2.9A Terranes (NOT terrain): What is the Difference? Mountain Building Plate Tectonics

Mountain belts can develop as a result of the collision and merger of an island arc, or some other small crustal fragment, to a continental block. The process of collision causes the accretion, or joining together, of comparatively small crustal fragments to a continental margin. This accretion process has generated many of the mountainous regions rimming the Pacific.

Playlist of Geology and Earth Science Videos from Snow Mountain
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Geologists refer to these accreted crustal blocks as terranes. Simply, the term terrane refers to any crustal fragment that has a geologic history distinct from that of the adjoining terranes. Terranes come in varying shapes and sizes.

The term terrane is used to designate a distinct and recognizable series of rock formations that has been transported by plate tectonic processes. Since geologists who mapped these rocks were unsure where they came from, these rocks were sometimes called exotic, suspect, accreted, or foreign terranes.

What is the nature of these crustal fragments, and from where do they originate? Research suggests that prior to their accretion to a continental block, some of the fragments may have been micro-continents similar to the present day island of Madagascar, located east of Africa in the Indian Ocean. Many others were island arcs similar to Japan, the Philippines, and the Aleutian Islands. Still others may have been submerged crustal fragments such as those occurring on the floor of the Western Pacific. More than 100 of these comparatively small crustal fragments are presently known to exist. Their origins vary. Some are submerged fragments consisting mainly of continental crust, whereas others are extinct volcanic islands, such as the Hawaiian Island and Emperor Seamount chain. Still others are submerged oceanic plateaus created by massive outpourings of basaltic lavas associated with hotspot activity.

What are some modern-day examples of material that may wind up as Terranes in the future? The southwestern Pacific Ocean is a good place to find pieces and fragments of land that may one day become terranes. Here there are many island arcs, oceanic plateaus, and microcontinents that will likely become accreted to the edges of a continent.

One of the best known terranes is the area west of the San Andreas fault, which includes southwest California and the Baja California peninsula of Mexico. Already named the California Terrane, it is moving to the northwest and will probably detach from North America in about 50 million years. Continued movement toward the northwest will bring it to Southern Alaska, where it will become the next in a long procession of terranes that have rafted toward and docked against Alaska during the past 200 million years.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ