Volcano Field Trip


Explore of traces of a primitive submarine volcanic activity
 
Many years ago submarine volcanic activities were prosperous all over the seashore from the Shakotan Peninsula to Otaru district of Southewest Hokkaido and their traces have been preserved very well. Furthermore, since this area has upheaved and become land now, we can observe the area of eruption in the seabed without moving from our seat. 
 
The Oshoro Peninsula(@)
 
Here, a variety of occurrences are observed such as the pillow lava of basaltic andesite, hyaloclastite, feeder dyke and many others of the Miocene Tertiary period about 6 million years ago.
When less viscous (fairly fluid) and high temperature magma extrudes onto the seafloor, it will become spherical (round) because of surface tension and forms pillow lava with a chilled margin. During formation, the outer surface of each lobe chills instantaneously. However, the constant injection of lava may cause a single lobe to swell, thus cracking the glassy rind, allowing a new lobe to bud forth. As the interior of a pillow cools, it forms radial joints (cooling joints that radiate inward, perpendicular to the cooling surface).

Pillow lava Hyaloclastite

Quenching and fragmentation of lava produces an accumulation of angular, glassy fragments called hyaloclastite. It may form the matrix surrounding pillow lavas.
A mushroom-shaped intrusion is called a Feeder dyke which is an exit of the seafloor when magma is extruded.

Feeder dyke

 

The main volcanoes in Hokkaido


Active volcanoes are those that have erupted or are showing signs of unrest (such as unusual earthquake activity or significant new gas emissions) within the past 10,000 years. There are about 800 active volcanoes all over the world and 10 % or more of them, 108 active volcanoes, are in Japan. 19 of them are in Hokkaido and therefore Hokkaido belongs to an active volcanic zone. We can see 5 active volcanoes in this field trip, namely Niseko, Yotei, Usu, Eniwa
and Tarumae. Usu and Tarumae are considered A rank volcanes and thus are most likely to be active in the future and must be under constant observation. These volcanoes woke up from their long sleep about a thousand to several thousand years ago (in the 17th century) and resumed activity together. 

I hope you will be able to understand the frightfulness of the volcanic activity of present and the past through this field trip. Then, we will simply introduce the main volcanoes which are seen from a car window along the route.


Niseko(Niseko Annupuri 1308m, Iwaonupuri 1116m)(A)

About 2 million years ago, Niseko volcano group became active and was characterized by eruptions of andesitic lava flow and lava domes. The highest peak of Niseko volcano group is Niseko Annupuri which is at the easternmost end and the foot of the mountain is a famous winter mountain resort. The most recent eruption took place at Iwaonupuri estimated about 6000 years ago. In contrast with other volcanoes, it shows its skin and a sulphuric crystal is observed everywhere.
Niseko Annupuri
 

Yotei(1898m)(B)

Yotei is called "Ezo Fuji" and is an almost perfectly symmetrical stratovolcano. There is a crater with a diameter of 700m at the top. Volcanic activity was started from about 50,000 〜 60,000 years ago and pumice, volcanic ash and lava flow were repeatedly erupted. It was built mainly by the 3 eruptions of lava flow. Although there is no record of an eruption, it is considered that a volcanic activity took place within the past 10,000 years. There is no gas emission activity now.
Yotei

Here, a lava flow erupted at the 2nd stage can be seen. You can observe a columnar joint, formed by cooling and contraction, of the lava flow. Column size generally decreases as cooling rate increases, thus, the upper and lower side of the lava has small column.
Lava flow of Yotei


Lake Toya

Toya is a caldera with a diameter of about 10km and has Nakajima lava domes in the center of the lake. The maximum depth is 179km. About 100,000 years ago a large eruption took place and Toya erupted a large amount of pyroclastic flows. The caldera was partially filled by a lake after it collapsed. The time of pyroclastic flows formed extensive pyroclastic platforms around the Yotei and Date district.
About 40,000 years ago volcanic activity resumed at the center of the caldera and formed Nakajima lava domes which consists of andesitic lava domes. 4 of the lava domes seen in the center of the caldera are extinct volcanoes.
The height of Nakajima lava domes from the surface of the lake is 455m but the height becomes about 630m if it is measured from the bottom of the lake. Now you can understand that the Nakajima lava domes themselves are a big volcano.


Usu(732m)(CD)
 
Usu is a post-caldera cone situated on the south of Toya caldera facing Volcano Bay. It consists of a stratovolcano(a base diameter is about 6km with an elevation of about 500m)having a somma(a diameter of 1.8km)with 3 lava domes(Ko-Usu,O-Usu,Showa-Shinzan) and many cryptodomes at its summit and on its flanks and pyroclastic cone at the northeastern foot.

Usu Showa-Shinzan

@ Somma Formation
From 20,000 to 10,000 years ago there were many eruptions on the south of Toya caldera forming the stratovolcano like Yotei. This volcanic edifice consists of basaltic-andesitic lava flow and scoria. The pyroclastic cone called Doncoro-yama at the northeastern foot of Usu is considerd to be formed during this period.
 
A Collapse Of A Volcanic Edifice And Formation Of Mud-flow Hill
At the eruption of from about 7,000 to 8,000 years ago, the summit of the stratovolcano collapsed violently and generated a large debris avalanche into the south foot of Usu and formed many mud-flow hill of various sizes around Usu district.
 
B Activity In History
Usu woke up from a long sleep in the 17th century and resumed activity. The composition of magma has changed to more viscous rhyolitic-dacitic magma and its eruptive activitiy has come to repeat a plinian-type explosive eruption generating pyroclastic flows. The culmination of this activity was the formation of a lava dome. Usu has erupted 9 times since 1626.
The Nishi-yama crater walk path has preserved the disaster in order to use a part of the Usu 2000 eruption as teaching materials of volcano study. The threat of the volcano (such as a crater which is rising ash plume), a disrupted road, buildings which were destroyed, etc. can be seen directly there.
 
Date of eruption Dormant period(year) Dome which is formed
  1663   Ko-Usu(Lava dome)
 17th century? about 30
  1769 about 70
  1822 52 Ogari-yama(Cryptodome)
  1853 31 O-Usu(Lava dome)
  1910 57 Meiji-Shinzan(Cryptodome)
  1943〜45 33 Showa-Shinzan(Lava dome)
  1977〜78 32 Usu-Shinzan(Cryptodome)
  2000.3.31 22 Cryptodome



B Eruption 2000
Submerged R230
A swamp formed by damming up
a river after the upheaval of land
A town road changed to resemble
stairs due to a crustal movement
The first observation platform of
Nishi-yama crater
Trees which were bended and buried
Factory of Wakasaimo which
collapsed
a road which was split into different levels by a crustal movement



Lake Kuttara(Hiyori 377m)

Lake Kuttara Hiyori
About 40,000 years ago a large-scale pumice eruption was repeated and the nearly perfect circular caldera (Kuttara) with a diameter of about 3km was formed. Hiyori is a post-caldera cone which was erupted about 10,000 years ago and is situated on the west of the lake. The Noboribetsu thermal features (Jigokudani and Oyunuma) are an explosion volcano group, which are formed by phreatic eruption. They currently exhale vigorous hydrothermal activities. It turns out that a single phratic eruption occurred from a flank vent in 1820. The eruption was dated by tephrochronology.



Lake Shikotsu

Lake Shikotsu & Eniwa
Shikotsu is a caldera with a diameter of about 12km. The altitude of the lake is 248m above sea level, the maximum depth is 360m. This means the bottom of the lake is 112m deeper than the surface of the Pacific. The quantity of water is equal to Lake Biwa and is the 2nd deepest lake in Japan. It is situated at the northern limit of a none-frozen lake(ice-free lake). About 30,000 years ago the volcano erupted a large amount of ashfall and pyroclastic flows. Most of the caldera is now filled with a lake. The large pyroclastic flows at this time had surged over the southern part of Sapporo and formed a rock called a welded tuff. This rock is mined by the name of "Sapporo-Nanseki".

Eniwa(1320m)
 
About 15,000 years ago a large plinian eruption generated extensive pumice and ash fall. Eniwa became dormant after eruption 2000 years ago untill the phreatic eruption of 17th century. Ash plume is still rising from here. The steep slope became the downhill course in the Sapporo Olympics in 1972 and the destruction of the virgin forests by logging became a big problem.
 
 
Tarumae(1041m)
The top is surrounded by the somma about 1.2km N-S and 1.5km E-W and a low central cone is in it. A lava dome with a height of about 130m was created by the 1909 volcanic activity. In the Holocene, Tarumae has had three pre-historic and 34 historic eruptions. The eruptions in 6950 BC and 800 BC were very large. There were two more very large historic eruptions in 1667 and 1739. Both of these historic eruptions produced pyroclastic flows. The most recent eruption, in 1982, was a phreatic eruption.



Explore of the past of lake shikotsu

Tarumae (an active volcano of A rank) is close to lake Shikotsu. All of the eruptions in history have been explosive at the summit. What do you think will happen to this district where Doo expressway, Tomakomai and Chitose Airport are adjoining if a large-scale eruption occurrs in the future? I think it is not so difficult to imagine because we could see an example of the Usu 2000 eruption. We are anxious about a nationwide unprecedented city type disaster that would span Ground, Marine and Air.
Here we are going to see a scratch of the past eruption of Shikotsu caldera 30,000 years ago. We can see again how fierce the nature was compared to the Usu 2000 eruption. 
 
Gozensui area (E)  

Here, a large-scale outcrop of volcanic ash with a height of 20m or more can be seen. All the volcanic ash here was generated by the eruptions of Shikotsu caldera and volcanoes associated with Shikotsu about 30km away from here. The lower part with a little bit pink, the upper with yellow and the top with reddish volcanic ash was from Shikotsu caldera, Eniwa and Tarumae respectively.

You can find several blackish charcoal fragments which are completely buried under the lower part of the volcanic ash and pumice signified by being a little bit pink. This is called a carbonized wood. This material has an exceedingly interesting story to tell. The fact that the logs were changed to natural charcoal forces the conclusion that the ash and pumice generated by the eruption of Shikotsu was still exceedingly hot when it came to rest and a very terrific volcanic explosion occurred.


Shimamatsu area(F)

Here, an outcrop of rock about 10m thick sandwiched between two ash layers can be seen. If you observe the rock carfully, you can find flattened pumice fragments in it. This rock is called a welded tuff mentioned in the topic of "Lake Shikotsu". This rock was formed during a violent eruption of Shikotsu caldera. When the pyroclastic flow came to rest, it was still partially molten because it was still extremely hot, so that the droplets became attached, or "welded" to one another in the center of the overburden with the weight of overlying tuff flattening the pumice fragments.