Viscous andesite and dacitic lava flows are found on the northern flank of Tacaná. The aerial photographs indicate that lava flows also occur on the southern flank. These lava flows are 20 to 40 m thick and blocky, up to 2500 m long and 1000 m wide on the northern flank and up to 4000 m long and 1500 m wide on the south flank, with associated avalanche and block and ashflow deposits downslope.
The areas likely to be affected by future lava flows (see map on previous page) were defined on basis of the extent of observed lava flows. Lava flows are likely to be slow moving, thick, blocky flows, accompanied by avalanching and block and ash flows downslope and mudflows and floods along streams draining areas below the flow. The populations of Chocabj, Vega del Volcán, La Vega, San Rafael, and La Laguna, located at the foot of the Tacaná cone, are likely to be affected by future lava flows from Tacaná volcano. Sibinal is topographically shielded from lava flows.
The flows produced by Tacaná are geomorphologically, chemically, and petrographically similar to those produced by Tolimán, Guatemala and Arenal, Costa Rica. The closely-observed lava flows show three dynamic stages: (1) early phase, beginning when the lava is erupted at the crater, the flow is 0.5 to 1 m thick and thickens to 10-15 m with a width of 25-30 m, temperature ranges from 980° to 1020°, and moves at a rate of 50-70 m/day, (2) mature phase, observed as a flow approaches the base of the cone, the flow becomes continuous and relatively uniform, enlarges to 50-80 m and up to 400-500 m maximum width, up to 100 m thick, and moves at a rate of 30-40 m/day, and (3) collapsing phase, which occurs as magma ceases to erupt from the crater, and moves at a rate of 50 m/day. The total volume of magma emitted from Arenal between 1968 and 1980 is reported as 304 x 10^6 m³ (dense rock equivalent). The analogy between Tacaná's surface and the 1968-1987 lava flow activity at Arenal Volcano has influenced us to construct the "Arenal Scenario" of possible future activity.
(Mercado and Rose, 1992)