2003 Volume 109 Issue 7 Pages XIII-XIV
In southern Andes back-arc region, behind the South Volcanic Zone (SVZ, 33°S-46°S) and Austral Volcanic Zone (AVZ, 49°S-52°S), hundreds of monogenic basaltic volcanoes take place (Fig.1), forming numerous cinder cones and lava flows (e. g. Ramos et al, 1982; Ramos and Kay, 1992). The eruption age ranges from the Oligocene to the Recent. The lava flows cover top of the sedimentary plateaux, forming table mountains, so-called "meseta" (Fig.2). In many cases, the plateau top lava is composed of only one cooling unit, less than 10 m total thick. Accumulated lava flows are also observed, and they are generally made up of less than four flow units of as much as of 30 m total thick. Some young lava flows fill glacial valleys, showing lava cascade (Fig.3). Certain lava flows and scoria falls layers are very fresh (Fig.4).
Weathering, wind erosion, and glacial denudation expose cross section of the cinder cones and their underground structure, which makes possible the elaboration of their general structure (Fig.5). Young cones show fresh volcanic morphology almost without degradation (Fig.5A). Old cones loose surface non-welded scoria blanket, and remain their ba-cal core part made up of welded scoria and spatter (Fig.5B). Glacial erosion sometimes shows vertical section of the cinder cones (Fig.5C). Advanced denudation exposes silllike subvolcanic intrusive rock bodies (Fig.5C, 5D) and feeder dykes (Fig.5E).