The rise of rooted upland vascular plants during the Devonian probably had a major effect on the rate of weathering of silicate rocks and consequently on levels of atmospheric CO 2 at that time. In order to better understand the role of plants in rock weathering, an electron microscope/microprobe study of the root/rock interface has been done on several young (100-3000 year old) Hawaiian basalts. We observe the development of porosity due to dissolution of minerals by solutions secreted by symbiotic microbiota associated with plant roots. In nearby plant-free parts of the same basalt flows, similar porosity development is essentially absent. Also, in nearby areas covered by lichens the extent of rock dissolution is much less. In these early stages of weathering, dissolution is complete and voids are not infilled by fine-grained weathering products. Quantitative measurement of porosity around roots developed on flows of known age enables the calculation of a rate of basalt weathering that is in order-of-magnitude agreement with results based on the study of global volcanic weathering based on the chemistry of river water.