The Intermontane Plate was an ancient oceanic

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

that lay on the west coast of North America about 195 million years ago. The Intermontane Plate was surrounded by a chain of

volcanic island Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

s called the

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 ...

, which had been accumulating as a volcanic chain in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contin ...

since the

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

period, 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 The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

are located on the Philippine Mobile Belt, one of the few places on

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

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

Slide Mountain Ocean The Slide Mountain Ocean was an ancient ocean that existed between the Intermontane Islands and North America beginning around 245 million years ago in the Triassic period. It is named after the Slide Mountain Terrane, which is composed of rocks f ...

. The name comes from the Slide Mountain Terrane, a region made of rocks from the floor of the ancient ocean.

Collision of the Intermontane Islands

During the early

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

period, the Intermontane Islands and the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Tho ...

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 () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

-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 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 ro ...

accreted to the edge of the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions In geography, regions, otherwise referred to as zones, lands or territories, are areas t ...

, 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
{{Tectonic plates Tectonic plates Historical tectonic plates Historical geology Natural history of North America