ICE, ERUPTIONS, AND AVALANCHES: DISTINGUISHING LAHAR ORIGINS WITH EXAMPLES FROM MOUNT SHASTA, CA

Mount Shasta in northern California provides an ideal setting for studying lahars of three primary origins: climatic fluctuations, eruptions, and debris avalanche transformations. Though multiple factors could be involved, there are characteristics in lahar deposits that are indicative of the triggering event. Climatically- induced lahars occur almost exclusively in drainages headed by glaciers, resulting from the rapid retreat of ice, or the failure of a moraine-dammed lake. As non-cohesive lahars, they become diluted as they progress downstream.  Holocene lahars along Mount Shasta’s largest drainage, Mud Creek, have formed levees containing 1 – 2 meter boulders; downstream the deposits form alluvial fans with smaller clasts.  Headed by the Konwakiton glacier, Mud Creek has produced lahars into historic times.  West of Mud Creek, unglaciated drainages have not produced lahars since the end of the last glaciation. One of these drainages contains a latest Pleistocene lahar that likely occurred as the ice receded, possibly resulting in the breach of a moraine-dammed lake.

Lahars with eruptive origins have large concentrations of pumice clasts several centimeters in size. A lahar on the Mud Creek fan contains pumice clasts up to 10 cm belonging to the Red Banks eruption 9700 ¹⁴C years ago. Another characteristic of eruptive lahars is intensive oxidation from high heat, resulting in a red or pink color. The extent of eruption-induced lahars is dependent on the amount of ice and snow in contact with pyroclastic deposits, and they are often non-cohesive in behavior.

Debris avalanches transform into lahars through disintegration of avalanche blocks. High clay content in the avalanche matrix creates a cohesive lahar able to travel long distances. Deposits from an enormous debris avalanche which occurred ~ 350 ka on the north side of Mount Shasta have been recognized for years (Crandell, 1989), but there is evidence that this avalanche triggered lahars on the south flank of the mountain. A south-flank lahar similar in age to the avalanche contains large amounts of pumice, evidence that an eruption occurred prior to or coincident with the avalanche. Whether this avalanche occurred during a glacial period is unknown, though the large amount of water required to mobilize it would be readily available during extensive glaciation.