The Intermontane plate was surrounded by a chain of volcanic islands called the Intermontane Islands, which had been accumulating as a volcanic chain in the Pacific Ocean since the Triassic period, beginning around 245 million years ago.
The name comes from the Slide Mountain terrane, a region made of rocks from the floor of the ancient ocean.
On the continent, subduction supported a new volcanic arc that again sent intruding granite-type rocks into the ancient continental sediments.
Eventually, about 180 million years ago in the middle Jurassic, the last of the microplate subducted, and the Intermontane Islands collided with the Pacific Northwest.
The subduction zone of the Intermontane plate shut down, ending the volcanic arc.