Sunday, 28 June 2015

Day 17 - Volcan de Teneguia and Llano del Banco flows (at last!)

Today was a great day. I was responsible for the soil gas sampling and our main area to sample was on and around the volcano responsible for the 1971 eruption, Teneguia.

The first image Doesn't show Teneguia from her best angle but it does show the clear contrast between the colour of the young lavas of Teneguia (and some of San Antonio) and Roque Teneguia.

It's more a moonscape but with Blue skies. Teneguia has three craters and covers the field of view with the contrasting pale Roque Teneguia to the far right.

Roque Teneguia is much older than the surrounding landscape at ~56,000 years. This pale colour is due to the lava responsible for forming this rock having a much greater evolved composition. This outcrop is what remains of a phonolite dome, now devitrified and spherulitic due to chemical weathering.

This site is of importance for other reasons also. It is only one of two sites, not just within the Fuencaliente municipality, but the island as a whole, where the plant species centaurea (Cheirolophus junoianus) is found. Evidence suggesting this area may have been a sacred place for the pre-Hispanic population of the island is presented in funeral remains and rock carvings. This area would have appeared very different during these times.  

The two main highlights of today were sampling/climbing to the top of Volcan de Teneguia and walking around the Llano del Banco lava flows. 

Volcan de Teneguia, 1971 eruption, second active crater

Teneguia has 3 craters, all of which contributed to the 1971 eruption. The eruption began with the smallest crater in the northwest of the volcanic field and progressively moved south with the second crater that in the above image and the third below. 

The first crater is towards the top of this image among the black lavas, taken from above the second crater, looking roughly north west. The reddy-brown colour in the foreground is due to oxidisation of  some of the individual minerals in the lavas at this location on the volcano

Crater three

Our final sample site on the volcano was the end of the crater rim in the upper right of this image. From this point looking inland, there is a great view of San Antonio volcanic cone.

San Antonio volcanic cone as seen from the rim of crater three of Teneguia

Soil gas sampling on the rim of Teneguia crater three

Making our way back to crater two for the climb back to the car, we found a great example of a small fossil fumarole. We took a small sample of the white rock back to the car with us and put a small amount of 10% dilute HCl on it. If it effervesces (fizzes) the rock, or at least the alteration minerals within the sample, contain CaCO3. This proved to be absent of CaCO3, meaning the white is likely to be associated with chemical weathering to clay containing magnesium - probably the smectite group. 

Fossil fumarole (hammer for scale) on the rim between crater two and three

The next highlight of the day involved finally having a walk around the Llano del Banco (1949) lava flow. As stated in an earlier post, the flow started high up on the western flank of Cumbre Vieja, just below Pico de Birigoyo, it flowed all the way to the sea just over 7 kilometers away, before effusion rates dropped, resulting in the formation of an ocean platform, adding more land to the west of the island. 

I have to admit, after seeing just a tiny snippet of these stunning flows on filming day, I was pretty desperate to find time to go back and explore just a little.

The flows combine blocky, a'a' and pahoehoe morphologies and there are channels, collapsed tubes, windows, tubes that become channel and channels that become tubes everywhere you look. In previous posts I have already presented images of pahoehoe lava from this flow, but below there are examples of some of the other features observed.

 This is probably a lava tube that has collapsed simple because the debris on the surface within the channel is broken up, but not in the style of block/a'a' morphology, with pahoehoe below the debris

 The sunlight made photographing some of the features a challenge. In the centre of the image is a window. These are often seen in Hawaii. They are small areas of a lava tube where the roof is yet to form. It is possible to look through (carefully) and watch molten lava move beneath

This is one of many examples observed of a lava tube transitioning to a channel

 In some areas the lava has formed small topographic highs that we were soon to discover indicated the location of lava tubes larger than previously seen.

Walking to the end of the constructed walk way, Hannah got to a point before me and excitedly called back to tell me to hurry up. We were looking down on a pretty big opening - a lava tube.

A large lava tube, the photo doesn't really do it justice

We walked down in to the lava tube following the constructed route where it became apparent that this was in fact an area where the lava tube started as a channel and a tube roof had started to develop, but due to a drop in effusion rate had never completely formed. This appears to have happened at least three times during the eruption phase, with the final drop in effusion remaining consistent for a long enough period to form a tunnel roof. 

The lava tube was thiisssss biiggggggg......

We were not expecting this!

Every day is an adventure!

Buenos noches de La Palma!

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