Distribution by grain size
Coarse bombs and blocks are dropped close to the vent and therefore found in a circular pattern around the vent. The finer pyroclastic material will be carried furhter away from the volcano and could cover hundreds or even thousands of kilometres. The ash gets finer and thiner with distance away from the vent.

Distribution by wind
Where the wind blows in a prevailing direction during an eruption, the ash will be deposited mainly on the leeward side of the volcano. The ash cloud can be carried by the wind over huge distance and, if it gets up to the jetstream, can go right around the world. The higher the wind velocity, the further the ash particles can be carried.

Distribution by energy of blast
The most obvious relationship is that the greater the energy of the blast, the further away from the volcano the pyroclasts will be found. However, it is likely that the blast energy will be directed in one direction, so that even very close to the volcano there could be very little effect if the blast was in a different direction. The best recent example of this was the 1980 eruption of Mt St Helens, which produced a lateral blast due to a landslide on the northwest side of the volcano. This reached anout 12km from the summit, with the shockwabe tragelling at speeds above 1000km/h. Pyroclastic flows of gas and roc, reaching up to 800 degrees, raced down the mountainside at speeds up to 320km/h. Everything in the blast zone - natural or man made - was oblittered within seconds. In a zone 13-27 km from the summit, millions of trees were flattened.

Lahars
A volcanic disaster occured at a Columbian volcano called Nevado del Ruiz in 1985. The lahar began when hot ash and pyroclastic flows erupted, just after 9pm, from the vent of the volcano and rapidly melted the snow and ice. The water and ash surged down, picking up sediment and rocks, forming lahars. These travelled 74km down river valleys to the town of Armero. The lahar, moving at speeds of up to 50km/h entered Armero at 11:30pm as a wall of muddy water nearly 40m high. Armero was inundated and 23000 people were killed, about 5000 injured and more than 5000 homes destroyed, covered by 2-5m of mud.
Scientists were monitoring Nevado del Ruiz at the time of the eruption. Likely lahar pathways were included on the maps, and nearby communites were warned of the damger. but not in time for the people of Armero. The town was built on top of of the 1845 mudflow deposit.

Distribution of lavas by viscosity
The viscosity or resistance to flow of lava is important because it determines how the lava will behave. It controls the length of lava flows, the velocity of the flow and the shape of the volcano.
Highly viscous rhyolite and andesite lava means that:

the volcano will have steep sides
eruptions are infrequent
lava flows move slowly, are short in length and tend to be black
trapped gases may escape explosively
there are explosive eruptions, tuffs and pyroclastics flows form
a degassed viscous lava or one that it hotter than usual may form a lava flow

Basalt lava with low viscosity means that:

the volcano will have shallow sides, but wherever the slopes are slighlty steeper the flow will speed up
eruptions will be frequent
lava flows easily, forming long flows in channels and tubes
gases are easily released
eruptions are rarely pyroclastic and are usually quiet
cooler basalt lava is quite viscous

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Undergraduate Geology - Part 2 (Igneous Processes and Products) Notiz am Where to find the pyroclasts, erstellt von siobhan.quirk am 21/05/2013.