Pulse of the Planet: Picking up stones in Tanzawa, I

 

Tanzawa’s rock covered by, I think,
Riverina jigongshanensis, an endangered species
 … Really?

It is well-known that Tanzawa Mountains 丹沢 are one of the exemplar fields in Japan where we can observe Plate Tectonics. The main body of today’s Honshu Island was an edge of the Eurasian Continent where the North American Plate went under the Plate that the continent sit. More than 25 million years ago, the crush of these two titans ripped off the edge and created Sea of Japan / East Sea. By the power created by the collision of the Plates, the separated landmass that was still on the Eurasian Plate was cracked into 3 parts, the north, the center, and the south. The north rotated counterclockwise that became today’s Tohoku 東北 area. The south rotated clockwise and became today’s Kinki 近畿 and the southern area of Japanese archipelago. There are several theories why it was so, but in any case the center became a hinge for a double door of the north and south lands. It “stayed” as such with massive cave-in and volcanic activities. The rift became Fossa-magna area (; my post for October 5, 2018). Then about 20 million years ago, the center bedrock started to feel real pressure from the south by the Philippines Plate that had coral reefs and underwater volcanos. About 5-4 million years ago, these reefs and volcanoes reached the contact point with the remnants of Eurasian Continent, could not go under the Plate, and puckered there. The Tanzawa Mountains were born.

As a mass, mountains of Tanzawa are not so huge.
 But …

In this way, the surface of Philippines Plate was uplifted and became mountains. But even now beneath the mountains the Philippines Plate continues crawling down under the Eurasian Plate. After the first contact, the larger volcanic mass that was the beginning of Izu Peninsula 伊豆半島 followed bumping. The shock kept pressuring the original puckered coral reefs and volcanoes to go more inland and become taller. In addition, the energy of banging of two tectonic plates caused numerous terrestrial volcanic activity that spewed out new lava from the mantle. Among them were primitive Mt. Fuji, and mountains of Hakone area 箱根. Such volcanic products piled up over the raised undersea rocks from the south. Have you noticed something in this story? Yeah. These reefs and volcanoes were on the same Philippines Plate, but the plate is HUGE. It has many kinds of coral reefs and volcanoes since eternity. In addition, the original soil of Eurasian Continent and continuing volcanic eruptions can leave marks on the southern coral reefs and underwater volcanos. So, when we move around Tanzawa area which is not at all a large area, we can observe many kinds and histories of these activities in the Planet Earth’s evolution. That’s the reason why Tanzawa is good place to see tectonic movements.

This summer, I had a chance to observe such varieties in Tanzawa’s ground with experts. One theory for the origin of the name Tanzawa is, it was from old Korean that means Tan + Zawa = lots of small streams. Thanks to many small torrents running down steep rocky slopes of Tanzawa, it is easy to pick up many kinds of pebbles from riverbeds where people can safely approach. I made small specimen boxes of pebbles from 3 different points in Tanzawa. My 3 points were sides of (now familiar) Yadoriki やどりき沢, of Mizunashi River 水無川 in Togawa Park 戸川公園 (; my post for February 14, 2020), and of Nakagawa River 中川川 next to Nishi-Tanzawa Visitor Center 西丹沢ビジターセンター. I tell you they are all different. And they are more or less another side of a ridge each other. From next week, I will return there one by one. It’s really interesting that neighboring rivers can tell a different story of Planet Earth. Please stay tuned!

If you find environmental issues in Kanagawa Prefecture, please make a contact with

Kanagawa Natural Environment Conservation Center 神奈川県自然環境保全センター
657 Nanasawa, Atsugi City, 243-0121
〒243－0121　厚木市七沢657

Phone: 046-248-0323

You can send an enquiry to them by clicking the bottom line of their homepage at http://www.pref.kanagawa.jp/div/1644/