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Home » Disaster Prevention » Kozu Island Field Trip – Introduction on Geomorphic, Geologic Field Condition, and Mitigation Facilities

Kozu Island Field Trip – Introduction on Geomorphic, Geologic Field Condition, and Mitigation Facilities

1. Introduction

1.1 General

Kōzushima is a volcanic Japanese island in the Philippine Sea, administered by Tōkyō and located approximately 30 kilometers (19 mi) northwest of the Miyakejima and 14 kilometers (8.7 mi) southwest of the Niijima. It is one of the Izu Seven Islands groups of the seven northern islands of the Izu archipelago.

Fig. 1 Kozu island location

Fig. 1 Kozu island location (source: google map)

Below is the geographic data of Kozushima:

Fig. 2 Geographic data (source: wikipedia)

Fig. 2 Geographic data (source: wikipedia)

The Kozushima Island is located in the 180km south of Tokyo, and it is a volcanic island of 18 square km.This island is covered by the thick rhyolite lava and pyroclastic flow deposits from several volcano eruptions. A representative volcano of Kozushima Island is the Tenjo mount and it is about 570m above sea level. The latest eruption of a Tenjo mount is A.D.838. And a large scale landslide is occurring now.

Tenjo volcano has many hiking courses around, which dominates the island and is also a great place for diving. Its white sandy beaches make it an excellent place to swim in summer, since it receives considerably fewer visitors than the other islands in the chain. Winter visits are discouraged, as you are likely to get stranded on the island due to bad weather.

1.2 Landscape Characteristic and History

A cluster of rhyolitic lava domes and associated pyroclastic deposits forms the small 4 x 6 km island of Kozu-shima in the northern Izu Islands. Kozu-shima lies along the Zenisu Ridge, one of several en echelon ridges oriented NE-SW, transverse to the trend of the northern Izu arc. The youngest and largest of the 18 lava domes, 574-m-high Tenjo Mt., occupies the central portion of the island. Most of the older domes, some of which are Holocene in age, flank Tenjo Mt. to the north, although late-Pleistocene domes are also found at the southern end of the island. Only two possible historical eruptions, from the 9th century, are known. A lava flow may have reached the sea during an eruption in 832 AD. Tenjo Mt. lava dome was formed during a major eruption in 838 AD that also produced pyroclastic flows and surges. Earthquake swarms took place at Kozu-shima during the 20th century.

1.3 Materials

Successive eruptions of rhyolite formed many lava domes on this island. 4 lava domes are Tenjo Mt., Kobe Mt., Ohsawa Mt., and Awano-mikoto Mt. These lava domes have similar dimensions. They are all composed of biotite rhyolite, with a mineral composition of 10-15% phenocrysts such as quartz, plagioclase, and biotite, and 85-90% glassy groundmass with flow structures, and chemical composition of SiO2 and Al2O3.

2. Geomorphic and Geologic Features of Mt. Tenjo

i. Stopping point 1
Fig. 3 Lava dome of Tenjo volcano

Fig. 3 Lava dome of Tenjo volcano

Fig. 4 Rock Hardness Test by Schmidt rock hammer

Fig. 4 Rock Hardness Test by Schmidt rock hammer

Crumbling breccia was resulted from Mount Tenjo lava dome. Rhyolite is the kind of this lava with sticky and stiff characteristics. Schmidt-rock hammer rebound value (R-value) was measured randomly with value of about 30%.

ii. Stopping point 2
Fig. 5 Cave at Lava dome (about 300 m from point 1)

Fig. 5 Cave at Lava dome (about 300 m from point 1)

This lava dome is same with stopping point-1 (also Mount Tenjo Lava dome, see pic. 1.3). But a cave found here is created by natural wind process. Thermometer is also available here with the value of 31 deg. Celsius (at that time). The hardness value is about 25-30% here.

iii. Stopping point 3
Fig. 6 Hardness test (elevation +500 m)

Fig. 6 Hardness test (elevation +500 m)

At this stopping point, after hiking a little bit steep of mount Tenjo, we consolidated about our actual position from learning the available map. The rock has about 25% of hardness value.

iv. Stopping point 4
Fig. 7 Rocks at elevation +500 m

Fig. 7 Rocks at elevation +500 m

At this place, Mount Tenjo lava dome, there are so many rocks found around here and absolutely came from Mount Tenjo lava.

v. Stopping point 5
Fig. 8 One of Tenjo Mt. Lava domes with beautiful view

Fig. 8 One of Tenjo Mt. Lava domes with beautiful view

Fig. 9 View from Tenjo Mt. (one of the New 100 views of Tokyo)

Fig. 9 View from Tenjo Mt. (one of the New 100 views of Tokyo)

We stopped at this point where we can see a beautiful view, extending beyond the peak of Mt Kushigamine to the Izu islands (Pic. 1.8), which has been selected as one of the “New 100 view of Tokyo” on account for its exceptional beauty. Pic 1.7 showed the Kushigamine pumice cone.

vi. Stopping point 6
Fig. 10 Dried Fudoike Pond

Fig. 10 Dried Fudoike Pond

At Mount Tenjo there is a pond, called Fudoike Pond, that has a shrine at the center of the pond. It dries up from time to time. The scarcity of water in the pond serves as indicator of the year’s weather condition.

vii. Stopping point 7 (Land slide)
Fig. 11 Slope near the peak of Tenjo Mt.

Fig. 11 Slope near the peak of Tenjo Mt.

Fig. 12 Unique rock at the peak of Tenjo Mt.

Fig. 12 Unique rock at the peak of Tenjo Mt.

A unique rock was found at around top of Mount Tenjo. It has black color at top, a thin white layer below the black one, and gray layer at bottom of rock.

3. Repair and Maintenance Work after Natural Disaster

Kozushima Island has a good management system in mitigating and maintain the possibility occurred during and after natural disasters. We can see from the availability of channel (pic. 3.1), used to flow the material dissolved in water from higher place to the ocean, steel frame layer support (pic. 3.2), a good retaining wall (pic. 3.3 and 3.4), and also water breaker for high wave (pic. 3.5).

Fig. 13 Lava flow channel

Fig. 13 Lava flow channel

Fig. 14 steel frame layer support attached at cliff

Fig. 14 steel frame layer support attached at cliff

Fig. 15 Strong retaining wall

Fig. 15 Strong retaining wall

Fig. 16 Another type of retaining wall

Fig. 16 Another type of retaining wall

Fig. 16 Wave breaker structure

Fig. 16 Wave breaker structure

All of those have to be checked or repaired after the natural disaster. Bellows are derived some explanation of maintenance works:

a. Debris cleanup: Emergency cleanup of trees, branches, leaves and other debris from council maintained public areas such as roads, access ways, parks, drains (including mud from man-made surfaces). It does not include debris from private property (including rubbish piled on public property from residences and businesses).

b. Tree replacement: Trees need to be replaced to maintain the trees-cape and ambiance of an area.

c. Storm-water/breakwater assets: Restoration of storm-water/breakwater assets is eligible.

d. Retaining walls and rock protection: Retaining walls and rock protection are eligible items.

e. Recreational facilities and playground: Facilities and equipment at council owned parks and reserves and sporting fields is eligible for assistance.

f. Emergency works to prevent public health risk: Emergency repair work of a temporary nature in order to restore essential services (such as water supply and sewerage works) may be eligible if undertaken to prevent an immediate and extreme public health risk to the public.

g. Other drainage works: Restoration of head-walls and man-made structures such as channel or tunnel is needed.

4. Closing

Field trip or outside learning activity is a kind of brief observational activity or a longer more sustained investigation or project offering an opportunity for people to get exposure to real nature, society, and events that they are not normally exposed to during daily activity. Absolutely, Kozu Island field trip is an example. Indonesia, consisting of many similar places, needs to promote more and more to its people. Not only to enrich the knowledge but also to arise the sensitivity to nature or human.

Mitigation system founded in Kozu Island is very well-managed and well-organized. It can be verified from facilities at shoreline (tripod or water breaker) to retaining wall or Steel frame layer support attached at cliff. These comprehensive facilities reveal the government responsibility in maintaining and developing this island to not only protects people but also a good tourism or education destination.

written by: -FHP-


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