According to a recent study by Chang et al. (2007) the Yellowstone caldera has begun a rapid uplift (as large as 7 cms/yr) in 2004. The uplift is three times larger then previous measurements. It is interpreted to be caused by recharging of the Yellowstone magma chamber below the caldera. The Yellowstone caldera itself was produced by a super volcano eruption.
Chang, W-L., et al. (2007), Accelerated Uplift and Magmatic Intrusion of the Yellowstone Caldera, 2004 to 2006, Science 318, 952, DOI: 10.1126/science.1146842.