TThe lithosphere is the Earth's rocky outer shell. Lithospheric 'plates' drift over the Earth, carrying the continents, and large earthquakes and volcanic eruptions occur where the plates interact. Hence, understanding the lithosphere is central to our understanding of how the Earth works. The oceanic lithosphere, which covers some 70% of the Earth, is central to our understanding of global geodynamics and plate tectonics, but our understanding of its character (including composition and thickness) and the factors that control them is poor. Variations of seismic and electrical properties of the Earth are indicators of compositional variations. The aim of this project is to use the most up-to-date seismic and electromagnetic methods to address two fundamental questions about the lithosphere that lies beneath the Pacific basin: What factors control the seismic structure of the lithosphere, and what defines the base of the lithosphere? The broader impacts of this work include improved understanding of factors such as the composition of the lithosphere that affect volcanic eruptions, great earthquakes and tsunamis that threaten heavily populated areas in Alaska, the Pacific Northwest, and elsewhere around the Pacific Rim.
Recent advances in laboratory measurements and theoretical models of the seismic properties of mantle rocks predict seismic velocity profiles for mature oceanic lithosphere (the strong and thick outer layer of the earth) that are fundamentally inconsistent with the best observations of seismic velocities in two important ways. First, the theoretical models consistently display the velocity should decrease with depth in the lithosphere, while observed velocity profiles show increases. Second, the theoretical models predict a that the base of the thick strong layer should be defined by a critical temperature. However, this predicted transition is much deeper and broader than the transition suggested by seismology . These inconsistencies suggest that factors such as bulk composition, mineral fabric, the presence of volatiles (such as water), and the presence of melt play important roles in controlling the formation of the lithosphere. Dubbed NoMELT, our experiment targeted two questions: (1) What factors control the velocity structure of the lithosphere?, and (2) What is the origin of the unique boundary at 70-80 km depth and does it define the base of the lithosphere? We hypothesize that these features are fundamentally controlled by melting processes deap in the Earth. We are now completing an integrated analysis of the seismic and electrical conductivity data from this experiment. Owing to delays in the data aquisition parts of the project, we have not yet finished the analyses. Our colleagues at Lamont (Columbia University) and Wood Hole Oceanographic Institution received extensions to complete the project. If you are interested in seeing the final results, please refer to their Outcomes report in the near future.
