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Gravitational force acting on a deformed lithosphere
You probably have tried to push a balloon to the bottom of a swimming pool. This is not on easy task as the balloon, because it has a density lower than water, resits the push. On the contrary a rock will drop to the bottom of the pool without any help. The lithospheric mantle of modern lithospheres acts like a stone in a swimming pool. What prevents the lithospheric mantle to drop to the bottom of the astenosphere is the continental crust attached to it which has a density much lower than that of the asthenosphere. However upon convergence and thickening, the pull exerced by the heavy keel of SCLM will tend to enhance convergence and thickening.
In contrast the lithopheric mantle of Archaean continental lithosphere acts more like a balloon. Therefore upon convergence and thickening the light keel of SCLM resists convergence and thickening.

The force that enhances or opposes thickening it the gravitational force. It is a force produced by lateral variation in density. This force is equal to the integration with depth of the difference between the lithostatic pressure underneath two lithospheric columns with different density distribution: a column of thickened continental lithosphere (Column B) with a column of undeformed continental lithosphere (Column A). The gravitational force is equal to the surface area of the coloured regions. The bleu areas correspond to positive gravitational forces that oppose thickening, whereas pink areas produce negative gravitational forces that promote thickening. In the Archaean, thickening was accompanied by a large gravitational force opposing thickening, whereas in modern lihtosphere there is a balance between opposite gravitational forces in the crust and the mantle.