The Hawaiian-Emperor Seamount Chain is arguably the world's best known example of hotspot magmatism, where volcanic activity and earthquakes occur far from plate boundaries. Nevertheless, questions remain about the fundamental processes that control such magmatism and seismicity along the 5800-km-long, 0-80 Ma, chain, in part because the volume and compositions of frozen magma that has been added to the surface and base of Pacific oceanic crust is too poorly known. The aim of this study is to use 'state of the art' marine seismic imaging techniques to constrain the thickness and composition of the magmatic material created by the Hawaiian hotspot, how it varies along the seamount chain, and how the Pacific oceanic plate has deformed in response to volcano loading. This study, which is a collaborative one with US scientists at Lamont-Doherty Earth Observatory, will utilize reprocessed seismic reflection and refraction data acquired on previous research cruises (e.g. R/V Robert D. Conrad C2308, R/V Thomas Washington Roundabout 2, and R/V Maurice Ewing EW9801), together with a new data set that will be acquired onboard R/V Marcus G. Langseth during late summer, 2018 and early summer, 2019. The Langseth cruises, which have been funded by the National Science Foundation (Marine Geology and Geophysics Division), will acquire deep penetration seismic reflection data using a 15 km long streamer and a large tuned airgun array and wide-angle reflection/refraction data using 70 Ocean Bottom Seismometers spaced at 15 km intervals along four 500-km-long transects of the chain. The transect locations have been carefully chosen to represent variations in the timing of magma emplacement and volume flux, the age of oceanic lithosphere at the time of loading and the presence/absence of a mid-plate topographic swell, and are sufficiently long to capture the response of the lithosphere to volcano loading out to the flexural bulge. The reprocessed and processed seismic reflection profiles and velocity models created from wide-angle seismic data will constrain the volume and distribution of magmatic addition to the surface and base of the crust, the nature of the stratigraphic fill in the flanking flexural moats and the relative role of faulting within the flexed volcanic edifice and underlying oceanic plate. The seismic constraints will be integrated with swath bathymetry and potential field data, compared to other marine geophysical studies of hotspot magmatism and used as the basis for thermal and mechanical modeling in order to gain fundamental insights into crust and lithosphere rheology and stress state and to inform potential geohazards along the chain such as large-scale slope failures, fault slip and tsunamigenic earthquakes. The study proposed here is central to NERC's strategy especially as it involves discovery science that impacts on how planet Earth works, how it deforms in response to surface and sub-surface loads and how it might deform in the future.