Usu Volcano, Japan
Most geothermal fields in Japan are developed around young volcanoes or intrusives. Very few of these fields are associated with basaltic volcanoes. As was mentioned earlier, Ishikawa (1970) states that geothermal areas commonly develop around lava domes of viscous, silicic compositions and that these extrusions plug a crater vent and then prevent gases and heat from freely escaping into the atmosphere. Figure 5.33 indicates the locations of volcanoes on Hokkaido island; Usu volcano is in the southwestern part of the island. The Toya hot springs are of particular geothermal interest because their development at the foot of Usu volcano was well documented during the emergence of the Meiji-Shinzan cryptodome in 1910.
Fig. 5.28
Temperatures at depths of (a) 5 m and (b) 10 m for shallow heat flow boreholes in the Coso geothermal
area. The colored numbers and symbols refer to boreholes referenced in Fig. 5.27, and the numbers
below the symbols are measured temperatures (°C). Note that isotherms shown by solid lines are
concentric to the Sugarloaf Mountain and Devil's Kitchen thermal areas.
(Adapted from Combs, 1980.)
Fig. 5.29
Heat flow contour map for depths <65 m in boreholes and generalized geology of the Coso
geothermal area (taken from Duffield et al., 1980). Borehole heat flow values are shown beside each
borehole (
to Duffield et al. (1980). Fumarole areas are stippled; heavy solid lines are faults (bar with ball on
downthrown side); and hachures outline areas of internal drainage (Moyle, 1977).
(Adapted from Combs, 1980.)
Fig. 5.30
Number of earthquakes vs depth for Coso volcanic field. The · and
geothermal-related earthquakes in the geothermal area shown in Fig. 5.29.
Most earthquakes are at depths between 5 and 6 km.
(Adapted from Walter and Weaver, 1980.)
Geology
Usu volcano is a truncated composite cone (Katsui et al ., 1981) located on the southern margin of Toya caldera (Fig. 5.34). During historic times, three dacite domes and seven cryptodomes formed on Usu. The 1910 activity included phreatic explosions that produced a zone of 45 craters on the northern foot of the volcano, and a number of hot springs formed along this zone just after the eruptions (Fig. 5.35). The 1910 activity was apparently initiated when magma intruded into a shallow region of abundant groundwater. Subsequent eruptions at Usu from 1943 to 1945 (Showa-Shinzan dome) and from 1977 to 1978 (Usu-Shinzan cryptodome) were well documented. Repeated hydrovolcanic activity during these eruptions resulted in numerous vulcanian explosions as well as new fumarolic areas, and magmatic bursts produced widely dispersed
pumice falls (Fig. 5.36). The 1977 to 1978 activity consisted of two major stages: (1) paroxysmal magmatic pumice eruptions in August 1977 and (2) phreatic and phreatomagmatic bursts beginning in November 1977 and continuing intermittently for nearly a year.
All of the historic eruptions involved calcalkalic rhyolite that, in general, gradually decreased in silica content to dacitic compositions. Katsui et al . (1978) has interpreted this change in composition as evidence of progressive downdraw of a compositionally zoned magma chamber.
Geophysical Properties
Regional gravity studies (Yokoyama, 1964) mainly show the anomaly of the Toya caldera (Fig. 5.37), but seismic observations such as those related to the Showa-Shinzan eruption (Minakami et al ., 1951) revealed shallow hypocenters beneath Usu (Fig. 5.38). Minakami et al . (1951) distinguished three types of volcanic tremors—all of low frequency. "A-type" tremors have a predominant period of ~0.3 s, clear S waves, nearly constant amplitudes at various epicentral distances, and hypocenters deeper than 0.5 km. "B-type" tremors are recorded for shallow hypocenters; they have predominant periods from 0.2 to 0.6 s and unclear S waves. "C-type" tremors, which occur during extrusion of lava, are also called harmonic tremors .
Hydrogeochemistry
The first hot spring produced at Usu after the 1910 activity had a temperature of 42°C, but wells drilled later show higher water temperatures, which increase from near the lake to the explosion crater zone where water temperature is 85°C and the Cl- content is 4.261 g/l (Ishikawa, 1970). Fukutomi (1960) recognized two types of hot springs: Type I, found near Meji-Shinzan, has Cl > SO4 > HCO3 and Type II, at greater distance from the young cryptodome, has SO4 > HCO3 > Cl. The major cationic components of each are very similar; that is, Na >> Ca > Mg. Because the water level in the explosion craters is nearly the same as that of Lake Toya, Nakamura (1962) suggested that the hot-springs water originated from the lake and was heated by the cryptodome Meiji-Shinzan. Definite changes in the temperature and chemistry of hot springs were documented before the most recent activity at Usu, and these changes apparently coincide with the monthly frequency of volcanic earthquakes (Fig. 5.39).
Differences in tephra chemistries noted in the eruptions from 1977 to 1978 are also geochemically significant and useful (Kondo et al ., 1979). Pumices from the first-stage magmatic eruptions are grayish-brown in color (SiO2 = 53.91 to 57.55 wt%), weakly alkaline, and rich in water-soluble Ca2+ and Na+ , but poor in water-soluble Cl- and
Fig. 5.31
East-west electrical cross sections from Rose Valley across Devil's Kitchen, Coso Hot Springs,
and Coso Basin (see Fig. 5.20). (a) Audiomagnetotelluric (AMT) at 7.5 MHz (heavy line) and Telluric J
(dashed line). (b) Pseudosection skin depths and apparent resistivities from AMT soundings, whose
locations are shown by numbered arrows. (c) Interpreted depths and true resistivities from
Schlumberger vertical electrical soundings, whose locations are shown as HR (Furgeson, 1973)
and USGS (Jackson and O'Donnell, 1980). A section of conductive alluvial deposits occurs in
Rose Valley (west side) and Coso Basin. A secondary resistivity low, occurring in the geothermal
area between Devil's Kitchen and Coso Hot Springs, is caused by a shallow conductive zone that is
interpreted as hydrothermally altered basement rocks containing saline geothermal water.
(Adapted from Jackson and O'Donnell, 1980.)
Fig. 5.32
Schematic east-west cross section of the Coso Range through Sugarloaf Mountain. The horizontal
scale is equal to the vertical scale, and there is some surface topography exaggeration. This section
summarizes the geological and geophysical interpretations for the origin of high heat flow from a
rhyolite magma chamber under a horst of crystalline basement rocks on which rhyolite domes have
been extruded. This interpretation emphasizes the fact that the geothermal heat emanates not from
the young domes themselves, but from the postulated underlying magma chamber.
(Adapted from Duffield et al ., 1980.)
Fig. 5.33
Volcanoes of Hokkaido, showing the Toya caldera and Usu volcano in the southcentral
portion of the island just west of the volcanic front.
(Adapted from Katsui et al ., 1981.)
Fig. 5.34
Geologic map of Usu, located on the southern margin of the Toya caldera, which is occupied by Lake
Toya. Usu is a collapsed composite volcano with a small summit caldera that is filled by several dacitic
domes and flanked by the Showa-Shinzan dome and cryptodome. The dome eruptions of Ko-Usu,
O-Usu, and Showa-Shinzan, as well as 7 other cryptodomes of Usu, occurred in historic times.
(Adapted from Katsui et al ., 1981.)
Fig. 5.35
Map of phreatic explosion craters formed during the 1910 activity at the base of Usu volcano.
The Toya hot springs, formed in 1962, became major geothermal direct-use features. This map was
drawn before the eruption of Showa-Shinzan volcano just south of Nishi-Kohan.
(Adapted from Ishikawa, 1970.)