The main island is a tilted fault block, characterized by the contrast between the eastern two-thirds, consisting mostly of five rugged mountain ranges parallel to the east coast, and the flat to gently rolling plains of the western third, where the majority of Taiwan's population reside. There are several peaks over 3,500 m, the highest being Yu Shan at , making Taiwan the world's fourth-highest island. The tectonic boundary that formed these ranges is still active, and the island experiences many earthquakes, a few of them highly destructive. There are also many active submarine volcanoes in the Taiwan Straits.