The Insular Islands were a giant chain of active volcanic islands somewhere 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...

 during the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 time that rode on top a microplate 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...

, beginning around 130 million years ago. The Insular Islands were surrounded by two prehistoric ocean

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s; the Panthalassa Ocean

Panthalassa

Panthalassa , also known as the Panthalassic Ocean, was the vast global ocean that surrounded the supercontinent Pangaea, during the late Paleozoic and the early Mesozoic years. It included the Pacific Ocean to the west and north and the Tethys Ocean to the southeast...

 to the west and the Bridge River Ocean

Bridge River Ocean

The Bridge River Ocean was an ancient ocean that existed between North America and the Insular Islands during the Paleozoic time. Like the earlier Slide Mountain Ocean the Bridge River Ocean had a subduction zone on the ocean floor called the Insular Trench...

 to the east. About 115 million years ago, these islands collided with the North American continent, fusing onto the North American Plate

North American Plate

The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...

 and closing the Bridge River Ocean during the Mid-Cretaceous time.

The Insular Islands' formation had occurred at least 210 million years before the collision. Like the earlier 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 large size of the islands prevented them from being pushed under the North American Plate. Instead, the Insular Islands were crumpled and crushed along the shoreline of North America to become part of the continent. The Insular Plate stopped subducting under North America; the subduction zone moved to the Farallon Trench

Farallon Trench

The Farallon Trench was an ancient oceanic trench on the west coast of North America during the Late Cretaceous period. When the trench disappeared, it turned into the San Andreas Fault. Since then, it has spread out to the north and south....

 further westward. The rocks that comprised the Insular Islands now form a large geologic feature along the coast of North America known as the Insular Belt

Insular Belt

The Insular Belt is a physiogeological region on the north western North American coast. It consist of three major island groups and many smaller islands and stretches from southern British Columbia into Alaska and the Yukon...

.