The current major explosive volcanic eruption in southern Chile presents an immediate opportunity for scientists to measure the impact of fine ash fallout during and after an eruption. We wish to use this opportunity to collect detailed measurements of the thickness and grainsize of ash which has fallen from Chaiten across a very large area of prime grazing land in southern Argentina. It is important that this work is carried out quickly - before the ash has been moved around on the ground by winds, and before the ash has been leached by rain water. Ash fallout is the major hazard from volcanic eruptions to humans, and their life-support systems (agriculture, transport, communications). Even only a thin deposit of ash can have a devastating effect on grazing animals, since they either refuse to eat ash-dusted grasses (and starve); or they consume the toxic salts deposited along with the ash (and die of fluorosis). At the moment, we so not have sufficiently sophisticated models of where fine ash ends up after eruptions - mainly because we do not have the measurements to test these models. Eruptions such as this, which are major explosive eruptions and which deposit fine ash across accessible land areas (rather than the sea), only happen about once in a decade or more. This is the first such opportunity since the major explosive eruptions of Pinatubo (Philippines) and Hudson (Chile) in 1991.