Intermontane Plate

The Intermontane Plate was an ancient oceanic tectonic plate that lay on the west coast of North America about 195 million years ago. The Intermontane Plate had a chain of volcanic islands called the Intermontane Islands. The Intermontane Islands had been accumulating as a volcanic chain in the Pacific Ocean since Triassic time, beginning around 245 million years ago. The volcanism records yet another subduction zone. Beneath the far edge of the Intermontane microplate, another plate called the Insular Plate was sinking. This arrangement with two parallel subduction zones is unusual. The modern Philippine Islands are located on the Philippine Mobile Belt, one of the few places on Earth where twin subduction zones exist today. Geologists call the ocean between the Intermontane islands and North America the Slide Mountain Ocean. The name comes from the Slide Mountain Terrane, a region made of rocks from the floor of the ancient ocean.