Seamounts, Tectonic Plates, and Other Fun Stuff near SE Alaska
Southeast Alaska is on the western edge of the North American Tectonic Plate, and is very close to the world’s largest tectonic plate, the Pacific Plate. In the waters near SE Alaska, these plates slide against each other along the Queen Charlotte Fairweather Fault. This is a transform fault, more specifically a right lateral strike-slip fault. It is similar to the notorious San Andreas fault in orientation, length, and type. Several other faults are also involved.
Recent earthquakes on Queen Charlotte Fairweather Fault
Year Magnitude Location
------- ------------ --------------------
1927 7.1 N. Chicagof Island
1949 8.1 Haida Gwaii
1958 7.9 Lituya Bay
1972 7.4 SitkaYakutat Terrane is a key tectonic feature adjacent to SE Alaska. Its southern end is near Cross Sound at the northwest tip of Chicagof Island.
A terrane is huge mass of rock transported on top of an unimaginably humongous tectonic plate towards a subduction zone. An accretionary wedge is similar to a terrane but it is much smaller. During subduction, the accretionary wedge is scraped off (accreted) onto the edge of the continental plate. In the case of a terrane, when it contacts the continental plate during subduction, it is so massive that it causes immense friction and disrupts the subduction process. It becomes partially subducted under the edge of the continental plate but also blocks the plates from moving freely.
Imagine a hypothetical oceanic plateau that is riding on an oceanic plate and moving toward a subduction zone. As the oceanic plate is subducted under the continental plate, the oceanic plateau gets wedged and stuck at the subduction zone between the oceanic plate and the continental plate. Now it has become a terrane. The accretionary wedge that is already present on the edge of the continental plate will be uplifted, creating a mountain range.
This process is playing out in south central Alaska. Yakutat Terrane, covered by the Pacific Ocean, is moving north and colliding with the continental plate. This terrane (also called a block) is bounded on the north by the Chugach and St. Elias Mountain Ranges and is uplifting these mountains. The collision is also rupturing the St. Elias Chugach thrust fault and is linked to the huge Aleutian mega thrust fault. It is one of the primary forces driving the Denali Fault and causes earthquakes across the southern half of Alaska.
The rapid uplift of the St. Elias Mountains by Yakutat Terrane triggers an increase in volume of materials eroded from the mountains and glaciers. This process, continuing over eons, has deposited large amounts of gravelly sand along the continental shelf and ocean floor south of Yakutat Bay.
Other interesting features found near continental margins are submarine canyons. There are six major submarine canyon systems offshore of SE Alaska: Noyes Canyon and Gulf of Alaska 1 through 5. These areas have been closed to bottom trawling since 1998 to protect deep sea corals. Landslides are frequently triggered in these canyons especially during earthquakes. The landslides often enlarge into debris flows that surge down the canyons.
There are three spreading center ridges near the coast of North America. The farthest north and closest to SE Alaska is Explorer Ridge which is about 134 km long. The eastern edge of this ridge is about 114 km west of the northern tip of Vancouver Island. Juan de Fuca Ridge (500 km long) and Gorda Ridge (134 km long) extend to the south, reaching into the waters well offshore of Northern California. All these ridges have vigorous hydrothermal vents known as “black smokers”.
There are two seamount chains in the Gulf of Alaska: Cobb-Eickelberg Seamount Chain and Kodiak-Bowie Seamount chain. The Cobb-Eickelberg Seamount Chain originates from the Cobb Hotspot which coincides with Axial Seamount on Juan de Fuca Ridge. This chain extends northwest with the oldest one being Patton Seamount southeast of Kodiak. The nearby subduction zone shows scars from previous seamounts that were subducted millions of years ago.
Kodiak-Bowie Seamount Chain runs parallel to Cobb-Eickelberg chain and is closer to SE Alaska. It crosses the Gulf of Alaska, running about 280 km southwest of Baranof Island. This chain originates at Bowie Hotspot which is about 170 km west of Haida Gwaii. Kodiak Seamount is located at the northwest end of the chain, about 390 km east of Kodiak. It’s the oldest seamount in this chain.
Kodiak-Bowie Seamount Chain includes five seamounts that are relatively close to SE Alaska and Haida Gwaii. Closest to SE Alaska is Dickens Seamount which is about 260 km west of Craig and 305 km south of Sitka. This seamount is unusually close to a submarine canyon, Noyes Canyon. About 60 km further south is Denson Seamount. Also in this vicinity, there’s a cluster of three seamounts: Pierce, Hodgkins, and Bowie. Bowie Seamount is at the southeastern end of the chain and as such, is the most recently active volcano in the chain. Scientists believe it last erupted some 16,000 years ago.
The last feature covered in this overview is extremely important for biological productivity of the ocean: the continental shelf. In the waters near SE Alaska, the shelf varies in width from about 100 km near Yakutat to just 25 or 30 miles near Baranof Island. To the south, it widens out to 85 or 90 km near the Outer Islands, west of Prince of Wales Island. West of Haida Gwaii, the continental margin is almost nothing. But to the east and southeast is Hecate Strait which is entirely continental shelf and widens out to more than 100 km. This wide continental shelf extends through Queen Charlotte Sound to Vancouver Island.