New observations from Surtsey, the definitive surtseyan volcano

The eruption that formed Surtsey lasted from 1963 into 1967, and provides the name used for emergent eruptions from the seafloor, and sometimes even more generally for any eruption involving explosive interaction of magma with water. New work on Surtsey's eruption is allowing us to extend our understanding of many aspects of its evolution and the processes that took place both prior to emergence and after, when it was so well observed by Sigurdur Thorarinsson and others. In 1979, drilling through Surtsey was accomplished, and a core acquired that extends almost to the pre-eruption seafloor. Near the base of the hole, unlithified pyroclastic deposits were encountered, and sampled as drill cuttings. These are highly vesicular, and many show large populations of small, spherical to sub-spherical vesicles. Examination of the core and dozens of thin sections reveals strong palagonite rims on pyroclasts at many intervals in the core, developed particularly well on highly vesicular and originally glassy pyroclasts. In the uppermost several meters armoured lapilli are present, along with "vesiculated tuff". We see no evidence for deep subsidence of surficial deposits at the site cored, and our working hypothesis is that an eruption stratigraphy can be established from the drillsite. On the emergent cone, a notable feature not previously emphasized is an abundance of blocks from the pre-eruption seafloor. These blocks have been identified as lithified volcaniclastic material deposited as turbidites largely from the Vestmann Islands. It will be important to quantify the abundance of this seafloor sedimentary rock as clasts in Surtsey's deposits, because these lithic clasts imply excavation, perhaps substantial, of the pre-eruption seafloor. No fragments of pillow lava have been identified in Surtsey's ejecta, but there are abundant fragments of dikes characterized by parallel bands of vesicles and, on some fragments, paired chilled margins. Many of these exhibit strong cracking and a cauliflower-like appearance on one side, but they are not true cauliflower bombs. Juvenile bombs are also abundant, and display ubiquitous composite textures. Typical juvenile bombs have glassy weakly fractured surfaces and a contorted internal structure in which pyroclasts are entwined with stretched and bubbled coherent basalt. These textures are interpreted to have developed through strong 'recycling' processes that allowed capture of older pyroclasts within new ones through in-vent welding and agglutination, or in some cases by capture of particles within magma that was subsequently disrupted. Taken as a whole, these new observations challenge existing models for Surtsey's eruption. A new assessment of eruptive processes will take into account evidence for both ubiquitous hot-state particle recycling, and excavation and ejection of subvolcanic sedimentary strata at times in the eruption, including during the last explosive phase.