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