Vertical surface displacements at Yellowstone Caldera, Wyoming, 1976–1986

Abstract

Between first‐order leveling surveys in 1976 and 1986, the east central floor of Yellowstone caldera rose 15.4 ± 1.4 mm/yr and the west central floor rose 17.0 ± 2.0 mm/yr with respect to points on the south caldera rim. The same pattern has prevailed since an initial survey in 1923, except that the western caldera rose 30% faster during 1976–1986 than during 1923–1976. The 1976–1986 uplift extends beyond the northwest caldera rim near the adjacent Hebgen Lake fault zone, site of a magnitude 7.3 earthquake in August 1959. Marginally significant displacements in the eastern caldera suggest that uplift continued during 1983–1984, stopped during 1984–1985, and reversed to subsidence during 1985–1986; thus long‐term uplift may accumulate episodically. Historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward. The pattern and dimensions of the uplift indicate a source in the upper crust beneath Yellowstone caldera, probably a reservoir of rhyolite magma within a cooling granitic pluton. A simple point source model would suggest a source 10–15 km deep, but the reservoir is probably lens‐shaped and closer to the surface. Uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir. Subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids. Magmatic inflation, volatile accumulation, and crustal extension may culminate eventually in renewed eruptive activity, but there is no evidence of an imminent volcanic hazard.