We reveal the detailed syn-and post-caldera eruptive history of Izu Oshima Volcano, Japan, by tephra and loess stratigraphy. Twenty-four tephra layers, which overlie the slope outside the caldera, show that 24 eruptions occurred since the formation of the caldera (about 1, 450 years ago). These eruptions are separated by 10-200 years clear dormant periods, which can be identified by eolian dust (loess) interbedded with tephra layers. The 24 eruptions can be classified into three types : 1) eruption with scoria and ash falls (12 eruptions), 2) eruption only with scoria falls (7 eruptions), and 3) eruption only with ash falls (5 eruptions). While tephra discharge mass of the type 1 is generally large (1.5×1010 to 7×1011 kg), that of the type 2 or 3 is small (0.6 ×109 to 1 ×1011 kg). The 1986 eruption is classified into the type 2. Debris avalanches, which occurred just before the caldera formation and covered almost all of the Izu Oshima island, demonstrate that the present caldera wall was formed by slope failure of an old edifice. The tephra-discharge stepdiagram, which shows a relationship between time and cumulative discharge volume / mass of magma, shows : 1) the average tephra-discharge rate is constant (92 kg/ s before the N1.0 eruption and 25 kg/s after the N1.0 eruption), showing an abrupt decrease of the rate at about the time of the N1.0 eruption, which occurred about 900 years ago and was the most voluminous eruption for the past 1, 450 years, 2) both before and after the N1.0 eruption, the type 1 eruption shows volume-predictability, that is, the discharge volume / mass of a next type 1 eruption can be predicted, 3) a type 1 eruption should occur sometime in the future again, and when it occurs, the discharge mass of tephra should attain to as much as 2×1011 kg or more.
A special groundwater collector system, termed “Lianhuanjing”, exists in Hangjin Qi, Inner Mongolian Autonomous Region, on the Ordos Plateau, China. The author investigated “Lianhuanjing” on its development history, size, function, water quality, etc. More than 60 “Lianhuanjing” exist on the top of the hill at elevations of 1, 450 to 1, 600 meters above sea level, where little groundwater recharge occurs and only fissure water is expected. Hangjin Qi has only 150-300 mm of annual precipitation, about 3, 000 mm of potential evapotranspiration and is an arid climate typical of deserts and steppes. Major droughts have very often occurred there as shown in Table 1. As it was very difficult to obtain water, farmers living there developed a unique groundwater collector system known as “Lianhuanjing”. “Lianhuanjing” may be described as a “chained collector well system” from a functional point of view. The length of the first “Lianhuanjing”, constructed in 1959 and completed in 1962 at Ganhaiz, Shengli Village, Hangjin Qi shown Fig. 2, was 1, 200 meters and contained 300 vertical holes. Each 4 m-interval-hole was the hole from which earth and sand was thrown away. An ideal arrangement of “Lianhuanjing” is linear and goes direct to groundwater flow for collecting water efficiently. Water volume brought from “Lianhuanjing” was limited, but extracted water contributed to the local development of agriculture. After the favorable performance of the first “Lianhuanjing”, a lot of “Lianhuanjing” were constructed. But during and immediately after the “Cultural Revolution”, new “Lianhuanjing” were constructed. Several reasons, such as the collapse of the people's commune system in China, the privatization of agricultural land, and the introduction of new technology for drilling wells encouraged farmers to construct many new, small-scale “Lianhuanjing” systems after 1989. The arrangement of the “Lianhuanjing” constructed after 1989 was a geometrical distribution as shown in Fig. 6. This was the reason why farmers thought such an arrangement of the wells could collect groundwater more efficiently than the linear arrangement in case of getting fissure water in limited agriculture land. Quality of groundwater withdrawn from “Lianhuanjing” was Ca-HCO3 type, but contained quite high nitrate concentrations, over the drinking standard. Sodium Adsorption Ratio (SAR) was less than 1, and the value was judged as good only for irrigation. “Lianhuanjing” is a special and unique groundwater collector system combining vertical holes with lateral ones, found only in Hangjin Qi, to obtain fissure water. There are two other similar irrigation facilities, such as the Kanat system and the so-called “Manchulian well”, but “Lianhuanjing” is quite different from others as shown in Table 3. However, the technology of “Lianhuanjing” is quite valuable even for Japan as it is applicable to the places where the sandstone bedrock is rather shallow and groundwater resources is limited.
The Paleogene of Hokkaido is dominated in coal-bearing formations. Four stages are discriminated in evolution of the Paleogene sedimentary basins, above all, coal basins. A subsiding deep trough extended in the axial part of Hokkaido and southeastern Sakhalin between the East Asian continent and the Okhotsk microcontinent, and was filled with a 5 km-thick, clastic turbidite facies in the Paleocene. At the stage I in the latest Paleocene to earliest Eocene, the Haboro coal basin formed as a fore-arc basin in north-central Hokkaido on the west side of the trough. By contrast, the Nemuro shelfal clastic basin existed on the east side of the trough and encircled the southwest margin of the microcontinent. A drastic paleogeographic change occurred in the Early Eocene. The axial part of Hokkaido, which had been occupied by the deep trough, turned to upheaving associated with the Hidaka metamorphism, probably due to collision of the Okhotsk microcontinent against the East Asian continent. As a result, the trough and the sedimentary basins on its both sides were converted into a land. At the stage II in the middle Middle Eocene, the Ishikari-Uryu coal basin formed as an inter-arc basin in south-central Hokkaido where the Late Cretaceous continental fore-arc basin preexisted. The coal basin was first restricted to the Ishikari coalfield in the south, and later it enlarged to the Uryu coalfield in the north by the northward Wakanabe-Shiraki transgression. At the stage III in the late Middle Eocene, the Ishikari-Uryu coal basin extended to the west due to the subsidence of the Kabato-Rebun Cretaceous volcanic terrane. Basal fanglomerates, a few hundreds meters thick, overlapped the Cretaceous basement rocks in the west part of the coal basin. At the same stage, the Kushiro coal basin of the Kushiro coalfield formed incisely within the Nemuro shelfal basin in southeast Hokkaido, although it extended to the west and overlapped the Cretaceous basement rocks of the Tokoro terrane with conspicuous fanglomerates. At the stage IV in the Late Eocene to Early Oligocene, all coal basins of the previous stages subsided to the site of the deposition of shallow-marine to upper bathyal muddy sediments. In central Hokkaido, the Late Eocene marine basin of the Poronai Formation and its equivalents extended northwards from the Ishikari-Uryu coal basin to the Haboro coal basin of the stage I, and further to the north, to Sakhalin. However, the Early Oligocene mudrocks of the Momijiyama Formation was recognized only in the southern part of the Ishikari coalfield, and showed a restricted embayment environment. In east Hokkaido, the Early Oligocene continental sediments of the Wakamatsuzawa Formation overlapped the Cretaceous basement rocks of the Tokoro terrane on the northwest of the Late Eocene to Early Oligocene marine basin of the Onbetsu Group. The Paleogene of Hokkaido has a total thickness of two to five kilometers. The Paleogene strata of central Hokkaido are commonly extensively folded and overthrusted. The thick and folded Paleogene in central Hokkaido is undoubtedly traceable to Sakhalin, and possibly to the west side of central and northern Kamchatska. The third-ordered eustatic curves of Haq et al. (1987) indicate that, as a whole, Late Eocene is low stand while Early Oligocene is high stand. In Hokkaido, however, marine basins are most widespread in the Late Eocene, whereas they are restricted in the Early Oligocene. Local tectonic movements are therefore essential for the evolution of the Paleogene sedimentary basins in Hokkaido.
Flume experiments are conducted to examine the effects of plan geometric factors, such as confluent planform and junction angle, on channel morphologic adjustments at stream confulences. Because a receiving stream aligns with an axis bisecting an initial angle of junction, channel morphology can be classified into three types based upon an initial confluent planform symmetrical, transitional, and asymmetrical. Channel capacity is directly related to junction angle regardless of confluent planform. However, changes in channel slope and form ratio are different depending on confluent planform. They are related to junction angle inversely for asymmetrical confluences, but directly for symmetrical confluences. It indicates a mutual adjustment of channel slope and form to exogenous conditions. Channel morphologic adjustments to the changes of junction angle (A) and symmetrical degree of confluent planform (S) can be explained in terms of combined effects of these variables as follows : A+S+≈c+ (w+d-) f+s+ A-S+≈c- (w-d+) f-s- A+S-≈c+ (w-d+) f-s- A-S-≈c- (w+d-) f+s+ where c is the channel capacity, f the form ratio of width (w) to depth (d), and s the channel slope. The plus and minus superscripts represent an increase and decrease in channel characteristics. These relations qualitatively show how changes of channel characteristics occur with the plan geometric factors at stream confluences.