 As the Intermontane Plate drew closer to the pre-existing continental margin by ongoing subduction under the Slide Mountain Ocean, the Intermontane Islands drew closer to the former continental margin and coastline of western North America, supporting a pre-existing volcanic arc on the former continental margin of North America.As the North American Plate drifted west and the Intermontane Plate drifted east to the ancient continental margin of western North America, the Slide Mountain Ocean eventually closed by ongoing subduction under the Slide Mountain Ocean.The subduction zone eventually jammed and shut down completely about 180 million years ago, ending the volcanic arc on the ancient continental margin of western North America and the Intermontane Islands collided, forming the Intermontane Belt.