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The planet we live on: The beginnings of the Earth Sciences

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Figure 5.10: A hazard z<strong>on</strong>e map <strong>of</strong> <strong>the</strong> Nevado del Ruiz volcano showing volcanic hazards,including <strong>the</strong> high mudflow (lahar) hazard areas (which included Armero) and pyroclasticflow (nuée ardente) risk areas; <strong>the</strong> map was produced after <strong>the</strong> Armero disaster.Destructive volcanoes with intermediate and silica-rich magmas not <strong>on</strong>ly erupt columns<strong>of</strong> ash into <strong>the</strong> air, but <strong>the</strong> column may collapse and flow sideways as red hot (up to1000 ◦ C) clouds <strong>of</strong> ash and gas called nuées ardentes (glowing clouds) or pyroclasticflows. Since <strong>the</strong>se clouds <strong>of</strong> ash in air are denser than <strong>the</strong> surrounding air, <strong>the</strong>y flowdownhill as density currents at speeds <strong>of</strong> hundreds <strong>of</strong> kilometres per hour, incineratinganything in <strong>the</strong>ir paths (Figure 5.9). <str<strong>on</strong>g>The</str<strong>on</strong>g>y tend to become funnelled down valleys andspread out <strong>on</strong> <strong>the</strong> plains beneath, ra<strong>the</strong>r like lahars, but much hotter and faster, andcan even flow across water. <str<strong>on</strong>g>The</str<strong>on</strong>g> hazard map <strong>of</strong> Nevado del Ruiz (Figure 5.10) showspotentially hazardous pyroclastic flow paths as <str<strong>on</strong>g>we</str<strong>on</strong>g>ll as lahar flow paths.<str<strong>on</strong>g>The</str<strong>on</strong>g> landslide at Mount St Helens which triggered <strong>the</strong> 1980 erupti<strong>on</strong> occurred after m<strong>on</strong>ths<strong>of</strong> activity had built a huge bulge <strong>on</strong> <strong>the</strong> side <strong>of</strong> <strong>the</strong> mountain. An earthquake triggered<strong>the</strong> largest landslide in recorded history at <strong>the</strong> steep edge <strong>of</strong> <strong>the</strong> bulge. This released<strong>the</strong> lateral blast that was follo<str<strong>on</strong>g>we</str<strong>on</strong>g>d by <strong>the</strong> full erupti<strong>on</strong>. Although this landslide was anunusual event, most o<strong>the</strong>r major landslides <str<strong>on</strong>g>we</str<strong>on</strong>g>re triggered by earthquakes too, althoughsome <str<strong>on</strong>g>we</str<strong>on</strong>g>re also triggered by storms.Landslides are <strong>the</strong> result <strong>of</strong> <strong>the</strong> collapse <strong>of</strong> unstable slopes under gravity. <str<strong>on</strong>g>The</str<strong>on</strong>g> stability<strong>of</strong> slopes depends up<strong>on</strong> how steep <strong>the</strong>y are, <strong>the</strong> <str<strong>on</strong>g>we</str<strong>on</strong>g>aknesses in <strong>the</strong> material (from faults,116

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