Variations in Lahar Matrices at Cotopaxi Volocano and their Hazard Implications (Invited)

Most lahars produced at Cotopaxi volcano have been matrix-poor, non-cohesive and clast-supported. The 1877 lahars, in which gravel and sand-sizes predominate, were of this nature. By mixing with stream water they transformed to hyperconcentrated stream flow (HCSF) some 40 km downstream from the vent. In contrast, cohesive lahars are not easily diluted by water and rarely transform to HCSF since attenuation is muted by the strong internal, inter-particle attractive forces. However, there are a few exceptions seen in the stratigraphic record. These examples serve to remind us that while a snow-clad volcano is known to produce similar types of events, it may indeed have a record of generating other, less-frequently occurring phenomena, which in turn results in broad hazard implications. For example, the Chillos Valley Lahar (CVL) generated 4500 yBP by a rhyolitic pyroclastic flow and sector collapse on Cotopaxi’s N flank has a bulk volume of about 3 km3. At its widest, the CVL’s flow was 11 km across and about 100 meter deep. Along its flow trajectory of 326 km, little transformation occurred in its lithology, due to the fines-rich, cohesive matrix. More recently, lahars associated with the 1742 Cotopaxi eruption (VEI= 4), that occurred after 200 yrs of repose, also have higher fines contents. A beige pumice lapilli (marker) layer fell over the volcano’s W flank and was followed by the transit in nearby stream channels of a matrix-rich lahar, whose deposit directly overlies this pumice “marker” unit. The matrix-rich 1742 lahar deposit found in the Cutuchi Quarry (25 km from vent) is notable for its ability to have transported large 2-3 m diameter clasts in suspension. This characteristic continues down valley to Mulalo (30 km from vent), implying minimal water entrainment. Nonetheless, at Salcedo, 65 km downstream, less silt has been retained, implying that some mixing with water had occurred, although large clasts are still present. Silt concentrations in these deposits are about 20%. Since there is an absence of clay-size particles in the matrix, water miscibility is greater than with a true matrix-rich lahar, such as observed in the Osceola or Electron deposits. Thus, the 1742 lahar retained “matrix-rich” characteristics for more than 50-60 km downstream. The generating factor of the Cotopaxi “pseudo” cohesive lahars has probably been pyroclastic flows with a high concentration of fine sand to silt grains. These fine-grained components may have formed from abrasion while the pyroclastic flow descended 2000 vertical meters from the crater rim. At Cotopaxi hydrothermal alteration and clay formation is not observed. Given that matrix-rich lahars are slow to transform downstream to more water-rich flows, the potential impact to essential infrastructure, property and life is greater due to the solid nature of the flow, greater discharge and higher velocities. It may be difficult to know during a future Cotopaxi eruption process if a fine-grained pf is in transit and that a cohesive lahar may form. While water is becoming less available on Cotopaxi due to receding glaciers, the now highly-fractured and mushy ice makes the remaining ice cap more easily melted, and thus future lahar generation is still a likely scenario.