Volcano Watch — Cycles of eruption style change our long-term view of Kīlauea
Some of these new findings are summarized in a recent, illustrated USGS Fact Sheet "Kīlauea—an Explosive Volcano in Hawai‘i," available online at http://pubs.usgs.gov/fs/2011/3064/. But things happen quickly around here, and a new way to view Kīlauea, taking into account its explosive activity, dawned after the Fact Sheet was out. Here’s the story.
Over the past 40 years, more than 90 radiocarbon ages have been obtained on charcoalized vegetation burned by lava flows on Kīlauea. More than 80 of these ages are for flows erupted in the past 2,500 years; we can’t look back earlier than that very well, because younger flows cover and obscure older ones. Remarkably, the ages show that these lava flows were not erupted evenly with time, but, instead, are mostly clustered into three periods: 2,500 (or older) to 2,200 years ago; 1,000 to 500 years ago; and the past 200 years. What was happening during the gaps between these age clusters?
Well, the gaps are the very times when explosive eruptions were relatively frequent. Geologic research shows that episodic explosive activity took place throughout the 2,200–1,000-year period and the 500–200-year period. We used to think the periods of explosive activity were brief hiccups in the otherwise consistent production of lava flows. They're more than hiccups, though. Periods of explosive activity lasted fully 60 percent of the past 2,500 years, and periods of frequent lava flows only the remaining 40 percent.
To be clear, this doesn't mean that explosive activity was continuous during these periods. Far from it. The explosive eruptions themselves were probably brief, lasting hours to days. But research suggests that they require a deep caldera—with its floor at or below the water table, today about 615 m (2,000 ft) below the caldera rim—to enable water to mix with magma and generate steam-powered violent eruptions. So, an explosive period really means a time when the volcano has a deep caldera; explosive eruptions are relatively common, then, but lava flows are not.
Why are lava flows infrequent if Kīlauea has a deep caldera? When the summit of the volcano collapses, most of the magma reservoir, such as that which exists today, may be destroyed. Apparently then, it takes several hundred years for the reservoir to rebuild itself; until that happens, few lava flows erupt along the rift zones. Only when the reservoir is substantial and pressurized can magma inject into the rift zones and start a period of frequent surface flows.
Geochemical data support this idea. Most lava erupts at Kīlauea's summit after cooling slightly and undergoing chemical changes while stored in a reservoir 1–4 km deep. But, occasionally, lava arrives at the surface with characteristics indicating little cooling or chemical change and only brief, if any, underground storage. Such "primitive" lava appears only during explosive eruptions—just what one would expect if the reservoir were mostly destroyed during caldera collapse.
The periods of frequent lava flows build the volcano and, as such, are the most obvious to us. In contrast, not much change happens when there is a deep caldera, though hazards from explosive eruptions are high. Does the supply of magma to the volcano decrease when the caldera is deep? It probably does decrease, because, if it didn't, the caldera would be filled in a few decades, and lava flows would erupt at the summit. A big unanswered question is whether this decrease occurs when the magma supply from deep in the earth is cut off completely or when magma actually starts to rise toward Kīlauea but is diverted laterally into the crust before reaching the surface.
Robin Holcomb, HVO geologist in the 1970s, was the first to suggest a cyclic model for Kīlauea. Robin had only a few radiocarbon ages at his disposal and believed that the explosive periods were short-lived, but his thinking was spot on. To honor his pioneering effort, we will propose, at the December meeting of the American Geophysical Union in San Francisco, that the pattern of alternating behavior at Kīlauea be called the Holcomb Cycle.
Volcano Activity Update
On August 3, just after 2:00 p.m., HST, the floor of Pu‘u ‘Ō‘ō collapsed and lava began erupting from new vents on the west flank of Pu‘u ‘Ō‘ō cone, sending flows towards the north and south of the cone. Since that time, lava has continued to erupt from these west-flank vents, but the active lava has retreated closer to the vent area and is largely contained within a perched lava pond. A very small amount of lava has returned to the collapsed floor of Pu‘u ‘Ō‘ō over the past week.
Since the new Pu‘u ‘Ō‘ō vents opened on August 3, the lava lake within the Halema‘uma‘u Overlook vent has dropped substantially over the past week as the summit deflated. As of Thursday, August 11, the Halema‘uma‘u lava lake was small, sluggish, and deep within the vent cavity.
Three earthquakes beneath Hawai‘i Island were reported felt this past week. A magnitude-1.85 earthquake occurred at 8:59 a.m., HST, on Friday, August 5, 2011, and was located 1 km (1 mi) northeast of ‘Opihikao at a depth of 3 km (2 mi). A magnitude-2.1 earthquake occurred at 10:24 a.m. on the same day and was located 3 km (2 mi) southeast of Pāhoa at a depth of 1 km (1 mi). A magnitude-2.2 earthquake occurred at 1:58 p.m. on Monday, August 8, and was located 5 km (3 mi) southeast of Kapoho at a depth of 3 km (2 mi).
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