4: Hawaiian Hotspots...a guest post from Gary Griggs

Early morning moon setting over Moana Kea (with the UCSC Keck Observatory visible at the right) in Hilo, Hawaii

Seeing the full moon set over Moana Kea as we sailed into Hilo, Hawaii, was not only a welcome sight after 7 days at sea, but was also a reminder that these islands are all volcanic, created over five or six million years as the massive Pacific Plate has moved northwest at a few inches a year over a hot spot or thermal plume rising from deep within the Earth. Six million years ago Kauai sat over the hot spot and was actively erupting, building that island up nearly 30,000 feet from the seafloor. Each island gradually formed and then died in a progression to the southeast as it moved away from the hot spot or plume. Kauai, Oahu, Molokai, and Maui each had their day, and now Hawaii sits over the plume, with Kilauea the most active volcano on Earth directly connected to a source of magma deep in the mantle.

While many of the people on the ship expected a classic steep-sided cone like Fujiyama, Mt. Saint Helens, or Shasta, the Hawaiian volcanoes are all much more subdued, being shield volcanoes, or shaped like a Roman shield resting on the ground. The difference in their shapes lies in the nature of their eruptions, which results from different lava compositions. The Hawaiian Islands consist entirely of basalt, lava low in silica. As a result the eruptions produce very fluid lavas, which flow considerable distances at only moderate speeds and are generally not a hazard to people- unless the people are moving very slowly or in the wrong place at the wrong time

The Cascade volcanoes, on the other hand, as well as Mt. Fuji, which loomed on the horizon on Yokohama as we sailed in early on the 29^th of January, have a classic steep-sided, symmetrical shape. The Chain of Fire, or the volcanoes that surround the entire Pacific Ocean, contain a higher silica content, and produce sticky, explosive eruptions, which build up steep, cone shaped peaks. Our best view of Mt. Fuji came on our entry to Japan and as the weather changed, we sadly never saw it again. But Deepika did get some nice pictures (stay tuned...)

Kilauea steaming (photo by G. Griggs)

Hiking across Kilauea Iki crater (photo by G. Griggs)

I did lead a class field trip to Kilauea on a hike down into and across Kilauea Iki, which was a red-hot, molten lava lake in the not too distant past, and which is still releasing sulfur oxide fumes and hot steam. As the lava lake gradually cooled and hardened the surface subsided and left a surface looking an asphalt highway broken and cracked by an earthquake.

Site of 1959 Kilauea Iki 2000ft high eruption (photo by G. Griggs)

Kilauea Iki following the lava lake cooling and collapse (photo by G. Griggs)

Broken and collapsed lava from former molten lava lake (photo by G. Griggs)

Walking across the now hardened lava brought thoughts of walking over hot coals and led us to wonder when the next eruption was likely to occur.  Kilauea is monitored virtually continuously by the U.S. Geological Survey’s Hawaiian Volcano Observatory, through a combination of the earthquake detection beneath the volcano, GPS measurements of the slopes around the volcano to see if there is any swelling, as well as daily sampling of the gases being discharged for any changes. Fortunately, the eruptions of Kilauea are somewhat more predictable and less devastating than the volcanoes surrounding the Pacific Basin.

Lava Tree - molten lava rushed around a living tree trunk, and immediately cooled, creating a mold of the tree within
(photo by G. Griggs)