PYROCLASTIC FLOWS
Originally termed nuée ardente or glowing ash cloud, the term pyroclastic flow is used to denote all flows where rapidly expanding hot gases transport particles in a fluidised mass down valley and across surfaces of low gradient until the flow loses mobility by dissipation of the constituent gases. Such flows are dry and extremely mobile fluids. If the seed, volume and momentum are sufficient, pyroclastic flows may flow uphill or across areas of irregular relief, but usually then tend to flow along river courses and into depressions where they fan outwards. On Egmont Volcano pyroclastic flows have been directed along catchments to the northwest, northeast and east where they have extended up to 15 km from source.
Pyroclastic flows travel at speeds of up to 200 km/hour. They are usually extremely hot; temperatures up to 715°C have been measured in pumice flow deposits erupted from Mt St. Helens in October 1980.
LAHARS
A lahar is a rapidly flowing mixture of rock debris and water originating from a volcano. Lahars resemble the behaviour of wet concrete as they flow. Lahars have low viscosities and are thus readily influenced by relief, being guided along stream channels and into deep gorges or even along shallow incised stream channels at low gradients. Close to source a lahar may be erosive and scour underlying soft materials on steep slopes, often incorporating loose material within the flow. The velocity of a lahar is dependent on the density of the flow, the gradient of the ground surface and the volume. If large enough volumes of material are incorporated on initiation of a lahar, and there is sufficient vertical drop, then it may travel great distances. Some mudflows are reported to have attained speeds of up to 180 km/hour on very steep slopes similar to the upper section of Egmont Volcano, and speeds of 20-40 km/hour can be reasonably expected on lower gradients. As lahars descend valleys, sufficient momentum may be attained for a flow to climb the walls at bends in valleys and even flow uphill over topographic barriers directly in their path.
Lahars are created by a variety of mechanisms. Generally water is necessary for lahar formation and the sources of this water are many and varied. Large volumes of water are often stored in crater lakes and when portions of a crater-lake rim collapse or when eruptions occur through the lake, much water is released.
If the crater lake is sited high on the flanks of a volcano or at the summit, a fast moving lahar may be generated due to the large vertical drop. Pyroclastic flows are also recorded as having generated lahars by becoming mixed in with river or lake waters. Heavy rains on the flanks of a volcano may lead to saturation of loose material which become unstable and flow as a lahar. These may be particularly common after eruptions when a thick blanket of ash coupled with vegetation destruction may lead to widespread remobilisation of the erupted products. Melting of snow and ice by a variety of processes including high heat flow from the ground or lava flows may create large volumes of water which may be temporarily stored in depressions or directly create lahars. Collapse due to hydrothermal alteration of summit rocks has also been invoked as a mechanism for some large lahars. Steam explosions have also triggered collapses of sectors of volcanoes that have generated lahars.
To log this Earthcache, please answer the following questions...
Stage 1
1. At this location describe the landscape above and below Wiremu road.
2. In what direction do the principal landforms lie? i.e. north-south, east-west, or...?
Three kilometres away South-East on Wiremu Road is Stage 2
3. There is a cutting beside the road. Describe what you see, specifically the composition of this feature.
4.i Estimate, or measure the size of the biggest of the features visible. (There’s one low down).
4.ii. Estimate, or measure the size of the smallest of the features visible.
5. So would you describe this feature as a Pyroclastic flow, a Lahar, or something else? Why do you think this?
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Neall, V.E.; Alloway, B.V. 1993 Volcanic hazards at Egmont volcano. 2nd ed. [Palmerston North, NZ]: Ministry of Civil Defence. Volcanic hazards information series 1. 31 p.
Institute of Geological and Nuclear Sciences, Taranaki/Egmont Volcano Geology
Available at: https://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/New-Zealand-Volcanoes/Volcano-Geology-and-Hazards/Taranaki-Egmont-Volcano-Geology Accessed: October 10, 2017