Bowie Seamount

The volcano has a flat-topped summit rising about 3,000 m (10,000 ft) above the seabed, to 24 m (79 ft) below sea level. The seamount lies at the southern end of a long underwater volcanic mountain range called the Pratt-Welker or Kodiak-Bowie Seamount chain, stretching from the Aleutian Trench in the north almost to Haida Gwaii in the south.

Bowie Seamount lies on the Pacific Plate, a large segment of the Earth's surface which moves in a northwestern direction under the Pacific Ocean. It is adjacent to two other submarine volcanoes; Hodgkins Seamount on its northern flank and Graham Seamount on its eastern flank.

Geology

Structure

Seamounts are volcanic mountains which rise from the seafloor. The unlimited supply of water surrounding these volcanoes can cause them to behave differently from volcanoes on land. The lava emitted in eruptions at Bowie Seamount is made of basalt, a common gray to black or dark brown volcanic rock low in silica content (the lava is mafic). When basaltic lava makes contact with the cold sea water, it may cool very rapidly to form pillow lava, through which the hot lava breaks to form another pillow. Pillow lava is typically fine-grained, due to rapid cooling, with a glassy crust, and has radial jointing.

With a height of at least 3,000 m (10,000 ft) and rising to within only 24 m (79 ft) of the sea surface, Bowie Seamount is the shallowest submarine volcano on the British Columbia Coast, as well as in Canadian waters, and one of the shallowest submarine volcanoes in the northeast Pacific Ocean. Most seamounts are found hundreds to thousands of metres below sea level, and are therefore considered to be within the deep sea. If Bowie Seamount were on land it would be about 600 m (2,000 ft) higher than Whistler Mountain in southwestern British Columbia and 800 m (2,600 ft) lower than Mount Robson, the highest mountain in the Canadian portion of the Rocky Mountains.

Bowie Seamount is about 55 km (34 mi) long and 24 km (15 mi) wide. Its flat-topped summit is made of weakly consolidated tephra and consists of two terraces. The lowest terrace is about 230 m (750 ft) below sea level while the highest is about 80 m (260 ft) below sea level, but contains steep-sided secondary summits that rise to within 25 m (82 ft) below sea level. From a physical perspective, the effective size of the submarine volcano is considerably large. The effects of other submarine volcanoes along the Pacific Northwest, including Cobb Seamount off the coast of Washington, affect the composition and abundance of plankton up to 30 km (20 mi) away from the seamount summit due to the upwelling of nutrients from deeper waters. Because of its similar size, Bowie Seamount most likely has a similar effect on its adjacent waters.

Eruptive history

Bowie Seamount was formed by submarine eruptions along fissures in the seabed throughout the last glacial, or "Wisconsinian", period, which began about 110,000 years ago and ended between 15,000 and 10,000 years ago. While most submarine volcanoes in the Pacific Ocean are more than one million years old, Bowie Seamount is relatively young. Its base was formed less than one million years ago, but its summit shows evidence of volcanic activity as recently as 18,000 years ago. This is recent in geological terms, suggesting the volcano may yet have some ongoing volcanic activity.

Close to Bowie's submerged summit, former coastlines cut by wave actions and beach deposits show that the submarine volcano would once have stood above sea level, as either a single volcanic island or as a small cluster of shoals that would have been volcanically active. Sea levels during the last glacial period, when Bowie Seamount was formed, were at least 100 m (300 ft) lower than they are today. It would have had a land area similar to, if not larger than, Midway Atoll in the central Pacific Ocean.

Origins

There is some disagreement about the origin of Bowie seamount: Geological studies indicate that the Kodiak-Bowie Seamount chain may have formed above a center of upwelling magma called a mantle plume. The seamounts comprising the Kodiak-Bowie Seamount chain would be formed above the mantle plume and carried away from the mantle plume's magmatic source as the Pacific Plate moves in a northwesterly direction towards the Aleutian Trench, along the southern coastline of Alaska.

The volcanic rocks which make up some of the seamounts in the Kodiak-Bowie Seamount chain are unusual in that they have an acid-neutralizing chemical substance like typical ocean-island basalts but a low percentage of strontium as found at mid-ocean ridge basalts. However, the strontium-bearing volcanic rocks comprising Bowie Seamount also contain lead. Therefore, the magma mixtures that formed Bowie Seamount seem to have originated from varying degrees of partial melting of a depleted source in the Earth's mantle and basalts which had distinctly high lead isotopic ratios. Estimates during geological studies indicate that the abundance of the depleted-source component ranges from 60 to 80 percent of the erupted material.

Some aspects of the origin of the Kodiak-Bowie Seamount chain remain uncertain. The volcanic rocks found at the Tuzo Wilson Seamounts south of Bowie are fresh glassy pillow basalts of recent age, as would be expected if these seamounts are located above or close to a mantle plume south of Haida Gwaii. However, the origin of Bowie Seamount is less certain because even though the seafloor which Bowie lies on formed 16 million years ago during the late Miocene period, Bowie's summit shows evidence of recent volcanic activity. If Bowie Seamount formed above a mantle plume at the site presently occupied by the Tuzo Wilson Seamounts, it has been displaced from its magma source by about 625 km (390 mi) at a rate of about 4 cm (2 in) per year. The geologic history of Bowie Seamount is consistent with its flat-topped eroded summit, but the source for Bowie's recent volcanic activity remains uncertain. Still others, such as Dickens Seamount and Pratt Seamount further north of Bowie Seamount, fall a little to the side of the chain's expected trend. Another hypothesized origin of some or all seamounts in the Kodiak-Bowie Seamount chain is that they formed on top of the Explorer Ridge, a divergent tectonic plate boundary west of Vancouver Island, and have been displaced from it by seafloor spreading.

Although some of the seamounts in the Kodiak-Bowie Seamount chain appear to follow the expected age progression for a mantle plume trail, others, such as Denson Seamount, are older than that hypothesis would suggest. As a result, the Kodiak-Bowie Seamount chain has also been proposed by geoscientists to be a mix of ridge and mantle plume volcanism.