Teaching in Tenerife day 2 – building a shield volcano

Day 2 was spent at Masca, in the northwestern corner of Tenerife. The sheer cliff exposures here cut through the remnants of an old shield volcano, the Teno massif. The island of Tenerife has been through several stages of volcanism, beginning with a shield building phase, which produced the Roque del Conde (south), Teno (north-west) and Anaga (north-east) massifs. This was then followed by construction of the Las Canadas stratovolcano in the centre of the island between the three shield volcanoes. The Las Canadas edifice has since been through several episodes of caldera collapse and the current caldera is home to the Teide-Pico Viejo volcanic edifice.

map tenerife
Simplified geological map of Tenerife, showing the stages of volcanic eruptions mentioned above. The Teno massif is circled. Map from Hernandez et al, 2017.

The Teno shield volcano section at Masca revealed many layers of lavas, scoria cones and columnar jointed sills, all cross cut by thousands of dykes. The students were tasked with analysing these dykes by looking at their geometry and composition, before presenting their findings to the group. Those looking at the compositions uncovered two different types – crystal rich, forsterite (Mg-olivine) and Ti-augite (clinopyroxene) bearing ankaramite dykes, and mostly aphyric basaltic dykes. Many of the dykes have obvious chilled margins where they contact the baked country rock.

IMG_4593
View along the length of one of the basaltic dykes at Masca.

The groups studying dyke geometry concluded that most of the dykes showed a similar orientation. This orientation was not radial from the central Teide-Pico Viejo edifice, as may be expected, which led them to the interpretation that these dykes may instead be radiating from what was the centre of the shield volcano. Some unusual “handshake structures” were found, where the interacting stress fields at the tips of propagating dykes have caused them to curve inwards towards each other (see below).

Dyke handshake structures
Plan view diagram of a “handshake structure”

After lunch and some ice cream, we took a relaxing boat trip along the coast at Los Gigantes, to view the base of the shield volcano. This allowed close-up observation of the layered lavas, sills, scoria cones and dykes, along with some friendly local dolphins! This exposure demonstrated perfectly how a shield volcano is built by repeated eruptive events and intrusions.

masca 2

Cliff section seen from the boat, showing the different components of the Teno shield volcano.

References: Hernández, P.A., Padilla, G., Barrancos, J. et al. Bull Volcanol (2017) 79: 30. https://doi.org/10.1007/s00445-017-1109-9

 

 

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