A continuation of the project started two years ago is proposed. We recall that it was initially proposed to investigate in details the internal structure of a well-exposed caldera, the Las Cañadas caldera of Tenerife, in the Canary archipelago. Geophysical techniques using electromagnetic waves proved to be particularly convenient in such volcanic areas with strong contrasts of the electrical resistivity. We employed EM methods available to us, such as: audio-magnetotellurics (AMT), very low frequency (VLF-R, VLF-EM, VLF-GRAD) and PPM (proton precession magnetometer) measurements of the Earth DC magnetic field.
The highest volcanic edifice of the Atlantic ocean is the island of Tenerife, culminating at 3,718 metres with the Teide volcano. The caldera structure is the object of intense debates about its buildup, either by multiple vertical collapses or by giant sector collapses. Other first-order geological features affecting the topography of the island are landslides. The landslides truncated an older ridge and valley topography that is associated with the shield building basalts on Tenerife. The landslides are also though to have caused the disappearance of the northern margin of the caldera.
Stratigraphic, structural, volcanological, petrological, geochronological and geophysical data suggest that the Las Cañadas caldera resulted from multiple vertical collapse episodes that occurred during the construction of the Las Cañadas youngest edifice, the Upper Group. Three long-term ( ³ 200 ka) cycles of phonolitic explosive activity, each culminating with a caldera collapse, have been identified.
The exact development of the events described in the last two paragraphs is still controversial, largely due to the hiding effect from recent pyroclastic deposits. Although some relics from the first caldera wall are still visible (Roques de Garcia, a large spur of pre-caldera rocks), most of the detailed structure is now hidden. In the initial project we proposed to carry out the following investigations:
•to reveal the shallow (<200 m depth) caldera structures buried under the pyroclastics,
•to detect the hidden continuation of the most recent caldera rim (Diego Hernández),
•to circumscribe the extension of the three successive calderas and estimate the depth to the impermeable floor (hydrothermalized lavas).
After processing the sofar acquired data, the aims remain unchanged. However, we believe that it would be interesting to extend our field of investigation to the first 1-2 km outside the caldera.
This would allow us
•to better understand the roots of the landslides,
•to delineate the aquifers and the grounwater flowpaths.