Mixing Magmas at Krakatau

Throughout the early morning of August 27th, 1883 and into the late afternoon, the inhabitants of Rodriguez Island — a small speck of land in the southwest Indian Ocean – heard the sound of heavy guns firing in the distance. Or so they thought. What they had actually heard was a series of volcanic explosions emanating from another small island in the Indian Ocean, Krakatau (also known as Krakatoa), which lay nearly 5,000 km (3,000 miles) to the northeast. The noise from these explosions ranks as the loudest and farthest traveled sound in recorded history.

During its final cataclysmic eruption Krakatau, which had been an island of about 30 square kilometers (12 square miles), nearly vanished. A total of 25 cubic kilometers (6 cubic miles) of material, including both new magma and bits of the old island, were blown into the stratosphere, the eruption column reaching more than 30 kilometers (20 miles) high. A huge tsunami rushed from the exploding island and crashed against the nearby shorelines killing tens of thousands. At no time in history had more people been killed by a volcano.

About 50 years after Krakatau destroyed itself, a new island appeared from the sea in about the same place. Called Anak Krakatau, meaning “Son of Krakatau,” this young volcano now stands about 500 meters (1,500 feet) above the waves. On the flanks of Anak Krakatau one can find easily find, cutting through the nascent jungle, dark lava flows that strongly resemble the young flows of the Big Island. Indeed, the resemblance is more than skin deep. Chemically, some of Anak Krakatau’s lava flows are closely akin to Hawai`i’s.

Frequent readers of this column have learned that the explosivity of a volcanic eruption depends primarily on the type of lava being erupted. Hot, fluid lava (basalt), like that typical of Hawai`i, allows the easy release of volcanic gases, and hence, is not usually explosive. Cooler, more viscous and sticky lava (dacite or rhyolite) is much more reluctant to let the gases go; the result being that the pressure builds until released violently, like champagne from a shaken bottle.

The gigantic eruption that destroyed Krakatau consisted of this latter type of lava, rhyolite. Why, then is, Krakatau’s son covered with basalt?

It turns out that both kinds of lava are present on Anak Krakatau. The hot, runny basalt creates the Hawaiian-looking lava flows, while the sticky rhyolite explodes into piles of ash and pumice. This type of volcanism is referred to as bi-modal (i.e., two modes), and is now known to occur in various places throughout the world. Bi-model volcanism is thought to occur when large volumes of basaltic magmas intrude into a region and melt the pre-existing “country rock.” The two different magmas then co-exist side by side and, in some cases, may actually mix.

Some volcanologists have suggested that truly enormous explosive eruptions may be touched off by just this sort of underground mixing. A body of rhyolitic magma that is left alone has the opportunity to release its gases slowly and avoid catastrophe. However, if the same body of magma is suddenly intruded by a very hot tongue of basalt, then the additional heat might just be enough to send the whole system over the brink.

Recent chemical analysis of the material erupted by Krakatau in 1883 suggests that magma mixing did indeed occur. There is also evidence for magma mixing at other centers of explosive volcanism such as Long Valley in California and Yellowstone in Wyoming. The question remains, however, whether magma mixing is the primary means of sparking giant eruptions or just one of many possible mechanisms.

Whatever the ultimate cause of Krakatau’s 1883 eruption, the effects were felt literally around the world. The volcanic dust ejected into the stratosphere lingered for years, lowering global temperatures and creating livid sunsets that delighted and, in some cases, terrified onlookers. So crimson and vivid were some of these sunsets that, on more than one occasion, fire crews dispatched themselves to extinguish blazes thought to be raging at the western edge of town. Little did they know that the fire they wanted to fight was the sun itself.

Activity Update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Streams of lava are still visible on Pulama pali and Paliuli. The lava breaks out of the main Mother’s Day tube above Pulama pali and wends it way to the coastal flat in a series of open channels and tubes. Surface flows extend from the base of Paliuli to the coast at Highcastle, where a small new ocean entry began on May 19. The formerly active West Highcastle lava delta stopped last week

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has places signs in critical places. Do not venture beyond these signs and onto the lava deltas and benches.

One magnitude-3.3 earthquake, at 04:58 a.m. on May 19, was reported felt at Ocean View, Honaunau, and Wood Valley. It was located 10 km (6 miles) west-northwest of Pahala at a depth of about 10 km (6 miles).

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with no earthquake located in the summit area during the last seven days.