1. Rock Layers 280 Million Years Apart Sit Side by Side at Fossa Magna Park in Itoigawa
At the Fossa Magna Park in Itoigawa City, you can see rock layers from about 300 million years ago and about 16 million years ago sitting next to each other on either side of the great fault [1]. The Niigata Prefecture guide introduces this as a place to compare the two rock layers [1]. The gap between 300 million and 16 million years is about 280 million years. To visualize this, imagine a book where one page represents 1 million years; the gap across the fault would be like 280 pages. However, this is just an analogy to help see the time span; it does not mean rock layers from the intermediate eras are lined up there.
This fault is part of the massive zone called the Fossa Magna, which divides the geology of the Japanese archipelago into East and West, marking the western edge of this zone [1]. In March 2021, it was designated as a National Natural Monument [1]. So, what can be seen if you walk east from this fault?
2. Pillow Lava, Evidence of an Ancient Sea Floor, Lies a 10-Minute Walk East of the Fault
About 10 minutes east of the fault, you can visit pillow lava [1]. Pillow lava forms during underwater volcanic eruptions, and the Niigata Prefecture guide states that this site teaches us that this location was once the bottom of the sea [1]. Today, it is on land, within walking distance. Yet, if the shape of the stones indicates an underwater eruption, it suggests that this area was once a sea within the large trough known as the Fossa Magna.
Within walking distance of the fault, which features rocks from about 300 million years ago, there are also traces of submarine eruptions. Objects from vastly different eras and formed in different places are lined up in this area. So, what lies on the older western side? There, you will find stones used by people of the Jomon period.
3. Jade from 500 Million Years Ago Became Jomon Ornaments in Itoigawa
On the Southwest Japan side of Itoigawa, old rocks from the Paleozoic and Mesozoic eras are distributed. Representative examples include Mt. Myojin and Mt. Kurohime, which are made of limestone from about 300 million years ago and jade from about 500 million years ago [6]. Itoigawa also has steep terrain with a height difference of 5,000 meters (about 16,400 feet), ranging from sea floors at a depth of about 2,000 meters (about 6,560 feet) to mountains at an elevation of about 3,000 meters (about 9,840 feet) [6].
People of the Jomon period processed and used this jade. At the Daikakuji site in Itoigawa City, jade hammerstones from the early Jomon period (about 6,500 years ago) have been found. At the Chogahara and Teraji sites, evidence shows that the processing of jade ornaments was active during the middle Jomon period (about 5,000 years ago) [6]. Processed items were distributed widely to various parts of Japan and even to the Korean Peninsula [6].
It is important to note that these two dates tell different stories. The jade rock itself formed about 500 million years ago, while people began using it about 6,500 to 5,000 years ago. 500 million years is 100,000 times longer than 5,000 years, meaning human history appears only at the very end of the stone's history. Why Jomon people chose jade is not clear from the available materials. Today, the Otaki River Jade Gorge has been a National Natural Monument since 1956, and collecting stones from the riverbed is prohibited [7]. Jade that flows out is washed up on the coast via the Hime River and can be found at "Jade Beach" [7].
4. How German Geologist Naumann's Horseback Journey Revealed the Great Trough
Naumann was a German geologist born in 1854. He arrived in Japan in 1875 (Meiji 8), became the first professor of geology at the University of Tokyo, and stayed in Japan for 10 years [3]. On the early morning of November 4 of his arrival year, Naumann left Tokyo on horseback. Crossing the Usui Pass, he reached the village of Hirazawa at the foot of Mt. Yatsugatake. It is said that the grand view he saw there became the origin of the idea of the Fossa Magna [3].
Fossa Magna means "great groove" in Latin. Naumann showed that the geology of the Japanese archipelago is divided into East and West by this groove [3]. He also named it. On the other hand, the name "Itoigawa-Shizuoka Tectonic Line" was given 43 years after Naumann’s arrival, in 1918, by the geologist Hisakatsu Yabe [4]. Thus, the name of the groove and the name of the line at its western edge were given by different people in different eras. So, is this line truly a single fault?
5. The Itoigawa-Shizuoka Tectonic Line Is Two Faults with Different Dips Joined Together
According to an explanation by the Geological Survey of Japan (GSJ), the Itoigawa-Shizuoka Tectonic Line is formed by two faults with completely different dips and origins that moved again under current east-west compressive forces, connected by a strike-slip fault near Lake Suwa [4]. The northern half is a normal fault dipping east, while the southern half is a west-dipping boundary of an ancient subducting plate [4]. Although it looks like one line, it is essentially a connection of two different faults.
The Fossa Magna itself is currently divided into northern and southern regions with completely different origins. The southern part is a collision zone formed by the Izu-Ogasawara island arc continuously colliding with Honshu since about 15 million years ago, with the Izu Peninsula still colliding with the Tanzawa Mountains today. Such places where island arcs collide are described as rare globally. However, how the northern part was formed is not yet understood [4]. In the south, at Niikura in Hayakawa Town, Yamanashi Prefecture, a fault is visible where the west side is black slate and the east side is tuff. It was designated a National Natural Monument in 2001 [2].
The Headquarters for Earthquake Research Promotion evaluates this line as an active fault zone that may still move. As an active fault zone, it spans about 158 km (about 98 miles) from near Otari Village in Nagano Prefecture in the north to near Hayakawa Town in Yamanashi Prefecture in the south, and is divided into four segments for evaluation [5]. Even if it looks like one geological line, whether this is truly Japan’s "East-West boundary" is a separate question.
6. Why Some Geologists Say the True Boundary Is Elsewhere: Southwest Japan's Belts Continue into the Kanto Mountains
The GSJ explanation notes that the geological belts of Southwest Japan (such as the Sanbagawa Belt) continue into the Kanto Mountains. Therefore, it argues that the geological discontinuity between East and West Japan, based on basement rocks, is not the Itoigawa-Shizuoka Tectonic Line but lies under the Kanto Plain (near the Tone River middle lowland) [4]. This is the view of one researcher, and other ideas exist. It does not mean this line is meaningless. The location of the boundary can change depending on whether you measure by geology, people's lives, or language.
Meanwhile, the Itoigawa city plan describes the town as an area where Eastern and Western cultures mix. Because the Kaga Highway and Matsumoto Highway intersected and shipping was active, information and culture from a wide range flowed through [6]. However, the materials do not state that the "tectonic line created a cultural boundary." It is reasonable to interpret this as them overlapping. So, how does this "boundary" look from where you live?
7. Trace the Itoigawa-Shizuoka Line on a Map and Compare Your Town
Find Itoigawa City and Shizuoka City on a map and trace a line with your finger passing through both cities. You will see that Lake Suwa and Yamanashi Prefecture are along the way. You can also check if your town is on the east or west side of this line. Next, change your measuring stick. Will the boundary be in the same place if you look at it by geological line, language, or lifestyle? Finding one thing that differs between East and West in your surroundings can help you understand that there is more than one way to define a boundary.
When visiting, check the Niigata Prefecture guide page for locations where faults are visible and rules for observation [1]. Collecting stones is prohibited in the Otaki River Jade Gorge riverbed [7]. Itoigawa City was certified in 2009 as one of Japan's first UNESCO Global Geoparks (along with Toya Lake Usu and Shimabara Peninsula) [6]. As a place to learn about the formation of the earth, visitors come from many places.