The South Island is the largest landmass of New Zealand. It is divided along its length by the Southern Alps. There are 18 peaks over , the highest of which is  / Mount Cook at . Fiordland's steep mountains and deep fiords record the extensive ice age glaciation of this southwestern corner of the South Island. The North Island is less mountainous but is marked by volcanism. The highly active Taupō Volcanic Zone has formed a large volcanic plateau, punctuated by the North Island's highest mountain, Mount Ruapehu (). The plateau also hosts the country's largest lake, Lake Taupō, nestled in the caldera of one of the world's most active supervolcanoes.