Volcanism apparently began during the Miocene with the release of extensive lavas of andesite and dacite that capped the older metamorphic and intrusive rocks. Subsequent periods of effusive activities resulted in an extensive sequence of differentiated lava flows and pyroclastic units consisting mainly of andesites and dacites that were erupted from distinct central vents. These flows seem to have occurred mainly along two series of fractures: one set of fractures strikes in a N2O°E direction parallel to the right-lateral Palestina fault that passes near the summit of the volcano; the other series of fractures is oriented in a N12°E to N130°E direction approximately orthogonal to the first fracture set. Ruiz volcano is composed of flows and pyroclastics that extend 30 km to the east and more than 35 km west of the vent area.
Extensive glaciers formed in the Nevado del Ruiz area during late Pleistocene and Holocene times. At the height of Pleistocene glaciation, an ice cap blanketed the summit region and terminated near the 3,600-m elevation on the west flank and near the 3,200-m elevation on the east (Herd, 1982). Valley glaciers drained from the ice cap downstream along river valleys to an approximate elevation of 2,700 m to the east and 3,200 m to the west as indicated by glacial erosion features (striae, grooving), moraines, and “V”-shaped valleys. Some of the preexisting tephra cover was stripped away by the advance of these glaciers.
A postglacial eruptive phase of the volcano began during Holocene time. Episodes of explosive volcanism generated a number of lahars and pyroclastic flows in several directions from the volcano. To the north they are intercalated with the late Quaternary tephra mantle, while to the east the lahar deposits fill most of the upper Azufrado River valley, remaining as terraces above the present river level. The observed degree of soil development between lahars indicates that the mudflows are of substantially different ages. A more detailed description of the lahars in the Nevado del Ruiz area, including two historically recorded events, is given in the body of this report (see Chapter 1 ).
The Magdalena River Valley, west of the river, includes a relatively flat plain 20-25 km wide between the Mulatos fault along the eastern foothills of Nevado del Ruiz and the north-bound river. The surficial deposits of this region are Quaternary floodplain deposits of stratified sand, silt, and gravel. Mesas, composed of Upper Tertiary sedimentary rocks, stand prominently above this plain and are composed of alluvial conglomerates, sandstones, and siltstones. The various lahars generated during late Pleistocene and Holocene times are intercalated with the Holocene alluvial deposits and both can be seen overlying the Tertiary rocks.