Because air-fall tephra covers the landforms of the
mountains and plains and constitutes the present surface, it is
also important as a soil parent material. Many active volcanoes of the Japanese Islands produce andesitic and basaltic
ejecta and erupt about once every several decades or several
centuries. As a result, the tephra accumulating from the foot
of a volcano to the leeward plain forms a thick stratum.
A large number of these tephra layers blown by wind and
deposited on the surface are weathered as time elapses after
deposition, lose their original sedimentary structure and
become seemingly non-stratified. The Kanto Plain is a typical example for such deposits, and it is difficult to distinguish individual layers of many thin scoria ejected from Mt.
Fuji (Machida et al. 1971; Fig. 2.24). With increasing distance from a volcano, the apparent sedimentation rate
decreases and the chance of contamination by eolian dust
and other particles moving from the adjacent slope becomes
greater, resulting in a stratum with massive facies. Such
strata are universally found around volcanic bodies in the
Japanese Islands and have been called “loam layer” in the
field of Quaternary science in Japan. However, “loam” was
originally defined as a term referring to a soil texture with an
equal mixture of sand and silt, and normally the particle size
of the “loam layer” decreases with the distance from the
volcano. Thus, it is important to note that “loam layer” as a
stratum name is not meaningful.
(2) Eolian dust
Strong wind induces dust to rise from the ground. It also
transports fine grains of landform materials formed by running water, waves, mass movement, etc., and redeposits
them to cover another geomorphic surface. Fine-grained
substances transported by wind are generally called eolian
dust. Among eolian dust, sand mainly comes from continental sandstorms, and that which continues to be transported to a distant place such as Japan is called “tropospheric
dust.” In order for eolian dust to be fixed on a ground
Fig. 2.23 General distribution of the representative marker-tephras of
the Late Pleistocene and volcanic ash soil (“loam layer”) with total
thickness of more than 2 m in Japan and adjacent areas (stipples)
(reprinted from Machida(2002), Copyright 2019 Japan Academic
Association for Copyright Clearance, with permission from Japan
Academic Association for Copyright Clearance). Volcano symbol: name
(location): A: Aira (Southern Kyushu), Aso: Aso (Central Kyushu), Ata:
Ata (Southern Kyushu), B: Baekdusan (North Korea/China), D: Daisen
(Western Honshu), U: Ulreung (Korea), K: Kikai (Southern Kyushu),
Kc: Kutcharo (Eastern Hokkaido), On: Ontake (Central Honshu), S:
Shikotsu (Central Hokkaido), Sb: Sambe (Western Honshu), Toya: Toya
(Central Hokkaido). Tephra symbol: name (age) Aso-4: Aso-4 (80–
90 ka), AT: Aira–Tanzawa (26-29 ka), B-Tm: Baekdusan–Tomakomai
(1 ka), DKP: Daisen–Kurayoshi (> 55 ka), K-Ah: Kikai–Akahoya
(7.3 ka), K-Tz: Kikai–Tozurahara (95 ka), Kc-Hb: Kutcharo–Haboro
(115-120 ka), Kc-Sr: Kutcharo–Shoro (35-40 ka), On-Pm1: Ontake-1
(100 ka), SK: Sambe-Kisuki (110-115 ka), Spfa-1: Shikotsu-1
(40-45 ka), Toya: Toya (112-115 ka), U-Oki: Ulreung–Oki (10.7 ka)
36
K. Tamura et al.
mountains and plains and constitutes the present surface, it is
also important as a soil parent material. Many active volcanoes of the Japanese Islands produce andesitic and basaltic
ejecta and erupt about once every several decades or several
centuries. As a result, the tephra accumulating from the foot
of a volcano to the leeward plain forms a thick stratum.
A large number of these tephra layers blown by wind and
deposited on the surface are weathered as time elapses after
deposition, lose their original sedimentary structure and
become seemingly non-stratified. The Kanto Plain is a typical example for such deposits, and it is difficult to distinguish individual layers of many thin scoria ejected from Mt.
Fuji (Machida et al. 1971; Fig. 2.24). With increasing distance from a volcano, the apparent sedimentation rate
decreases and the chance of contamination by eolian dust
and other particles moving from the adjacent slope becomes
greater, resulting in a stratum with massive facies. Such
strata are universally found around volcanic bodies in the
Japanese Islands and have been called “loam layer” in the
field of Quaternary science in Japan. However, “loam” was
originally defined as a term referring to a soil texture with an
equal mixture of sand and silt, and normally the particle size
of the “loam layer” decreases with the distance from the
volcano. Thus, it is important to note that “loam layer” as a
stratum name is not meaningful.
(2) Eolian dust
Strong wind induces dust to rise from the ground. It also
transports fine grains of landform materials formed by running water, waves, mass movement, etc., and redeposits
them to cover another geomorphic surface. Fine-grained
substances transported by wind are generally called eolian
dust. Among eolian dust, sand mainly comes from continental sandstorms, and that which continues to be transported to a distant place such as Japan is called “tropospheric
dust.” In order for eolian dust to be fixed on a ground
Fig. 2.23 General distribution of the representative marker-tephras of
the Late Pleistocene and volcanic ash soil (“loam layer”) with total
thickness of more than 2 m in Japan and adjacent areas (stipples)
(reprinted from Machida(2002), Copyright 2019 Japan Academic
Association for Copyright Clearance, with permission from Japan
Academic Association for Copyright Clearance). Volcano symbol: name
(location): A: Aira (Southern Kyushu), Aso: Aso (Central Kyushu), Ata:
Ata (Southern Kyushu), B: Baekdusan (North Korea/China), D: Daisen
(Western Honshu), U: Ulreung (Korea), K: Kikai (Southern Kyushu),
Kc: Kutcharo (Eastern Hokkaido), On: Ontake (Central Honshu), S:
Shikotsu (Central Hokkaido), Sb: Sambe (Western Honshu), Toya: Toya
(Central Hokkaido). Tephra symbol: name (age) Aso-4: Aso-4 (80–
90 ka), AT: Aira–Tanzawa (26-29 ka), B-Tm: Baekdusan–Tomakomai
(1 ka), DKP: Daisen–Kurayoshi (> 55 ka), K-Ah: Kikai–Akahoya
(7.3 ka), K-Tz: Kikai–Tozurahara (95 ka), Kc-Hb: Kutcharo–Haboro
(115-120 ka), Kc-Sr: Kutcharo–Shoro (35-40 ka), On-Pm1: Ontake-1
(100 ka), SK: Sambe-Kisuki (110-115 ka), Spfa-1: Shikotsu-1
(40-45 ka), Toya: Toya (112-115 ka), U-Oki: Ulreung–Oki (10.7 ka)
36
K. Tamura et al.