The nature and rate of weathering by lichens on lava flows on Lanzarote

Samples of lichen-covered and bare lava surfaces from Lanzarote, dating from the 1730–1736 eruption, have been analysed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) to investigate the relative roles of biological and inorganic weathering processes. Lichens here grow preferentially on N and NE facing surfaces and create a range of nanomorphological features as a consequence of their weathering activities. The fruticose lichen Stereocaulon vesuvianum and a mixture of crustose lichen species are found to be particularly effective agents of weathering. Comparison of the thickness of the weathering rind on bare and lichen-covered samples (mean thickness 15.7 and 253.9 μm, respectively) shows a significant difference at the 99% confidence level. Following previous studies in Hawaii by Jackson and Keller [Am. J. Sci. 269 (1970) 466], these results are used to suggest that lichens on Lanzarote lava flows cause a 16 times increase in weathering rates over those found on bare surfaces. Comparison of these results with those from similar lava flows in Hawaii indicates that under the wetter climate of Hawaii both biological and inorganic rates are over double those found in Lanzarote.