The Intermontane Plate was an ancient oceanic tectonic plate that lay on the west coast of North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 about 195 million years ago. The Intermontane Plate had a chain of volcanic islands called the Intermontane Islands

Intermontane Islands

The Intermontane Islands were a giant chain of active volcanic islands somewhere in the Pacific Ocean during the Triassic time beginning around 245 million years ago. They were 600 to long and rode atop a microplate known as the Intermontane Plate...

. The Intermontane Islands had been accumulating as a volcanic chain in the Pacific Ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 since Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 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

Insular Plate

The Insular Plate was an ancient oceanic plate that began subducting under the west-coast of North America around the early Cretaceous time. The Insular Plate had a chain of active volcanic islands that were called the Insular Islands...

 was sinking. This arrangement with two parallel subduction zones is unusual. The modern Philippine Islands are located on the Philippine Mobile Belt

Philippine Mobile Belt

The Philippine Mobile Belt is a complex portion of the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate, comprising most of the country of the Philippines. It includes two subduction zones, the Manila Trench to the west and the Philippine Trench to the east, as well as the...

, one of the few places on Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 where twin subduction zones exist today. Geologists call the ocean between the Intermontane islands and North America the Slide Mountain Ocean

Slide Mountain Ocean

The Slide Mountain Ocean was an ancient ocean that existed between the Intermontane Islands and North America in the Triassic time beginning around 245 million years ago. Its name origin comes from the Slide Mountain Terrane, a region made of rocks from the floor of the ancient ocean...

. The name comes from the Slide Mountain Terrane

Slide Mountain Terrane

The Slide Mountain Terrane is a late Paleozoic terrane made of a complex of oceanic rocks in northern and southern British Columbia, Canada. These oceanic rocks were derived from the seafloor of the prehistoric Slide Mountain Ocean, the Intermontane Plate, which was subducted under the North...

, a region made of rocks from the floor of the ancient ocean.

The Intermontane Islands collide

Over early Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 time, the Intermontane Islands and the Pacific Northwest

Pacific Northwest

The Pacific Northwest is a region in northwestern North America, bounded by the Pacific Ocean to the west and, loosely, by the Rocky Mountains on the east. Definitions of the region vary and there is no commonly agreed upon boundary, even among Pacific Northwesterners. A common concept of the...

 drew closer together as the continent moved west and the Intermontane Microplate subducted. On the continent, subduction supported a new volcanic arc that again sent intruding granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

-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 Intermontane Islands were too big to sink beneath the continent. The subduction zone of the Intermontane Plate shut down, ending the volcanic arc. As the Intermontane Belt

Intermontane Belt

The Intermontane Belt is a physiogeological region in the Pacific Northwest of North America, stretching from northern Washington into British Columbia, Yukon, and Alaska. It comprises rolling hills, high plateaus and deeply cut valleys. The rocks in the belt have very little similarities with the...

 accreted to the edge of the continent

Continent

A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

, the subduction zone of the Insular Plate became the active subduction zone along the edge of the continent.

External links

• Burke Museum - University of Washington