Crash of Titans in our backyard: Looking stones in Togawa Park 秦野戸川公園

 

Let’s start our Tanzawa stone hunting from the backyard of suburban Kanagawa. This summer, we did a geological hunting session in the bank of Mizunashi River 水無川 for Tokawa Park 戸川公園. The place is popular with families in Hadano City 秦野市. When weather is right, the river does not have much flow, or even no water. Its reason is exactly from Plate Tectonics. Hadano City sits on Hadano Basin that is created by earth and sand accumulated from collapsing strata by crustal shortening. Coral seabeds and islands on Philippines Plate crinkled when the plate crushed with the Eurasian Plate. Inevitably lots of earth broke down along the active faults. Such sediment from the crash of Titans came down along the rivers of old Tanzawa and piled up in the area now called Hadano Basin. Then, volcanic ashes from Mt. Fuji and Hakone Mountains heaped on the crumbled soil. The result is a Basin on lots of active faults and more than 150m thick strata made of gravel, sand and volcanic ashes. The structure lets the water from Tanzawa mountains become groundwater, not river. The Hadano City has lots of famous springs here and there, but the visible waterflow on the ground is limited. Mizunashi, = “No Water” in Japanese, River does not have much regular flow, unless the underground water system reaches its limit due to torrential rain.

Tokawa Park in summer has
lots of families who enjoy breeze from the “water.”

Not much water for Mizunashi River

The Park itself is on the line of contact between Hadano Basin and “Oyama Subgroup 大山亜層群” for Tanzawa Group of geological feature 丹沢層群. We can regard the place as the point where the wrinkles of Philippine Plate start at the aboveground level. This birth story of Hadano tells Tokawa Park is a good place for collecting many kinds of rocks tumbling down from Tanzawa Mountains especially from the surface of shortened crust. In addition, the water systems of steep Tanzawa slopes carry rocks not only from Oyama Subgroup but also from a bit far Togatake and Susugaya Subgroups.

Mt. Oyama over there from Tokawa Park.
Rolling stones come this far from there, baby.

The sample map I made from the pebbles collected
from Tokawa Park and Yadoriki Stream.
The black arrow shows the point we did stone hunting.
The blue area is Oyama Subgroup,
and light green area is Togatake Subgroup.
The point itself is on the white area for Hadano Basin.

The birth order of these geolotical groups is, the oldest Togatake 塔ヶ岳亜層群, the middle Oyama, then the youngest Susugaya 煤ヶ谷亜層群 Subgroups. Togatake and Oyama Subgroup areas have rocks evolved from volcanic magma, mainly Andesites, that cooled rapidly near the surface. They were originally from very active underwater volcanoes so that the chemical reaction with the sea water lets them be colored green. In large rocks procured from the area we can often observe chunks of volcanic bomb trapped and fixed in the volcanic ashes by tectonic pressure. Some good and polished example is situated artfully in the slope of Tokawa Park when we come down from the visitor center to the river. Some are good for using as benches. 😉 Actually, Togatake Subgroup has a strong connection with Yadoriki area 寄. I tell you about it when we look the pebbles in Yadoriki Stream.

Mr. Kiyoshi Sudoh
of Kanagawa Earth Science Association
was our instructor for stone hunting.
This rock has volcanic bombs inside, like

Bomb trapped, or so called tuff breccia, of Tanzawa.
By the way, the rock itself looks perfect
 to have lunch on it, don’t you think?

The dry riverbed of Mizunashi River in Tokawa Park is wide. We walk gravelly fields for some time to reach to the water. Though, for stone hunters of Tanzawa’s tectonic journey it’s a fairly convenient space. We stroll there, even without rainboots, and pick up interesting pebbles. Each doubtlessly has marks of Crash of Titans. The stones here were formulated some 15 to 17 million years ago 240km offshore from the point as undersea volcanic tuff (and occasionally basalt). i.e. The rocks here were the memory of active volcanoes older than Mt. Fuji (; the last eruption was in 1707) or Hakone Mountains (; it’s still erupting). They were carried by the Philippines Plate and arrived here about 5 million years ago. Larger and thicker Eurasian Plate with heavy basalt and peridotite did not yield easily to the pressure from the Philippines Plate. Before came here, the uplift from the bottom of the ocean started to wrinkle at the point of Crush of Titans. Around Tokawa Park area the crustal shortening become ferocious on shore and many fault lines created crumbling rocks. Oh, yes. It’s in present tense. Philippines Plate continues advancing 10cm per year. Do the math. 1 million years produce steady and never- conceding 100km progress ... In the process, many kinds of chemical reactions occurred for tuff and basalt. These straight volcanic products turned into another kind of rocks with different textures and colors. The pebbles we find here underwent such transformations in the relatively shallow part of the Plate, compared with the other areas of Tanzawa. Next week, I’ll show off my catch here. 😊

The field for hunting

Lots of catch here!

At the entrance of Tokawa Park, there is Tanzawa Visitor Centre. The contact address is

Hadano Visitor Center for Tanzawa-Oyama Quasi-national Park
丹沢大山国定公園・自然公園秦野ビジターセンター

1513 Horiyamashita, Hadano, Kanagawa Prefecture, 259-1304
〒259-1304 秦野市堀山下1513

Phone: 0463-87-9300

https://www.kanagawa-park.or.jp/tanzawavc/