Geomorphologists believe that the island of New Guinea is part of the Australian continent, both lies on Sahul Shelf and once joined via a land bridge during the Last glacial period. The tectonic movement of the Australian Plate created towering, snowcapped mountain peaks lining the island's central east-west spine and hot, humid alluvial plains along the coasts. The New Guinea Highlands range some east to west along the island, forming a mountainous spine between the northern and southern portion of the island. Due to its tectonic movement, New Guinea experienced many earthquakes and tsunamis, especially in its northern and western part.