Volcanic Mudflows (Lahars)

A volcanic mudflow is a mixture of juvenile and nonjuvenile pyroclasts, lithic clasts picked up on the volcano surface, and a silt or mud matrix. Lithic blocks can range from pebble size to slabs that are tens of meters in diameter. Within the fine-grained matrix may be vesicles formed by trapped air or steam. Fisher and Schmincke (1984) and Crandell (1971) provide excellent detailed descriptions of volcanic mudflows.

Volcanic debris flows are nearly always associated with composite cones in submature or mature stages of growth. These flows can be initiated in several ways: (1) condensation of water vapor in cooling pyroclastic flows or surges (Wohletz, 1986), (2) phreatic eruptions of muddy tephra, (3) expulsion of water from crater lakes, and/or (4) snow and ice melt following emplacement of small pyroclastic flows. Lahar is a commonly used Indonesian term for a coarse, poorly sorted volcanic debris flow (van Bemmelen, 1949). Lahar deposits may coat the flanks of a cone, but they are generally confined to the canyons and valleys of the watershed. They may travel for distances of > 100 km if the volume is great enough and the stream gradient steep enough. Where there is a change of gradient—for example at the head of a plain below the volcano—lahars spread out to form fan-like sheets. Because they are generally confined to drainages, volcanic mudflows are interbedded with reworked pyroclastic deposits and eroded lava flows. Surfaces of debris flow breccias of any type can be smooth, but they can also be hummocky when they contain >1-m diameter blocks that have been carried along with the flow, as shown in Fig. 7.11. In most field situations it is difficult to distinguish between laharic deposits and debris avalanche deposits from volcanoes.

Fig. 7.11
A laharic breccia and a small, valley-filling ignimbrite exposed along the east coast of the
island of St. Lucia in the West Indies. Most of the outcrop consists of a matrix-supported
boulder-rich breccia deposited by a volcanic mudflow (lahar). The steep-walled, flat-floored
ravine cut into the breccia has been filled by a massive ignimbrite (the Qualibou Tuff).

Laharic breccias are massive and consist of boulders in a fine-grained, muddy matrix. Most are reversely graded, but some are normally graded; this feature depends on the bulk density of the fluid as well as the velocity and strength of the mudflow. All such breccias are very poorly sorted and thick-bedded deposits. Trees, shrubs, and grass, frequently ripped up by the mudflows in the upper reaches of a drainage, are carried along with the flow. The plants may or may not be charred, depending upon the volume of hot juvenile pyroclasts in the deposit; if the plants are charred, it might be possible to use carbon-dating techniques on the mudflow.