In-situ study of volcanic terrain and processes during the past 20 years revolutionized the study of submarine volcanism. Technology and methods evolved rapidly with the advent of GPS navigated surface ships with higher-resolution multibeam mapping systems, improved ultra-short baseline bottom-tracking of vehicles, widespread adoption of remotely operated vehicles (ROVs), and autonomous underwater vehicles (AUVs) equipped with high-precision navigation, CTDs, multibeam and sidescan sonars, and sub-bottom seismic reflection profilers. These technologies now allow scientists to map and sample select areas of the seafloor at scales comparable to working on land.

ROV-push coring led to recognition of widespread pyroclastic debris on the MOR, and now offers a means to obtain minimum eruption ages through radiocarbon dating of foraminifera. The cores provide chemostratigraphy of glass shards of eruptions near the sampling site. Historical lava flows mapped at 1-m resolution include the 1993 and 1982-1991 CoAxial; 1986 North Cleft; 1996 North Gorda; 1998, 2011, and 2015 Axial Seamount; and 2008 NE Lau Spreading Center. Regional-scale mapping has been done at Endeavour Segment, Gorda Ridge, and Alarcon Rise. The maps define individual flows and their morphologies, and are combined with minimum flow ages and compositions to reconstruct volcanic and tectonic history.

Three examples of application of these new methods and technologies are presented. The axial graben at Endeavour formed ~4.3 kyr ago as the ridge shifted from a magmatic to an extensional tectonic phase. Magmatic activity slowly resumed ~2 kyrs ago and continues. The tectonized crust supports intense hydrothermal circulation. At Axial, caldera formation ~1.1-1.3 kyr ago coincided with eruption of voluminous flows on the deep S rift, and likely with summit phreatomagmatic eruptions as seawater penetrated into the volcano along caldera-bounding faults. Crystal-rich, primitive, depleted lava erupted when effusive activity resumed a few hundred years later. Lava compositions shifted to less-depleted, less-primitive, aphyric compositions ~400 yrs ago. The 2015 lava is the most primitive to erupt in 400 yrs. At Alarcon Rise, the northern third of the ridge erupted evolved andesite, dacite, and rhyolite from 19-2 kyr, but then returned to more typical mid-ocean ridge basalt eruptions.

Dave Clague’s research interests are nearly all related to the formation and degradation of oceanic volcanoes, particularly Hawaiian volcanoes, mid-ocean ridges, and isolated seamounts. Topics of interest include: compositions of mantle sources for basaltic magmas and conditions of melting; development and evolution of magma chambers beneath spreading centers and Hawaiian volcanoes; volatile and rare-gas components in basaltic magmas and their degassing history; chronostratigraphic studies of eruption sequence and evolution of lava chemistry during volcano growth; subsidence of ocean volcanoes and its related crustal flexure, plate deformation, and magmatic activity; formation of cumulate xenoliths during different stages of Hawaiian volcanism; transport of volcaniclastic sediment on submarine slopes of volcanoes; geologic setting of hydrothermal activity; origin of isolated seamounts; monitoring of magmatic, tectonic, and hydrothermal activity at submarine and subaerial volcanoes; emplacement dynamics of subaerial and submarine lava flows; slope instability on volcanoes. (http://www.mbari.org/clague-david/)