Text Box: Alaska Introduction | Pacific Mountain System | Earthquakes | Pacific Mountain National Parks  | Volcanoes | Interior and Western Alaska | Brooks Range | Arctic Slope | Resources
Alaska, meaning Great Land or Mainland, is the largest of the 50 states:  its area of more than 586,000 square miles is nearly one-fifth that of the conterminous states.  The east-west extent of Alaska is equivalent to the distance from the Atlantic coastline of South Carolina to the  Pacific shore of California (2,100 miles), and it extends the same north-south distance as that between Canada and southern New Mexico (1,100 miles).  Alaska is more than twice the size of Texas and has about 10 times the area of Florida.
The topographical variation and geological structure of Alaska are perhaps the most complex of any state.  Occurring within its borders are extremely flat lowlands, 39 mountain ranges, the tallest mountain in North America, the 16 highest peaks in the United States and 17 of the tallest 20, and rocks belonging to nearly every period of geological time.  The state contains more than 100 volcanoes and volcanic fields–more than any other state, many of which have been active within the past 1.5 million years, including nearly 50 that have been active within historic time; the volcanoes within the state account for approximately 80 percent of active volcanoes in the United States and nearly 10 percent of active above-sea-level volcanoes on Earth.  (The Alaska Volcano Observatory, with offices in Anchorage and Fairbanks, was established in 1988.)  Alaska is considered to be the most seismically active and fastest deforming state in the country.  The state’s 33,900 miles of coastline (along the mainland and around the major islands) is 50 percent longer than that of the conterminous United States.  Several massive icefields and approximately 100,000 glaciers together cover approximately 30,000 square miles–about 5 percent–of the state’s surface. 
The continental shelf in Alaska is also complex and varies greatly in width.  Along the Alaska panhandle (southeastern portion of the state) and around the Aleutian Islands the shelf  is only a few miles wide.  It is about 120 miles wide in much of the Gulf of Alaska.  The shelf varies from a few miles to nearly 100 miles in width along much of the northern edge of the state.  Along the western and northwestern coastlines the shelf is wider than anywhere else in the country, in places extending outward more than 500 miles.
Geomorphic History:  Terranes
The topographical and geological complexity of the state relates primarily to the many terranes that have been assembled in the region over the past 200 million years.  Approximately 50 terranes have been identified within the state, with more probably waiting to be discerned, ranging in size up to several thousand square miles, one being perhaps about the size of California.  Some of the terranes may have originated from Asia, the Pacific Ocean, South America, and from south of the equator.  Most of these terranes have been brought to Alaska by the movement of the Pacific plate as it subducts beneath the state, which is on the North American plate,  in the Aleutian trench at a rate of about 2.6 inches per year.  Most of the terranes were added to the state during the Mesozoic, although the process continues in the southernmost portions of the state.  The Yukon-Tanana terrane, in the east-central portion of the state, docked about 200 million years ago and may contain the oldest rocks in Alaska, which are 600-800 million years old and mostly metamorphic, and resulted from metamorphism of shale and sandstone that were deposited about 2.2 billion years ago.  This section is extremely complex and the rocks indicate they may have been subjected to six separate metamorphic episodes.  The terrane that borders most of the southern edge of the Yukon-Tenana terrane in Alaska–Wrangellia–originated perhaps at 15° north latitude.  Much of this terrane consists of a very old volcanic island chain; today this volcanic material is overlain by shales and limestones.  It probably merged 150 million years ago with at least one other crustal mass, surmised to be what is now the Alexander terrane that composes much of the panhandle, before colliding with Alaska about 120-85 million years ago, producing significant metamorphism and intrusions of plutons.  Remnants of this terrane are also found in southwestern Canada and west-central Idaho.  One particularly complex terrane, the Chulitna terrane, west of Wrangellia, apparently arrived from Asia about 90 million years ago; it contains tropical fossils, oceanic limestone and basalt, as well as several non-contiguous layers of sandstone. 
The terranes that comprise the southern two-thirds of Alaska were transported mostly from the south, southeast and southwest.  The terranes in the northern third of the state, which are generally larger than those farther south, apparently had a very different origin.  About 145 million years ago an oceanic plate in the Arctic Ocean rotated counterclockwise away from the Canadian arctic islands and met the northern edge of Alaska where the present-day Kobuk River flows.  The interaction compressed the Yukon Valley in central Alaska by more than 100 miles and raised the Brooks Range within the state as well as mountains in Canada. 
Alaska Introduction | Pacific Mountain System | Earthquakes | Pacific Mountain National Parks  | Volcanoes | Interior and Western Alaska | Brooks Range | Arctic Slope | Resources