Looking back to our future: Is the IPCC optimistic on climate change?

Claude Jaupart

Institut de Physique du Globe de Paris

A summary of recent advances in our understanding of volcanic eruptions is presented focusing on physical mechanisms which are specific to volcanic flows. During ascent towards Earth's surface, magma undergoes dramatic physical changes as it exsolves volatiles and involves a highly compressible gas phase. The gas phase behavior is critical for eruption behavior, as illustrated by examples from two types of eruptions, involving slow effusive flow of viscous bubbly lava or the extremely rapid eruption of a mixture of gas and fragmented magma.

The flow of bubbly magma involves three different pressures: those of the gas and magma phases, and that of the exterior. Expansion of gas bubbles is inhibited by viscous magma implying that gas is over-pressured when it oozes out of a volcanic vent. This explains why lava domes may become unstable and expand catastrophically. In the conduit and at the vent, there are large horizontal variations of gas pressure, and hence of exsolved water content. Lava batches which get erupted simultaneously may have different exsolved water contents and internal gas pressures. These principles account for the complex internal variations of gas content observed in fossil lava-filled eruption conduits and ancient lava flows.

Explosive volcanic eruptions may occur in two end-member regimes. One is such that an atmospheric column rises buoyantly at high altitude above the vent, generating fall deposits cover large areas. Another is such that the atmospheric column collapses at small altitude and feeds an avalanche of pyroclastic material. A marginally stable atmospheric column in transitional conditions between the buoyant and collapse regimes generate fall and flow deposits simultaneously. A regime diagram delineates these regimes as a function of the eruptive mass flux and of the mass fraction of gas carrying the flow. The size distribution of pyroclasts has a strong effect on atmospheric column behavior because it determines the effective amount of continuous gas phase in the erupted mixture. In coarse pyroclast populations, a large amount of magmatic gas remains in bubbles trapped within the pyroclasts and is not involved in the bulk volcanic flow.

About the Lecturer

Claude Jaupart, a native of Paris, earned his Ph.D. in Geophysics from Massachusetts Institute of Technology. He returned to Paris in 1980 and was both Research Fellow and a Professor of Geophysics at the Institute de Physicque du Globe de Paris and the Université Paris 7. Jaupart is currently the Chairman of the Institut de Physique du Globe de Paris.

Included among Jaupart's accomplishments is the contribution of new data and concepts to constrain the distribution of radioactive elements in Earth's crust and the thermal structure of continental roots. In addition, he developed scaling laws for various mantle convection phenomena, including strongly temperature-dependent viscosity and continents at Earth's surface. Jaupart also contributed new insights into the dynamics of magmatic and volcanic systems, including convection in partially crystallized magmas, foaming in basaltic reservoirs and magma degassing during ascent.

Claude Jaupart