Posted by: tsubakuro | April 21, 2010

Elevation Gain

The Japan Archipelago is a chain of mountain islands created by the tectonic forces that push the Pacific Ocean Plate and Philippine Plate under the Eurasian Plate and part of the North American Plate. The mountains are either a result of uplifting or volcanism and generally run the length of the archipelago.

One exception to this is the Japan Alps. The three ranges of the Hida, Kiso, and Akaishi Ranges run roughly through the middle of the main island of Honshu. The reason for their atypical orientation is found the tectonic line that separates the Eurasian Plate part of Japan from the North American Plate part (yes, I too was incredulous when I read “North American Plate” but it’s true). Splitting the island from Itoigawa in Toyama to Shizuoka, it is known as the Itoigawa/Shizuoka Tectonic Line. Pressure from the two plates pushing against one another caused the three ranges to be uplifted, the Hida and Kiso Ranges (North and Central Alps respectively) being largely composed of igneous rock like granite and andesite with sedimentary rock being also found in the North Alps, and the Akaishi Range (South Alps) being more sedimentary as parts of the range were once a seabed.

It is interesting to note that the Kiso and Akaishi Ranges lie almost parallel to each other and that neither range has as many high mountains as the Hida Range, even though the Akaishi Range covers more area. The explanation lies deep in the Tenryu River Valley of Nagano. Here a second tectonic line, the Chuo Tectonic Line, branches off from the Otoigawa/Shizuoka. As pressure from the plates was spread over two tectonic lines, two ranges were formed – one short and under 3,000 metres and the other broad but with only a single spine of mountains clearing 3,000 metres flanked by many minor subranges mostly below 2,500 metres – while to the north all the pressure was applied to a single tectonic line and as a result a high, broad range was created with sub-ranges all reaching above 2,900 metres at at least one peak. (The above explanation is my own conjecture. I am searching for documented supporting evidence.)

Previous to the uplifting of the Alps, large volcanoes already existed where the Hida Range is now, most notably around the Yari/Hotaka Range and around Jiigatake. These volcanoes collapsed and the remaining shells of their calderas were pushed up by tectonic forces. The uplifting of the Alps took place mostly between 1.7 million years ago and 0.6 million years ago. The igneous intrusion of the Kurobe pluton in the Hida Range was formed at depths of 4 to 10 kilometres below the surface, but due to rapid tectonic processes and consequent uplifting, followed by erosion, the rocks are now exposed at 700 to 2,900 metres*. Compared to most other ranges in the world, the mountains of the Japan Alps were uplifted in a geologically short time. Following the collapse of a volcano that resulted in a huge caldera, the Yari/Hotaka Range was originally uplifted to around 4,000 meters; however, erosion, perhaps mostly from glacial periods, has since reduced them to their current elevation of less than 3,200 meters.

In these three ranges stand 92 of Japan’s 100 highest peaks with Kitadake in the Akaishi Range being the highest at 3,193 metres, second in the country only to the much higher Fuji San (3,776 metres). Fujisan, a composite volcano, is an anomaly here because it reaches heights far greater than any other mountain in Japan, while the succeeding 99 highest peaks follow a very gradual decrease in elevation as the list goes down. Consider that Fujisan is 583 metres higher than Kitadake but the next nine highest are all between 3,193 metres and 3,101 metres, a mere 92 metres in difference.

Kitadake – 3,193m

Oku Hotakadake – 3,190m

Ainodake – 3,189m

Yarigatake – 3,180m

Arakawa Higashidake (Warusawadake) – 3,141m

Akaishidake – 3,120m

Karasawadake – 3,110m

Kita Hotakadake – 3,106m

Obamidake – 3,101m

The difference in elevation between Kitadake and the 100th highest mountain, Nokogiriyama (2,685m), is 508m, less than the difference between the first and second highest mountains in the country. This list of the 100 highest peaks includes uplifted mountains as well as volcanoes, so Fujisan’s superlative height is clearly is an isolated exception, at least in the present day.

Ainodake in the Akaishi Range is Japan’s fourth highest peak and stands across a col from Kitadake. However, due to the amount of erosion rubble observed around the mountain, it is believed that Ainodake may have been much higher** during the peak of the last glacial period and at the time was higher than Fujisan.

Though the forces of erosion and weathering are at work wearing down the mountains, the tectonic forces are also pushing up the mountains. Kitadake is increasing in height at an average rate of 6.25mm a year. If the rate remains constant and the summit of the mountain does not collapse in a rockslide, Kitadake will reach an elevation of 4,000 metres in about 129,120 years.

Until the end of the 19th century, the elevations of Japan’s mountains were not precisely known. As Japan opened its doors to foreign trade, and engaged in warfare with other nations, it became a necessity to have an accurate map of the country with mountain elevations included. But it took time to do this using triangulation teams, and even up to the beginning of the 1900’s, it was believed that Yarigatake was Japan’s second highest mountain*** (it is actually the fifth).

Triangulation measurements reached their final target, Tsurugidake, in July of 1909. The map of Japan was complete. Yet the question of true mountain elevations has continued to fall under scrutiny and revisions and re-measurements have been made. For example, Tsurugidake’s elevation was given as 2,998 metres around 1915 but later increased to 3,003 metres on a map in 1932, and then was later reduced back to 2,998 metres in 1970. The current revised elevation as of 2007 is 2,999 metres. Other mountains have had their elevations revised after improved measurement systems were used. Most notably, Kitadake was upgraded from 3,192.4 metres to 3,193 metres.

Sources:

The geological history of the Yari/Hotaka Range and Kamikochi:

Yama-to-Keikoku magazine, May 2007

Measuring Tsurugidake:

Yama-to-Keikoku magazine, June 2009

Japan’s hundred highest mountains. I have found another list that is slightly different from the one on the link below but my point about Fuji being an anomaly when elevation differences of the hundred highest peaks are considered still holds with either list.

Japan’s Hundred Highest Mountains

Kitadake’s growth:

Feature: Kitadake

Tectonic lines of the Japan Alps:

「信州山岳大展望台 日本の屋根」 Shinshu Sangaku Daitenbodai: Nihon no Yane (The Grand View of Alpine Shinshu: The Roof of Japan) a book

For a good map that shows the areas of the three ranges and information about their uplift see the site linked below:

Uplifting of the Alps

*I currently cannot find the source of these numbers in the italicized text, however, the link below reports similar information.

Rapid uplift in the Japan Alps

**I had read this about Ainodake on another site but when searching for that site again I found the same information on Wikipedia.

***I have lost the URL of the site where I got this information. Or maybe it was from a book…

The Yari/Hotaka Range from Sugorokudake in the Kita(North) Alps, a.k.a. the Hida Range

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Responses

  1. This is fascinating: thanks for posting. I’m delighted to read that Kita-dake will soon (geologically speaking) become a 4000-er, thus putting in a truly alpine class. Its rate of uplift is already several times faster than that of the Swiss Alps – way to go. On a more serious note, this is the first time I’ve seen the explanation of why the Central/Southern Alps are different in height and topography from the Northern Alps. What was your source for that item?

    • Hi Hyaku! Thanks for reading. Yes, the Japan Alps are world class alps in the making it seems. Funny how time seems to slow down so much once you look at it from a present human scale.

      I am trying to be more meticulous with the sources. I can appreciate the effort you put in to your site. My sources for the physical geography and topography of the Alps come from the sites and book I mentioned in my source list, but specifically the remark about the North Alps receiving the full force of the tectonic push and thus having more high peaks while the Central and South Alps split the force over two tectonic lines and thus creating less extensive high peaks is my own conjecture. I will add that into the post.

      Actually, this post is a work in progress. There are a couple of other things I want to double check too.

      Did you see my source link on your latest post?

  2. Your conjecture about the forces on the N and S Alps sounds convincing to me – maybe you should run it past a geophysicist….

    As for a “source link”, er …um, forgive my ignorance, but what’s that?

    • Not your ignorance. Mine. I meant the link I provided to my source. Or reference might be the better word.

      Thanks for the vote of confidence regarding my hypothesis of the reason for two ranges with fewer high mountains. First I will see what I can find if the idea hasn’t already appeared in writing somewhere. If nothing turns up I’ll ask someone in the field.


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