EarthRef.org Digital Archive (ERDA) -- Deep Mantle Convection and Hotspot Formation

Description
This image shows convection that occurs throughout the mantle. At 670km there is a seismic discontinuity of increasing temperature and pressure that is most likely due to the presence of an abundance of perovskite. The 670km boundary separates the upper mantle and lower mantle, believed by many scientists to be the lower boundary for upper mantle convection. In this theory, cold subducting slabs are believed to accumulate at the 670km phase boundary, until they eventually penetrate the layer and continue down towards the core-mantle boundary. Seismic data supports the presence of cold rock at the core-mantle boundary underneath subduction zones. From the figure, convection is shown to occur in both the upper and lower mantle, with the subduction of oceanic crust down into the mantle. This occurs because hot mantle material rises at divergent boundaries (spreading center) pulling tectonic plates apart, while the colder, more dense oceanic crust subducts. The oceanic island in the image has formed due to a plume rising from within the deep mantle. The plume forms because mantle material, which is extremely hot at the core-mantle boundary, becomes more buoyant than surrounding mantle material and begins to rise upward. The plume rises up towards the surface until it reaches the cold, rigid lithosphere, where it flattens and spreads outward, forming an oceanic (volcanic hotspot) island over time.