If the global estimate of one million submarine volcanoes is correct perhaps many thousands of these volcanoes are active. In contrast, few submarine volcanoes are caught in the process of erupting. Of the nearly 8,000 known volcanic eruptions in the last 10,000 years only about 300 were submarine. From 1975 to 1985, 160 volcanoes erupted but only 24 of these were submarine. Most of these submarine eruptions were in shallow water. Recent technological advances have allowed volcanologists to detect and respond to eruptions, even eruptions in very deep water. This close-up photo shows a drip of the 1991 lava flowing over the irregular surface of the margin of the axial summit collapse trough, depth 8,216 feet (2505m). Photography courtesy of Woods Hole Oceanographic Institution and members of the Adventure dive (Principle Investigators: D. Fornari, R. Haymon, K. Von Damm, M. Perfit, M. Lilley, and R. Lutz).
Argo I photograph taken in 1989 showing contact between new, glassy lobate lava flow on East Pacific Rise crest near 9 degrees 18'N estimated to be ~ 1-2 years old, and older sediment dusted lobate lava (in center of photo), depth ~8,690 feet (2650m). Photo courtesy of D. Fornari, Woods Hole Oceanographic Institution.
Volcanologists use the following types of observations to recognize very recent eruptions at the East Pacific Rise:
- lava surfaces completely devoid of sediment cover,
- lava surrounding recently extinct hydrothermal chimneys that are still colonized by vent animals,
- uncolonized lava partially covering older lavas that are colonized by anemones and other vent animals,
- the presence of organisms covering some of the freshest lava surfaces,
- evidence for changes in axial morphology since previous dives, and
- the presence of warm to hot water shimmering directly off of lava surfaces. From the SEPR Final Report
The following are descriptions of a few of the recent and well documented submarine eruptions.
Several lines of evidence indicate that Axial volcano has erupted in the last few years. Photos and sidescan sonar data show fresh lava at the summit and along one rift. Subsidence in the summit caldera, possibly coincident with an intrusion into or eruption on a rift, was measured in 1988. Three different geophysical methods suggests a magma body only 1-2 km below the caldera floor (see Applelgate and Embley, 1992). This photo shows a flat "lineated" sheet flow in the summit caldera of Axial Volcano. The grayish material is pelagic sediment deposited since time of eruption. Photo is from a towed camera system and courtesy of NOAA.
A large submarine eruption on the East Pacific Rise produced the 8 degrees S lava field, probably in the last 25 years. The estimated volume of the eruption is about 15 cubic km. The great size of this eruption is illustrated by comparing it to a few other volumes. For example, the average annual volume of lava erupted from all of Earth's volcanoes is estimated to be 4-5 cubic km, about 3 cubic km erupts at mid-ocean ridges. The 1783 eruption of Laki in Iceland, the largest historic basaltic eruption, had a total volume of 12.3 cubic km. Macdonald and others (1989) point out that the volume of the 8 degrees S lava field is enough to bury the entire U.S. Interstate freeway system to a depth of 10 m. The eruption originated at either the axial summit graben or from a fissure and line of cones just 2.5 km east of the graben. Timing of the eruption is constrained by the high reflectivity of the flow, earthquakes in the area in the 1960's, and the presence, volume, and location of a plume of 3He. Map from Macdonald and others (1989).
In February of 1996 a swarm of earthquakes were detected on the northern Gorda Ridge. Geologists visited the area shortly after the earthquakes and found warm water and fresh lava (Haymon and others, 1991). The photo below shows the end of the fresh, new black lava flow on top of an older gray lava flow. Photo courtesy of the NOAA/VENTS Geology Program.