Looking back at my path over the past few weeks, I have been to quite a few island volcanoes, each with its own distinct volcanic-island-chain style. from the broad slopes of Hawai’i to the cone-studded volcanoes of the Canaries…and then here to the overturned soup-bowls of the Galapagos. islands of Ecuador…Fernandina Island, to be more specific, or as the volcano is sometimes popularly known, La Cumbre.
La Cumbre is the youngster of this hotspot island chain, but also the most active. She is a 1,476 m high shield volcano with an elongated 5 X 6.5 km summit caldera. The outer slopes of the volcano are fairly inhospitable to life, so plants and animals only occupy a small percentage of the island. Inside the caldera, the walls are steep, and there are lava benches along the inner slopes, most notably at the northwest. These denote a volcano’s history of lava entering and retreating from the crater. A lake temporarily occupies the caldera floor, though frequent eruptions within the caldera have affected its shape and size.
There have been 29 confirmed eruptions at La Cumbre, 24 of which are confirmed by historical observation. However, because of the volcano’s remoteness from human populations, the dates of the beginning and/or end of the eruptions are often uncertain. Most eruptions are smaller effusive events, often occurring within the caldera along the rim or walls at faults. Flank eruptions are less frequent, although there have been two recently; one in 1995 and again in 2009. The latter eruption produced the small cones seen in my 3rd postcard clue.
La Cumbre’s eruptions are usually VEI 0-2, with one exception. In the summer of 1968, the first news of a new eruption arrived in the form of these two bulletins, though it appears the source of the quake was not yet named.
[from GVP bulletin reports]
Seismic Report from Quito, Ecuador “P wave 05h 56m 28s UT 11 June 1968. S wave missing. 600 miles west of Quito”
Infrasonic Report from Boulder, Colorado “Acoustic wave reached Boulder at 0329 UT on 12 June. (Origin of wave at Galapagos approximately 2230 UT/1730 local time on 11 June). Stupendous explosion in the multi-megaton range, with gravity waves in atmosphere arriving about the same time as acoustic waves. Periods as long as 9 minutes amplitudes may be 20-40 dynes/cm2.
The explosive June 11 eruption of La Cumbre commenced with a phreatomagmatic bang. It is believed the cause was groundwater entering the system as the magma column retreated, possibly into a new dike or submarine flank vent. A radio report received on 14 June describes the explosive eruption in more detail, though there still appeared to be some confusion regarding just where the eruption was happening.
[from GVP bulletin reports]
“Heard eruption for first time, even though the island is 75 miles away. Quite a bit of lightning in the eruption clouds. A lot of lightning. No flames could be seen from Santa Cruz, but smoke seen pouring from area to the west and started again later on. Not sure which crater it came from on Isabela, but it came from the south end of Isabela. The night before last (12 June) there was quite a bit of activity. There may be a lava flow. There was a lava flow on the SE slopes of Fernandina a few weeks ago…about two or three weeks ago. The lava flow was seen on Fernandina from a ship at sea. This one may be Santo Tomas, or it may very well be Blue Mountain.”
A few days later, upon learning that a Chilean ship had observed a lava flow on Fernandina Island, a team was sent from Darwin Station to investigate the impact on wildlife, but they found something quite unexpected. The caldera was collapsing.
[from GVP bulletin reports]
The following report was received from Santa Cruz Seismic Station at 2115 GMT on 23 June 1968. Darwin Staion members visited the rim of the crater of Fernandina on 19 June and report the following observations: “The caldera is collapsing. There is continuing activity and it is causing the rim of the crater to break down. All around the crater the rim is collapsing, especially the south southeast rim. Very strong shocks have been felt. We had great difficulty in standing up because of heavy dust created by the thousands of tons of rock that was breaking off the rim and falling onto the floor. there are platforms that have also broken down and are continuing to break down. The platform on the SSE side of the crater has broken down. No lava flow was seen (a visit was not made to the SE of the crater where a lava flow was seen by the Chilean ship Navarino last month).
Before 11 June, the caldera had been almost flat except for a single tuff cone. Following the explosion, a 2 km wide section of the caldera floor in a series of around 75 incremental slips, had lowered much like a trap-door hinged at the northwest, and carried the intact tuff cone down 250 m with it. During that time, swarms of earthquakes ranging from M4 to M5.2 accompanied the collapse. Strangely, the volume of material erupted during that time only accounts for a small percentage of the void below the caldera, lending further support to the theory of the magma moving into a dike or elsewhere.
Thank you Jesper and Granyia for coming to find me this week. This volcano, like many others, has proven to be a learning experience and I have gained some new insight into caldera formation. However, I STILL haven’t figured out how to avoid being found by the Search Party. Still, it’s fun to try. Be here this Monday (Halloween..at midnight EDT…mwahahaha)..for a new adventure at a new volcano.