Phase III – Stage 7: Afar mantle plumevolcanism

Mid Oligocene

Flood basalts

Phase III – Stage 8: Late volcanic activity

Miocene - Quaternary

Shield volcanoes

Gani et al. (2008) Geological Journal

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

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

0.55 km of uplift unaccounted for!