Region 6: Alaska

Alaska is a mosaic of accreted terranes, each with its own history (Figure 2.27). Due to the accretion of these terranes over time, Alaska today showcases a diverse array of mountain ranges and physiographical regions (Figure 2.28). The first rocks of Alaska were put in place when the Yukon-Tanana terrane collided with the western edge of North America 200 million years ago. These rocks are late Precambrian in age and are mostly composed of schist. The greenschist and blueschist of the southwestern Brooks Range show clear signs of having been formed at this convergent plate margin, as they contain minerals that form at high pressure but low temperature. The Brooks Range, Alaska’s northernmost mountain range, is heavily composed of the Yukon-Tanana’s ancient seabed, and contains many ancient marine fossils. Rocks on the northern side of the Alaska Range, Alaska’s central mountain chain, have also been correlated with schists from the Yukon-Tanana.

Figure 2.27: A cross-section of Central Alaska. Each section delineates a separate terrane.

Other late Precambrian and Paleozoic rocks of Alaska include gneisses of the Seward Peninsula, which extends from northwest Alaska and is a remnant of the Bering land bridge that connected Alaska to Siberia during the Pleistocene. The Seward Peninsula is actually composed of two terranes: the Seward terrane, which is made up of Precambrian schists, granites, and marbles, and the York terrane, which consists of Ordovician through Mississippian limestone, dolostone, and phyllite. The sedimentary rocks of the York terrane would have originally been deposited in a shallow marine environment before their accretion onto the North American plate. During the Cretaceous, thrust faulting deformed the York terrane, allowing the formation of intrusive granites.

In the late Cambrian to early Devonian, undersea volcanism resulted in large quantities of igneous material being mixed with fossiliferous limestone. As these units were transported to finally dock against North America, they accreted as part of the northern Alaska Range, and metamorphism created large quantities of carbonaceous schist.

Figure 2.28: Regions and ranges of Alaska.

The next major terrane, Stikinia, began as a volcanic island arc that formed on the rifted margin fragments of the Yukon-Tanana. Its formation would have been similar to the volcanic arc that makes up the islands of Japan today. As this island arc traveled towards a collision with Alaska during the Cretaceous, it scooped up marine shales, sandstones, and limestones that became jumbled up with the volcanic rhyolites, andesites, and basalts that formed the islands. This collection of varied, jumbled rocks eventually became part of the Alaska Range. The Taku terrane and the Tracy Arm terrane were the next to crash into Alaska, adding more metamorphic rocks such as gneiss, schist, and marble.

The Wrangellia terrane, born near the equator about 300 million years ago, didn’t dock with Alaska until 120 million years ago, so it contains rocks that tell a complex story of many years of volcanism, erosion, deposition, sinking, and uplift. Wrangellia’s defining unit is a 2500-kilometer (1553-mile) Triassic flood basalt, which would have been extruded onto the terrane’s landmass about 230 million years ago. Shallow seas later inundated the region, covering these volcanic rocks with layers of limestone and other marine sediments. Today, these flood basalts and their overlying sediments extend across the southern portion of the Alaska Range and the Wrangell Mountains. As Wrangellia moved toward North America, it also collided with a variety of other smaller terranes, compressing seafloor rocks against it and forming a complex fault system. This fault is expressed today as the Border Ranges Fault, which occurs throughout the Chugach Mountains at the southern edge of Alaska.

Each terrane was brought to Alaska on a subducting plate, and subduction is still occurring along the Aleutian Trench, resulting in the formation of the Aleutian Islands. The subduction creates stratovolcanoes and plutonic igneous intrusions (Figure 2.29) along the entire Aleutian Range. The stratovolcanoes produce rocks that tend to be intermediate in silica content, so andesite and dacite are common. Pyroclastic rocks from explosive events are also quite common. One spectacular example is found at Novarupta in Katmai National Park, where a volcanic eruption thirty times as large as Mt. St. Helens occurred in 1912. Geologists who investigated the aftermath of the eruption were awe-struck by the still-smoking tuff that covered the surrounding area, and named it the Valley of Ten Thousand Smokes (Figure 2.30).

Figure 2.29: Intrusive and extrusive igneous features.

Today, the Yakutat Block is the terrane currently accreting to Alaska, along the state’s south-central coast. Its convergence with Alaska is responsible for the volcanism evident in the Wrangell Mountains today. The largest volcano in the range, Mt. Wrangell, is unusual because it formed from massive andesitic lava flows that gave it a shield shape rather than the cone shape of a typical stratovolcano. The process that allowed andesite to form a shield volcano is poorly understood, but is likely related to the volume of ejected magma. Besides these andesitic lava flows, the rocks of the Wrangell Mountains include scattered cinder cones and rhyolite domes (Figure 2.31).

Figure 2.30: Novarupta Volcano in the Aleutian Range.

During the late Jurassic and early Cretaceous, rotation of the North American plate caused basins to open up in the Arctic, creating shallow seas that filled with sediments eroded from the Brooks Range. Today, these sediments form the upper source rocks of the Arctic Foothills and Arctic Coastal Plain, north of the Brooks Range. The shales here are rich with oil, formed as the sediments were deposited in a marine environment with high biological activity. Prudhoe Bay, at the edge of the Coastal Plain, is estimated to hold nearly 25 billion barrels of oil.

Figure 2.31: The Wrangell Mountains, showing (left to right) Mt. Drum, Mt. Blackburn, and Mt. Wrangell.

Southeastern Alaska, also known as the Alaskan Panhandle, expresses a slightly different geologic history than does the main body of the state. The core of the Coast Mountains is a granitic batholith, parts of which have been deformed by metamorphism to form schist, gneiss, and marble. The region contains a large fault system, which has increased uplift in many areas to expose more metamorphic rock. In addition, a jumble of marine sedimentary rock and conglomerates was accreted to the southeastern coastal area as terranes continued to dock and subduct.