Geology of Yakushima
Yakushima, a small island, roughly 130km in circumference has the highest mountains in Kyushu, while less than 20km away the neighbouring Tanegashima barely rises above sea level. Why the extreme difference? The answer lies in the geological structure of the island.
Both Yakushima and Tanegashima share the same base rock – the Kumage group. The Kumage group is a sedimentary group of rocks that began to accumulate 60 million years ago from sand and mud deposits built up on the ocean floor.
For millions of years the islands of Tanegashima and Yakushima were formed in similar ways until around 15.5 million years ago when molten granite 10km below Yakushima was formed and began to be forced upwards on a tectonic plate movement. The granite began to intrude into the Kumage group stratum and pushed it up just below the island. The subterranean granite rose at a steady 1 metre per thousand years (and continues to rise, but at a slower rate), eventually creating the dramatic mountain landscape that exists today. 15 million years later, Tanegashima remains low and is still made up of the older shale and sandstone, while the majority of Yakushima consists of granite. Granite is a hard, granular, igneous rock characterised as containing at least 20% quartz. However, the Yakushima granite is soft and is (fortunately!) not a good building material due to its very rough consistency and large, white potassium feldspar crystals.
If you make the hike up to the 'Okudake' central mountains (see the Miyanouradake hike) you will be stunned by the surreal landscape of huge rocks that seem to have fallen on to the mountain tops. These monoliths are actually formed from granite that has been exposed to millions of years of erosion and weathering.
Closer to the coast you can visit the Senpiro waterfall to see how its force has shaped the huge granite banks on either side of the waterfall. So impressive were/are some of these monoliths that they were/are deemed to contain the gods of the island and annual pilgrimages undertaken by the islanders make their way to these sacred rocks .
Yakushima is not a volcanic island, but there are a few lava fields on the island, particularly on the Tashiro Coast (田代海岸) on the east of the island. The lava outcrop has been given the name 'Pillow shaped lava field' due to it apparently looking like bundles of pillows. The lava came from a huge eruption to the south and became added on to the Kumage group due to tectonic plate movement.