A field trip to Mt Omuro makes a good first introduction to volcanoes and volcanism for children. It is a small volcano, its crater peak is easy to get up to (via chairlift) and it has a neat cone shape and grassy slopes that makes it a beautiful volcano to look at. Click on the tabblo below to see all the photos of the volcano.
Mt Omuro or Omuro-yama is a tidy cone-shaped 300 m-diameter dormant volcano covered by pasture grasses with the charming appearance of an inverted miso-soup-bowl (it is said). The crater is hollowed out like a mortar (in which there is now an archery range).
Height of Omuro-yama: 580 meters above sea level. The crater is 70 meters deep, 300 meters across, and 1000 meters around. Visitors to the volcano take a chair lift to the peak from which there is a hiking trail that follows the rim of a large volcanic crater.
Is the Omuro-yama volcanic crater a caldera? What is a caldera?
The word caldera is derived from the Portuguese word for cauldron. A caldera is a circular depression in the earth that is larger than an ordinary crater. Craters usually do not exceed 1 kilometer. A caldera is much larger than that.
A caldera is distinguished from a crater because the large depression is not thought to have been created by a simple eruption or explosion. There are three types of calderas classified by the way in which they were created: explosive, non-explosive, and non-volcanic.
The Aso caldera is a famous example of a caldera in Japan, which extends 17 kilometers from east to west and 25 kilometers from north to south. It is a non-explosive caldera. It is one of the largest calderas in the world, roughly equal in size to the Aira caldera.
The Omuro-yama crater is only 300 meters across and thus cannot be called a caldera.
The 360 degree panoramic view that can be seen of Oshima, Izu Shichito Islands, the Amagi Chain, Mt. Fuji and the mountains extending to Hakone is touted to be the best in Izu peninsula. We were lucky to have fair weather and so the peaks around us were visible as was the expanse of lava plateau that stretched out below us.
The volcanics behind Omuro-yama volcano
Izu-Tobu volcanic field is a group of pyroclastic beds, a lava platform, shield volcanoes, stratovolcanoes and lava domes in Japan. Izu-Tobu consists of six or seven volcanoes and several partly destroyed craters or vents. All this lies within an area about 5 by 6 miles (8 by 10 km) wide just south of the city of Ito. Many vents erupted lava and pyroclastic material in this area building many small volcanoes instead of one large cone like nearby volcanoes.
If you take the trouble to take it all in during your hike of the crater rim, you should be able to easily see the lava plateau South of Ito city that was formed by flat lying lava and pyroclastic beds. Two cones, Komuro-yama as well as Omuro-yama the one we are standing in, rise above this area.
To the North, west, and southwest of the plateau, steep mountains made of Pleistocene volcanic rocks are found.
Most of the craters or vents of Ito are found on a thin zone running N 20 degrees E.
How the Izu volcanoes were formed
Volcano-building of Izu peninsula occurred under shallow sea.
The Quaternary volcanoes of the Izu Peninsula, northern Izu-Bonin arc, are aligned into two parallel, eastern and western, chains.
To the east: volcanic rocks of the Shirahama Group and the Umegi Formation are composed mainly of calc-alkali dacite or andesite and partially with basalt.
The Taga Volcano Group of the Izu Peninsula has four volcanoes (Older Taga, Atami, Shimo-Taga and Himenosawa) – each volcano consisting of basalt-andesite lavas and volcaniclastic sediments, mainly of low alkali tholeiite composition.
The activity of the Shimo-Taga Volcano is divided into three stages (early, middle and later):
During the first two stages of formation – rocks erupted from a zoned magma chamber consisting of lavas and volcaniclastic sediments of basalts and andesites.
During the later stage, the eruption was made up mainly of mudflow deposits consisting of volcaniclastic sediments.
The volcanic field of this group is scattered over a broad, plateau-like area of more than 400 sq km on the east side of the Izu Peninsula.
Omuro-yama is a scoria cone formed about 5,000 years ago as a result of the later stage and fourth volcanism. It is the most well-known feature of the Izu-Tobu volcano group (also known as the Higashi-Izu volcano group) and like the other volcanoes of the group is composed chiefly of basalt and andesite.
Omuro-yama volcano is considered by vulcanologists to be among the so-called Quaternary Volcanoes including the Hakone, Fuji, Taga, Yugawara, Amagi volcanoes.
Further notes:
While only one recent volcanic eruption has taken place in Izu-Tobu, many earthquakes have occurred in historic times in the Izu peninsula region. 4,880 earthquakes shook the area between February 13, 1930 and the end of April in the same year.
Most of the earthquakes originate in the northeast area of the thin Ito volcanic crater zone running N 20 degrees E. zone described earlier. Izu-Tobu has had five historic eruptions. The last of these eruptions occurred in 1989.
Earthquakes are also caused by plate tectonic reasons. Additionally, there occasionally occur so-called earthquake swarms offshore of Ito. These are coupled with movements of magma and volcanic eruptions.
The latest earthquake swarm occurred in April 2006. The one before was accompanied by the eruption of a submarine volcano just in front of Ito city on 9th July 1989. This was the first ever eruption in the history of Izu and the volcano was named as “Teishi kaikyu”.
The mountain-grasses-burning matsuri (festival) event that is held every 2nd Sunday of February is a spectacle that is popular with tourists.
There is also a park at the foot of the mountain called ‘Sakura-no-sato’. It contains over 3000 cherry trees of 35 different varieties, which bloom off and on from September to April.
Access: Omuroyama is located halfway down the east coast of Izu Peninsula. 50-60 minutes from Atami to Izu-Kogen. Mt. Omuro can be reached by bus from Ito Station (35 minutes) or Izu Kogen Station (15 minutes). By car, you can drive down Route 135 and take a sideway toward Omuroyama at the Gran-Pal park traffic light. There is ample parking for those who drive.
References:
Kuno, Hisashi, “Catalogue of the Active Volcanoes of the World Including Solfatara Fields: Part XI, Japan, Taiwan and Marianas,” International Association of Volcanology, Rome, Italy, 332 pp., 1962.
See aerial photo of Izu peninsula, mountains and volcanoes here
Aso Volcano Disaster Prevention Council (on “What is a caldera?”)
EDUCATION IN JAPAN COMMUNITY Blog 
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