The First Few Days

The schedule for the first few days of the cruise was thrown off a little because the AUV was delayed coming into Apia. We got on board on Tues 11/8, but instead of sitting around in port and waiting for it to arrive, we decided to go out to sea for two days and get some mapping, so we didn’t waste any precious and expensive ship time. We transited about 12 hours to the south to a site that was not extremely critical to the Nautilus people, and did 2 camera tows and 3 dredges. We were looking at 2 volcanoes that are very near the volcanic arc, where the lavas have a high water content. This has a number of effects, but two primary ones are that it causes many explosive eruptions and the lavas have a higher silica (SiO₂, aka glass) content than those erupted along spreading centers. The explosive eruptions are caused by water and other gases in the magma expanding extremely quickly once they reach the seafloor. Within the magma chamber and the conduit through the crust, the pressure is very high, keeping the bubbles very small, but once that pressure is released, the bubbles expand extremely quickly, which causes the magma to fragment and explode instead of calmly flowing out of the vent. The high silica content also causes the magma to be more viscous, so that it more effectively traps the gases and they can’t just escape as a plume of volcanic gas. Silica content is the primary factor that determines whether an eruption will be explosive (like Mt. St. Helens, Krakatau, or Pinatubo), or effusive (like Kilauea on the Big Island in Hawaii, where the lava flows out relatively calmly over the surface and rarely explodes). That’s why it’s relatively safe to walk around near the lava flows on the Big Island, but you would not want to be anywhere nearby when a volcano with high-silica magma erupts. So, because the volcanoes we dredged and did the camera tows on in the first few days had high-water content and high-silica lavas, we recovered samples of rhyolite (a broad term for high-silica lava, the same chemical composition as granite) pumice (very light weight rock with tons of gas bubbles). We got a few samples of another type of lava called boninite, but ~95% of the first three dredges was pumice. After the first few days, we transited back to Apia to pick up the AUV and run some tests in the harbor to make sure it was working fine, so we got to go out and have a few beers on Thurs 11/10 and Fri 11/11, which was nice. On Sat 11/12, we headed back out to the study area to start the main part of the research program.