We have only been able to detect submarine volcanic eruptions since about 1990, and even then in only a small part of the world's oceans. Before 1990, scientists knew submarine eruptions had to be happening frequently, but had no idea when or where.

HOW ARE SUBMARINE ERUPTIONS DETECTED?

We never see most of the earth's volcanic eruptions, because most of them occur undetected in the deep ocean. An estimated 75% of the average annual volume of magma reaching the earth's surface is emplaced along the mid-ocean ridges, the global rift system that is 60,000 km long and straddles the earth like the seams on a baseball. Tectonic plates spread apart at a mid-ocean ridge and mantle upwelling leads to intrusions and extrusions that form new oceanic crust. Spreading rates along the mid-ocean ridge system vary greatly, from 2 cm/yr along the Mid-Atlantic Ridge, to 6 cm/yr along the Juan de Fuca Ridge, up to 16 cm/yr along the East Pacific Rise. The most frequent and voluminous volcanism is presumably associated with the highest spreading rates.

For years, oceanographers have known that eruptions must be occurring frequently on the mid-ocean ridge, but until recently it has been impossible to know when and where they were actually taking place. The key to detecting these events was a new system specifically designed to measure the small earthquakes normally associated with magma migration and dike intrusion. The problem before now has been that seismometers on land are typically too far away from mid-ocean ridges to detect small earthquakes (those below magnitude 4). The solution was to listen for them in the ocean, instead of feeling for them on land.

When an earthquake occurs beneath the ocean, in addition to the usual P- and S-waves that are produced, some of the energy is transferred into a "T-wave", which is a water-borne acoustic wave. Basically, the shaking and rumbling of the seafloor produces sound that propagates outward from the source as a T-wave, and can be detected from great distances because sound travels so efficiently in the ocean. The US Navy has maintained sensitive hydrophone networks in the oceans for years, mainly to listen for enemy submarines. These once-secret hydrophone networks also hear T-waves from earthquakes, but these have always been just filtered out as extraneous "noise" by the Navy. In 1991, the NOAA EOI Program received permission to tap into the Navy's hydrophone system, and process the data specifically to detect and locate T-waves from the NE Pacific. Since then many intrusions and eruptions have been detected in the NE Pacific. This has opened up a new field of research because for the first time scientists can rush out and study mid-ocean ridge eruption sites soon after the events.

HISTORICAL ERUPTIONS DOCUMENTED IN THE NE PACIFIC

Below is a list of the submarine volcanic eruptions that have been documented in the NE Pacific.

~1986 - Cleft segment, Juan de Fuca Ridge

This was the first documented eruption on the global mid-ocean ridge system, but was inferred years after the fact. It was documented by a combination of repeat bathymetric surveys and seafloor mapping in an area that had previously released unusual event plumes (or "megaplumes"). This event was described in a special issue of Journal of Geophysical Reseach (v. 99, n. B3, p. 4741-4776, 1994).

June 1993 - CoAxial segment, Juan de Fuca Ridge

This eruption was the first to be detected in real time as it was happening by the US Navy's SOSUS hydrophone network and the first to be responded to within weeks of the event. A ship with a remotely operated vehicle was able to investigate the site of the eruption within 3 weeks of the earthquake swarm. This eruption was described in a collection of 14 papers published in a special issue of Geophysical Reseach Letters (v. 22, n. 2, p. 129-182, 1995). After this eruption, evidence was found that two other recent eruptions had occurred on this segment between 1981 and 1991, based on repeated bathymetric surveys and seafloor mapping. All three eruption sites are described in detail in two papers of the Journal of Geophysical Research (v. 105, n. B7, p. 16501-16525, 2000, and v. 106, n. B8, p. 16075-16099).

February 1996 - North Gorda segment, Gorda Ridge

The second eruption detected in real time was on the northern Gorda Ridge, which is south of the Juan de Fuca Ridge. It was the first eruption in which the hydrothermal event plume that was released with the eruption was tracked for months after the event. This eruption was described in a collection of 13 papers published in a special issue of Deep-Sea Research Part II, v. 45, n. 12, p. 2503-2766, 1998.

January 1998 - Axial Seamount, Juan de Fuca Ridge

Axial Seamount erupted in January 1998 and is the site of a multi-year seafloor observatory called NeMO. On-going research is studying the effects of the eruption and the response of the site's hydrothermal and biological systems to the eruption. The eruption was described in a collection of 12 papers published in a special issue of Geophysical Reseach Letters (v. 26, n. 23, p. 3425-3456 and n. 24, p. 3637-3652, 1999). The NeMO web site has updates on continuing research at the site.

April 2001 - Jackson segment, Gorda Ridge

This event included an earthquake swarm indicative of a dike intrusion, but and event response effort found there was no evidence that lava was erupted onto the seafloor. Apparently magma was intruded along the segment at depth but it never made it to the surface.

September 2001 - Middle Valley, Juan de Fuca Ridge

This was apparently another example of a large seismic swarm with thousands of earthquakes over several weeks, but with no evidence for an eruption on the seafloor or an event plume in the water column.

February 2005 - Endeavour Segment, Juan de Fuca Ridge

Another large seismic swarm with thousands of earthquakes over several weeks, but with no evidence for an eruption on the seafloor or an event plume in the water column. However, there were chemical and thermal changes detected in the Endeavour vent fields and pressure changes observed on boreholes off-axis.

April 2011 - Axial Seamount, Juan de Fuca Ridge

Axial Seamount erupted for a second time in April 2011 (13 years after the previous eruption in January 1998). This time bottom pressure recorders and ocean-bottom hydrophones were in place during the eruption. High-resolution bathymetric surveys before and after the eruption made it possible to map the new lava flows in unprecedented detail. The eruption was described in a collection of 3 papers published in Nature Geoscience (v. 5 ,p. 474-488, July 2012).

View a map of these eruption and intrusion sites.