Although the land seems unmoving over a short period, it changes greatly over long periods through uplift, subsidence, folding, and faulting.
Basic Knowledge
When great forces act on strata, they form folds(wave-like bends) and faults(displacements caused by breaks). Faults include normal faults, in which one side drops because it is pulled apart; reverse faults, in which one side rises because it is compressed; and strike-slip faults, in which the sides move horizontally.
When land is uplifted, it forms mountains and terraces; when it subsides, it forms bays and ria coasts. The Japanese archipelago is located in a tectonically active zone and is one of the most tectonically active regions in the world.
It is believed that the Japanese archipelago separated from the Eurasian continent about 20 million years ago, forming the Sea of Japan. Later, subduction of the Philippine Sea Plate and Pacific Plate brought it close to its present shape. The Itoigawa-Shizuoka Tectonic Line is a major fault separating eastern and western Japan, while the Median Tectonic Line is a major fault running through southwestern Japan.
Folding(A phenomenon in which strata deform into wave-like shapes)
Fault(A crack where rock masses have shifted. Normal, reverse, and strike-slip faults)
Uplift and subsidence(Movements in which the crust rises / falls)
Tectonically active zone(A region with active earthquakes and volcanic activity. The Japanese archipelago is a representative example)
Itoigawa-Shizuoka Tectonic Line(A major fault marking the western edge of the Fossa Magna)
Median Tectonic Line(One of Japan's longest-class faults, extending east-west through southwestern Japan)
In Depth (Background and Significance)
The crust continues to rise and sink by several millimeters each year. The Japanese Alps(the Hida, Kiso, and Akaishi mountain ranges) rise by about 2〜4 mm per year and have reached elevations of about 3000 m over several million years. Coastal terraces are evidence of uplift, while ria coasts(Sanriku and Wakasa Bay) are evidence of subsidence.
The Fossa Magna is a great rift valley in the central part of the Japanese archipelago, where sediments more than 6000 m deep have accumulated. It explains why eastern and western Japan have different geological structures, separated by the Itoigawa-Shizuoka Tectonic Line and the Shibata-Koide Tectonic Line.
Because Japan is located in such an active zone, it has many disasters, but it also has great benefits, including hot springs, geothermal energy, diverse landforms, and abundant water resources. We need the wisdom to coexist with earthquakes and volcanoes.
- Folding = bending; faulting = shifting
- Normal fault = pulling / reverse fault = compression / strike-slip fault = horizontal movement
- Uplift → mountains and terraces / subsidence → bays and ria coasts
- The Japanese archipelago separated from the continent about 20 million years ago
- Itoigawa-Shizuoka Tectonic Line = boundary between eastern and western Japan
- Median Tectonic Line = one of Japan's longest-class faults
- The Japanese Alps rise by 2〜4 mm per year
Points to Note (Common Confusions)
① The direction of force is opposite in a normal fault(pulled apart and dropped) and a reverse fault(compressed and raised). ② Folding and faulting are different(bending/breaking). ③ The Itoigawa-Shizuoka Tectonic Line and the Median Tectonic Line are both major faults, but they are in different locations. ④ Ria coasts are landforms formed by subsidence, while coastal terraces are landforms formed by uplift.
Practice
- Name all three types of faults.
- What is the name of the major fault separating eastern and western Japan?
- Are ria coasts formed by uplift or subsidence?