The eruption . . . began on 19 April and ended in the early morning hours of 24 April. It was observed by several groups both on and near Fernandina, providing documentation that is unusually detailed for this uninhabited island volcano.

The start of the eruption was witnessed at about 1300 by Kirstin and Feo Pitcairn while sailing towards Fernandina ~30 km to its N. A "towering column" developed within only a few minutes, and one hour later a second plume, from a source N of the first, was recognized. David Day. . . reported that the main vent was near the base of the ESE caldera wall at the 1988 eruption site, with another vent ~3 km to the NW, also on the main caldera boundary fault and near the easternmost 1978 eruption vent. At 1500, Day, then sailing near Isla Santiago, noted that the leading edge of the cloud had already reached that island's high point, ~ 90 km ENE of its source.

Shortly after 1500, cloud development accelerated. Kirstin Pitcairn described a "big white mushroom cloud above the N plume" and estimated the height of the rapidly rising S plume at 4-6 km. Day described the distant cloud as building slowly after 1510, and both observers remarked on the increased density of the ash cloud. At 1535 a new plume joined the other two, nearer the S plume, and rose very rapidly, but the S plume remained dominant and Pitcairn saw pink coloration to its top in daytime. Starting about 1600, ash fell at Cabo Hammond, on Fernandina's SW corner, where Markus Horning and assistants were studying fur seals. Ashfall was continuous for 3 hours and intermittent until about 2230, with an estimated accumulation of 5-10 mm for the full eruption. At 2015 Horning first heard noise from the eruption, a strong continuous rumbling without booms or explosions, that continued until well after midnight. A single explosion was heard by Milton Friere, 50 km E on Volcán Alcedo, at 1630 ( ± 15 minutes).

At 1830 David Day, then 110 km ESE, saw "the first of 3 large dark clouds punch up quickly above the low cloud covering Isabela . . . over a 10-minute period," and estimated the cloud height at 3-4 km.

That night the Pitcairns watched and videotaped the eruption from Punta Espinoza on Fernandina's NE coast. They described a varying spectacle including "flame-shaped jets shooting high into the billowing column," alternation of brightness between the two main plumes, and cessation of the central plume at 2043. At Cabo Hammond, Horning routinely measured incident light intensity at sea level every night, and his readings indicated maximum light emission/reflection that night from about 2000 to 2200. He noted that this was the only night in which glow from two vents was visible (only the S vent being active in later nights). Although it was a dark night (new moon 14 April), the peak glow corresponded to roughly 2/3 the light measured on clear full-moon nights.

The eruption was quieter on the early morning of 20 April, but zoologists N.P. and M.J. Ashmole, also at Espinoza, described renewed activity around 0845, including audible explosions, ash, and reappearance of the central column. On the opposite corner of the island, Horning experienced a heavy, dense fog that obscured the summit, but he heard strong explosions at 0857 and 1116. The Pitcairns described a huge dark cloud forming at 0910, and in late morning they sailed W to circle the island, but encountered heavy ashfall off the WNW coast. At 1152 the Nimbus-7 . . . TOMS instrument measured a strong SO₂ plume to the SW, with the greatest concentration 500-600 km SSW and trace values to the W. A preliminary estimate of the total mass of SO₂ was 1.7 x 10⁵ metric tons. The combination of ash and aerosol that stung the eyes caused the Pitcairn group to turn back about 1500. Ashfall increased to the N in late afternoon, and they experienced (decreasing) ashfall all the way back to Punta Espinoza. Very little ash fell at Cabo Hammond.

Activity had declined by the morning of 21 April, with only the S plume continuing and at decreased height. By mid-morning the summit was obscured by low cloud cover, but at 1120 Pitcairn saw all three plumes active (although the N one was small). From the summit of Sierra Negra, 65 km SE of Fernandina, David Day photographed "a medium-size eruption cloud" at noon. At the same time, however, the TOMS instrument detected virtually no SO₂ over Galápagos but a low concentration 600 km W, on the equator. That night, Day sailed around Isabela and briefly saw faint glow over Fernandina as he approached it from the S.

On the morning of 22 April, . . . Day landed at NW Fernandina and noted 1 mm of fresh ash. At about 1040, while still low on the NW flank, he heard roaring from the vent, then roughly 12 km distant. This apparently marked a renewal of activity, for the TOMS instrument measured a strong concentration of SO₂ immediately over Fernandina at 1046. Day reached the rim at 1730 and described 50-100-m fountains from the 1988 vent area, low on the opposite caldera wall. Fresh aa flows covered an estimated 80% of the low caldera floor, with only the higher lobes of the 1988 debris avalanches still visible. Most flows were to the NW, but a smaller flow went W below the SE bench. The aforementioned northerly vent, on the E side of the NW bench, had fed "a small flow" to join the others on the NW floor, and fumarolic activity was vigorous at the vent.

Day reported that the eruption continued with the same intensity all night, and the next day he explored to the S, finding that the maximum thickness of new tephra on the W rim was 1 cm at a point WNW of the main vent. Pele's hair was "fairly abundant." On this day (23 April), the GOES satellite detected a 105-km plume at 0900 that grew to 320 km SSW at 1300 and had dissipated by 1600 (16:3). At 1103 the TOMS instrument detected a strong SO₂ concentration ~ 90 km SW and lower values to ~ 225 km SW; a preliminary estimate of the total mass was ~4 x 10⁴ metric tons. Day was on the S rim of the caldera at 1205, when he saw "a mass of landslides round and above the main vent" that was immediately followed by increased activity at the vent. Fountain height increased by almost 50% and his group (~ 3 km SW of the vent) experienced light scoria fall 10 minutes later that lasted for 15 minutes. Noise and fountaining, after almost ceasing, resumed at 2006 that evening and Day saw additional flareups at 2019, 2037, and 2100. Day observed a small flow NW from the main vent from 2100 to 2122, with no noise, but reported no further observations or sounds overnight.

Horning had reached the SW rim at 1700 and watched the S vent continue producing lava until at least 0100 on 24 April, but it had ceased by 0530. Day also noted no activity between dawn and his leaving the rim at 0630 that morning. Horning's SW-rim camp received 1 mm or less of ash overnight, but when they returned to their coastal camp that evening ~ 1-2 mm had accumulated in their absence. No glow was observed during the nights of 24 and 25 April.

Geologist Dennis Geist was on the summit of Alcedo from 24 April and reported that the only sign of a Fernandina eruption was a small (~ 3 km diameter) white cloud above the caldera. No glow was observed that night, either from Alcedo or N of the volcano (where Day was sailing around N Isabela). The small white cloud persisted over Fernandina at least until 27 April when Geist left Alcedo.

Geological Summary. Fernandina, the most active of Galápagos volcanoes and the one closest to the Galápagos mantle plume, is a basaltic shield volcano with a deep 5 x 6.5 km summit caldera. The volcano displays the classic "overturned soup bowl" profile of Galápagos shield volcanoes. Its caldera is elongated in a NW-SE direction and formed during several episodes of collapse. Circumferential fissures surround the caldera and were instrumental in growth of the volcano. Reporting has been poor in this uninhabited western end of the archipelago, and even a 1981 eruption was not witnessed at the time. In 1968 the caldera floor dropped 350 m following a major explosive eruption. Subsequent eruptions, mostly from vents located on or near the caldera boundary faults, have produced lava flows inside the caldera as well as those in 1995 that reached the coast from a SW-flank vent. Collapse of a nearly 1 km3 section of the east caldera wall during an eruption in 1988 produced a debris-avalanche deposit that covered much of the caldera floor and absorbed the caldera lake.

Information Contacts: D. Day, Isla Santa Cruz; F. Pitcairn and K. Pitcairn, Bryn Athyn, PA, USA; M. Horning, Seeweisen, Germany; S. Doiron, GSFC; N. Ashmole and M. Ashmole, Univ of Edinburgh, Scotland; D. Geist, Univ of Idaho, USA.