Alaska lies on the North American Plate. The deep Aleutian Trench lies a few hundred miles to the south of Alaska. This is where the basaltic rocks of the Pacific Plate are subducting underneath North America. This collision zone is responsible for much of the geography of Southcentral Alaska: its active volcanoes, massive earthquakes, crumpled mountain ranges, and indirectly, some of its climate patterns.
To the north of Alaska, in the Arctic Ocean lies the rift zone that is separating North America from the Eurasian Plate. To the west, in an ill-defined location in Siberia, lies another relatively quiet border between the North American and Eurasian Plates.
North Pacific Plate Tectonic and Holocence Volcanic Activity Map
In the accompanying map, we can clearly see the path the Pacific Plate has followed as it has moved over the Hawaiian Hot Spot, a fixed location of intense heat in the underlying mantle. As the Pacific Plate moves over this spot, the plate melts and the Hawaiian volcanoes form. A chain of extinct volcanoes, the Emperor Seamounts, can be followed along the Pacific floor all the way to the Aleutian Trench, where they subduct under Alaska.
Alaska is made up of numerous geologic terranes. Terranes are blocks of continental crust that did not originate at the same time or place as the continent to which they are now attached. Almost all of Alaska originated elsewhere and over time these bits and pieces were moved by plate tectonics until they accreted onto the North American Craton; the ancient continental core that began forming over 3.5 billion years ago.
Most of Southcentral Alaska’s terranes originated far to the south as either volcanic island arcs or sedimentary material that was eroded off of these island arcs. These island arcs journeyed northward along the coast of North America, first being carried by the Farallon Plate, then by the Kula Plate, which was formed when the Farallon Plate split along a northeast-southwest axis. The Kula Plate has almost entirely been subducted underneath Alaska and its successor, the Pacific Plate, continues to bring future terrane material northward.
In Southcentral Alaska, the oldest terranes are located in the north, closer to the North American Craton, Generally the farther from the Craton, the younger the age of the terrane. The youngest terranes, the Prince William and Yakutat Terranes, are still in the process of accreting onto Alaska.
Major geologic faults such as the Denali Fault and the Border Ranges Fault are found along the edges of many of these terranes.
Southcentral Alaskan Terranes Map
Alaska Beneath Your
Feet
The geologic situation under Southcentral Alaska is fairly complicated. In the accompanying diagram, we can
observe several different things occurring. This diagram is a geologic cross-section along a line that
runs from Mount Iliamna, one of the Cook Inlet volcanoes, eastward across Cook
Inlet and the Kenai Peninsula and to the northeasternmost portion of the
Aleutian Trench in the Gulf of Alaska.
The Pacific Plate is subducting underneath the continental masses of the Peninsular, Chugach, and Prince William Terranes. At this location it subducts at a relatively shallow angle, so the trench is not very distinct. Because of the shallow subduction angle, the Pacific Plate does not heat up enough to melt until it is far beyond the subduction trench. The resulting volcanoes have formed in the Alaska Range on the east side of Cook Inlet, hundreds of miles away.
Large amounts of sediment accumulate on the ocean floor in a subduction zone. We can see how this sediment, rather than subducting is being crunched back up against the continent as an accretionary wedge, forming the Chugach and Kenai Mountains. Other sediments can be seen filling the Cook Inlet Basin between the Chugach Mountains and the Alaska Range. This basin has been filling up with continental sediments from rivers and glaciers since early Tertiary times.
In this diagram we can also see the remaining portion of the Kula Plate. This ancient and stagnant plate used to subduct under Alaska much as the Pacific Plate does today, but now nearly all tectonic activity has been relegated to the Pacific Plate. A portion of the Kula Plate remains at the surface in th southern Bering Sea.
Cook Inlet/Kenai Peninsula/Prince William Sound Geologic Cross Section