After Crashes: beautiful rocks found in the stream of Tanzawa Mountains 2

 

So, the base of Tanzawa Mountains has the ancient volcanic origin. “Volcanic” can be ashes, lapilli, or lava itself. The rocks that become rocks straight from lava are magmatic/igneous rock. In the riverbed of streams in Tanzawa, we can find several igneous kinds. Roughly speaking we can categorize such volcanic rocks into two families. One is extrusive rocks. They were rapidly cooled lava when it came out of the earth’s crust. In our neighborhood the familiar extrusive ones are andesite and basalt. Andesite contains more silicic acid than basalt. Andesite is often created on the edge of tectonic crash. Actually, andesite is the major component of earth’s continent, so it could be a familiar rock for some of you reading this post somewhere away from the sea. It can be found easily in riverbeds of Tanzawa too. Here is one of my collections.

Andesite

The microscopic view of the above pebble.
The white mineral is zeolite
penetrated in the lava’s holes
at the time of formation and crystalized later.

In contrast, basalt is made of smoother lava with less silica. The amount of silicic acid changes the way lava flows at the time of eruption. The less silicic, the smoother the movement of lava. It affects the way a volcano erupts. In general, the less silicic, the calmer an explosion is. At the time of burst basalt lava can be splashed over and flow rapidly that creates a volcano of gentler slope. Come to think of it, such a volcano is situated very near to Tanzawa, i.e. Oshima Island floating at the mouth of Sagami Bay. Mt. Fuji, famous for its basalt, has the origin of volcanic activity from 7 hundred thousand years ago. Compared to this, the crash of tectonic plates for Tanzawa is far older. For Tanzawa the estimated tectonic undersea eruptions began about 1.5 to 1.7 million years ago. The age of basalt we find in and around the streams of Tanzawa Mountains are definitely older than for Mt. Fuji. There is another difference between these two neighbourhood mountains. Mt. Fuji stands always on land since its birth. Tanzawa was once under sea. At the time of undersea eruption, the lava enters the mass of water directly from the crust. The surface of the lava is cooled immediately. When its texture is fluent, the instant check of water creates flow of lava like whipped cream squeezed from piping. Such lava can be solidified in a form similar to pillows, and so it is called Pillow Lava. When we walk along streams in Tanzawa Mountains, we sometimes find a pillow-like boulder. That’s a Pillow Lava containing ancient memories of the earth. It has smooth surfaces. Please touch them calmly, close your eyes, and feel the essence of the planet …

Oshima Island seen from the top of Mt. Ogusu 大楠山, ASL 241.1m (; my posts on 1, 16, 24 of April).
Could you see a wispy shadow in the middle of the ocean?
That’s Mt. Mihara, ASL 758m, famous for its very gentle ridge line.

Pillow Lava in Yadoriki Stream.

When lava is cooled slowly, it is called plutonic or intrusive rock. The creation of intrusive rock is easier to understand when we talk the origin of Tanzawa Mountains. Tanzawa is by the crash of tectonic plates, right? Philippines Sea Plate is crawling under the Eurasian and the North American Plates. Imagine the pressure inside the planet there. By the force, the lava stayed deep down is pushed up through the crack of crust near the surface, like toothpaste squeezed out of tube. It does not necessarily end up with volcanic eruption, but sometimes stays underground for a very very long time. The humongous compression continues affecting such mass of lava but in the environment of lower heat temperature, I mean, compared with that for the core of the planet. The wedged lava slowly becomes rocks. This is basically the way diamonds are created. This kind of phenomenon often occurs at the edge of tectonic collision and creates large underground geology, called metamorphic rock. Tanzawa is not an exception. About 5 to 4 million years ago, pools, or rather lakes of lava were created underground, cooled slowly, and formed metamorphic rocks. The rocks made of this process were continuously pushed up from the deep down and eventually reached the surface. The rock is ginormous. Do you remember we visited Mt. Yagura 矢倉岳, ASL 870m, located at the edge of Hakone Mountains (; my post on August 4, 2017)? When we watch the mountain from the bank of Sakawa River 酒匂川 near Odakyu Shin-Matsuda Station, don’t you think it has a particular shape, like a bump on earth? It is indeed a hump 100% made of one metamorphic rock pushed up by the pressure of tectonic crash.

Mt. Yagura is here.

The base rock of Mt. Yagura is called quartz diorite, or tonalite. In 2016, Kanagawa Prefecture designated the rock as the official rock of the Prefecture. As it has such a celebrated status, it is not so difficult to find one in the streams of Tanzawa Mountains. Here is my collection.

Tonalite. White part is plagioclase,
and black dots are amphibole and the others.

Microscopic view of the pebble.

Casually quartz diorite shows its huge face
in a riverbed of Tanzawa’s stream.
This one had a span of at least 1m.

Not all intrusive rocks are tonalite in Tanzawa. The sedimentary rocks of vocanic ashes and lapilli do not necessarily come soon to the cooler surface of the planet after they became rock. It stayed for a while near magma, and baked for a harder rock called hornfelse. The mass of hornfelse can be found along the western ridge of Mt. Nabewari, ASL 1272.4m. Inevitably, when it crumbles, it tumbling down Yadoriki Stream, and we can find hornfelse there. It is a hard rock, and broken into peaces as if it produces sharp knives. I‘ve heard the rock was an important tool for the European Stone Age. I guess that would be the same for Japan.

There is a hornfelse in this photo.

In this blog, I several times told you Tanzawa Mountains have very fragile geology, and the forests stand on a very thin soil covering the scree beneath. By studying the tectonic origin of the area, I realized it‘s no wonder the place has such feature. Tanzawa can never have rich soil as Mississippi, but the place has its own charm, including beautiful rocks witnessing the tectonic crashes from long ago. When you hike there next time, please enjoy the scenery of rocks and boulders in Tanzawa. It tells the life of the planet earth.

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/