If you ever get a chance to go to the island of Yakushima off the southern coast of Kyushu, Japan, you will immediately notice the geology. With over 46 mountains over 1,000 metres, it is no wonder that it is called the “Alps of the Sea”. If you are interested in how this island was formed, you do not have to look any farther than the rocky southern coast.

Formation of Yakushima

Yakushima and the nearby island of Tanegashima were covered by the ocean approximately 40 million years ago. As the Asian continent moved around, sediment flowed from the land into the sea, creating layers of sand and mud. These layers were compressed and eventually had enough material to rise above the sea.

Approximately 15 million years ago, they started to differ. While Tanegashima remained unaffected by the resulting changes, 18 km (11.2 miles) away, Yakushima was affected by various eruption events. A fault had developed in the formation layers, and magma moved closer to the surface, hardening into granite. It is believed that every 1,000 years, the island rose another metre (3 feet) as the granite pushed the original layers towards the sky.

Geology

While granite sounds very mundane, Yakushima’s granite is actually a rarity. Yakushima has no active volcanoes, and this type of granite, much of which is now exposed to the open air due to erosion, is one of the rarest types due to its composition. Within the granite are rare feldspar mega crystals that are anywhere from 6 to 15 cm (2.4 to 5.9 inches) long. These, along with the abundance of quartz and normal feldspar, give the granite a white colour.

Granite monoliths have also formed on the island. In fact, granite covers over 90 percent of the island, with sandstone and shale (the original layers) and hornfels (rock formed from sandstone and shale coming into contact with granite and heated to over 700 degrees Celsius (1292 degrees Fahrenheit) covering the rest.

Rocky coasts

If you like stunning rocky cliffs, parts of the southern and eastern coasts are where you should head. One example is Tashiro Coast, where the lava outcroppings look like pillows. An underwater volcano is believed to have erupted, spewing lava that eventually hardened. Over time, the Pacific Plate transported the hardened lava from its original location to Yakushima’s coast. People say the outcropping resembles pillows, hence the name “Pillow-shaped Lava Field” in English.

The southern, especially in the less-settled western section of the coast, has many rocky cliffs along which you can walk, feeling the power of the ocean and the solidity of the rock beneath your feet.

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