Eruption ends 30 October; some lava on NE flank, more on E caldera floor

Our last report (BGVN 30:09) described the first five days of this eruption, and was taken largely from a valuable joint report of Ecuador's Instituto Geofísico and Parque Nacional Galápagos. Here we report information from several sources on these topics: (a) initial observations of the eruption, (b) caldera-floor deformation prior to the eruption, (c) observations of the eruption's progress during 26 to 30 October (when it ended), and (d) satellite infrared observations of thermal fluxes associated with the eruption.

Eruption's start and subsequent plumes. As noted previously (BGVN 30:09), the eruption began around 1730 on 22 October 2005, when an explosion was heard by many residents of the volcano's S flank. Satellite images showed no activity at 1715, but revealed a large eruption at 1745 local time (2345 UTC). The eruption cloud reached an estimated altitude of at least 15 km (50,000 ft) and was moving SW.

At about this time, passengers and crew on Lindblad Expeditions' 80 passenger vessel M/N Polaris had an excellent view of the eruptive plume (figure 5). Lucho Verdesoto, the expedition leader, reported that the ship was then at Cerro Dragon, Santa Cruz island. Sunset was at 1753. As night fell they sailed to a position ~ 18 km NE of the volcano, where they had clear views of flows descending the volcano's upper NE flank (figure 6).

Naturalist Carman Guzman wrote, "After sunset the show was fascinating so we decided to move the Polaris to a much closer location. After dinner, we were only eleven miles from the eruption itself. What a thrill! The darkness of the night enhanced the beauty of the fiery reds and oranges that were seen at the top of the caldera. We spent several hours enjoying this rare and fantastic event. Rivers of lava were running down the slopes of the volcano and enormous flames were lighting up the sky."

According to NASA MODIS imagery and VAAC/NOAA reports, on 25 October 2005 a large plume of gases and steam was observed in GOES 12 imagery for 1545 local time (2145 UTC). The plume extended ~ 460 km W and SW of the summit at an altitude of ~ 4.6 km. Figure 7 shows the average concentration of SO₂ over the Sierra Negra plume as imaged by NASA's Aura satellite for the period 23 October-1 November.

Deformation monitoring. In the early stages of this eruption, Bill Chadwick (NOAA) submitted a report on pre-eruption deformation (figure 8). The plot shows both Synthetic Aperture Radar (InSAR) and GPS data on vertical deformation of the caldera floor. Chadwick wrote that he, Dennis Geist (University of Idaho), and Dan Johnson (University of Puget Sound, recently deceased) installed a 27 station GPS network at Sierra Negra in 2000, that was reoccupied in 2001 and 2002 (Geist and others, in press). With help from UNAVCO (a consortium supporting high-precision deformation measurements), the group then added a 6-station, continuous GPS network in 2002. Since then, there occurred a change from caldera subsidence to caldera uplift in March 2003. During this uplift, an M 4.6 earthquake on 16 April 2005 marked trapdoor faulting. The continuous GPS network measured a surface displacement of 85 cm within 10 seconds. Both this event and the previous case of trapdoor faulting in 1997-8-documented by satellite measurements using Interferometric Synthetic Aperture Radar (InSAR) (Amelung and others, 2000)-were preceded by over a meter of inflation (Jónsson and others, 2005). Both the 1997-98 and 2005 trapdoor movements occurred along the caldera floor's S side.

Aside from its immediate affects, the April 2005 earthquake left the later inflation rate unchanged. Caldera-centered uplift has continued since then without pause at about the same high rate. During the interval from March 2002 to April 2005 there was about ~ 1.2 m of uplift. Rates after the April 2005 earthquake are not plotted but were roughly the same as those during the interval March 2002-April 2005. The only other large earthquakes at Sierra Negra in the last year were an M 4.0 on 23 February 2005, which was associated with a small (2 cm) displacement near the trapdoor fault, an M 4.6 on 19 September 2005 that caused no obvious displacements, and an M 5.5, just 3 hours before the 22 October eruption started. The GPS data has not yet been processed.

Field descriptions of the eruption. The eruption began on 22 October with venting along a 2-km fissure near the caldera's N rim (figure 6). The fissure descended the caldera's inner wall at its E end. Flows were fed both northward down the outer N flank and southward onto the NE caldera floor. Although flows reached 5 km down the outer flank, flow into the caldera soon dominated, with strong channels descending inner caldera slopes before combining to form a wide aa flow banked against the caldera's E wall and moving steadily southward (see figures and discussion, BGVN 30:09).

Figure 9 is a photo taken by Greg Estes on 24 October. It highlights the vigorous venting and intracaldera flows at that point in the eruption. Figure 10, a post-eruption satellite photo, illustrates the broad pattern of still-cooling, erupted lavas (which appear as light colored areas on this 2 November thermal-infrared image). Although this may represent the best overview of the new lavas at this time, some of the thinnest flows or chilled flow features may not appear on this image.

By 26 October, fissure activity had narrowed to one major vent very near the N rim, but at 0830 on the 27th, eyewitness Godfrey Merlin reported that a second vent opened downslope and SE of the first. This new vent did not diminish the activity of the first, meaning that the total flux of erupting lava nearly doubled.

By about 1400 on the 27th, a team including Dennis Geist (University of Idaho), Terry Naumann (University of Alaska), and Karen Harpp (Colgate University) had arrived at the E caldera rim and began sending back a series of valuable reports. Their first report noted a major vent immediately below GPS station SN12 on the rim NE of the caldera's center. This vent emitted a large intracaldera aa flow.

Some active N-flank vents stood about 300-400 m NW of a station (GV01) on the caldera's N rim . There, two major vents fed lava fountains up to 50 m high. Most lava being erupted was flowing into the caldera, although some of the scoria from the fountains was falling outside the caldera and then forming a short, sluggish flow. Lava inside the caldera was cascading from the vents down the slope on the N edge of the caldera in 3 main channels, each 30-40 m across, with lava flowing at ~10 m/s (36 km/h) and in some cases over 10 m/s, and coalescing into a major aa flow to the S. On the caldera floor these channels merged into one big aa channel about 100 m wide that flowed more slowly both to the S, clockwise along the base of the E caldera wall, and into the moat along the S edge of the caldera floor. Pahoehoe outbreaks occurred along the margins of the major aa flow. New aa lava covered an estimated one-third of the caldera floor.

The report for 28 October noted that the eruption was still going strong. There were no significant new events on this day, but it appeared that the lava flux had increased because the vents looked wider and there seemed to be a lot more gas emitted. The lava continued to feed from the vents to the caldera floor in two large streams, each ~ 20 m across with lava traveling at 5-10 m/s, adding up to probably hundreds of millions of cubic meters of lava per day. The aa field continued to grow. The group reached the caldera floor and were able to sample both lava and tephra.

By 0700 on the 29th some of the vents had shut down and the two lava channels to the W (previously fed by the upper vent) stopped moving. The lower vent still emitted lava and fed one channel E of the others. The team estimated the channel to be ~ 10 m wide and moving ~5 m/s. Assuming a 2-m depth, the lava flux was 5 to 10 million cubic meters per day, about half that seen the morning of the 29th

The emission rate continued to diminish throughout the 29th and by the evening it was only 10-20% of that seen on the 28th. In addition, the amount of gases emitted decreased such that the gas plume only rose ~ 1 km, whereas earlier plumes had risen to several kilometers. The lower vent was no longer fountaining continuously as it had on the 28th; instead the fountaining came in bursts at intervals of about 1-30 seconds. A lava lake sloshed around in the lower vent's crater; some lava escaped this crater along a breach in the crater rim. The upper vent (the one that shut off) was still incandescent with a lot of gas coming out, so it was possible that there was a lava lake there too.

The eruption appeared to end on the 30th. Glow was observed at 0200, but had ceased by 0400. The vents still emitted gas, but not fresh lava. However, it was possible that there was still N-flank activity. There were reports of lava flows there, and while it was certain that at least some of these flows were clastogenic (composed of spatter from fire fountains that accumulated and then began to flow), it was uncertain whether there were also actively erupting flank vents. The team remained separated from this area by hot lava, thwarting reconnaissance. Initial estimates of the coverage of the caldera floor were an area of ~ 14 km². Assuming a 3-4 m average flow thickness, this was ~ 0.05 km3 (50,000,000 m³) of lava. There were obviously high error bars on this estimate, but it was clearly much less than the ~ 1 km³ extruded in the 1979 eruption.

MODVOLC Thermal Alerts. A large set of thermal hotspots in multispectral imagery was observed beginning late 22 October (local time and date) and continuing through 16 November 2005 (figure 11). Although MODVOLC data were missing for some days and reduced for others (presumably due to cloud cover screening the radiation from the satellite) these hot-spot pixels dramatically document the course of the eruption. Data on figure 11 appear consistent with in situ observations, in that by the second day, lava was at least 5 km down the outer N slope and covering much of the E caldera floor. By the 8th day (30 October), the outer slope flows had cooled significantly, but flows inside the caldera had continued their clockwise advance, filling all low points to the extreme SW corner of the caldera. Ten days later (9 November), the eruption had ended and only flows from the vents to the SE caldera floor were still emitting detectable heat. The last pixels observed, two above the original vent area on the N rim, were on 16 November.

References. Amelung, F., Jónsson, S., Zebker, H., and Segall, P., 2000, Widespread uplift and 'trapdoor' faulting on Galápagos volcanoes observed with radar interferometry: Nature, v. 407, p. 993-996.

Geist, D.J., Chadwick, W.W., Jr., and Johnson, D.J., in press, Results from new GPS monitoring networks at Fernandina and Sierra Negra volcanoes, Galápagos, 2000-2002: Journal of Volcanology and Geothermal Research (in press).

Jónsson, S., H. Zebker, and F. Amelung, 2005, On trapdoor faulting at Sierra Negra volcano, Galápagos; Journal of Volcanology and Geothermal Research, v. 144, p. 59-71.

Information Contacts: Lucho Verdesoto and Carman Guzman, M/N Polaris, Galápagos Islands, Ecuador (Email: explead.polaris@expeditions.amosconnect.com); Francisco Dousdebes, Galápagos Expedition Manager, Metropolitan Touring, Ecuador (Email: fdousbedes@metropolitan-touring.com); Lindblad Expeditions (URL: http://www.expeditions.com/); U.S. National Aeronautical and Space Administration (NASA), (URL: http://earthobservatory.nasa.gov/; http://www.nasa.gov/vision/earth/); Bill Chadwick, Cooperative Institute for Marine Resources Studies (CIMRS), National Oceanic and Atmospheric Agency (NOAA) Pacific Marine Environmental Laboratory (PMEL), Oregon State University, 2115 SE OSU Drive, Newport, OR 97365, USA (Email: William.W.Chadwick@noaa.gov); Dennis Geist, Department of Geological Sciences, University of Idaho, Moscow, ID 83844-3022 USA (Email: dgeist@uidaho.edu); Terry Naumann, Department of Geology, University of Alaska at Anchorage, Anchorage, AK 99598, USA (Email: aftrn@uaa.alaska.edu, URL: http://geology.uaa.alaska.edu/); Karen Harpp, Department of Geology, Colgate University, 408 Lathrop Hall, Hamilton, NY 13346, USA (Email: kharpp@mail.colgate.edu, URL: http://classes.colgate.edu/kharpp/khwebpage/); MODVOLC Alerts Team, Hawaii Institute of Geophysics and Planetology (HIGP), University of Hawaii at Manoa, 1680 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

November 1979 (SEAN 04:11) Cite this Report

14-km-high cloud; lava flows to sea

An eruption began on 13 November at Sierra Negra, the only historically active volcano in the Galápagos Islands that is presently inhabited. The location was Volcán Chico, a circumferential fissure zone 1 km N of, and 100-200 m below, the caldera rim of Sierra Negra. Volcán Chico was also the site of the last two eruptions from this volcano, in 1963 and 1953, and has been described by Delaney and others (1973). At the time of the last report lava was still flowing down the volcano's uninhabited N slope 3 weeks after it began. The residents of the S slope, evacuated on the first day of the eruption, had returned to find damaged crops and livestock.

At 0730 on 13 November a local earthquake was registered on the CDRS seismograph (90 km E of Sierra Negra on Isla Santa Cruz), part of the WWSSN maintained by the USGS [see also 4:12]. A second, larger, earthquake followed in 20 minutes, and at 0845 the first explosion was heard by people living on the S flank. Within 20 minutes of the first explosion, tephra (including scoria and Pele's hair) fell on the villages of Santo Tomás and Villamil, 13 and 26 km SE of the eruption site. Residents of Santo Tomás began evacuation to the coastal village of Villamil.

The eruptive cloud was large enough to be seen on NOAA's SMS-1 weather satellite, which transmits images of the whole hemisphere every 30 minutes from its equatorial geostationary orbit. The cloud first appeared on the 0930 image and grew rapidly to an area estimated to be 220 x 130 km within 2 hours. The cloud soon separated into two lobes and measurements on the infrared imagery by Arthur F. Krueger indicate a maximum elevation of at least 14 km for the main lobe at 1500. This lobe moved SE at 25 km/hour while the other lobe, estimated to be about 8 km high, moved SW at 30 km/hour. By 2200, the main lobes were quite diffuse, but a low, thin plume (near the 4-8 km resolution of the imagery) extended about 50 km S from Sierra Negra. No prominent eruptive clouds were visible on SMS-1 imagery during the following days.

A CDRS team (including P. Ramón, J. Budris, and T. & A. Moore) reached Volcán Chico 24 hours after the eruption began. Upon arrival at Villamil, they noted a thin coating of tephra with some light scoria fragments to 5 cm in diameter. Tephra at the caldera rim was 2-3 cm thick, with some 15-20-cm bombs, but nothing larger than 1 cm was falling at the time. Up to 20 vents were active along the 1-2-km [8 km, see 5:1] circumferential fissure and individual fountains reached 100-m heights. As many as 13 lava flows coalesced downslope to the N and probably reached the sea on the first day. Billowing clouds were visible at the coast of Elizabeth Bay on the morning of 15 November, and two biological groups were reported to be investigating the lava/sea interface.

At 1500 on 14 November, a higher resolution NOAA satellite, VHRR Tiros 6, returned an infrared image showing the lava flow on the N flank and a high vapor plume heading S. The CDRS seismograph recorded 8 local events on 14 November and 6 on the following day. Observers at the site felt many tremors, but noted that eruptive intensity seemed to decrease after a peak around 2200-2300 on 14 November. Local haze increased during the next 2 days and vent activity declined steadily. Only four vents were active and fountaining had almost stopped by sunset on 16 November. At 0400 the next morning, however, observers were awakened by the largest explosion of their 5 days at the site. Fountaining increased immediately and continued to build to a peak around 5 hours later. That same night the wind changed, carrying haze ENE over Isla Santa Cruz and reducing visibility there to 2 km on 19 November.

Activity at the vents remained low on 18 and 19 November, when the group left. The flows continued, however, and on the morning of 19 November, NASA's Landsat C satellite returned a high-resolution near-infrared image clearly showing the main flow as a 100 m band bearing 030° for about 12 km from the vent area (figure 1). On that same day, tephra fragments to 5 mm in diameter were collected on the ship Delfin, over 100 km E of the volcano.

As the CDRS group left the volcano, they noted an average tephra thickness of 3-5 cm on the caldera rim and serious damage to the biota on the upper SE flank. Acid rain and haze killed much vegetation and the group noted dead birds and rats. Residents of the S flank, who returned to their farms after the eruption's first day, reported damage to crops and livestock.

H. Hoeck was on the volcano 1-2 December and reported that the vegetation was beginning to recover. One large lava flow continued down the N slope, and several vents were steaming. Six small vapor vents were observed on the S floor of the caldera nearly 10 km S of the eruptive fissure. This caldera, the widest (10 x 7 km) and shallowest in the Galápagos, was not otherwise affected by the eruption, and Arnaldo Tupiza (CDRS representative on Isabela and very familiar with the volcano) recognizes these six vents as new.

None of the eruption earthquakes have yet been located on the WWSSN [but see 4:12], but a magnitude 5.0 event was located in the Galápagos at 1006 on 28 October. Its preliminary epicenter determination is 60 km NW of the eruption site.

Information Contacts: P. Ramón, J. Budris, H. Hoeck, P. López, and J. Villa, CDRS, Galápagos; M. Hall, Escuela Politécnica, Quito; A. Krueger, NOAA/NESS; C. Wood, NASA/GSFC; B. Presgrave, USGS/NEIS, Denver, CO.

December 1979 (SEAN 04:12) Cite this Report

Eruption ends; additional earthquakes located

The fissure eruption was continuing on 5 December. LANDSAT imagery on 7 December, however, showed no thermal anomaly like the lava flow on the 19 November LANDSAT image (figure 1), although a vapor cloud similar to that of 19 November was present. When Arnaldo Tupiza visited the eruption site on 29 December, the lava flow had ceased, but the vents continued to steam and the solfataric area had grown. The new fumaroles on the S caldera floor remained active.

Two earthquakes connected with this eruption have been located by USGS/NEIS. The first, at 0751 on 13 November [4:11], was magnitude 4.4 and its hypocenter (0.89°S, 91.23°W) was located 18 km SW of the eruption site. This was followed at 0817 by another earthquake that wrote a very similar, and only slightly smaller, record on the CDRS seismograph 90 km to the E. This event has not been located by NEIS, but was followed in less than 30 minutes by the first explosion recognized on the island. Seismicity increased on the eruption's second afternoon, and at 1858 on 15 November, a magnitude 4.8 event was recorded. Its hypocenter (0.96°S, 91.25°W) was located by NEIS 24 km SSW of the eruption site. These hypocenters are in the area of the February 1979 flank eruption of Cerro Azul [4:1-2], but study of the 1968 caldera collapse earthquakes from Fernandina (Filson et al., 1973), where true epicenters were known, showed that hypocenter mislocations for events of this magnitude range averaged about 50 km.

Reference. Filson, J., Simkin, T., and Leu, L.-K., 1973, Seismicity of a caldera collapse: Galápagos Islands 1968: JGR, v. 78, no. 35, p. 8591-8622.

Information Contacts: A. Tupiza and H. Hoeck, CDRS, Galápagos; USGS/NEIS, Denver, CO.

January 1980 (SEAN 05:01) Cite this Report

Last month's eruption obituary was premature

On 14 January the new lava delta, formed at Bahia Elizabeth by the 13 November eruption, was visited for the first time in 2 months. Hendrik Hoeck, inspecting the flow at night with binoculars, saw red spots along the flow length and some active feeding at vents 12 km inland. Motion was not visible along the 1-1.5-km flow front in daylight, but water temperatures of 50-60°C at distances of 30 m from the shore provide further evidence that the flow remains active. Pelicans and Franklin's gulls were concentrated in the area, presumably feeding on killed marine organisms, and two dead green turtles were observed. A pack of feral dogs appears to have been cut off by the new flows from more vegetated parts of Sierra Negra, and it is feared that they may cross the barren older lava flows to reach volcán Alcedo to the N. Alcedo has not been threatened with feral dogs and it is the home of the largest population of giant tortoises in the Galápagos.

Arnaldo Tupiza walked about 6 km inland on 14 January, noting flow thickness of 3-5 m, two 10 m-diameter "pools" of molten lava, and another major flow diverging toward the NE. The most active vents were farther W, along the circumferential fissure, than any previously reported, and it is not yet clear whether activity has resumed after migrating westward or whether the westerly vents were not recognized during previous observations from the vigorously steaming E end of the fissure system. The mid-January observations indicate an active fissure length of 8 km.

Information Contacts: H. Hoeck and A. Tupiza, CDRS, Galápagos.

September 2004 (BGVN 29:09) Cite this Report

12 June-29 August 2004, GPS data indicate 77 cm of caldera-floor uplift

Sierra Negra volcano contains a six-station, continuously monitored GPS network. The instruments were installed in collaboration with a research consortium (UNAVCO) in May 2002 (figure 2).

Starting on 12 June 2004 and continuing through at least 29 August 2004, the rate of uplift of the caldera floor, as measured by this network, had accelerated to 77 cm/year (table 1). This rate was comparable to that inferred from InSAR data in the late 1990s (Amelung and others, 2000). That late 1990's uplift was attributed to trap-door faulting of the caldera floor along its southern margin, a process presumably driven by a shallow (< 2 km) intrusion of magma. Deflation occurred during 2001-2002, and slower uplift of about 12 cm/y prevailed during March 2003-May 2004 (table 1). The 12 June-29 August interval was noteworthy for the high rates of uplift (table 1).

Table 1. A summary of the geophysically derived movement of the caldera floor at Sierra Negra. Courtesy of Dennis Geist, William Chadwick, and Dan Johnson.

    Interval              Comments                                Measurement Technique

    1992-1998             240 cm of uplift punctuated by          InSAR
                            trapdoor uplift in 1997 or 1998         (Amelung and others, 2000)
    1998-1999             Inflation at 65 cm/year                 InSAR
                                                                    (Amelung and others, 2000)
    2000-2001             Deceleration of uplift to 7 cm/year     Campaign GPS
                                                                    (Geist and others, submitted)
    2001-Feb 2003         Deflation of ~ 9 cm/year                Campaign and continuous GPS
    Mar 2003-May 2004     Inflation at ~ 12 cm/year               Continuous GPS
    12 Jun-29 Aug 2004    Inflation at 77 cm/year                 Continuous GPS

The Instituto Geofísico in Quito, Ecuador monitors seismic activity in the Galápagos, using a network that includes a single station on Sierra Negra. Unfortunately, that network was down for the past year, in need of a variety of hardware, including the seismometer at Sierra Negra. Thus, for the interval of interest, seismic data were absent. Hugo Yepes estimated that to repair the Galápagos system would require about $9,000 (USD) in equipment and $4,000 (USD) in personnel transport and field expenses. He also said that the region requires more stable long-term logistical support.

The 12 June-29 August 2004 uplift was symmetrical about the caldera's center. The pattern and rate of uplift was well modeled as a 2.1 km deep sill intruded by about 12 x 10⁶ m³ of magma since June 2004.

Sierra Negra last erupted in 1979, when nearly 1 km³of lava erupted from a circumferential fissure near the summit, covering its N flank./p>

References. Amelung, F., Jonsson, S., Zebker, H., Segall, P., 2000. Widespread uplift and "trapdoor" faulting on Galápagos volcanoes observed with radar interferometry. Nature 407, 993-998.

Geist, D., Chadwick, W.W., and Johnson, D., Results from new GPS and gravity monitoring networks at Fernandina and Sierra Negra volcanoes, Galápagos, 2000-2002 (submitted to the Journal of Volcanology and Geothermal Research in 2004).

Information Contacts: Dennis J. Geist, Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022 USA (Email: dgeist@uidaho.edu), William W. Chadwick, Jr., Cooperative Institute for Marine Resources Studies (CIMRS), NOAA Pacific Marine Environmental Laboratory (PMEL), 2115 SE OSU Drive, Newport, OR 97365 USA (Email: bill.chadwick@noaa.gov); Daniel J. Johnson, University of Puget Sound, Department of Geology, 1500 N. Warner, Tacoma, WA 98416, USA (Email: djjohnson@ups.edu); Hugo Yepes, Geophysical Institute (IG), Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/).

September 2005 (BGVN 30:09) Cite this Report

Caldera erupts starting 22 October 2005 at fissure on caldera's inner N wall

At about 1730 on 22 October 2005 Sierra Negra began erupting. This shield volcano with a large oval-shaped caldera is located at the S end of Isabela Island. Circumferential fractures define the northern edge of the caldera. Volcán Chico, noted for its 1963 and 1979 eruptions, is comprised of a series of scoria cones and other vents aligned along several prominent fractures on the outer slope of the N caldera rim. The present activity is not related to the Volcán Chico fracture system, but is venting from fractures along the N inner caldera wall. The most prominent fracture can be traced westward ~ 3 km where it lies along the rim. This initial report was provided by a scientific team from the Instituto Geofísico.

The eruption was preceded by a seismic event at 1438 on 22 October, felt in the coastal village of Villamil (20 km SE of the caldera border) and by Park Wardens on Cerro Azul. Others reported single earthquakes on 19 October and two weeks earlier. At 1730 the eruption began with an explosion heard by many people in the Villamil area. Hikers in the area of the subsequent lava emission in the mid-afternoon of both 21 and 22 October witnessed no unusual activity. By 1745 the eruption column had reached an estimated altitude of 5 km, and the setting sun illuminated the light gray eruption column. At 1815 the team observed the column after sunset from Point (Punto) Ayora, Santa Cruz Island (80 km E) and estimated its height at 10 km. The still-rising column was 4-6 km wide, not spreading laterally, and a small lenticular cloud was beginning to form a cap over the column. As night fell, the western sky above the caldera was a burgundy red, suggesting that lava had covered an extensive area of the caldera floor. Satellite imagery of the eruption at 1745 showed an eruption cloud at an estimated altitude of at least 15 km moving SW. A very large hotspot in the multispectral imagery was also observed and continued on 27 October.

Observations at 1945 from the Santa Cruz highlands (75 km away) employing a camcorder with night vision capabilities confirmed extensive lava fountaining estimated to be 200-300 m high along a segment of the caldera rim, as well as the incandescence from a lava flow several kilometers long descending the NW outer flank. Although the complete eruption column was not visible, it may have reached an altitude close to 20 km and had spread out. Tourist boats between Isabela and Fernandina Island reported seeing two lava flows descending the N flank.

During an overflight between 0715 and 0900 on 23 October the team did not witness active lava flows or evidence of lava having entered the sea. A thin khaki-colored ash cloud layer was observed, between about 1,200 and 1,500 m altitude, that had spread out laterally and extended E as far as St. Cruz Island and N to Santiago Island. Later in the day the plume was directed NNW in agreement with satellite information. From the plane the team confirmed that the main eruption was venting from four craters along a 500-m-long fracture at the base of the NNE inner caldera wall. The highest lava fountaining (up to 200 m high) was being generated at the two middle vents, while the end vents were feeding many lava flows S onto the caldera floor. The fracture apparently extended W along the inner wall, but then climbed to the caldera rim where its trace was not obvious. However, small vents with fountaining and incandescent lava were observed on the rim along this general fracture system, implying that the active fracture extended ~ 2 km W of the main vents.

During the mid-day hours of 23 October the team ascended the S flank, followed the E rim of the caldera, and reached a point ~ 800 m from the active vents, from which the following description was made. From the four principal vents the lava flowed S with exceptional force, volume, and speed downslope in three main channels (figures 3 and 4). Based on the apparent speed of the lava, and the more than 10-m height of the waves in the stream of passing lava, the team estimated that the main lava flow was traveling nearly 20 m/second as it left its vent. The W channels, some 30-50 m wide, maintained their red incandescent color and high speeds, albeit less than that near the vent.

By 1500 the E channel was slowing and cooling to a gray surface color; this thin solid veneer was subsequently fragmented when the flow went over the edge of the bench and cascaded to the caldera floor. On the caldera floor the incandescent lavas of all three channels disappeared under the black solidified a'a lava that already covered about 12% of the caldera. In the 22 hours since the eruption had begun, the lavas had formed one large flow 1-1.5 km wide that traveled SE along the base of the E interior caldera wall, then W along the S wall reaching a point almost halfway across the caldera. As such it had traveled a total distance of 7 km and had started small brush fires on the floor and interior walls of the caldera. With an estimated thickness of no more than 3 m, the volume of the lava ejected by 1530 on 23 October was calculated at about 25 million cubic meters.

Along the trail leading to the vent area an increasing amount of scoria fragments was observed on the rim's edge. Fragments ~ 1 cm in size were first observed ~ 4 km SE of the active vents, and they increased in size (up to 15 cm) and abundance towards the vents. Very little fine ash was in the air or on the ground along the E caldera rim. The scoria was black, exceedingly vesiculated, with vesicles from millimeter to many centimeters in diameter; it seemed comparable in density to popcorn. No crystals were observed in the glassy scoria material. At their closest approach to the vent, scoria fragments formed a deposit 3-5 cm thick.

An explosion heard at 1900 on 25 October was accompanied by a dark eruptive column and minor ashfall along the E rim of the caldera and probably elsewhere. By early 26 October the Park Wardens were reporting that one of the four principal vents had shut down. Observations made late on 26 October indicated that the a'a flow on the caldera floor had slowed and was still several kilometers from the sulfur mine area. Civil Defense officials also reported that apparently less lava was leaving the vents and that lava extrusion might have shifted to the outer N flank, possibly to the Volcán Chico fracture system.

The only inhabited areas include the small town of Villamil, located 20 km SE of the caldera's border on the S coast, plus several other small populated areas about halfway between the caldera and Villamil. There was no immediate threat to those residents, given the fact that in order to spill out of the caldera and descend the S flanks the entire 100-m depth of the caldera would have to fill with lava. The southern caldera border has not been active in the recent geologic past.

Information Contacts: Minard Hall and Patricio Ramón, Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/, Email: mhall@igepn.edu.ec, pramon@igepn.edu.ec); Washington Tapia and Oscar Caravajal, Parque Nacional Galápagos, Pto. Ayora, Santa Cruz Island, Ecuador (Email: oscar_caravajal@yahoo.es).

October 2005 (BGVN 30:10) Cite this Report

Eruption ends 30 October; some lava on NE flank, more on E caldera floor

Our last report (BGVN 30:09) described the first five days of this eruption, and was taken largely from a valuable joint report of Ecuador's Instituto Geofísico and Parque Nacional Galápagos. Here we report information from several sources on these topics: (a) initial observations of the eruption, (b) caldera-floor deformation prior to the eruption, (c) observations of the eruption's progress during 26 to 30 October (when it ended), and (d) satellite infrared observations of thermal fluxes associated with the eruption.

Eruption's start and subsequent plumes. As noted previously (BGVN 30:09), the eruption began around 1730 on 22 October 2005, when an explosion was heard by many residents of the volcano's S flank. Satellite images showed no activity at 1715, but revealed a large eruption at 1745 local time (2345 UTC). The eruption cloud reached an estimated altitude of at least 15 km (50,000 ft) and was moving SW.

At about this time, passengers and crew on Lindblad Expeditions' 80 passenger vessel M/N Polaris had an excellent view of the eruptive plume (figure 5). Lucho Verdesoto, the expedition leader, reported that the ship was then at Cerro Dragon, Santa Cruz island. Sunset was at 1753. As night fell they sailed to a position ~ 18 km NE of the volcano, where they had clear views of flows descending the volcano's upper NE flank (figure 6).

Naturalist Carman Guzman wrote, "After sunset the show was fascinating so we decided to move the Polaris to a much closer location. After dinner, we were only eleven miles from the eruption itself. What a thrill! The darkness of the night enhanced the beauty of the fiery reds and oranges that were seen at the top of the caldera. We spent several hours enjoying this rare and fantastic event. Rivers of lava were running down the slopes of the volcano and enormous flames were lighting up the sky."

According to NASA MODIS imagery and VAAC/NOAA reports, on 25 October 2005 a large plume of gases and steam was observed in GOES 12 imagery for 1545 local time (2145 UTC). The plume extended ~ 460 km W and SW of the summit at an altitude of ~ 4.6 km. Figure 7 shows the average concentration of SO₂ over the Sierra Negra plume as imaged by NASA's Aura satellite for the period 23 October-1 November.

Deformation monitoring. In the early stages of this eruption, Bill Chadwick (NOAA) submitted a report on pre-eruption deformation (figure 8). The plot shows both Synthetic Aperture Radar (InSAR) and GPS data on vertical deformation of the caldera floor. Chadwick wrote that he, Dennis Geist (University of Idaho), and Dan Johnson (University of Puget Sound, recently deceased) installed a 27 station GPS network at Sierra Negra in 2000, that was reoccupied in 2001 and 2002 (Geist and others, in press). With help from UNAVCO (a consortium supporting high-precision deformation measurements), the group then added a 6-station, continuous GPS network in 2002. Since then, there occurred a change from caldera subsidence to caldera uplift in March 2003. During this uplift, an M 4.6 earthquake on 16 April 2005 marked trapdoor faulting. The continuous GPS network measured a surface displacement of 85 cm within 10 seconds. Both this event and the previous case of trapdoor faulting in 1997-8-documented by satellite measurements using Interferometric Synthetic Aperture Radar (InSAR) (Amelung and others, 2000)-were preceded by over a meter of inflation (Jónsson and others, 2005). Both the 1997-98 and 2005 trapdoor movements occurred along the caldera floor's S side.

Aside from its immediate affects, the April 2005 earthquake left the later inflation rate unchanged. Caldera-centered uplift has continued since then without pause at about the same high rate. During the interval from March 2002 to April 2005 there was about ~ 1.2 m of uplift. Rates after the April 2005 earthquake are not plotted but were roughly the same as those during the interval March 2002-April 2005. The only other large earthquakes at Sierra Negra in the last year were an M 4.0 on 23 February 2005, which was associated with a small (2 cm) displacement near the trapdoor fault, an M 4.6 on 19 September 2005 that caused no obvious displacements, and an M 5.5, just 3 hours before the 22 October eruption started. The GPS data has not yet been processed.

Field descriptions of the eruption. The eruption began on 22 October with venting along a 2-km fissure near the caldera's N rim (figure 6). The fissure descended the caldera's inner wall at its E end. Flows were fed both northward down the outer N flank and southward onto the NE caldera floor. Although flows reached 5 km down the outer flank, flow into the caldera soon dominated, with strong channels descending inner caldera slopes before combining to form a wide aa flow banked against the caldera's E wall and moving steadily southward (see figures and discussion, BGVN 30:09).

Figure 9 is a photo taken by Greg Estes on 24 October. It highlights the vigorous venting and intracaldera flows at that point in the eruption. Figure 10, a post-eruption satellite photo, illustrates the broad pattern of still-cooling, erupted lavas (which appear as light colored areas on this 2 November thermal-infrared image). Although this may represent the best overview of the new lavas at this time, some of the thinnest flows or chilled flow features may not appear on this image.

By 26 October, fissure activity had narrowed to one major vent very near the N rim, but at 0830 on the 27th, eyewitness Godfrey Merlin reported that a second vent opened downslope and SE of the first. This new vent did not diminish the activity of the first, meaning that the total flux of erupting lava nearly doubled.

By about 1400 on the 27th, a team including Dennis Geist (University of Idaho), Terry Naumann (University of Alaska), and Karen Harpp (Colgate University) had arrived at the E caldera rim and began sending back a series of valuable reports. Their first report noted a major vent immediately below GPS station SN12 on the rim NE of the caldera's center. This vent emitted a large intracaldera aa flow.

Some active N-flank vents stood about 300-400 m NW of a station (GV01) on the caldera's N rim . There, two major vents fed lava fountains up to 50 m high. Most lava being erupted was flowing into the caldera, although some of the scoria from the fountains was falling outside the caldera and then forming a short, sluggish flow. Lava inside the caldera was cascading from the vents down the slope on the N edge of the caldera in 3 main channels, each 30-40 m across, with lava flowing at ~10 m/s (36 km/h) and in some cases over 10 m/s, and coalescing into a major aa flow to the S. On the caldera floor these channels merged into one big aa channel about 100 m wide that flowed more slowly both to the S, clockwise along the base of the E caldera wall, and into the moat along the S edge of the caldera floor. Pahoehoe outbreaks occurred along the margins of the major aa flow. New aa lava covered an estimated one-third of the caldera floor.

The report for 28 October noted that the eruption was still going strong. There were no significant new events on this day, but it appeared that the lava flux had increased because the vents looked wider and there seemed to be a lot more gas emitted. The lava continued to feed from the vents to the caldera floor in two large streams, each ~ 20 m across with lava traveling at 5-10 m/s, adding up to probably hundreds of millions of cubic meters of lava per day. The aa field continued to grow. The group reached the caldera floor and were able to sample both lava and tephra.

By 0700 on the 29th some of the vents had shut down and the two lava channels to the W (previously fed by the upper vent) stopped moving. The lower vent still emitted lava and fed one channel E of the others. The team estimated the channel to be ~ 10 m wide and moving ~5 m/s. Assuming a 2-m depth, the lava flux was 5 to 10 million cubic meters per day, about half that seen the morning of the 29th

The emission rate continued to diminish throughout the 29th and by the evening it was only 10-20% of that seen on the 28th. In addition, the amount of gases emitted decreased such that the gas plume only rose ~ 1 km, whereas earlier plumes had risen to several kilometers. The lower vent was no longer fountaining continuously as it had on the 28th; instead the fountaining came in bursts at intervals of about 1-30 seconds. A lava lake sloshed around in the lower vent's crater; some lava escaped this crater along a breach in the crater rim. The upper vent (the one that shut off) was still incandescent with a lot of gas coming out, so it was possible that there was a lava lake there too.

The eruption appeared to end on the 30th. Glow was observed at 0200, but had ceased by 0400. The vents still emitted gas, but not fresh lava. However, it was possible that there was still N-flank activity. There were reports of lava flows there, and while it was certain that at least some of these flows were clastogenic (composed of spatter from fire fountains that accumulated and then began to flow), it was uncertain whether there were also actively erupting flank vents. The team remained separated from this area by hot lava, thwarting reconnaissance. Initial estimates of the coverage of the caldera floor were an area of ~ 14 km². Assuming a 3-4 m average flow thickness, this was ~ 0.05 km3 (50,000,000 m³) of lava. There were obviously high error bars on this estimate, but it was clearly much less than the ~ 1 km³ extruded in the 1979 eruption.

MODVOLC Thermal Alerts. A large set of thermal hotspots in multispectral imagery was observed beginning late 22 October (local time and date) and continuing through 16 November 2005 (figure 11). Although MODVOLC data were missing for some days and reduced for others (presumably due to cloud cover screening the radiation from the satellite) these hot-spot pixels dramatically document the course of the eruption. Data on figure 11 appear consistent with in situ observations, in that by the second day, lava was at least 5 km down the outer N slope and covering much of the E caldera floor. By the 8th day (30 October), the outer slope flows had cooled significantly, but flows inside the caldera had continued their clockwise advance, filling all low points to the extreme SW corner of the caldera. Ten days later (9 November), the eruption had ended and only flows from the vents to the SE caldera floor were still emitting detectable heat. The last pixels observed, two above the original vent area on the N rim, were on 16 November.

References. Amelung, F., Jónsson, S., Zebker, H., and Segall, P., 2000, Widespread uplift and 'trapdoor' faulting on Galápagos volcanoes observed with radar interferometry: Nature, v. 407, p. 993-996.

Geist, D.J., Chadwick, W.W., Jr., and Johnson, D.J., in press, Results from new GPS monitoring networks at Fernandina and Sierra Negra volcanoes, Galápagos, 2000-2002: Journal of Volcanology and Geothermal Research (in press).

Jónsson, S., H. Zebker, and F. Amelung, 2005, On trapdoor faulting at Sierra Negra volcano, Galápagos; Journal of Volcanology and Geothermal Research, v. 144, p. 59-71.

Information Contacts: Lucho Verdesoto and Carman Guzman, M/N Polaris, Galápagos Islands, Ecuador (Email: explead.polaris@expeditions.amosconnect.com); Francisco Dousdebes, Galápagos Expedition Manager, Metropolitan Touring, Ecuador (Email: fdousbedes@metropolitan-touring.com); Lindblad Expeditions (URL: http://www.expeditions.com/); U.S. National Aeronautical and Space Administration (NASA), (URL: http://earthobservatory.nasa.gov/; http://www.nasa.gov/vision/earth/); Bill Chadwick, Cooperative Institute for Marine Resources Studies (CIMRS), National Oceanic and Atmospheric Agency (NOAA) Pacific Marine Environmental Laboratory (PMEL), Oregon State University, 2115 SE OSU Drive, Newport, OR 97365, USA (Email: William.W.Chadwick@noaa.gov); Dennis Geist, Department of Geological Sciences, University of Idaho, Moscow, ID 83844-3022 USA (Email: dgeist@uidaho.edu); Terry Naumann, Department of Geology, University of Alaska at Anchorage, Anchorage, AK 99598, USA (Email: aftrn@uaa.alaska.edu, URL: http://geology.uaa.alaska.edu/); Karen Harpp, Department of Geology, Colgate University, 408 Lathrop Hall, Hamilton, NY 13346, USA (Email: kharpp@mail.colgate.edu, URL: http://classes.colgate.edu/kharpp/khwebpage/); MODVOLC Alerts Team, Hawaii Institute of Geophysics and Planetology (HIGP), University of Hawaii at Manoa, 1680 East-West Road, Post 602, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).