A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock,[1] which makes it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.
The speed of pyroclastic density currents has been measured directly via photography only in the case of Mount St. Helens, where they reached 320-470 km/h, or 90–130 m/s (200–290 mph). Estimates of other modern eruptions are around 360 km/h, or 100 m/s (225 mph).[2] Pyroclastic flows may generate surges. For example, the city of Saint-Pierre in Martinique in 1902 was overcome by one. Pyroclastic surge include 3 types, which are base surge, ash-cloud surge, and ground surge.
First recognized after the Taal Volcano eruption of 1965 in the Philippines, where a visiting volcanologist from USGS recognized the phenomenon as congruent to base surge in nuclear explosions.[3] Very similar to the ground-hugging blasts associated with nuclear explosions, these surges are expanding rings of turbulent mixture of fragments and gas that surge outward at the base of explosion columns. Base surges are more likely generated by the interaction of magma and water or phreatomagmatic eruptions.[4] They develop from the interaction of magma (often basaltic) and water to form thin wedge-shaped deposits characteristic of maars.[5]
These are the most devastating. They form thin deposits, but travel at great speed (10–100 m/s) carrying abundant debris such as trees, rocks, bricks, tiles etc. They are so powerful that they often blast and erode material (like sandblasting). They are possibly produced when conditions in an eruption column are close to the boundary conditions separating convection from collapse. That is, switching rapidly from one condition to the other.[5]
Estos depósitos suelen encontrarse en la base de flujos piroclásticos. Son de capa fina, laminadas y, a menudo, cruzadas. [6] Normalmente miden aproximadamente 1 m. de espesor y consisten principalmente en fragmentos líticos y cristalinos (cenizas finas elutriadas). Parecen formarse a partir del flujo mismo, pero el mecanismo no está claro. Una posibilidad es que la altura del flujo se expanda mediante el arrastre de aire (que luego se calienta). Esto da como resultado que el frente de flujo avance, que luego es superado por el resto del flujo. [5]
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