Cal Geographic

Pillow Basalt of the Black Mountain submarine volcanic accreted terrane, Marin County, R. Forest

Accreted Terrane:

Accreted terranes are the blocks of continental fragments and oceanic islands that have collided with a continent and are now permanently attached. All continents, including North America, tend to be older in their interiors and grow outward over geologic time, as terranes are added to the edges of the continent.

A portion of a plate added to a larger block of crust along a convergent plate boundary

Far traveled geologic blocks accreted to a continent

Accretion can refer to the theory that continents have increased their surface area during geological history by the addition of marine sediments at their boundaries via tectonic collision with other oceanic or continental plates

(Definition references are listed at the bottom of the Geomorphology Word of the Day home page)

Where in California:

For millions of years exotic terranes accreted on to the western boundary of the North American plate with the convergence and subduction of the Farallon Plate beneath it. From the Jurrasic to the Miocene, California was constructed from east to west as geologic terranes, such as chunks of oceanic crust and sediments, as well as islands and other landforms, conveyed-in and deposited to the continent’s edge.

Today these unique geologic masses can be identified, classified, and linked to disparate but related terranes across the state, from the Sierran foothills (such as the Foothills and the Northern Sierra Terranes; the Shoo Fly and Calaveras Complex); the Klamath Mountains (made up almost entirely of distinct accreted terranes such as the Galice, Rattlesnake Creek, Josephine Ophiolite, Sawyers Bar, the Yreka, the Trinity, and the Eastern Klamath Terranes); to the Coast Ranges (Terranes of the Franciscan and Great Valley Complex).