High mudline mark from May 18, 1980 debris flow from Mount St. Helens, about 40 miles from the volcano. Photograph by Harry Glicken.

What is a lahar?

For field geologists who need to interpret the origin of a layer of rock from from its field characteristics, a lahar may be defined as a debris flow composed of a significant component of volcanic materials (>25%) (Fisher and Schmincke, 1984), a descriptive definition that can be applied in the field from observations of deposits without requiring a judgement about synchroneity of volcanism. Many volcanologists prefer to define a lahar as caused by a volcanic eruption. But in ancient deposits, it is not always possible to determine if the lahar was caused directly by eruption or by remobilisation of volcanic rubble long after an eruption.

For geologists who study modern deposits, lahars may be defined in terms of visible characteristics of witnessed flows. The following definitions of lahar come from research geologists that study flows in action and their deposits: definitions of lahars from the U.S. Geological Survey. This USGS link also leads to definitions of other volcanic terms.

Origin of Lahars

Three major categories of lahars by origin are, (1) those formed by the direct and immediate result of eruptions through crater lakes, snow or ice, and heavy rains falling during or immediately after an eruption on abundant unstable loose material. They may also form by dewatering of debris avalanches (Pierson and Scott, 1985; Scott, 1988) and by pyroclastic surges flowing over and melting snow and ice (Lowe et al., 1986; Major & Newhall, 1989). (2) Lahars may form indirectly from eruptions such as commonly occur shortly after eruptions by triggering of lahars from earthquakes or rapid drainage of lakes dammed by erupted products (Glicken, et al. 1989). (3) Many lahars are unrelated to contemporaneous volcanic activity, occurring by mobilization of loose tephra by heavy rain or meltwater on steep slopes of volcanoes by rain or meltwater from snow seeping into loose debris at any stage of volcano cone-building or cone degradation. Moreover, volcanic terrain in areas of extinct volcanic activity and without a volcanic edifice can give rise to debris flows composed mainly of epiclastic volcanic fragments.

Common non-volcanic processes by which lahars and other debris flows form are by heavy rains falling upon loose debris or by loose debris becoming saturated with water from melting snow, glaciers or heavy rains (Osterkamp et al., 1986). Water-saturated material can move downhill like wet concrete when its internal strength is exceeded.

Debris flows as fluids

Debris flows are granular fluids with high bulk density, and exhibit the property of strength resulting from particle interactions due to high concentration of particles. At volume concentrations of less than about 20 or 30 percent, particle support in a solids-water mixture is mostly by turbulence, but above that concentration (up to about 60% volume percent), particle interactions greatly modify flow behavior with particles being supported by a combination of turbulence and particle interactions. At still higher concentrations, particle support is primarily by particle interactions and the fluid may be described as plastic (Costa, 1984).

Internal resistance to flow results from (1) electrostatic forces causing cohesive resistance to flow arising from clay-water mixtures or (2) from friction caused by inertial interaction of large fragments (greater than medium silt-size) causing inertial resistance to flow or frictional resistance (cohesionless or density-modified grain flows; Lowe, 1982). The relative importance of these two causes of flow resistance depends upon the amount of admixed clay-size grains, a small amount (~5%) of which can cause major changes in flow behavior. Grains are supported in debris flows by mass effects caused by high concentration (e.g., cohesive strength, frictional strength, viscous resistance and dispersive pressure), and by turbulence and pore-fluid expulsion.

Debris flows consist of: (1) a continuous phase (matrix phase or fluid phase) composed of water mixed with particles <2 mm, and (2) a coarse-grained phase of large particles >2 mm (Fisher, 1971; Scott, 1988). Therefore, even with a continuum of grain sizes from clay to boulders, it is possible to conceptually consider mass properties of high concentration flows (e.g. viscosity, density and strength) without knowing the individual properties of the particles: the matrix phase can be considered to be the fluid that transports the large fragments, even though the large particles are part of the fluid. A coarse-grained debris flow can be most easily characterized by grain size parameters of the matrix phase, and matrix competence can be characterized by a measure of the largest fragments (e.g., the average of the five largest particles within a specified area, e.g. 1 square m). Complete analyses of coarse-grained lahar deposits containing boulders, cobbles and pebbles entail various methods of size determinations including wet or dry sieving of the sand-size and coarser parts of a sample (up to about 16 mm) and analysis of the silt- and clay-size fractions.

During downstream movement in water-rich environments, lahars may progressively mix with water and transform to hyperconcentrated flood flows. Hyperconcentrated flood flows lack the strength and cohesion of lahars, but can carry high sediment loads with fragments supported both by turbulence and particle interactions (Pierson and Scott, 1985; Scott, 1988; Smith, 1986). Such floods can move as far as 250 km or more down valleys, and because of their high loads, will impact an entire river system such as occurred at Mount St. Helens in 1980.

References Cited

Costa, J.E. (1984) Physical geomorphology of debris flow. In Costa, J.E. & Fleischer, P.J, eds., Developments and applications of geomorphology, Berlin, Springer-Verlag, 268-317.

Fisher, R.V. (1971) Features of coarse-grained, high-concentration fluids and their deposits. J. sedim. Petrol. 41, 916-927.

Fisher, R.V. (1983) Flow transformations in sediment gravity flows. Geology 11, 273-274.

Fisher, R.V. (1984) Submarine volcaniclastic rocks. In Kokelaar, B.P. and Howells, M.F. (eds), Marginal basin geology: volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins. Spec. Publ. geol. Soc. London 16, 5-27.

Fisher, R.V. & Schmincke, H.-U. (1984) Pyroclastic Rocks. Springer-Verlag, Berlin, 472 pp.

Glicken, H., Meyer, W., & Sabol, M.A. (1989) Geology and ground-water hydrology of Spirit Lake Blockage, Mount St. Helens, Washington, with implications for lake retention. U.S. geological Survey Bull. 1789, 1-33.

Lowe, D.R. (1982) Sediment gravity flows: II. Depositional models with special reference to the deposits of high density turbidity currents. J. sedimen. Petrol. 52, 279-297.

Lowe, D.L., Williams, S.N., Leigh, H., Connor, C.B., Gemmell, J.B. & Stoiber, R.E. (1986) Lahars initiated by the 13 November 1985 eruption of Nevado del Ruiz, Colombia. Nature 324, 51-53.

Major, J.J. & Newhall, C.G. (1989) Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods. Bull. Volcanol. 52, 1-27.

Osterkamp, W.R., Hupp, C.R. & Blodgett, J.C. (1986) Magnitude and frequency of debris flows, and areas of hazard on Mount Shasta, Northern California. U.S. geol. Survey Professional Paper 1396-C, 1-21.

Pierson, T.C. & Scott, K.M. (1985) Downstream dilution of a lahar: transition from debris flow to hyperconcentrated streamflow. Water Resources Research 21, 1511-1524.

Scott, K.M. (1988) Origins, behavior, and sedimentology of lahars and lahar-runout flows in the Toutle-Cowlitz system. U.S. geol. Survey Prof. Paper, 1447-A, 1-74.

Smith, G.A. (1986) Coarse-grained nonmarine volcaniclastic sediment: Terminology and depositional process. Bull. geol. Soc. Amer. 97, 1-10.

Copyright (C) 1999, by Richard V. Fisher. All rights reserved.