Teaching in Tenerife day 3 – a tuff task

Day 3 started with a trip to the coast to view the Montana Pelada tuff ring, a volcanic feature formed by the interactions between magma and water. A long climb up and over the steep circular outer wall led to an initially confusing exposure with layered pyroclastic deposits dipping in opposite directions. The students began by sketching the outcrops and identified 3 main units within the sequence – the lowermost lithic rich unit dipping towards the sea, a thinly bedded unit dipping in the opposite direction and a lithic poor upper unit with an erosional base.

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Typical exposure at Montana Pelada. The red dashed line marks the boundary between the two units.

After this exercise, we discussed what may have produced the characteristics of the different units and how this relates to magma-water interactions. The lower seaward dipping layers were interpreted to be produced by initial explosions where the water-magma ratio was high, which ripped apart the underlying basalt. This meant that the resultant pyroclastic density currents were rich in basaltic lithics.   The thinly bedded inward dipping layers showed cross bedding and bomb sags (where a lava bomb creates a depression as it lands in the flow deposit), suggesting that they were deposited by drier, more dilute currents as the magma-water ratio decreased. The uppermost lithic poor layer represents a later eruption where the magma had a much lower water content, producing a typical ignimbrite deposit. The yellow matrix colour of some of the rocks at Montana Pelada is due to the presence of palagonite (hydrated basaltic glass) which shows that these rocks underwent hydrothermal alteration (a change in chemical composition due to interaction with hot water) after eruption.

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Cross bedding in deposits from dilute pyroclastic density currents at Montana Pelada

After a lunch stop by the sea that involved getting continually sandblasted, we moved inland to continue the explosive theme at a roadcut through the deposits of the approximately 600 ka Granadilla eruption. This eruption was produced by the central Las Canadas volcanic edifice on Tenerife, which formed between the shield volcanoes after 3.3 Ma. The Granadilla deposits start with a lower layer of angular white pumice containing some perfectly preserved bubble walls, and a lithic rich, massive upper layer. The students quickly concluded that the well sorted pumice layer was produced by fallout from a Plinian eruption column, with the massive unit above it the product of a subsequent pyroclastic density current (PDC) after collapse of the column.

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The well defined fall and flow deposits from the 600 ka Granadilla eruption. Photo credit: my co-demonstrator Alex Iveson

The teaching and demonstrating team spent the evening providing feedback on the students’ summary sketches for the previous day. Having not been involved in the marking of student work before, I found this a useful exercise in learning how to give helpful feedback that they could then work on during the remaining field days. In the evening we enjoyed a trip to the local carnival, where we came away with a fluffy T-Rex for the department! Follow his adventures on the department Twitter feed: @DurUniEarthSci.

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