Cumbre Vieja and the San Juan eruption of 1949

/ 18/09/2021

The volcano Cumbre Vieja in the island of La Palma has been showing signs of unrest. The question on everyone’s mind is, will there be an eruption? Maybe, or maybe not. This is always hard to know.

The Spanish National Geographic Institute reports inflation, a total of 10 cm of deformation. As such it is evident that there is magma on the move under Cumbre Vieja, it has intruded underneath the volcano.

https://www.ign.es/web/ign/portal/noticias

There have been multiple swarms of earthquakes since 2017 in Cumbre Vieja, a total of nine. Previous swarms were probably magma intrusions too, but which did not reach the surface. The recent swarm however is more shallow and more intense than its predecessors which raises the possibility that the outcome may be different.

The current swarm started on September 12. A total of 4530 earthquakes have been detected at depths of mainly around 10 km, although there are a few which have been very shallow. The swarm commenced under the summit of Cumbre Vieja, where a magma conduit probably exists which is supplying the intrusion. Earthquakes have propagated to the northwest. This probably represents the propagation of magma filled fractures, possibly sills, radially from the centre of Cumbre Vieja. However the earthquakes only show a but a blur of what is going on down there. The exact shape and pathways used by the intrusions cannot really be known with much precision. It is somewhat similar to the prelude to the eruption of El Hierro in 2011 which also seems to have commenced with a sill that later propagated a crack towards the seafloor.

Image from the NASA.

The location of the earthquakes suggest a possible eruption in the NW sector of Cumbre Vieja. However there is a factor of unpredictability. The exact path that the intrusion takes may or may not connect with the surface, such being difficult to know if there will or will not be an eruption . The precise location where the intrusion will breach the surface is also difficult to know. The fissure could open in the middle of a town, in a forest, or it could open underwater, which are wildly different situations with wildly different consequences.

We can know however the style that the next eruption of Cumbre Vieja will take, whenever and wherever it happens. To do this we must look at the past history of this volcano.

 

La Palma

La Palma is one of the Canary Islands. It was formed due to volcanic activity. The oldest rocks of the island are 3-4 million years old and belong to a submarine volcano. These submarine lavas are now found at heights of up to 1500 meters above sea level in the Barranco de las Angustias, in the old northern part of the island, which shows the enormous uplift that the island has undergone. Probably numerous sill intrusions have pushed the volcano upwards.

La Palma. From the NASA.

La Palma is shaped like an arrowhead. The northern part is formed by the old Taburiente volcano. Deep gullies dissect the ancient lava flows exposing the overlapping layers of volcanic extrusions and the frozen dykes and sills which cut through them. The volcanic edifice was destroyed by a series of giant landslides, the last of which took place around  560,000 years ago. Activity continued inside the landslide scarp until 530,000 years ago. Afterwards activity died out in the northern half of the island.

Large scarp formed due to erosion of Taburiente volcano. Some sills and dykes are visible on the left. From Wikimedia, by Zyance.

Volcanic activity in the southern half of the island has been ongoing for at least 125,000 years and has constructed another volcano known as Cumbre Vieja, or also simply as Dorsal Sur, “Southern Ridge”. It is a shaped like a ridge in a N-S direction. Despite being different edifices it seems that Cumbre Vieja is part of Taburiente’s structure. Taburiente had 5 subtle radial rifts. This is much better appreciated in submarine shield volcanoes which are often shaped like ridges or like three to six-pointed stars. Knowing well the shape of submarine volcanoes I can see that Taburiente displays the same five-pointed star structure, although being subaerial it is not so easily visible. The longest, dominant rift goes southward, known as Cumbre Nueva. It can be seen that Cumbre Vieja is the southern continuation of Cumbre Nueva.

Topography of La Palma. Note the northern volcano Taburiente which is cut by deep gullies and a central erosional crater, and the southern volcano Cumbre Vieja which is dotted by young volcanic cones. There is a bow-shaped ridge connecting both volcanoes, this is Cumbre Nueva, the ancient rift zone of Taburiente, partly destroyed by a landslide. From maps-for-free.com.

 

The main magma erupted in La Palma, as well as in the Canary Islands, is basanite, which is relatively fluid, but not as much as say Hawaii. The fluidity is comparable to the more frequently active Mount Etna in Sicily. The magma is not so fluid that all of it would flow away upon landing on the surface, but it is not so viscous that it is entirely blasted into light pumice and ash carried away by the wind. The eruption style is known as “violent strombolian” or “violent hawaiian” depending on whether it produces explosions or sustained fountains. It is the middle ground between the blazing rivers of lava and the billowing columns of ash. This style is ideal for producing pyroclastic material that rains around the fountain, rapidly constructing a mountain around the vent, known as a scoria cone. These conical mounds of ejecta are everywhere over Cumbre Vieja. Because the volcano doesn’t have any central vent that erupts repeatedly, then it makes a new fissure each time it erupts. The pyroclastic material rapidly oxidices. This gives the terrain various hues ranging from black to red, which together with the abundant canarian pine trees gives the characteristic landscape of Cumbre Vieja.

Desertic volcanic landscape near the southern point of the island. San Antonio volcano visible in the centre formed in the 1677 eruption, The brighter cone to the left of San Antonio is Teneguia, which formed in 1971. From Wikimedia by Tony Hisgett.

Other magma types present in Cumbre Vieja are the tephrite and phonolite groups which are more silicic and viscous. They are  present in trace amounts making small lava domes. A small volume of phonolite was emitted in 1585 producing tiny cryptodomes and domes, although the eruption was mainly basanitic.

Types of volcanic rocks depending on silica and alkali content. From Wikimedia by Woudloper.

Cumbre Vieja last erupted in 1971, 1949, 1712, 1677, 1646, and 1585. It is the most active volcano in the Canary Islands. Eruptions have taken place at intervals of 20-60 years. The exception being the remarkable 237 years long dormancy between 1712 and 1949. Why did this happen? It is possible that the volcano follows cycles of more frequent eruptions separated by long dormancies. Another possibility is that the enormous 6-year long eruption of nearby Lanzarote Island, occurring in 1730, induced a long dormancy in Cumbre Vieja.

It would not be unexpected that now, 50 years after the last eruption, there was a new one.

The eruption of 1949

The eruption that took place in 1949 is an interesting example of a typical Cumbre Vieja eruption.

Swarms of earthquakes had been frequent since 1936 and leading until the eruption. The morning of June 24 some fumes were noticed, and soon afterwards a towering black column of ash was rising hundreds of meters, if not more, into the sky. A new volcano had formed along the crest of Cumbre Vieja. The fissure had opened a small distance north of the highest point of the ridge. The vent is known as Duraznero.

During the following days Duraznero continued to erupt, belching out ash and rocks. Earthquakes frequently rocked the nearby communities and steaming fractures opened in the  ground around Duraznero. Magma must have been making its way into growing fractures. Over the days the erupting fissure progressively grew to a length of 500 meters and developed 5 main vents, of which Duraznero 2, at the southern end, was the most active, creating a 170 meter-wide crater. The activity was entirely explosive but of a low intensity that must have been little more than a slight annoyance to the local population. The erupted lava was tephrite. Earthquakes were more impactful, they damaged houses, cracked roads, and occasioned rockfalls. On July 6 the ash was carried downwind over the island of Tenerife where it wrapped around the summit of El Teide in a menacing black cloud.

Fissures of Duraznero. Image by KrisNM.

On July 8 a stream of lava came out from a new location known as Llano del Banco, 3 kilometres north of Duraznero, and from the other end of a system of cracks that had opened up. It did so quietly with no explosive activity whatsoever. The lava must have been degassed by Duraznero, gone into cracks, and found an outlet at a lower elevation from Llano del Banco. The lava erupted was tephrite, same as that of the earlier phase of the eruption. The initial fissure died out at about the time the new vent opened.

It is common for eruptions of Cumbre Vieja to have some vents which are dominantly explosive while others are effusive. In the eruptions of 1646, 1677 and 1712 it also happened that the vents which opened at the highest elevations had explosive activity and built large cones of scoria, while other fissures opened at lower elevations, sometimes offrift, and even at sea level, producing solely lava flows. The eruption of 1949 shows how the process works. A vent that is high up degasses the magma and then it is carried laterally through fractures towards openings downslope from which it emerges effusively.

Lava descended in fiery tongues from Llano del Banco down the flanks of the mountain. People were being evacuated as the flow headed for populated areas. It took 10 hours for the lava to reach the main road of the south of La Palma. Later that day the flow had destroyed 20 structures, including houses, cellars, and barns.

On July 10 lava cascaded over a cliff into the ocean. From this day on the entry of lava into the sea became continuous, and a lava delta was gradually constructed. Cloud of steams rising over the waters were illuminated by the convoluted streams of incandescent rock.

A new change in the eruption took place on July 12. The composition of lava erupted from Llano del Banco changed from tephrite to basanite. It became less silicic. At a similar time a new vent opened 400 meters north of the initial vent of Duraznero in the location known as Hoyo Negro. Black cauliflowers of ash pierced with flashes of lightning rose rhythmically from the Hoyo Negro vent. It erupted various magma types including basanites, tephri-phonolites and phono-tephrites. Once again the vent uprift was explosive while the vent downrift was effusive. The basanitic magmas must have released their gas into the explosions of Hoyo Negro and then come out laterally through the opening in Llano del Banco.

Hoyo Negro projected bombs to a distance of 1 kilometre from the vent snapping the trees and setting portions of the pine forest on fire. Clouds of ash frequently dusted the western part of the island. The explosions excavated a 400-meter wide crater on sloping ground. This created a spectacular 200-metre cliff against the higher side of the slope, which exposed the many layers of ejecta painted in a variety of colours.

Hoyo Negro. Image by Rafael Medina.

A raging stream of lava continued to issue from Llano del Banco and cascade towards the coast. Despite erupting continuously for 18 days the vent produced no distinguishable ejecta, and shows how the gas had been entirely removed from the melt before erupting. The ground above the fissure collapsed among loud noises, the rocks fell into the stream and were carried away, a length of 150 meters of rock above the conduit was eroded away and disappeared leaving behind a deep chasm in the forest.

On July 22 the activity of Hoyo Negro was down to a solfatara. Llano del Banco was also dying down. By July 26 the eruption had fully stopped.

Early on the morning of July 30 the eruption suddenly resumed. Duraznero and Hoyo Negro exploded simultaneously. An hour later fluid basanite lavas emerged from the location of Duraznero 1 and poured into an old crater where it formed a lava lake which then overflowed and formed a narrow stream of lava that rapidly sped down the steep slopes of Cumbre Vieja, cutting the road of Santa Cruz de la Palma, and nearly reaching the sea after 11 hours of advance, when the eruption came to a stop. This was the last episode of the 1949 eruption.

The flow of July 30, although of rapid advance, it was fed at a rate of only 10 m3/s, which is very low. It was also similar to the mean eruption rate of Llano del Banco, which was approximately 14 m3/s. The explosive activity was of little volume so it probably does not change the overall numbers too much. As such the eruption of 1949 was of very low intensity, in both its effusive and explosive counterparts. Slow eruptions are typical of the Canary Islands. Such low intensity eruptions do not pose much of a hazard to the people, in fact no one died in the 1949 eruption, despite 120 houses or so being destroyed, and people having approached the eruption in order to view it. This doesn’t mean that the hazard is inexistent.

If someone stands very close to the vents he/she could be asphyxiated by the noxious gasses or may be impacted by a lava bomb or by lightning. Rarely when lava flows reach steep slopes they collapse into blistering landslides resembling small-scale pyroclastic flows that could potentially kill someone. Conditions around volcanic eruptions can change suddenly in unpredictable ways and become hostile to humans. Safety is not guaranteed.

If Cumbre Vieja erupts in the future it will probably resemble the 1949 eruption in many ways: an earthquake prelude to the eruption that may deal damage to structures, unpredictable opening of fissures, some vents producing mainly explosive activity while others feeding mainly streams of lava that destroy human properties, and also the likely entry of lava into the sea.

Of course if the current earthquake swarm will culminate in an eruption or not cannot be known for sure. Swarms before the 1949 eruption occurred as early as 1936 and did not culminate in eruption until 13 years later.

 

San Martin volcano, formed in a 1646 eruption. Image by Rafael Medina.

 

Interesting links

Eruption of 1949 (in spanish).

IGN news (spanish).

GRAFCAN visor (includes geologic and topographic maps).

 

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1,260 thoughts on “Cumbre Vieja and the San Juan eruption of 1949”

  10. Cold viscous alkaline low sillica lava
    Looks very much like Etna

    • Its a normal start in la palma could switch to a more fluid lava in the next days as the more “fresh” magma starts erupting like in 1949 or 1971

    • Yeah they raised it to orange, red would be if greater risk would be present, at least that is what i understood here in the news.

    • from long distance shots you can see how poorly placed this eruption is. There will be a lot of damage very soon.

  16. Hopefully the government prepares for some significant escalations in intensity, while it’s not a guarantee, this volcano is capable of much more dangerous eruptions then fagra-cone or Kilauea

    • Not sure where you are getting that idea from, eruptions here are rarely higher than a VEI 1. Kilauea is generally safe but has got some much more substantial numbers to its name. This is maybe not the volcano for you Tallis, save maybe where the eruption is occurring.

      • It doesn’t take much to be more dangerous then Kilauea or Fagra

        • If this was like the eruption I want, everyone on the Island would be dead, and we don’t want that to happen.

    • The lava is really cold basalt … Viscous.. the flows will be quite slow moving luckly

    • I believe you’re a bit off. Based on the images and the three words of Spanish that i did understand, I think it’s just uphill from El Paraiso.

      • Yes, that seems right. It would channel the lava through the town of El Paraiso

  18. Looks like two vents one more “explosive” and another somewhat lower more effusive

    • Oops, Charly91 I posted the same time. Sorry!
      I’m slightly bemused to see the TV correspondent on one of the above videos is called Vicky Palma.

  20. Looks like multiple vents opening with a lot of fountaining. Going to be messy for houses. Looks like a whole line of fissures.

  28. This is really ramping up! Getting bigger by the minute. Wow!

  29. Phreatic, it appears. Maybe a fissure crossed an old cone and that’s a small landslide. Looked like a small PF for a moment.

  30. Incredible that the fountains are so powerful with such a low eruption rate. Must be very gas rich, the eruption looks like a blowtorch.

    It also seems to already be creating a substantial hill even in just this short time, did it erupt through an older cone?

    • It looks like the lava is fairly viscous.
      This means it will pile up more around the vents, rather than flow away as a lava flow. That could explain the rapid build of the spatter cones.

      • They actually say it is quite fluid. There are older spatter cones in the area: what you see may predate the eruption

  31. This one is NOT a tourist eruption! Lava bombs everywhere…

    Are we sure they aren’t pyroclastic flows? And from what I understand of the Spanish they are saying 5 vents already.

    • I think the brown plumes are probably not even hot, just old material blows put of a fissure. It is likely there will be phreatomagmatic activity, most eruptions here show this effect, this might be why the fountains are so powerful in what is really a pretty small eruption otherwise.

    • No such flows yet, but I agree very gas rich its basically a jet engine, bombs must be falling everywhere .
      If the cone keeps growing fast we might see some partial collapse and that could cause some issues.

  33. That reporter is waaaaaayyyy to close!

    2 more explosions in the last 5 minutes and getting more intense according to the guy on the ground (I think)

  35. Looks like a fissure at least a couple of km long. Super impressively high fountains over a long length.

    Reporter is mentioning pyroclastics

    • That What happens When You gets relatively Viscous basaltic lavas
      Looks just like an Etna flank eruption

  41. What have these people been doing for the past 8 days? Some of them sound like they just became aware of this.

  42. I think the reporter on the road near the site just said that the lava has just crossed the road and is flowing slowly, but there is a lot of it.

    Found another live feed here https://www.youtube.com/watch?v=YKdIDajFI1I

    Wider view and can see the smoke where the lava is flowing. Looks like another vent lower down the mountain too.

  44. @involcan now talks about 7 vents. still throat clearing on some.

  46. Not to absolutely make everything about Iceland again 😉 But the eruption has now lasted half a year(184 days) and is the longest lasting this century(so far), Holuhraun lasted 181.

  48. Amazing viewing this live, I missed the beginning of Fagradallkallsfjall.
    Seems to be a mix of strombolian and hawaiian going on.
    Do worry about the affect on the western block though, all this activity is happening west of the fault.

    • Its strombolian ”violent strombolian”
      This is much more Viscous than Hawaiian and Icelandic basalts

      This is very much like an Etna flank eruption in temperatures and viscosity

    • The western flank is stable, perhaps in a couple 100k years its a different story

    • There are quite a few houses in the pathway of the biggest lava flow.

    • Tsunami is a non-issue. Rockfalls are possible – the landslide you think about is just hysterics from some news media. It is not on the risk register

      • But you had this flankcollapse/tsunami for Cumbre Vieja on your post about ‘The worlds 10 most dangerous volcanoes’ several years ago ?

        • Science has moved on. A flank collapse at La Palma may happen every 100,000 years or so.

          • And just like Hilina Pali in Hawaii, they aren’t always very fast on a human time scale. As for mass wasting, the Hawaiian islands are probably far more prolific than the Canaries.

            Lets look at it from a physics point of view.

            For the sake of argument, let’s suppose that a catastrophic mass wasting event occurs. Locally, it will be quite phenomenal. But once you account for the energy dispersion along the ever expanding wave-front, not a lot of energy will be left per linear unit of wave-front by the time it reaches the East Coast of the US, baring some focusing phenomena of features along the sea floor. The COMMON scenario expounded by the doom monger programs rarely take this into account and assume that source phenomena will be manifest several thousand kilometers away. Sorry, physics doesn’t work like that. Several years ago I ran the calculations specific to a La Palma collapse. I don’t wish to dig that stuff up again, If I remember correctly, for a 300 foot wave set at La Palma, it came out to about 3 INCHES by the time it reaches the East Coast US. Again, this is assuming a pure dispersion pattern with no focusing of energy.

            Caveat: I am not a physicist, Albert is. I put a lot of stock in his opinion.

          • Ok and interesting. Might there be new science in 2 weeks time? And how could the scientistS be that deadly wrong only less than 10 years ago?

      • “just hysterics from some news media”

        FACT!

        Just like “Super” volcano being invented by the BBC to hype a TV program.

        • This page has LOT of hype. WHY not now? And why just ‘blaming’ it on BBC ?

    • I read up on this recently. They have changed their mind or done some decent research. Before it was a tsunami for New York City. It probably was media hysteria. After the Indian ocean tsunami it was a click generator.

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