, where the oceanic Philippine Sea plate is bending upwards before it is pulled down into the mantle. This bending is why the islands exist in the first place – it pushes the coral reefs up to the surface. The islands are still moving upwards, so the ancient sea cliffs now form terraces several meters above the ocean today. Plants have occupied these terraces, making them visible from space. Each ring around the island is an ancient terrace, formed when sea levels stayed constant during an inter-glacial time.

-JBB

Image credit: https://twitter.com/OlegMKS/status/994591474678452226

Rice field forg