Figure 4. Schematic of mantle processes active within asthenosphere beneath Yellowstone swell. Buoyant and fertile mantle ascends beneath area of active hotspot magmatism, possibly supplied by mantle plume (see Fig. 5 for alternative model). Melt buoyancy drives convection in this mantle (large white arrows). Melt is expelled at top of convective roll (wavy red lines) and depleted residuum (blue areas) is pushed to sides, where it accumulates. When residuum buoyancy equals melt buoyancy, convective overturn ceases, leaving partially molten core (red areas). Buoyant mass then flattens (small white arrows) as it is carried southwest by North America plate motion. Effects of hotspot on North America are (1) magmatic modification of Snake River Plain (SRP) (basaltic underplating of crust, shown in yellow, and intrusion of basalt into midcrust, shown with blue tabular body), which loads and depresses SRP crust, and (2) uplift of region underlain by buoyant mantle (within the blue envelope), creating Yellowstone swell. With plate motion, Yellowstone encounters increasingly thick lithosphere of Wyoming craton.