PYROCLASTIC FLOW SKIN

PYROCLASTIC FLOW SKIN

This can cause burning of buildings, breaking of windows and toppling of walls, which make survival impossible 6.Д¬oncerning effects on humans, it is emerging that even in areas far from a volcano, where particle concentration, temperature and dynamic pressure strongly decrease, people engulfed in the flow have вЂњhigh probability of receiving fatal skin burns and inhalation injury of the upper and lower respiratory tract, unless the duration is very briefвЂќ 7. Currents moving in the vicinity of a volcano can have a high concentration of hot magmatic particles that confer high temperature and high dynamic pressure to the flow. Particle volumetric concentration represents an important parameter too because dynamic pressure is proportional to it. We conclude that in distal areas where the mechanical and thermal effects of a pyroclastic density currents are diminished, flow duration is the key for survival.Д®ngineering investigations 1, 4, 5 show that dynamic pressures higher than 5В kPa produce significant damage, while pressures under 1В kPa have minimal to no consequence on structures or infrastructures. Instead, our calculations demonstrate a flow duration of 17В min, long enough to make lethal the breathing of ash suspended in the current. Under such conditions, survival should have been possible if the current lasted a few minutes or less. At Pompeii, in the distal area, we use a new model indicating that the current had low strength and low temperature, which is confirmed by the absence of signs of trauma on corpses.

At Herculaneum, at the foot of the volcano, the temperature and strength of the flow were so high that survival was impossible. The AD 79 event of Somma-Vesuvius is used here to demonstrate the impact of pyroclastic density currents on humans during an historical eruption. Little emphasis has yet been given to flow duration, although it is emerging that the survival of people engulfed in a current strongly depends on the exposure time. The impact is generally associated with flow dynamic pressure and temperature. They move away from the volcano at high speed, causing devastation. Pyroclastic density currents are ground hugging gas-particle flows that originate from the collapse of an eruption column or lava dome. This is what makes these eruptions particularly dangerous. Eruptions sometimes change from one type to the other (like the famous eruption of Vesuvius in the year 79 AD) and are often in an intermediate regime with the two types of flow co-existing.

The volcanic mixture is concentrated and channelled by the valleys: it flows at high speed at heights that can reach hundreds of metres, devastating everything in its passage.ДЄlthough these two regimes are well defined, things are more complicated in practice. Pyroclastic surges are much more dangerous than plinian spines. In this case, the spine cannot contain large amounts of air: the mixture stays heavier than air, only rising a few kilometres over the erupting mouth, eventually falling to the ground, adding to dense flows of fragments and gas hurtling down the slopes of the volcano. Pyroclastic surge is one of the two main explosive regimes, the other being the plinian spine. This type of flow is often the result of the collapse of a volcanic plume. A pyroclastic surge is a relatively dense high temperature mixture (several hundred degrees Celsius) of volcanic gases, water vapour and solid particles (fragments of lava, scoria, pumice, rock etc.), which flows at high speed (several hundred km/h at first) next to the ground, highly influenced by gravity and guided by topography with a largely laminar flow.

Pyroclastic flow