 Castor, Christensen and Hertha are the only vents which show evidence of subaerial activity in form of lava flows; elsewhere the nunataks are constructed by hyaloclastite and pillow lavas which form beneath ice.The volcanoes appear to align along northwest-striking lines.The Antarctic Peninsula was a site of subduction from about 200 million years ago until 4 million years ago, when a number of spreading ridges collided with the subduction zone starting from 50 million years ago at Alexander Island and caused it to cease.