Day 4 provided the highlight of the trip for many – a walk around the towering spires of the Roques de Garcia formation in the spectacular barren landscape of the Las Canadas caldera. We kicked things off by asking the students to sketch the lower part of the Roques de Garcia, including the bizarre looking stump with many colourful rock layers. This prompted an interesting discussion about the origin of these deposits – a favourable interpretation is that they were formed in a lake that temporarily filled the caldera in the past. There were also some breccia deposits with large blocks up to a metre across, thought to be formed by collapse events on the old caldera walls.
A wander round to the back of the Roques de Garcia provided perfect views of Teide and the lava flow deposits snaking down from the edifice. The magmas erupted from Teide are unusual in that they are highly alkaline, meaning that they are lower in silica and enriched in sodium and potassium relative to most magmas. This gives them an unusual mineral composition, dominated by large white sanidine (K-feldspar) crystals, along with some kaersutite (Ti-amphibole). The students then had the opportunity to study the unusual morphology of the lavas, which have odd looking lobes and ropy pahoehoe sections.
We finished the day by discussing the process of caldera formation, and how recently models have changed from the idea of a “bung” collapsing into a central magma chamber which is emptied, to frequent episodes of partial collapse due to the emptying of multiple smaller magma reservoirs within a connected “crystal mush” system. The present day Las Canadas caldera is made up of three “nested calderas” which get progressively younger in a north-easterly direction. However, the origin of these “calderas” is controversial, with some authors claiming that they represent the headwall scarps of massive landslides that ended up in the ocean to the north. This led to some in depth discussions with the students as to how they would go about determining which theory was correct, or whether both processes could be operating. Personally, I feel that the voluminous ignimbrite deposits in the south of the island, which record multiple Plinian eruptions, provide strong evidence for caldera collapse. However, it is possible that such events may have triggered, or have been partially caused by, landslides.