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During RV “METEOR” cruise M51/1 in 2001, a hitherto unknown submarine ridge off Madeira island was discovered, hydroacustically mapped and sampled. Funchal ridge (working name) extends from near the capital Funchal over 50 km to the south and rises up to 600 m over the surrounding seafloor. At least 20 volcanic cones have been identified along the ridge, including a volcanic field at its southern end. Funchal ridge represents a classic rift zone and shows some similarities to the ridge forming the Desertas islands further northeast. Many of the alkalic basalts dredged are remarkably fresh and are highly vesicular despite sampling depths of up to 3600 m. We thus propose that Funchal ridge is the locus of most recent volcanic activity of Madeira Archipelago, marking the present location of the hotspot. This interpretation is in agreement with the concentration of small earthquakes around the ridge and with the occurrence of stratigraphically young cinder cones in prolongation of the ridge near the city of Funchal. The volcanic field at the southern end of the ridge where activity is concentrated may thus represent the early shield stage of a future island. Preliminary Sr, Nd and Pb isotope data support this interpretation since samples from Funchal ridge overlap with those from the isotopically enriched Madeira shield stage but differ from the less radiogenic post-erosional stage presently occurring on Madeira. According to our model, Funchal ridge could become the fourth large volcano of the Archipelago following Porto Santo (14.3 – 11.1 Ma subaerial age range), Madeira (4.6 Ma to <10 ka), and the Desertas islands (3.6 – <3.2 Ma). The reason for formation of a long ridge instead of a circular seamount at this early stage may be the overlap of the new edifice with Madeira’s lower slope. Gravitative stresses of such a geometry, combined with gravitative spreading of Madeira, can qualitatively explain the observed relations as we have modelled by analogue experiments.
Source: Discovery of a Young Submarine Rift Zone Near Madeira Island: the Leading Edge of the Madeira Hotspot Track? – NASA/ADS
