[Aztlan] Notes on the general geology of western Nicaragua

[Bruce Rogers]

Listeros,

A few general comments on the geology of western Nicaragua as it 
relates to the statuary under discussion here.

Western Nicaragua is largely volcanic in nature.  Volcanoes, many 
active, include Mombacho, Maysaya, Momotombo, Las Pilas-Cerro Negro, 
El Viejo, Consiguina, Zapatera, Conception, and Madera that dominate 
the landscape.  The entire western region falls into the Central 
American Pacific Volcanic Lowlands.  Being volcanic it is largely 
basaltic and andesitic in nature.  Both these rocks, as we all know, 
are excellent for carving large statuary from, albeit with some 
effort as the basalt is a tough rock.  The area is also the locus for 
many large earthquakes such as those in 1931 and 1972 that devastated 
the region.

For comparison to more familiar architectural elements, the large 
Olmec monuments of the Vera Cruz area were carved and re-carved from 
basalt while the impressive statuary at Copan is made from volcanic 
sandstone.

Much of the coastal plain is underlain with Tertiary Period (ca. 65 
to 2 million year old [Ma]) sedimentary rocks such as marl 
(clay-rich, usually brackish to fresh water limestone), marine 
limestone, and sandstone (possibly  including tuff [= consolidated 
volcanic ash]) made of volcanic fragments.  Also present are small 
areas of older Jurassic and Cretaceous Period (Age of Reptiles and 
following Age of Dinosaurs - ca. 208 -144 & 144-65 Ma) limestone, 
sandstone, siltstone, and shale.  Much of the sandstone and other 
clastic rocks (= sandstone, siltstone, and shale - the word derived 
from the Greek "klastos" = broken in pieces) are made of volcanic 
debris and are reddish in hue, both from the original material and 
from weathering staining.

The Isthmus of Rivas itself (between Lake Nicaragua and the Pacific 
coast) is made of Cretaceous to Eocene Epoch (ca 144 - 38 Ma) 
sedimentary rocks that have been bowed up (to form an anticline) and 
include older andesite volcanic rock as well as younger marl, 
sandstone, siltstone, travertine, and "silex" (an old French term for 
any hard rock such as basalt or dense limestone, but now applied to 
flint or chert).

The few accounts I've read imply that some of these statues were 
produced (most probably with obsidian or basalt tools) from hard, 
reddish sandstone, typical for "volcanoclastic" (= volcano debris) 
rock. The softer tuff, sandstone, and limestone could be literally 
carved using obsidian or basalt tools.  Other statues were carved 
from basalt.  The harder basalt can be shaped by pounding 
("percussed") by smaller, basalt hammer stones, then finished with 
"sanding" with basalt or sandstone (the quartz particles in the 
sandstone being harder than most of the materials basalt is made up 
of).

The soils described by E. G. Squier in his 1860 tome indicate that 
these were reduced to a sticky mass after rainfall; again, indicating 
volcanic materials weathered to clay-rich soil. These igneous rocks 
weather to clay-rich or sandy soils that are very fertile.

Cheers,
Bruce Rogers, earth scientist on a good day