We have updated our earthquake plots to 19.10.2021 8:41:10. Since our previous update there has been more seismic activity, mainly between depths of 9-15km and 32- 42km. The former is consistent with the initial and subsequent stages of the swarm; the latter is consistent with the deeper earthquakes which started on day 21 (1.10.2021).
Update earthquake plots are contained in the following video.
Magma Plumbing under Cumbre Vieja
We’ve Googled around to see what’s likely to be going on at these depths. The 1585, 1949 and 1971 eruptions may shed some light on this. Researchers have found that the erupted lavas are formed by fractional crystallisation and stored in the upper mantle; during ascent, these lavas stall in the lower crust or near the Moho; there are no long-lived shallow magma reservoirs.
Fractional crystallisation is an indicator of the depths at which magma stalls in reservoirs. Earthquakes tend to occur around magma reservoirs or during the ascent of magma in response to the stresses on rock that changes in magma produce. Hence our interest in them.
The 1585 eruption produced 0.03km3 of lava, which was composed of basanites, tephrites, tephriphonolites and phonolite. The eruption is famous for the extrusion of phonolitic spines, named “Devil’s horns” by eye-witnesses, at the start of the eruption. Examination of the 1585 lavas indicate that the more evolved lavas were the result of fractional crystallisation. Magma differentiated at three levels: in the deeper mantle, c.20km depth, basanite evolved to tephrite 1550 to 1750 years, collecting in more than one reservoir, before the eruption; basanite also stagnated at the base of the crust, c.14km depth, to differentiate to tephrite; and, differentiation also occurred in the edifice. Further evolution of to tephriphonolite / phonolite may have occurred in the lower crust and upper crust. The basanite erupted may have originated from a different batch of magma than the erupted tephrite. 14km is the depth of the Moho under La Palma.
The 1949 eruption started on 24 June 1949 and ended on 30 July 1949. It had been preceded by weak seismic activity from 1936. Seismic activity picked up in February 1949, being felt mostly at the southern tip of the island and accompanied by ground cracking. Stronger seismicity and ground cracking immediately preceded the opening of the first vent. The primary melt was sourced at depths between 80-100km. Fractional crystallisation occurred at 20 to 26km with some possibly at 26-36km. Magma was stored temporarily in the crust before eruption. Magma mixing occurred in the mantle three months prior to eruption, causing a dike to propagate southwards. A 3km long fissure eruption started with the Duraznero crater emitting tephrite for 14 days. This was followed by the Llano Blanco crater opening to erupt tephrite for three days, followed by basanite for three days. The Hoyo Negra crater opened 4 days later to erupt basanite, tephrite and phonotephrite, during which the Llano Blanco crater continued to erupt basanite. The Duraznero crater then erupted basanite. The eruption started on 24 June 1949 and ended on 30 July 1949. The primary melt was sourced at depths between 80-100km. Fractional crystallisation occurred at 15 to 26km with some possibly at 36km. Magma mixing occurred in the mantle three months prior to eruption, causing a dike to propagate southwards. Magma was stored temporarily in the crust between 7-14km before eruption. Later calculations put the depth of fractional crystallisation at 35-45km.
In 1971 Cumbre Vieja erupted again, this time at Teneguía, emitting 135,000 m2 of lava and created a 290,000 m2 lava platform – 40 million m3 of lava in total. This eruption produced basanitic to phonolitic lavas. The eruption was Strombolian and in two phases: initially a 300m fissure opened on 26th October 1971, producing effusive lava flows from vents; and, new vents opened on 8th November 1971, with rhythmic explosions, lapilli, scoria and lava bombs. CO and CO2 were emitted; these gases were thought to be the cause of death for the eruptions two fatalities. Examination of the lavas showed that magma stalled at two depths: clinopyroxene and plagioclase crystallised at depths of 20-45km; and the crystallisation of aluminium augite indicated that magma then ascended to 20-35km. Variations in the samples tested indicates that magma formed in batches over a range of depths in the lithospheric mantle to combine before ascent.
Taburíente, Cumbre Nueva and Bejenado each have zones of clinopyroxene crystallisation between 25-45km. Earlier Cumbre Vieja eruptions had shallower zones of 15-30km, before the deeper zones of the 1949 and 1971 eruptions (35-45km and 25-45km, resp.). In the earlier Cumbre Vieja eruptions magma stalled beneath the Moho and the in the later eruptions magmas depths were more in line with Taburíente, Cumbre Nueva and Bejenado.
How does this Compare to the Current Earthquakes?
To make any conclusions we need to wait until there is a detailed analysis of the erupted lavas. However, we can note that the current earthquakes are at two distinct depth ranges: 7 -16km and 30 to 42km, with not much in between. 7-16km correlates to a possible zone of magma storage beneath the crust and magma migration through the crust. 30 to 42km correlates to part of the lower zone of fractional crystallisation of the 1949 and 1971 magmas.
Time will tell how this eruption will pan out. In the meantime, the eruption is still going strong. Our thoughts continue to be with those affected.
© Copyright remains with the author; all rights reserved, 2021
Sources and Further Reading:
Raw earthquake data: Instituto Geográfico Nacional (ign.es)
Kursten Galipp, Andreas Klügel, Thor Hansteen, “Changing depths of magma fractionation and stagnation during the evolution of an oceanic island volcano: La Palma (Canary Islands)”, Journal of Volcanology and Geothermal Research Volume 155, Issues 3–4, 15 July 2006, Pages 285-306. Link: Source
T. S. Johansen F. Hauff K. Hoernle , A. Klügel, T.F. Kokfelt, “Basanite to phonolite differentiation within 1550–1750 yr: U-Th-Ra isotopic evidence from the A.D. 1585 eruption on La Palma, Canary Islands”, Geology; November 2005; v. 33; no. 11; p. 897–900. Link: Source
Andreas Klügel , Kaj A. Hoernle, Hans-Ulrich Schminck , James D. L. White, “The chemically zoned 1949 eruption on La Palma (Canary Islands): Petrologic evolution and magma supply dynamics of a rift zone eruption”, Journal of Geophysical Research, Vol 105, No. B3, Pages 5997-6016. Link: Source
Abigail K. Barker, Valentin R. Troll, Juan Carlos Carracedo, Peter A. Nicholls, “The magma plumbing system for the 1971 Teneguía eruption on La Palma, Canary Islands”, Contributions to Mineralogy and Petrology 170, Article number: 54 (2015). Link: Source