12.0 Karoo Mantle Plume (Fig. 12a, b)

The Karoo Mantle Plume is an upwelling of very hot mantle material, with a stem, hundreds of kilometres across, and a radiating top under or through the Earth’s crust. These are up to 3000 kilometres in diameter (Segev 2002), originating from the core-mantle boundary 2900 km below the surface. There are only about twenty of these plumes around the world, mostly welling up as wide mid-ocean ridges (Wyllie 1975). The giant Karoo Mantle Plume first erupted on the northern extremity of South Africa along the Limpopo River 204-205 Ma (Allsop & Roddick 1984; Kent, Storey & Saunders 1992).

There was no known reason for this plume to rise under Gondwana, which had been part of a stable Pangaea for the previous 100 million years. I propose that the Vredefort Meteorite Impact of 214 Ma supplied the shock and heat energy to upset the equilibrium in the mantle below, giving the impetus for the plume to begin moving. Once started it became self-sustaining as the hotter mantle material that had been trapped below began to rise with convection currents.

cross section of the karoo mantle plume

The Vredefort Impact shock wave travelled downwards until it reached the core-mantle boundary, about half way to the centre of the Earth, 2900 km in 10 minutes (Marcus, Melosh & Collins 2018). It then rebounded back up off the denser metallic core, like a billiard ball off the cushion, the turbulence leaving a disrupted path through the normally stable mantle

The second effect of the meteorite impact would be to dissipate energy into the mantle in the form of radiated heat directly below the shattered floor of the 50 km deep transient crater. This additional heat would cause the thick, molten magma in the mantle to become lighter than its surroundings resulting in the beginning of a slow upward movement. Once moving, the plume would continue to rise due to its hotter base. Mantle plumes, because of their sheer size and weight are very slow moving, a mere five centimetres per year (Wyllie 1975).

This would account for the time delay of about 10 million years between the Vredefort impact of 214 Ma and the start of a two kilometre uplifting of southern Africa. The mantle plume would have created the flood basalt of the Karoo system roughly 200 Ma (Storey, Leat & Ferris 2001). This was followed by the Drakensberg (185-180 Ma) capped by some 1.4 km of basaltic lavas (Veevers 1994; Johnson, van Vuuren, Hegenberger, Key & Shoko 1996).

karoo mantle plume location and ages

On a continental scale across Gondwana the wide top of the plume spread to Antarctica, southern South America, eastern India and south and eastern Australia (Fig.12b). Map from Moorbath 1977, note continental rotation of southern Africa, and plume locations from Segev 2002).

One hundred and thirty five million years after the Karoo Plume first erupt the core left a ‘breadcrumb’ trail of small volcano remnants from South Africa to Madagascar and then close to Reunion. This was the region where the south-west coast of India was situated about 65 Ma. It is therefore possible that the Deccan Traps were formed by the same mantle plume or an offshoot. As the crust then carried India northwards over the mantle the undersea mantle plume left its trail to the south, now at a small related hotspot under Marion Island (Courtillot 1990).




Rev 20180907 Copyright (c) 2018 dave (at) howcroft.co.za