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This figure shaws a calculation for thermal convection in the Yird's mantle. Colours closer tae reid are het auries an colours closer tae blue are cald auries. A het, less-dense lawer boondary layer sends plumes o het material upwards, an likwise, cauld material frae the tap moves dounwards.

Convection is the concertit, collective movement o groups or aggregates o molecules within fluids (e.g., liquids, gases) an rheids, aither throu advection or throu diffusion or as a combination o baith o them. Convection o mass canna tak place in solids, syne neither bulk current flows nor significant diffusion can tak place in solids. Diffusion o heat can tak place in solids, but that is cried heat conduction. Convection can be demonstratit bi placin a heat soorce (e.g. a Bunsen burner) at the side o a gless full o a liquid, an observin the chynges in temperatur in the gless caused bi the wairmer fluid movin intae cuiler auries.