Travel of pyroclastic flows as transient waves: implications for the energy line concept and particle-concentration assessment

We here consider the precept that pyroclastic flows travel as transient waves or pulses. We provide simple geometric models to estimate the wave height that may form under specified conditions and how it may vary with progression away from the vent. A main uncertainty is the length of the wave relative to the total travel distance, but the simple sequence of segregation layers observed in many ignimbrites may enable crude constraints to be placed on the wavelength. Application of these models to actual ignimbrites suggests wave heights of tens to hundreds of meters. A corollary is that a wave is able to surmount obstacles less high than itself, irrespective of its velocity. The wave may also possess momentum which confers an additional landscape-surmounting capability. Thus, a part of the obstacle-crossing ability of a pyroclastic flow is due to the height of the wave, and a part is due to the velocity of the wave. Another corollary is that the deposit will normally be much thinner than the wave height; estimates of the degree of expansion of pyroclastic flows (or surges) based on the relationships of the height of a trimline to the depth of a deposit may therefore be invalid. This is important, as at El Chichón, when diagnosing whether a deposit was laid down by a pyroclastic flow or surge. Care must also be taken not to confuse a trimline or upper limit of a veneer (which exists, and is a wave-height level) with an energy line (which is a theoretical concept). Application of the wave concept helps explain many observed features of pyroclastic flows and their deposits, for example, drainage of a wave off topographic highs to generate the “secondary” valley ponds, such as were observed in the Mount St. Helens 1980 blast flow deposit.