FIGURE 1.2 Regional map showing the location of towns and rivers referenced in this report. Numbers below physical locations indicate approximate elevations in meters. North is at the top of map.

A three-dimensional schematic diagram of the terrain and main population centers of the volcano’s eastern flank, where most of the destruction occurred and where the reconnaissance team’s field observations were made, is shown in Figure 1.3. The foothills at the base of the eastern side of the volcano are at an approximate elevation of 400-700 m, 45 km east of the summit.

A gently sloping floodplain extends 20 km east from the foothills to the bank of the north-bound Magdalena River. The floodplain stretches north-south along the Magdalena River for over 70 km. Within the floodplain a low ridge of sedimentary rocks runs parallel to the Magdalena River in a north-south direction, approximately 5 km west of the river. The ridge-top elevation is approximately 400 m above sea level, or about 100-150 m above the level of the Magdalena River.

The average mean annual temperature on the plain is about 25°C and the annual rainfall is approximately 180 cm. Annual precipitation in the highlands is estimated at 300-400 cm.

On the east side of the volcano, several towns have been built on the floodplain directly east of the foothills at elevations ranging from 400-500