Look strata head


Dec 15 2003

Before the eruption of Mount St. Helens, geologists considered models of strata formation within uniformitarian constraints. Thin stratification was thought to form very slowly, as sediment was delivered by rather sluggish agents. The boundaries between consecutive strata were often deemed to represent long-time breaks with no deposition. At Mount St. Helens, we saw a living laboratory for the rapid formation of strata. An abundance of coarse and fine particles was produced by the explosive eruptions. Pyroclatic flows, mudflows, and river floods distributed the particles widely and accumulated strata in a hurry. We learned that flow process creates a sweeping action as particles roll or bounce along at Earth’s surface, quickly separating the particles by size, shape, and density and forming even micro-thin laminae. Particles with similar size, shape, or density are deposited together at a specific horizon within the bed. Each moving particle requires a specific energy level to transport it. When that energy level is exceeded, the flow is able to winnow and segregate the larger particles from the finer ones within the moving flow. Below that energy level, deposition occurs.        Morris/Austin, Footprints in the Ash, Master Books 2003, pp. 50-51