Final Exam Book Notes - Final Exam Book Notes Volcanoes and...

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Final Exam Book Notes: Volcanoes and Civilization Chapter 7: Lave Flows A.   Nyiragongo, Zaire 1977 a) From about 1928 to 1977, Nyiragongo Volcano had filled with a fiery lave lake in a crater about 1.5 km. in diameter at 3,469 m. above sea level b) From time to time the lake up welled and overturned, bringing the hottest and reddest lava to the surface, which then cooled, darkened, and sank, then remelted and rose again c) In 1977, a fissure opened and the lava suddenly emptied from the crater; it poured out onto the surrounding countryside, moving at an astounding initial velocity of 100 kmph, and overwhelmed everything in its path killing 70 people and 800 lost their houses to the highly fluid lava d) Nyiragongo is perpetually active, and built of basaltic lava flows interspersed with some pyroclastic layers; before the lava drained out, lava flows from Nyiragongo had never been seen outside its crater although 13 eruptions occurred between 1894 and 1877 e) An estimated 20-22 million cubic meters of lava drained in half an hour and flowed for 20 km f) The lava’s hydraulic heads, its temperature, and its low viscosity account for the speed with which the lake emptied and for the lava’s high velocity B.   Stopping a Lava Flow a) Ditches, Walls, and Dams i) Lava usually moves slow enough that a ditch or a high wall may change the flow’s direction ii) In Hawaii in 1955, small debris dikes were plied up by bulldozers to divert a lava flow from Kilauea, but success was limited iii) Another more successful attempt was made in 1960 near the town of Kapoho (1) The height of a natural rise in front of a lava flow was increased to save a lighthouse, and other barriers were constructed to divert the flow; six dikes were erected and the lava moved over the dikes which caused a slight change in direction as the lava approached the lighthouse and it was saved (2) According to Gordon Macdonald, diversion barriers may work in a limited way for very fluid and thin pahoehoe lavas but are generally ineffective for the more viscous aa or block basalt lava flows (a) Macdonald also said that a barrier needs a very wide base and must be composed of denser material than the liquid lava, otherwise the lava could break through and carry the barrier away (b) Many of his conclusions were refuted at Mt. Etna in 1983 when a dam successfully diverted the lava into unoccupied zones around the volcano (dam built of low- density scoria, lapilli, and ash) iv) Purpose was to gain time like other types of intervention because any embankment will be overrun by lava if rate of flow is high for a long enough period of time b) Cooling Lava with Water i) Heimaey, Iceland 1973 (1) In 1973 a lava flow threatened to completely close off one of the most important harbors
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