 Analysis of the chemical composition of the lavas gives important clues about the source and dynamics of the hotspot plume . Where hotspots occur under thick continental crust , basaltic magma is trapped in the less dense continental crust , which is heated and melts to form rhyolites . These rhyolites can be quite hot and form violent eruptions , despite their low water content . Such rhyolitic magma can be found at the western part of the Anahim Volcanic Belt , which contrasts the more basaltic material in the Nazko Cone area . This suggests that as the North American Plate moves westwards , the Anahim hotspot underlies thinner continental crust . This hypothesis has been verified by observation of the crustal regime in other compression margins – the thickest granitic structures are found near the margin itself , with the North American Plate becoming less compressed in regions away from the margin . In this case , the thinner crust would represent briefer travel time , thus reducing the time available for magma differentiation , whose end products are rhyolites . As most magma is basaltic in origin , the eruption would therefore contain more basaltic materials . A few igneous rock types with composition unlike basalt , such as nephelinite , do occur at the small basaltic cinder cones and flows but are extremely rare . 
