As the Mesozoic Era dawned, 225 million years ago, only northern and northeastern Alaska were part of North America. Since that time, numerous terranes and sub-terranes, have accreted to that primitive coast, producing the vast and varied landscape of present-day Alaska.
Many of these terranes, carried in on oceanic plates, were once part of other continents or sub-continents, having rifted away as new seaways opened. Others formed as volcanic island arcs above subduction zones, either along the expanding coast of Alaska or elsewhere. Still other terranes were composed of oceanic crust, trapped and lifted between colliding continental segments or scraped from the upper surface of a subducting oceanic plate. Those terranes arriving from other regions of the globe were composed of rock much older than the sedimentary or volcanic rocks that formed along their margins at the time of accretion and possess fossils that provide clues to their site of origin. For example, the Alexander Terrane of southeastern Alaska contains Silurian rocks that harbor marine fossils also found in the northern Ural Mountains of Siberia, suggesting that the two land areas once bordered the same sea.
As the various terranes assembled, from the Triassic Period to the present day, the collisions lifted mountain ranges; on either side of these mountain corridors, downwarping of the crust produced basins that, over time, filled with erosional debris from the adjacent highlands. In other areas, volcanism along subduction zones (which shifted southward with each new terrane accretion), produced volcanic ranges, basalt flows, igneous plutons, ash sediments and lava dikes that, today, are interspersed with the strata of the terranes. Fault lines remain active between many of the terranes (triggering intermittent earthquakes) and volcanism continues to mold the coastline, especially along the Aleutian Chain. Needless to say, the geology of Alaska is highly complex, a jumbled collage of strata that range in age from ancient Precambrian rock to Pleistocene gravels, that originated anywhere from the tropics to polar latitudes and that have since been lifted, folded, incised, buried and eroded by tectonic forces, streams and glaciers.
