New Eddystone Rock, Behm Canal

New Eddystone Rock, Behm Canal

by | Oct 10, 2021

The New Eddystone Rock is a pillar of basalt 237 feet (72 m) high, situated in Misty Fjords National Monument and in the eastern arm of Behm Canal, about 87 miles (140 km) north-northwest of Prince Rupert and 32 miles (52 km) northeast of Ketchikan, Alaska. Archeological evidence suggests that human occupation of the outer islands in Southeast Alaska dates to approximately 10,000 years ago. But the earliest known human use of the Behm Canal area occurred less than 2,500 years ago. By the time of the first European exploration in the 18th century, it is probable that some 2,000 people, including the coastal dwelling Tlingit and the Tsetsaut Athabaskans, occupied the area in what is now Misty Fjords National Monument. In 1793, Captain George Vancouver was visited here by natives in canoes who tried to entice the British to join them at their village. However, since they had been attacked by natives the day before, Vancouver declined the offer. Vancouver named the rock formation because of its resemblance to John Smeaton’s design for the Eddystone Lighthouse off Plymouth on England’s south coast. Behm Canal is a fjord, about 108 miles (174 km) long, separating Revillagigedo Island from the mainland. From Clarence Strait, Behm Canal extends north and northeast, through the Behm Narrows and across the mouth of the Unuk River, then south to Revillagigedo Channel. Behm Canal was named after Magnus von Behm, who was the governor of Kamchatka in 1779 when Captain James Cook’s ships arrived at Petropavlovsk with the news of Cook’s death. It was Behm who carried the news to Europe. In 1978, Misty Fjords National Monument was established by President Jimmy Carter, and in 1980, the Alaska National Interest Lands Conservation Act designated almost all of the monument as a wilderness area.

The Coast Mountains encompassing Misty Fjords National Monument are a series of major mountain ranges extending from southwestern Yukon through the Alaska Panhandle and virtually through all of British Columbia south to the Fraser River. These mountain ranges consist of deformed igneous and metamorphosed structurally complex pre-Tertiary rocks that originated from diverse locations around the globe. The first formation event began 130 million years ago when a group of active volcanic islands riding on a tectonic plate called the Insular Plate, approached a pre-existing continental margin and coastline of North America. These volcanic islands were formed by subduction of the former Farallon Plate under the Insular Plate during the early Paleozoic era. The Insular Belt was then welded onto the pre-existing continental margin by magma that eventually cooled to form a large mass of igneous rock, and creating a new continental margin. The Farallon Plate continued to subduct under the new continental margin supporting a new continental volcanic arc called the Coast Range Arc about 100 million years ago during the Late Cretaceous period. Magma rising from the Farallon Plate under the new continental margin ascended through the newly accreted Insular Belt, injecting huge quantities of granite into older igneous rocks of the Insular Belt. This molten granite burned through the old oceanic sediments creating a glittering metamorphic rock called schist. The older intrusions were then deformed under the heat and pressure of later intrusions, turning them into a layered metamorphic rock known as gneiss. In some places, mixtures of older intrusive rocks and the original oceanic rocks were distorted and warped under intense heat, weight, and stress to create unusual swirled patterns appearing to have been nearly melted into a liquid in the process. Volcanism along the entire length of the Coast Range Arc shut down about 50 million years ago and many of the volcanoes disappeared by erosion. What remains of the Coast Range Arc today are outcrops of granite formed when magma intruded and cooled at depth beneath the volcanoes, forming the present Coast Mountains of Misty Fjords National Monument.

The New Eddystone Rock is a prominent landmark seen by thousands of people each year on cruise ships. The pillar and several of the surrounding New Eddystone Islands are remnants of a volcanic system extruded through the rock at the bottom of Behm Canal near the end of the last ice age or Last Glacial Period. The pillar rock is a geologically young plug of hardened magma that filled a volcanic vent about 15,000 to 13,800 years old. New Eddystone Rock was never eroded or scoured by Pleistocene ice since it is unlikely that the pillar could have withstood glacial ice erosion in its present form. During the Last Glacial Period, the landscape of Southeast Alaska was depressed by the weight of massive continental ice sheets and soon after deglaciation, when pressure on the Earth’s crust was relieved, the landscape began to rebound. The resulting isostatic crustal deformation created fissures in the rock that provided a conduit for magma to rise, creating the New Eddystone volcanic vent, as well as several other vents in the adjacent area. The deglaciated fjord was flooded with seawater, but the Earth’s crust continued to rise, and wave action over thousands of years eroded the outer layers of the volcano, leaving only the resistant core and a broad pedestal of fringing underwater shallows that are exposed at very low tides. Between 2009 and 2013, the National Oceanic and Atmospheric Administration conducted multi-beam surveys in Behm Canal that revealed several previously unidentified volcanic vents that formed during or after the last glaciation. All of the discovered vents likely erupted between 15,000 and 13,800 years ago. One vent was a perfect cinder cone with a summit about 150 feet (46 m) below sea level. Two other vents are dome-shaped and likely erupted underneath a glacier. Read more here and here. Explore more of the New Eddystone Rock and Behm Canal here:

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