Axial Seamount is a submarine volcano located on the Juan de Fuca Ridge, 480 km west of the Oregon Coast. It has been the subject of extensive study as it is extremely active and is of complex origins: it is on the intersection of the Cobb Hotspot Seamount Chain, and the Juan de Fuca extensional ridge; it is also close to the Blanco Fracture Zone. The seamount rises 700 m above the rest of the Juan de Fuca Ridge and about 1100 m above the surrounding sea floor; it is 1400 m bellow the sea surface. It is a biodiversity hotspot, hosting several 'black-smokers' - underwater hot springs producing super-heated mineral rich water, which support unique ecosystems.
The location of the Axial Seamount.
The Juan de Fuca Ridge, including Axial Seamount, was discovered by the SEASAT satellite using satellite alimentary, a technique which bounces radio-waves of the sea-floor to build up a three dimensional picture. SEASAT was a satellite launched by NASA's Jet Propulsion Laboratory in 1978. It carried a range of instruments intended to study and monitor the world's oceans. Its mission was short-lived, being cut off by a massive short-circuit 105 days after its launch.
In 1981 the National Oceanic and Atmospheric Administration (NOAA) Survey Ship Surveyor carried out a bathymetric study of the area using ship mounted sonar and thermal mapping equipment. This identified a number of thermal hotspots.
In 1983 the ridge was explored by the NOAAs submersible Pisces IV and 1984 by the Woods Hole Oceanographic Institution's DSV Alvin, between them discovering a number of black smokers. The Axial Seamount was named for its position on the intersection (Axis) between the Juan de Fuca Ridge and the Cobb Seamount Chain.
The DSV Alvin.
The seamount was further investigated by the NOASS Discoverer in 1991 and the German research ship RV Sonne in 1996. Later in 1996 the NOAA founded the New Millennium Observatory at Mount Axial, the first ever permanent monitoring station at a submarine volcano.
In January 1998 this reported a marine eruption on the site; when the site was visited in February it was found that there had indeed been an eruption, burying some of the station's equipment but confirming that the station was indeed able to detect eruptions.
In 2006 Bill Chadwick of the Hatfield Marine Science Center at Oregon State University and Scott Nooner of the Lamont-Doherty Earth Observatory at Columbia University published a paper in the Journal of Volcanology and Geothermal Research in which they analyzed movements on the sea-floor before and after the 1998 eruption, and found that there had been a long period of inflation prior to the eruption, followed by an abrupt deflation of the sea-floor during the eruption, with 3.2 m of subsidence. They then analyzed more recent data from the seamount and found that the sea-floor was once again swelling. From this they concluded that Axial Seamount was likely to erupt again within 8 years.
In April this year (2011) monitors on the seamount once again detected signs of an eruption, and in late July a visit by the Woods Hole Oceanographic Institutions ship RV Atlantis revealed that there had indeed been an eruption, with extensive lava flows, already showing signs of new colonization by benthic animals.
The Juan de Fuca ridge marks the extensional margin between the Juan de Fuca Plate and the Pacific Plate. The Juan de Fuca Plate is thought to be a remnant of an ancient plate known as the Farallon Plate, a once vast plate now largely subducted beneath the North American Plate. There are three remnants of this plate remaining off the west coast of the US; the Juan de Fuca Plate, the Gorda Plate to the south and the Explorer Plate to the north. The Juan de Fuca Ridge was once part of the longer Farallon Ridge, which ran along the eastern margin of the Farallon Ridge before it was fractured into three parts.
The former parts of the Farallon Plate.
The Cobb Seamount Chain runs from the Aleutian Trench off the south coast of Alaska south to Axial Seamount. It is gets its name from the Cobb Seamount, which is 500 km west of the Washington coast, and was named after the research vessel MV John M Cobb; this can cause some confusion as other objects on the seafloor of the NE Pacific are also named after this vessel (Cobb) and are not necessarily related to the seamount chain.
The Cobb Seamount Chain, top right.