The Mariana chain, of which Guam is a part, was created by collision of the Pacific and Philippine Sea tectonic plates. Guam is 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 Mariana chain of which Guam is a part was created by collision of the Pacific and Philippine Sea tectonic plates Guam is 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 339 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