About 5 km west of La Democracia a debris avalanche lobe about 5 km wide and 15 km long, which is herein called the La Democracia debris avalanche, underlies about 60 km² of the coastal plain. Parts of the avalanche were mapped by Hunter et al. (1984) as Tertiary volcanis; the shape, size, and distribution of the hummocks, the variable lithology of the hummocs, and the stratigraphic relations of the deposit suggest that it is a late Pleistocene debris avalanche. Assuming a 20 m thickness the deposit has a volume of 1.2 km³, and a considerable volume must now be buried or eroded by alluvial fan processes.
The deposit can be traced to witin 5 km south of Siquinalá where it is being buried by the actively aggrading Pantaleón fan system. In the area south of Siquinalá, isolated mounds protrude through the alluvium. Further south the surface is characterized by numerous hummocks up to 40 m high. The largest hummocks further up slope probably reflects burial of the deposit nearer the active part of the alluvial fan.
The mounds are formed of blocks of basalt, basaltic andesite, and andesite; very few good exposures were observed and most samples were taken from the surface of the hummocks. One hummock exposed to a depth of 6 m exhibits weathering to at least that same depth. Nevertheless, color variations typical of inhomogeneous avalanche blocks are visible.
Weathering to depths of 6 m or more suggest an age of tens of thousands of years; however, the lack of Los Chocoyos ash on the surface of avalanche suggest that it is younger than 85 ka. The Fuego-Meseta-Acatenango Complex is the only likely source for the avalanche. The hummocks contain lithologies similar to those of the Escuintla debris avalanche and it is possible that the La Democracia avalanche occurred at the same time. The more extensive soil on the La Democracia deposit suggest, however, that it is the result of a separate, older event.
(Vallance et al., 1988)