http://www.youtube.com/watch?v=MQ-7HwZyOVs (from Miguel Bravo’s youtube channel; well worth perusing)
You may want to mute it; but I kinda like the music…
The 1971 eruption of Teneguia, La Palma is well documented/ analysed, so presented here is an “abschtract”; with a list of such scientific papers as I could find freely available.
Tenguia is the most southerly, terrestrial expression of La Cumbre Vieja volcano; there is (not surprisingly) evidence of an undersea extension. Cumbre Vieja translates as “Old Summit” but it’s actually the active volcano driving construction of the island. There is a Cumbre Nueva; which is a scarp formed by a lateral collapse. Old and new refer to reforestation, not volcanic activity:
Caldera de Taburiente and Barranco de las Angustias are the dominant features; south of which are Cumbre Nueva, then Cumbre Vieja…
Cumbre Vieja has had several historic eruptions, with activity also documented in: 1585, 1646, 1677, 1712, and 1949, there is also solid evidence for an eruption at some time ~1430-80.
Cumbre Vieja: Eruptions, by Carracedo.
To give this some context; the Canaries have also had semi- historical eruptions in Tenerife: ~1341, ~1390, ~1444, 1492, 1704, 1706 and 1909; Lanzarote: 1730-36 and 1824 and El Hierro (questionably/ submarine) 1793 and (of course) 2011-present day?
That gives an average of ~ 39 years between (recent) eruptions, at widely spaced locations; it’s nearly 500km from NE Lanzarote to SW El Hierro. All powered by a single hotspot; edge driven convection is one of the current explanations for this, other theories are available…
Teneguia was an early tourist volcano:
The active phase lasted from 26th October until 19th November; there were two fatalities; a local fisherman working nearby and a photographer taking pictures of a lava flow. The cause of death was asphyxiation in both cases. There was some damage and loss of property; but the island is estimated to have gained ~29 hectares of land, much of which is now used for banana growing… The huge shade houses can (just) be seen at the bottom of the satellite photograph.
Intense earthquakes (up to MMI 5) were reported by locals immediately prior to the eruption, there were some quakes recorded during the eruption and a few more after; the last in the series was on 8/12/71. There is precious little in the IGN catalogue and no depths or magnitudes calculated…This is presumably due to lack of instrumentation; it’s hard to believe that an eruption of this scale had only 40 associated earthquakes. I generated an AVCAN map; but it showed only three points; ~30 EQs at Fuencaliente, a few at Los Llanos and one at El Paso.
Interestingly, given the quantum leap in seismic technology/ interpretation; La Palma has been virtually aseismic (I counted 4 in the IGN catalogue) since 1971…
During the eruption:
The Teneguia Tiger Team; sorry but I don’t know who is who…The main rift is fountaining in the background; the cone structure is entirely new, built by the eruption.
The eruption itself was extensively documented/ interpreted by The 3Ts; captained by Jose Maria Fuster Casas. Their morphology paper is the only article I’ve been able to find from: Volumen Teneguia, 1974. Estudios Geologicos (Volumen Especial), 85pgs.
A quick summary:
On 26th October, two eruptive centres opened at either end of a 300m long EW rift; producing spectacular strombolian lava fountains, they continued to emit lava virtually throughout…
Later, starting on 8th November; 4 smaller vents opened at the base of the new cone; of these, 3 went on to emit lava, also in a strombolian style:
The eruption has been classified as a schtrombolian type; but like the 1677, Fuencaliente eruption of Cumbre Vieja, (and others; including the 1949 multiple eruption) the emissions were more explosive at the upper vents and more effusive at the lower ones…
Teneguia was an impressive piece of mountain building, the main cone rises 100m above the pre- eruptive surface, the total erupted volume was ~40 million m^3, the lava flows have an average depth of ~12m and ~29 hectares of land was gained from the sea. All this in less than a month:
Teneguia’s lava consisted of basanites and tephrites; emitted in the form of lapilli, scoria/ lava-bombs and flows… This is a hand- sample of pumice; collected from near the path, halfway to the summit:
“All very interesting” I hear you mutter, “but it’s old news…”
Disclaimer 1: Please , there is no cause for panic at this time; remember we are talking geology here… Three of the papers referenced below discuss the following; in sensible, sober, scientific detail…
Some of the scientists who worked on the monitoring paper (see references) stated the folowing:
La Cumbre Vieja would collapse at the next eruption, which could happen at any time. They wildly exagerated the tsunami risk; saying that waves 25m high would reach the east coast of America in a few hours!!!
The press quite naturally went to town…
However, their own scientific paper concluded that movements detected were within the margin of error for the system they used…
I was going to leave this issue to one side; (I didn’t want the tsunami freaks showing up) until Lizzie pointed out that the article “fizzled out” around basanites and tephrites…
2012ers and scientists talking to the media aside… Does La Palma have the potential for a substantial landslide/ collapse? Remember La Cumbre Nueva? The black and white bathymetry above shows the debris avalanche quite well…
There is rifting along the summit ridge of Cumbre Vieja; new fissures and faults (up to 4 meters wide) opened during the 1949 eruption.
One can “join the dots” of cones along the ridge, so maybe some of these eruptions also produced fissures/ faults/ rifting…
Note that Teneguia did not noticably affect the situation; most likely because it was too far south, too low down and not on or near the very steep crest. The 1949 vents are close to the summit ridge at the far northerly end of Cumbre Vieja.
My amateur opinion: Other eruptions of La Cumbre Vieja will have contributed to rifting. (This would fit with the “general” theories of Canary Islands evolution.)
Please note my comment above: There have only been 4 EQs under La Palma since 1971; if the flank was moving and a landslide even remotely imminent, we should have some seismic indications…
Carracedo and others have done some work on this:
From Carracedo’s book, 2008:
“it is evident that if the western flank of Cumbre Vieja were detached and sliding, there should be observable ground cracks, seismicity and land deformation.”
“None of the above can be observed on La Palma.”
“There is no evidence whatever that the 1949 fractures are deep, nor is there any evidence for a detached block in danger of subsiding into the ocean.”
From Carracedo’s paper; below, 1999:
“lack of seimicity in the present inter-eruptive period indicates that the detachment; if present, is only activated by the intrusion of magma into the upper part of the volcano.”
I will of course be watching the GPS and seismograms closely if Cumbre Vieja erupts… As will the experts, the authorities and our old friends PEVOLCA.
To be very clear: (sorry regulars) The western flank is not at imminent risk of collapse.
Disclaimer 2: The author is an amateur journalist, and has absolutely no qualifications as a geologist… Please refer to the relevant government authority if you are at all worried about anything written above x
With thanks as ever to Lizzie x
Relevent Government Authority and EQ data:
Structure of La Palma using a 3d gravity inversion method; this is highly technical, but the pictures are interesting; (one for Geolurking?):
Carracedo et al on the 1677 eruption just up the (steep) hill from Teneguia; this reinterprets contemporaneous accounts in light of the geologic evidence:
The 1949 eruption:
Morphology of the 1971 eruption, by scientists who were there. Free; but you do have to download:
Carracedo et al on land slides and the evolution of La Palma and El Hierro, 1999:
Carracedo et al on the recent evolution of the Cumbre Vieja Volcano and the potential collapse:
Interesting work on monitoring of the potential collapse; authored in part by the scaremongers, 1999:
For more on edge driven convection and visiting Teneguia; see:
As ever; I have well thumbed copies of:
Carracedo’s Los Volcanes De Las Islas Canarias IV (ISBN 9788472071902) this particular edition has Spanish/ English/ Schpanglish text side by side by side; highly recommended…
Gates’ and Ritchie’s Encyclopedia of Earthquakes and Volcanoes (ISBN 0816063028) Carl would surely vouch for this one…
Right next to the clunkputer…