The Mariana chain of which Guam is a part was created by collision of the Pacific and Philippine Sea tectonic plates, with Guam located on the micro Mariana Plate between the two. Guam is the closest land mass to the Mariana Trench, the deep subduction zone that runs east of the Marianas. Volcanic eruptions established the base of the island in the Eocene, roughly 56 to 33.9 million years ago. The north of Guam is a result of this base being covered with layers of coral reef, turning into limestone, and then being thrust upward by tectonic activity to create a plateau. The rugged south of the island is a result of more recent volcanic activity. Cocos Island off the southern tip of Guam is the largest of the many small islets along the coastline. Guam's highest point is Mount Lamlam at above sea level. If its base is considered to be nearby Challenger Deep, the deepest surveyed point in the Oceans, Mount Lamlam is the world's highest mountain at .