Barren lava fields and flows

Hard or porous rocky surfaces and chaotic rock jumbles formed by solidified lava flows originating from volcanoes and fissures of the Palaearctic domaine, unvegetated, occupied by lichens or mosses, or colonized by specialised, relatively sparse herb- or shrub-dominated communities. They are restricted to the Mid-Atlantic ridge and its associated fracture zones, in Iceland and the Macaronesian islands, responsible for a large proportion of the volume of recent lava in the world, to the collision zones and constructive margins of southern Eurasia, in the Mediterranean and Tethyan basins along the folds of the Alpine system, to the Red Sea rim, on the Arabian peninsula, and to the western Pacific rim and island arcs. A range of surface characteristics is presented by lava of differing composition, viscosity and gas content, including pahoehoe or ropy lava, aa, scoria, pumice and pillow lava. A gradient of increasing acidity and viscosity extends from basalts through andesites and dacites to rhyolites.