Hello.
This is Takezawa.
Today, we present the second installment in our series on learning about Mount Fuji.
As before, we will follow the Fuji-san Certification as our guide.
*Note: Some details, such as dates, may differ from current common theories, but we will primarily reference this book. Please understand.
Today’s topic covers the geology of Mount Fuji and its parasitic volcanoes.
First, most of the volcanic ejecta covering the surface of the current Mount Fuji, known as the “New Fuji Volcano,” is basaltic lava, a type of volcanic rock. Basaltic lava is characterized by its low viscosity, allowing it to flow very quickly. As it cools and the flow slows down, the surface becomes flat with rounded, rope-like patterns, forming what is known as “pahoehoe lava” (also called “pāhoehoe lava”). Additionally, when pahoehoe lava flows over long distances, its temperature decreases, and gas escapes, causing the flow to become sluggish and solidify into what is called “aa lava.” In contrast to pahoehoe, aa lava has a rough and jagged surface. Both types can be observed when walking through the Aokigahara Jukai (Sea of Trees).
“Pahoehoe” and “aa” may sound like peculiar names, but they are legitimate scientific terms. Their origins are in the Hawaiian language. The island of Hawaii, exemplified by Kilauea Volcano, is a volcanic island, and the appearance of its lava surfaces led to these terms: “pāhoehoe,” meaning “smooth surface,” and “‘a’ā,” meaning “rough surface.” Both are catchy terms that even children can easily remember.
Among the many volcanoes in the Japanese archipelago, large-scale volcanoes like the current Mount Fuji, which are almost entirely covered by basalt, are rare; most are composed mainly of andesite. For instance, Mount Komitake was composed of andesite.
Incidentally, volcanic rocks (rocks that solidified rapidly on or near the Earth’s surface, such as at craters) include basalt, andesite, rhyolite, and dacite, classified based on the types of constituent minerals and chemical composition. This might be a bit confusing, so let’s gracefully skip over it.
Additionally, among the volcanic ejecta covering the current Mount Fuji, there is a material called “scoria.” Scoria is a type of volcanic gravel resembling small stones, typically black or dark brown, and can turn reddish when oxidized. These basaltic stones are commonly seen during climbs of Mount Fuji. Volcanic gravel like scoria is classified by size: particles between 2mm and 64mm in diameter are called volcanic lapilli, those smaller than 2mm are volcanic ash, and those larger than 64mm are volcanic blocks.
Exploring Mount Fuji’s geology while climbing or walking around the mountain can be quite interesting.
Next, let’s discuss “parasitic volcanoes,” which were once referred to as “secondary volcanoes.” These are small volcanoes that formed on the foothills or slopes during Mount Fuji’s formation process. A representative example is Mount Hoei.
When thinking of Mount Fuji’s eruptions, many might imagine eruptions from the summit. However, in reality, eruptions have repeatedly occurred on the slopes and foothills, far below the summit. The number of these eruptions, both large and small, is said to exceed 70, making it the highest in Japan. These parasitic volcanoes are concentrated from the north-northwest to the south-southeast of Mount Fuji. This distribution is believed to be significantly influenced by the forces exerted by the three tectonic plates subducting directly beneath Mount Fuji (the North American Plate, the Eurasian Plate, and the Philippine Sea Plate), particularly the Philippine Sea Plate, which is currently thought to be moving northwestward by about 3 meters.
To the north-northwest of Mount Fuji, there are small mountains that are immediately recognizable, such as Nagao-yama, which contributed to the formation of the Aokigahara Jukai, and Oomuro-yama, which boasts significant height following Mount Hoei, as well as Katafuta-yama. To the south-southeast, there are Mount Hoei and Futatsuzuka, among others.
The sheer number of these parasitic volcanoes is a testament to the vastness of Mount Fuji.
