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Phase III – Stage 7: Afar mantle plume volcanism

Mid Oligocene. Phase III – Stage 7: Afar mantle plume volcanism. Flood basalts. Miocene - Quaternary. Phase III – Stage 8: Late volcanic activity. Shield volcanoes. Gani et al. (2008) Geological Journal. Isostatic Subsidence.

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Phase III – Stage 7: Afar mantle plume volcanism

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  1. Mid Oligocene Phase III – Stage 7: Afar mantle plume volcanism Flood basalts

  2. Miocene - Quaternary Phase III – Stage 8: Late volcanic activity Shield volcanoes

  3. Gani et al. (2008) Geological Journal

  4. Isostatic Subsidence • Oligocene (~30 Ma) uplift of the Ethiopian Plateau due to the Afar Mantle Plume is ~2 km (Sengor, 2001 – GSA Special Paper) • Eruption of Afar Mantle Plume volcanism took only ~1 Ma - between 30 and 29 Ma (Hofmann et al., 1997) - covering ~600,000 km2,erupting ~350,000 km3 volume of basaltic material with a mean thickness of 1 km • This translates to ~980X1012 tons adopting density of 2800 kg/m3 for the basalts of the Ethiopian Plateau • Isostatic Subsidence = Thickness of material accumulated X (Crustal Density/Mantle Density) • Mantle Density = 3300 kg/m3 • Isostatic Subsidence = 1 km X (2800 kg/m3 / 3300 kg/m3) = 0.85 km

  5. Isostatic Rebound • 93,200 km3 of geological material was removed from the Ethiopian Plateau through Blue Nile incision from a catchment area of 250,000 km2 since ~30 Ma giving a mean thickness of 0.37 km (Gani et al., 2007 – GSA Today) • This translates to 250X1012 adopting density of 2750 kg/m3 • Isostatic Rebound = Thickness of material removed X (Crustal Density/Mantle Density) • Mantle Density = 3300 kg/m3 • Isostatic Rebound = 0.37 km X (2750 kg/m3 / 3300 kg/m3) = 0.30 km

  6. 0.55 km of uplift unaccounted for!

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