<P> Secondary forms of convection that may result in surface volcanism are postulated to occur as a consequence of intraplate extension and mantle plumes . </P> <P> It is because the mantle can convect that the tectonic plates are able to move around the Earth's surface . </P> <P> Mantle convection seems to have been much more active during the Hadean period, resulting in gravitational sorting of heavier molten iron, and nickel elements and sulphides in the core, and lighter silicate minerals in the mantle . </P> <P> During the late 20th century, there was significant debate within the geophysics community as to whether convection is likely to be "layered" or "whole". Although elements of this debate still continue, results from seismic tomography, numerical simulations of mantle convection and examination of Earth's gravitational field are all beginning to suggest the existence of' whole' mantle convection, at least at the present time . In this model, cold, subducting oceanic lithosphere descends all the way from the surface to the core - mantle boundary (CMB) and hot plumes rise from the CMB all the way to the surface . This picture is strongly based on the results of global seismic tomography models, which typically show slab and plume - like anomalies crossing the mantle transition zone . </P>

On which of the following are slow movements in the earth's interior dependent