1. Phase III – Stage 7: Afar mantle plume volcanism
Mid Oligocene
Phase III – Stage 7: Afar mantle plume
volcanism
Hummocks formed ~30m depth, below storm wave base (~60m).
Flood basalts

2. Phase III – Stage 8: Late volcanic activity
Miocene - Quaternary
Phase III – Stage 8: Late volcanic activity
Hummocks formed ~30m depth, below storm wave base (~60m).
Shield volcanoes

3. Gani et al. (2008) Geological Journal

6. 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

8. 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

10. 0.55 km of uplift unaccounted for!