These eruptions blanketed the area with thick layers of volcanic ash. This ash turned into a soft rock called tuff. Later, a harder basalt layer formed on top of the tuff. This created the unique ...

Jeju Volcanic Island and Lava ... the fortress-like Seongsan Ilchulbong tuff cone, rising out of the ocean, a dramatic landscape; and Mount Halla, the highest in Korea, with its waterfalls, ...