December 14, 2000
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
The origin of rejuvenation-stage volcanism still poorly understood
It's been 60 years since Harold Stearns first recognized that Hawaiian island volcanoes evolve through stages we call shield, postshield, and rejuvenated volcanism. A fourth stage, the preshield stage, has been added in the last 20 years. Yet his interpretation has remained nearly intact even as plate tectonics revolutionized our explanations of the Earth's dynamic lithosphere. Of the four stages, the hardest to explain is rejuvenation, in which volcanoes resume their activity 1-2 million years after the postshield stage ceases. Ko`olau volcano on O`ahu lay quiet about 1 million years before beginning its rejuvenated stage, a stage that during the past 500,000 years produced the famous features of Diamond Head, Salt Lake Crater, and Koko Head. Why should an oceanic island volcano become active after a lengthy pause when it is far removed from the hot spot?
Three theories have been suggested to explain rejuvenated volcanism. The first recognizes that the largest Hawaiian island, currently the Big Island, is so heavy that it bends the oceanic crust downward. If the bending is symmetric, then the crust will be bowed upward at some distance away from the Big Island. Islands on the crust may then ride up and over the bend as the Pacific plate drifts northwest. Hot parts of the crust, solid at the pressures of great depth, may melt when the pressure decreases during the uplift, fueling new eruptions.
A second theory is based on extent of heating as the Pacific plate passes over the hot spot. Presumably, heating related to shield-building volcanism is sufficient to cause additional melting near the base of the plate. For reasons that are unclear, some of this magma doesn't rise until later in the volcano's history. Models based on this heating theory differ from the bending theory, because the magma forms early and is stored as long as 2 to 5 million years until a change in an island's structure allows it to percolate upward and erupt. The change may result from fractures induced as an island rides over the upfolded arch. Perhaps landslides that displace large segments of an island onto the adjacent sea floor could induce fracturing favorable to volcanic eruptions. The third theory is derived from mathematical modeling of the upper mantle and lower crust as the Pacific plate passes over the hot spot. Most volcanism will be centered directly over the hot spot, but the model predicts that melting will also occur downstream from the hot spot.
Here's why. Shearing by the overriding plate drags hot material along with it as it drifts northwest. But the model also predicts that the material is dragged downward, just as smoke blown from a chimney on a windy day may sometimes be drawn downward before continuing its rise. According to this theory, as the hot material continues its rise, it passes from the high pressures of great depth into lower pressures at shallow depths, where it ultimately melts. By this point, it has been transported for horizontal distances of 300-550 km (200-350 mi) from the hot spot. Material dragged beyond 550 km (350 mi) from the hot spot is too cool to melt upon ascent. This third theory explains why rejuvenated volcanism occurs long after an island's main volcanic stages have been completed. Other questions remain unanswered, however, and parts of each theory fail to fit all islands. But the theories force us to reexamine our assumptions about the stratigraphy, age, and geologic history on those islands that seemingly violate a particular model. This refining of theories is the tug of war known as the scientific method. The goal is to find a better explanation of Earth processes. Obviously we're still seeking an adequate explanation for rejuvenated volcanism.
A slow deflation of the summit region commencing at 10 p.m. on Wednesday, December 13 and lasting 12 hours marked the only notable event during the past week. The deflation, which totaled 4 microradians, was accompanied by an increase in tremor amplitude recorded by the summit caldera seismic station. In spite of the summit subsidence, eruptive activity of Kilauea Volcano remained unchanged. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Lava is entering the ocean at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. Minor surface flows were observed, and the public is reminded that the active lava flows are extremely hot and have places with very thin crust.
The ocean-entry areas are also very hazardous, with explosions accompanying sudden collapses of the new land. Small sections on the distal end of the bench were calving into the sea this week. The steam plumes are highly acidic and laced with glass particles.
Two earthquakes were reported felt during the week ending on December 14. A resident of Pahala felt an earthquake at 10:03 a.m. on Friday, December 8. The magnitude-3.4 earthquake was located 13 km (7.8 mi) south of the summit of Kilauea Volcano at a depth of 31 km (18.6 mi). A resident of Waimea felt an earthquake at 2:31 p.m. on Saturday, December 9. The magnitude-2.7 earthquake was located 13.3 km (8 mi) southeast of Waimea at a depth of 13.6 km (8.2 mi).
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Updated: December 15, 2000