However lava flows from Egmont Volcano in the past 10,000 years have been of restricted distribution to within a 7 km radius of the summit. Such flows are dry and extremely mobile fluids. It is a stratovolcano (also called a composite cone volcano) made of layers of mostly andesite lava flows and pyroclastic deposits (tephra). View to southeast from the west Taranaki coast, near Pungarehu (to right centre). Hazard zones A, B, C and D in North Island from future eruptions of Egmont Volcano. This is due to the opaque nature of ash. The second area is to the south between Midhirst in the north to Ohawe Beach in the south and from Auroa in the west to Stratford in the east except where hazard zone C continues southeastwards along the Patea River. By tending to be channelled along lower ground, lahars are of greatest hazard in valleys or in depressions. If lava was extruded in winter much melting could occur generating explosions and small lahars that would rush down any of the catchments draining the mountain. At Egmont Volcano this would be most likely to occur in wintertime when much of the cone above 1500m is snow-covered. Of highest risk are the gorges and river channels within the confines of Egmont National Park where many lahars have travelled in the last 500 years. Pyroclastic flows travel at speeds of up to 200 km/hour. These vents are regarded as the likely source-areas for future lava flows in future eruptions of Egmont Volcano. If a small volume of viscous lava oozes to a gentle gradient surface or a depression, the lava may congeal as a hemispherical lava dome over its source vent. Close to source a lahar may be erosive and scour underlying soft materials on steep slopes, often incorporating loose material within the flow. If lava were to be extruded, the paths of flowage could be predicted very quickly after a flow is initiated, provided good visibility allowed immediate recognition of the hazard. Hazards associated with the spread of materials through the air. The zones take into account the most likely event of renewed activity in the existing summit crater, which is presently breached to the west, determining a principal westward component to the majority of flows likely to occur. Based on the thinning patter of Inglewood Tephra (which is representative of the larger eruptions from Egmont Volcano), together with known data for other tephra eruptions, four zones of tephra hazard are recognised (Maps 4 and 5). The speed with which the lava is most likely to flow and to solidify depends upon its viscosity and slope angle of the ground surface. The distance a lava flow travels is mainly dependent on the viscosity (or stickiness) of the lava. A cloud of gases and smaller ash particles often spread laterally onto the surrounding landscape and blanket surfaces irrespective of the relief. 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. The hazards presented by tephra may be considered as (1) the problem created by the physical presence of tephra and (2) the presence of potentially harmful substances adhering to tephra particles that create a poison or pollutant to water supplies and animal feedstuffs. / Learning It comprises those areas outside of hazard zone A that have been inundated by landslide and lahar deposits dated between 5000 and 20,000 years old. Those people involved in clearing the ash and scientists investigating the eruptions may be exposed to high dosage rates. The youngest deposit (at top) is younger than 1750 A.D. and contains charred tussock, evidence of its very hot mode of emplacement – V.E. This zone covers as area from NNE to SSE within 21 km of the present Egmont Volcano summit and includes the communities of Kaponga, Stratford, Tariki, Inglewood and Egmont Village. As a lahar grades downstream into a fine mudflow or flood, large quantities of silt may be deposited, or remobilised in subsequent rains, creating substantial floods. Buildings and vehicles may be smashed, buried or carried away and bridges are often lifted off their foundations to be carried along with the flow. No lateral blast deposits have been identified at Egmont Volcano, but this does not mean that they have never happened. Number Five ‘Auckland Volcanic Field’ This avalanche spread to the coastline between Okato in the north to Opunake in the south. This is because shallowly incised river beds will tend to be raised by debris deposited by lahars and flood waters. ; Alloway, B.V. 1993 Volcanic hazards at Egmont volcano. No lateral blast deposits have yet been recognised at Egmont Volcano. Within this zone safety increases with height about channel floors, with distance from river courses and towards the outer boundaries of the zone. These specifically include the Waiwhakaiho River where it flows through New Plymouth, Stony River and its tributaries adjacent to Okato, the Kaupokonui Stream and Kaponga, and the Waitara River at Waitara. The fourth district, to the northeast, extends from Tariki north to Inglewood. Simplified hazard maps are presented based on the known prior behaviour of the Volcano and the distribution and frequency of each volcanic hazard.
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