THE GROWTH AND EROSION OF CINDER CONES IN GUATEMALA: MODELS AND STATISTICS

A number of cinder cones in Guatemala have been dated by 40Ar-39Ar radiometric dating methods. These dates allow us to investigate the growth and erosion of cinder cones in more detail than previously possible. The cinder cones of Guatemala have an unusual statistical characteristic: the older cones are mostly steeper than the younger cones (until ages exceeding 1 Ma). Prior studies of erosion in cinder cones suggest cones should lose material from the top and gain at the base resulting in shallower slopes, wider bases, and reduced craters. Do Guatemalan cones erode differently? Or, do these slopes reflect primary differences in the cone shape just after formation and perhaps subtle differences in eruption processes? This study shows that Guatemalan cones experience little erosion in their early years (<1 Ma) due to the rapid cover by vegetation which stabilizes soil and reduces loss of material due to erosion. Further, we attribute the range in cone steepness from 20-40 degrees to primary eruptive processes. We will present several models to suggest this reflects the energy of the eruption and to explain the decrease in cone steepness (and presumably eruption energy) over the last 800 kyrs.