Southern Alaska is a patchwork quilt of diverse pieces of land.

Alaska is made of many separate pieces of land

Southern Alaska is a patchwork of geologically distinctive crustal fragments separated by major fault systems. These fragments exist in all shapes and sizes, but each has a history of its own. All are exotic—that is, they were formed elsewhere and transported to their present position by the motions of crustal plates. Some have been rotated relative to their neighbors, and some have been displaced vast distances compared to less traveled nearby fragments. Thus, adjacent fragments generally differ in the characteristics of the rocks which constitute them—their lithologies, and they differ in the structural modifications that those rocks have undergone—their tectonic histories. Thus, these fragments are called lithotectonic terranes—or, in shorthand, just terranes.

In southern Alaska, juxtaposition of disparate terranes has created a collage on a grand scale. Collectively, the processes by which the collage was assembled (by which south-central Alaska has grown by the addition of exotic terranes over the past 200 million years or so) are termed accretionary tectonics. This process continues in Alaska today: the latest terrane to arrive—the Yakutat terrane—still is being displaced, jamming against and beneath terranes to the north and west.

Diverse Origins

The makeup of these terranes is diverse: some represent pieces of old continental crust, whereas parts of others consist of oceanic crust. Some fragments represent the remains of volcanic island chains formed in the open ocean (like the Aleutian islands). Others represent volcanic chains formed on the edge of a continent (like western South America). Some terranes consist almost totally of old eroded material from the edge of North America.

Where these Alaskan terranes were formed originally and how they came to be fragmented and brought to Alaska is obscure. Most are presumed to have been derived from in and around the Pacific Ocean basin. Some may have been brought as “rafts” propelled on a conveyor belt of converging oceanic crust. Given sufficient time, oceanic crust created by seafloor spreading has the potential to convey exotic terranes great distances from their places of origin. Other terranes may have been brought northward along faults near the edge of the continent. The Yakutat terrane provides a prime example, having been transposed 375 miles along the Queen Charlotte–Fairweather transform fault system in the last 30 million years.

In fact, it has been shown that so much of Alaska consists of displaced terranes that less than 1 percent of Alaska’s total area is an original part of the North American continent. It is against this ancient continental edge that all terranes to the south accreted, including all the rocks of Wrangell–Saint Elias National Park and Preserve. The park includes parts of seven terranes. From north to south, they are the Windy, Gravina Belt, Wrangellia, Alexander, Chugach, Prince William, and Yakutat terranes.