North Gorda Ridge Segment

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  • Volcanic Region
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  • Last Known Eruption
  • 42.67°N
  • 126.78°W

  • -3000 m
    -9840 ft

  • 331031
  • Latitude
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Most Recent Bulletin Report: January 1998 (BGVN 23:01)


Seismic swarm on 8 January

On 8 January in the NE Pacific, the NOAA/PMEL T-phase Monitoring System recorded high seismicity from the E flank of the Gorda Ridge's N section (figure 7). The exact nature of the activity is unknown. Over 500 small earthquakes were recorded during a 6-day period (figures 8 and 9). The events were located along several en-echelon normal faults on the flank of the Gorda Ridge, E of the neovolcanic zone (figure 9). The moment-tensor solution for a M 3.8 earthquake within the swarm showed the event to be a normal fault earthquake.

Figure 7. Map of plate boundaries and other features associated with the Juan de Fuca Ridge in the NE Pacific. Courtesy of NOAA/PMEL.
Figure 8. Number of seismic events per hour at the N section of the Gorda Ridge during 6-14 January 1998. Courtesy of NOAA/PMEL.
Figure 9. Earthquake epicenters along the flank of the N end of Gorda Ridge recorded during 7-14 January 1998. The base map shows bathymetric contours shaded with darker fill patterns at greater depths. Courtesy of NOAA/PMEL.

Substantial data sets from the Gorda Ridge were collected during the 1980s, including water column surveys, full SeaBeam coverage, SeamarC I and II surveys, camera surveys, extensive dredges, and submersible dives. A submarine lava flow erupted in 1996 (BGVN 21:02 and 21:06) at the N end of the Gorda Ridge (~200 km W of Oregon's S coast). The eruption was initially detected based on acoustic T-waves (seismic waves transmitted through the ocean) from a seismic swarm and based on large thermal plumes in the colder ambient sea water. Subsequent camera tows revealed new lava flows ~100-200 m wide located along a fissure that was at least 3.5-km long.

Information Contacts: Chris Fox and Bob Dziak, NOAA Pacific Marine Environmental Laboratory (PMEL), 2115 SE Osu Drive, Newport, OR 97365 USA (Email: fox@pmel.noaa.gov, dziak@pmel.noaa.gov, URL: http://www.pmel.noaa.gov/).

The Global Volcanism Program has no Weekly Reports available for North Gorda Ridge Segment.

Index of Bulletin Reports


Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

02/1996 (BGVN 21:02) Eruption or intrusive event detected by acoustic signals

06/1996 (BGVN 21:06) Submarine plumes and a brief fissure eruption

01/1998 (BGVN 23:01) Seismic swarm on 8 January




Bulletin Reports

All information contained in these reports is preliminary and subject to change.


02/1996 (BGVN 21:02) Eruption or intrusive event detected by acoustic signals

In late February and early March a possible submarine eruption was detected on the Gorda Ridge. Seismo-acoustic T-waves established the epicenter at between 42.41 and 42.75°N. Vertical conductivity-temperature-depth (CTD) casts found a candidate plume at 42.67°N, 126.78°W.

Beginning at 0700 GMT on 28 February, intense seismicity was detected using the T-phase Monitoring System developed by National Oceanic and Atmospheric Administration's Pacific Marine Environmental laboratory (NOAA/PMEL) to access the U.S. Navy's Sound Surveillance System (SOSUS) in the NE Pacific. The event was located on the northernmost segment of the Gorda Ridge (figure 1), over 200 km W of the Oregon coast. The seismicity was very similar to that observed in June 1993 at the CoAxial Segment of the Juan de Fuca Ridge at 46.5°N (BGVN 18:07), which was later documented to be the lateral injection of magma with a subsequent eruption.

Figure 1. Bathymetric map of the northernmost Gorda Ridge, NE Pacific Ocean. White box shows the approximate area of the hydrothermal plumes found during 10-11 March 1996. The "narrow-gate" summit area is located just N of the plume location, around 42.75°N, 126.75°E. Inset bathymetric map shows the Blanco Fracture Zone and the Gorda Ridge, with the eruption site indicated by a white dot. Courtesy of the RIDGE Office.

For the first 42 hours of T-wave seismicity, two proximal SOSUS arrays were not operating, so the presence of seismicity in the general area of the northern Gorda Ridge was confirmed based on distant arrays. The proximal SOSUS array became operational on 6 March, allowing improved sensitivity and epicenter estimates. Seismicity continued during 6-8 March, located thoughout the S half of the ridge segment from 42°25' to 42°45'N.

The Gorda Ridge Eruption Assessment Team (GREAT), aboard the NOAA Ship MacArthur, reached the area on 8 March. They began a series of vertical CTD casts starting at 42°26.2'N, 126°55.3' W, and proceeding N along the ridge axis with measurements at ~2' intervals; only 2,900 m of wire was useable. No plume signals were detected on the first six casts, although up to 1 km of the water column remained below the deepest CTD depths reached. At 42°37.9'N, 126°47.8'W, temperature and particle plumes were found between 1,850 and 2,800 m above a bottom depth of 3,300 m. The main plume lens was centered at 1,850-2,300 m, with several thinner and less intense plumes below. Plume distribution was similar at the next two stations N, though the overall plume became thinner and less intense. A plume located 24 hours later was similar, perhaps indicating advection of the plume to the W.

On 9 March seismicity decreased to <10 events/hour. Only minor seismic activity was recorded on 10 March, mostly from the shallower "narrow-gate" (summit) area near 42°45'N. That day, GREAT detected a large hydrothermal plume centered near 42°40'N, 126°47'W that may have been due to recent magmatic activity. Initial survey work indicated that the plume may have been an agglomeration of more than one discharge. It had a maximum thickness of ~700 m, a maximum diameter of ~10 km, and a maximum temperature anomaly of ~0.12°C. Seismicity continued at a low level (<5 earthquakes/hour) during 11-14 March. Seismic activity increased again at 1625 GMT on 15 March to >25 events in the first hour. The nature of the seismicity appeared to be due to magma injection rather than eruption. Preliminary locations for the 15 March activity were in the summit area.

Based on their exceptional height above the axial valley, most of the major plumes detected through 15 March were thought to be event plumes. The capability to demonstrate the vertical and horizontal symmetry characteristic of event plumes was not available. Apparently, several distinct event plumes were mapped that differ in depth and in horizontal and vertical dimensions. One alternative hypothesis is that all, or some, of the plumes are chronic plumes originating high on the valley walls. No substantial near-seafloor plumes have been found. The source of the presumed event plumes may be S of their present position in water too deep for available equipment to reach, farther to the N where samples had not yet been taken, or beneath their present position but as yet undetected.

Remaining unanswered questions regarding the Gorda Ridge event, as well as mid-ocean ridge events generally include: spatial and temporal patterns of seismicity, intrusive vs. extrusive behavior, the origin of the event plumes, and patterns and rates of geochemical and microbiological processes associated with event plumes and resulting chronic plumes. A second response cruise on the UNOLS RV Wecoma during the first two weeks of April 1996 will focus on water column work and camera tows.

Substantial data sets have been previously collected in this area. Water column surveys collected by NOAA in 1985 and later surveys by Oregon State University showed water column temperature anomalies in the area, which was labelled GR-14. Full SeaBeam coverage has been collected by NOAA. SeamarC II surveys were collected in the area in 1983 by USGS/University of Hawaii. Detailed SeamarC I surveys were collected by NOAA/PMEL in the northern half of the segment in 1987. Camera surveys were conducted in 1985-86 by USGS and NOAA/PMEL. Extensive dredges were also collected by USGS. The Navy's SeaCliff submersible dove in the area in 1988.

Information Contacts: Chris Fox, Bob Embley, Bob Dziak, and Ed Baker, NOAA Pacific Marine Environmental Laboratory, 2115 SE Osu Drive, Newport, OR 97365 USA (Email: fox@pmel.noaa.gov, URL: http://www.pmel.noaa. gov/vents/eruption.html); RIDGE Office, Ocean Processes Analysis Laboratory, Morse Hall, 39 College Road, University of New Hampshire, Durham, NH 03824-3525 USA (URL: http://ridge.unh.edu).

06/1996 (BGVN 21:06) Submarine plumes and a brief fissure eruption

The following report describes preliminary results of investigations on the eruptive activity that began on 28 February along the Gorda Ridge (BGVN 21:02). On 10-11 March 1996 NOAA's RV MacArthur carried out a series of conductivity-temperature-depth (CTD) casts to study the plume(s) activity.

Figure 2 shows a N-S cross section of the temperature anomaly through the event plume(s) discovered above the ridge. The temperature anomaly was defined as the increase of the water temperature above that expected for a given density horizon. The contours are based on five vertical casts evenly spaced between 42°36' and 42°43'N. A similar pattern was observed on an E-W transect. Since previous event plumes were characterized by symmetry about a central core, the structure in this anomaly suggested an agglomeration of two or more separate plumes. The events that caused the plumes could have been separated in space and time or both.

Figure 2. N-S cross section of the temperature anomaly through the Gorda Ridge event plume(s), 18 March 1996. Courtesy of E. Baker, NOAA/PMEL.

Water samples taken from station 7 (42°37.9'N, 126°47.8'W) in mid-March showed a very high enrichment in Helium-3 (figure 3). The presence of this isotope, enriched in fresh oceanic volcanic rocks and in submarine hydrothermal fluids, suggested a hydrothermal input into the ocean. Station 7 was also characterized by an increase in temperature and in suspended particles (nephels).

Figure 3. Concentration of Helium-3 for station 7 along the Gorda Ridge. Courtesy of J. Lupton, R. Greene, L. Evans, and R. Kovar NOAA/PMEL.

The plot of Helium-3 concentration versus temperature anomaly at stations 7 and 13 (42°45.7'N, 126°44.8'W) suggested that each had sampled water columns with different plume characteristics (figure 4). The Helium-3/heat trend for station 7 (the event plume) had a flat slope of 0.34 x 10-12 cm3/cal, similar to other event plumes detected in 1986 and 1993 over the Juan de Fuca Ridge. In contrast, the plume detected at station 13 had a much higher helium-3/heat trend of 2.27 x 10-12 cm3/cal.

Figure 4. Plot of Helium-3 concentration vs. temperature difference for stations 5 and 7 along the Gorda Ridge. The y-axis shows 3He concentrations in units of cubic centimeters of gas at standard temperature and pressure per gram of seawater. Courtesy of J. Lupton, R. Greene, L. Evans, and R. Kovar NOAA/PMEL.

SEM analysis of the first sample from the megaplume site at GR-14 revealed the presence of Fe-oxides, Zn-sulfides, and bacterial aggregates. The Fe-oxides were found with and without phosphorus. It was suggested that Fe-oxides formed beneath the seafloor lacked phosphorus, whereas Fe-oxides formed within the megaplume were enriched in phosphorus. The Zn-sulfides were very pure (i.e., no Fe). This sample appeared similar to the plume samples collected over the flow site in 1993. Preliminary results from the dissolved concentrations of Mn and Fe suggested that the event plume had formed recently.

In April the RV Wecoma surveyed a new lava flow with five camera tows. Figure 5 is a digital camera image that shows the contact between the new lava flow at the ridge and the surrounding older lava. The eruption site was at least 3.5 km long and only ~100-200 m wide, based mainly on the distribution of near-bottom temperature anomalies above the cooling flow. With this shape, the flow was clearly the product of a brief fissure eruption, where a dike reached the surface. The lava flow was located directly under the event plume mapped by CTD casts during the RV MacArthur cruise. Figure 6 shows the new lava flow, the camera tows, the CTD casts, the event plume(s), and the epicenters detected in the area since 28 February (BGVN 21:02).

Figure 5. Contact between the new lava flow and the surrounding older lava. The image was taken by a Benthos digital camera developed for Woods Hole Oceanographic Institution (Dan Fornari, P.I.) (scale of image was undisclosed).
Figure 6. Map showing an overview of the investigated area with the general location of the event plume. Courtesy of B. Embley and B. Chadwick. The T-wave epicenters appear as triangles; the RV McArthur CTD casts are shown as crosses.

Information Contacts: Chris Fox, Bob Dziak, Bob Embley, Bill Chadwick, Ed Baker, John Lupton, Dick Feely, and Gary Massoth, NOAA Pacific Marine Environmental Laboratory, 2115 SE Osu Drive, Newport, OR 97365 USA (Email: fox@pmel.noaa.gov, URL: http://www.pmel. noaa.gov/vents/eruption.html).

01/1998 (BGVN 23:01) Seismic swarm on 8 January

On 8 January in the NE Pacific, the NOAA/PMEL T-phase Monitoring System recorded high seismicity from the E flank of the Gorda Ridge's N section (figure 7). The exact nature of the activity is unknown. Over 500 small earthquakes were recorded during a 6-day period (figures 8 and 9). The events were located along several en-echelon normal faults on the flank of the Gorda Ridge, E of the neovolcanic zone (figure 9). The moment-tensor solution for a M 3.8 earthquake within the swarm showed the event to be a normal fault earthquake.

Figure 7. Map of plate boundaries and other features associated with the Juan de Fuca Ridge in the NE Pacific. Courtesy of NOAA/PMEL.
Figure 8. Number of seismic events per hour at the N section of the Gorda Ridge during 6-14 January 1998. Courtesy of NOAA/PMEL.
Figure 9. Earthquake epicenters along the flank of the N end of Gorda Ridge recorded during 7-14 January 1998. The base map shows bathymetric contours shaded with darker fill patterns at greater depths. Courtesy of NOAA/PMEL.

Substantial data sets from the Gorda Ridge were collected during the 1980s, including water column surveys, full SeaBeam coverage, SeamarC I and II surveys, camera surveys, extensive dredges, and submersible dives. A submarine lava flow erupted in 1996 (BGVN 21:02 and 21:06) at the N end of the Gorda Ridge (~200 km W of Oregon's S coast). The eruption was initially detected based on acoustic T-waves (seismic waves transmitted through the ocean) from a seismic swarm and based on large thermal plumes in the colder ambient sea water. Subsequent camera tows revealed new lava flows ~100-200 m wide located along a fissure that was at least 3.5-km long.

Information Contacts: Chris Fox and Bob Dziak, NOAA Pacific Marine Environmental Laboratory (PMEL), 2115 SE Osu Drive, Newport, OR 97365 USA (Email: fox@pmel.noaa.gov, dziak@pmel.noaa.gov, URL: http://www.pmel.noaa.gov/).

The northernmost of five segments of the Gorda Ridge lies immediately south of the Blanco Transform Fault that offsets the Gorda and Juan de Fuca oceanic spreading ridges. The 65-km-long segment is located about 200 km W of the southern Oregon coast and has deep 5- 10-km-wide valleys at either ends with a shallower narrow axial valley at the center. This morphology, which in plan view resembles an hourglass, is typical of magmatically active spreading segments. A submarine lava flow was erupted in late February and early March 1996, near the center of the segment. The eruption was initially detected through acoustic T-waves from a seismic swarm and the emission of large thermal plumes. In April submarine cameras revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. A seismic swarm of uncertain origin also occurred at this location in January 1998.

Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
1996 Feb 28 (?) 1996 Mar 15 (?) Confirmed 0 Historical Observations Near center of North Gorda Ridge segment
3020 BCE (?) Unknown Confirmed 0 Uranium-series
4840 BCE (?) Unknown Confirmed 0 Uranium-series

The Global Volcanism Program has no synonyms or subfeatures listed for North Gorda Ridge Segment.

A fresh lava flow was erupted in 1996 along the North Gorda Ridge, the northernmost of five segments of the Gorda Ridge located immediately south of the Blanco Transform Fault, which offsets the Gorda and Juan de Fuca spreading ridges. Submarine cameras revealed new lava flows about 100-200 m wide along a fissure that was at least 3.5 km long. The 65-km-long North Gorda Ridge segment is located about 200 km west of the southern Oregon coast and has an hourglass shape in plan view, with a shallower narrow axial valley at the center.

Image courtesy of National Oceanic and Atmospheric Administration, 1996 (http://www.pmel.noaa.gov/vents/home.html).

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Chadwick W W Jr, Embley R W, Shank T M, 1998. The 1996 Gorda Ridge eruption: geologic mapping, sidescan sonar, and SeaBeam comparison results. Deep-Sea Res II, 45: 2547-2569.

Goldstein S J, Murrell M T, Janecky D R, Delaney J R, Clague D A, 1992. Geochronology and petrogenesis of MORB from the Juan de Fuca and Gorda ridges by 238U-230Th disequilibrium. Earth Planet Sci Lett, 109: 255-272.

Perfit M R, Chadwick W W Jr, 1998. Magmatism at Mid-Ocean ridges: constraints from volcanological and geochemical investigations. In: Buck W R, Delaney P T, Karson J A, Lagabrielle Y (eds) Faulting and Magmatism at Mid-Ocean Ridges {Amer Geophys Union Geophys Monograph} 106: 59-115.

Rubin K H, Smith M C, Perfit M R, Christie D M, Sacks L F, 1998. Geochronology and geochemistry of lavas from the 1996 North Gorda Ridge eruption. Deep-Sea Res II, 45: 2571-2597.

Smithsonian Institution-GVN, 1990-. [Monthly event reports]. Bull Global Volc Network, v 15-33.

Volpe A M, Goldstein S J, 1993. 236Ra-230Th disequilibrium in axial and off-axis mid-oecan ridge basalts. Geochim Cosmochim Acta, 57: 1233-1241.

Volcano Types

Submarine
Fissure vent(s)

Tectonic Setting

Rift zone
Oceanic crust (< 15 km)

Rock Types

Major
Basalt / Picro-Basalt

Population

Within 5 km
Within 10 km
Within 30 km
Within 100 km
0
0
0
0

Affiliated Databases

Large Eruptions of North Gorda Ridge Segment Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.