東尋坊（とうじんぼう）は、勝山市にある平泉寺の僧侶の名前で、悪行が過ぎ、仲間の僧侶達にここから突き落とされたことからその名がついたと言われています。海岸線に約１キロ続く輝石安山岩の柱状節理は、約1,300万年前、堆積岩層に入り込んだマグマが冷えて固まり、柔らかい堆積岩だけが日本海の荒波に削られてできたもので、国の天然記念物や日本の地質百選に選ばれています。
柱状節理の断面は六角形のものが多く見られます。六角形は、ハチの巣のハニカム構造で知られるように最小の面積で最大の強度をもつ形で、逆に岩の収縮時の割れ目形成には最小の力で済むため、この形が多いと考えられています。
タスク１．あなたが見つけた最も美しい六角形の写真を送ってください。
柱状節理の断面積は冷却速度が速いほど小さくなります。マグマは周囲の堆積岩によって冷やされるので、柱状節理の断面積からマグマ流入時の中心部を推定できます。
タスク２．座標のある大池と遊覧船乗り場のある千畳敷のどちらが中心部に近かったと考えられますか？
世界で有名な柱状節理には、アメリカのデビルズタワーやアイルランドのジャイアンツ・コーズウェイがありますが、これらはいずれも玄武岩でできています。玄武岩は、安山岩と同じく地上近くで形成された火山岩ですが、有色鉱物の含有率が多く黒っぽいです。
タスク３．東尋坊の柱状節理を観察して、その色を教えてください。
Tojinbo is the name of a monk from Heisenji Temple in Katsuyama City. The name came from the story that Tojinbo was pushed off from the cliff by his fellow monks after committing some bad deeds. The columnar joints of pyroxene andesite that extend about 1 km along the coastline were created about 13 million years ago when magma that entered the sedimentary rock layer cooled and solidified, and only the soft sedimentary rock was carved away by the rough waves of the Sea of Japan. It is a national natural monument and one of Japan's top 100 geological sites.
The cross section of columnar joints is often hexagonal. A hexagon is a shape that has the greatest strength with the smallest area, as is known from the honeycomb structure of a beehive, and conversely, it requires the least amount of force to form cracks when a rock contracts.
Task 1. Send a picture of the most beautiful hexagon you find.
The cross-sectional area of columnar joints becomes smaller as the magma cools faster. Since magma is cooled by the surrounding sedimentary rocks, the central area at the time of inflow can be estimated from the cross-sectional area of the columnar joints.
Task 2. Which do you think was closer to the center, Oike where the coordinates are located or Senjojiki where the pleasure boat pier is located?
The Devils Tower in the United States and the Giant's Causeway in Ireland are also famous for columnar joints. However, both of which are made of basalt. Basalt is a volcanic rock that was formed near the ground like andesite, but it is dark in color due to the high content of colored minerals.
Task 3. Please look at the columnar joints at Tojinbo and answer their color.