Hello, this is Takezawa.
Welcome to the first installment of our series on Mount Fuji.
Let’s dive right into the main topic.
In this inaugural article, we’ll explore “The Formation of Mount Fuji.” How did the current Mount Fuji come into existence? Let’s delve into this topic.
Mount Fuji is believed to have formed between 500,000 and 100,000 years ago, roughly during the same period as the volcanic activities that created mountains like Hakone and Ashitaka.At that time, not only were there no humans in Japan, but our species, Homo sapiens, had either just emerged or had yet to appear.
Volcanic activities during this period led to the accumulation of andesite and pyroclastic rocks (ejecta other than lava), eventually forming a mountain as the volcanic activity subsided This initial structure is known as the “Pre-Komitake Volcano” and “Komitake Volcano,” which are estimated to have had an elevation of around 2,400 meters.
Interestingly, until 2004, it was believed that the Komitake Volcano was the original mountain of Mount Fuji. However, a drilling survey in 2004 revealed even older strata, leading to the identification of the Pre-Komitake Volcano as the oldest structure.
It’s noteworthy that the initial elevation was around 2,400 meters. Given that the current Mount Fuji stands at 3,776 meters, the original mountain was significantly smaller. This 2,400-meter elevation corresponds to about the 5th station on the current Mount Fuji. In fact, at the Komitake Shrine near the new 5th station on the Yoshida Trail, you can observe andesite rocks believed to be remnants of the Komitake Volcano.
The progression towards the current Mount Fuji can be summarized as follows:
- First structure: Pre-Komitake Volcano
- Second structure: Komitake Volcano
- Third structure: ???
Let’s continue.
About 100,000 years ago, volcanic activity intensified on the southern slope of the Komitake Volcano (near the current summit of Mount Fuji). Repeated explosive eruptions covered the Komitake Volcano with lava and ash, leading to the formation of the third structure, the “Old Fuji Volcano.”
During this period, the world was in the midst of an ice age, with mammoths still roaming. There were likely no humans in Japan at this time. Lava from these eruptions mixed with snow and glaciers, creating the “Ko-Fuji Mudflow,” which now contributes to Mount Fuji’s underground water system. The Old Fuji Volcano is believed to have reached elevations of over 2,700 meters, possibly even 3,000 meters.
Notably, volcanic ash from the Old Fuji Volcano reached the Kanto region, forming the upper layer of the Kanto Loam, known as “red soil.” Some of you might recall learning about this in middle school.
The progression now looks like this:
- First structure: Pre-Komitake Volcano
- Second structure: Komitake Volcano
- Third structure: Old Fuji Volcano
- ???
We’re almost at the final form!
Time passed, and about 10,000 years ago, Mount Fuji began another phase of intense volcanic activity. This was during the Jomon period, after the end of the ice age, when our ancestors had started living in Japan. Unlike the explosive eruptions of the Old Fuji Volcano, these eruptions produced more fluid basaltic lava. The abundant lava flows covered the Old Fuji Volcano, leading to the birth of the “New Fuji Volcano,” which is the Mount Fuji we know today.
Some theories suggest that instead of being completely covered, the Old Fuji Volcano had twin peaks, which collapsed due to certain factors, leading to its disappearance.
In any case, the current Mount Fuji was finally born!
Even after becoming the New Fuji Volcano, Mount Fuji continued to erupt, with lava reaching cities like Mishima in Shizuoka Prefecture and Otsuki in Yamanashi Prefecture, both over 50 kilometers away. These eruptions also led to the formation of features like the Aokigahara Sea of Trees and the Hoei Crater. The last known eruption was the Hoei Eruption in 1707 (Hoei 4), and Mount Fuji has remained dormant for the past 300 years, though it is still considered active.
Thus, Mount Fuji became Japan’s tallest mountain by expanding and growing upon three foundational structures.
A happy ending indeed.
