Bimodal volcanism Bimodal volcanism Bimodal volcanism is the eruption of both mafic and felsic lavas from a single volcanic centre with little or no lavas of intermediate composition. This type of volcanism is normally associated with areas of extensional tectonics, particularly rifts. Most occurrences of bimodal volcanism are associated with thinning of the crust and the presence of such rocks in metamorphic sequences has been used to provide evidence for past rifting events. Most examples come from areas of active continental rifting such as the Basin and Range Province. Bimodal volcanism has also been described from areas of transtension, the early phases
Ringing rocks that retained the olivine and pyroxene crystals (OPM) is extremely resistant to weathering and is the material which forms the tors. The altered rock (AM) conversely is very weak and readily breaks down to a coarse soil. The inner felsic core is a medium grained quartz monzonite that gradually grades into granite at the center. The transitional unit between the mafic zone and the felsic core units is completely obscured, and is represented by distinctive low-lying swales. A series of radial dikes punctured the mafic units, beginning in the felsic zone and terminating at the outer border of the intrusion.