Saturday, April 30, 2005
Super volcano eruptions
The scale of the Toba eruption is difficult to comprehend. Pyroclastic flows (hot flows of ash and pumice) covered an area of at least 20,000 square kilometers (7,700 sq mi), with deposits as thick as 600 m (2,000 ft) near the vents. Ash fall was widespread over much of southeast Asia. An ash layer approximately 15 cm (6 in) thick was deposited over the entire Indian subcontinent. Our appreciation of the magnitude of this eruption continues to grow as Toba ash is recognized farther and farther from the source.
The volume of the Toba eruption is estimated at 2,800 cubic kilometers (670 cu mi). To give some comparison with more recent eruptions, the 1980 eruption of Mount St. Helens produced less than 1 cubic kilometer (0.25 cu mi). Vesuvius (A.D. 79) erupted about 5 cubic kilometers (1.2 cu mi), and Krakatoa in Indonesia (1883) about 12 cubic kilometers (3 cu mi). Closer to home, the volume of Kilauea's ongoing eruption is about 2.6 cubic kilometers (0.6 cu mi), erupted over the last 22 years. The most widespread hazard from such an eruption is its effect on global climate.
Large, explosive eruptions eject huge amounts of volcanic ash and gas that reach the stratosphere. Sulfur dioxide gas reacts with atmospheric moisture to form tiny droplets of sulfuric acid. The droplets and ash particles both absorb heat and reflect solar radiation, cooling the lower atmosphere. While ash tends to settle out of the stratosphere within months, the aerosol of sulfuric acid can remain in the stratosphere for 2-3 years before dissipating. We have no direct knowledge of the length or severity of global cooling caused by the Toba eruption.
Some scientists, however, have speculated that a severe "volcanic winter" triggered by the eruption, combined with the effects of ash fallout, may have brought about the near extinction of early humans in the path of the Toba fallout. You can still see the remains of this eruption on Sumatra, where beautiful Lake Toba fills the caldera formed when the magma chamber emptied.
The area has been rattled by several major earthquakes in the last century, but there have been no eruptions at Toba in historical time. In light of recent events, it may seem that Sumatra is particularly prone to natural disasters. In fact, so-called "super eruptions" have occurred in many parts of the earth, including several in the western United States. The largest known explosive eruption in the world originated from the La Garita caldera in the San Juan Mountains of Colorado approximately 28 million years ago.
The ash-flow deposit from this eruption, known as the Fish Canyon Tuff (tuff is consolidated ash), has an estimated volume of 5,000 cubic kilometers (1,200 cu mi). At Yellowstone (featured in "Volcano Watch" and a television special a few weeks ago), the largest eruption produced a volume of ash only slightly less than that of Toba about 2 million years ago. As noted in our earlier column, the odds of such an event occurring in our lifetime are vanishingly small, but that needn't lessen our desire to learn more about these amazing eruptions. Activity Update Eruptive activity at Pu`u `O`o continues. Glow is visible from several vents within the crater on clear nights.
The PKK lava tube continues to produce intermittent surface flows from above the top of Pulama pali to the ocean. Surface flows are active on the coastal plain inland of East Lae`apuki and Kamoamoa, within 30 m (100 ft) of the sea cliff. Lava has been entering the ocean at East Lae`apuki since April 25. This is the closest activity to the end of Chain of Craters Road, in Hawai`i Volcanoes National Park, and is located about 4.7 km (3 miles) from the ranger shed. Expect a 2-hour walk each way and bring lots of water.
Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs. During the week ending April 28, two offshore earthquakes were felt on Hawai`i Island. A magnitude-4.2 quake occurred 52 km (32 miles) southeast of Na`alehu (beyond Lo`ihi) at a depth of 9 km (6 miles) at 3:01 a.m. on Saturday, April 23; this earthquake was felt at various places on the island.
A magnitude-3.4 quake occurred 65 km (40 miles) northwest of Kailua at a depth of 13 km (8 miles) at 8:13 a.m. on Tuesday, April 26; it was felt at Kailua and Waimea. Mauna Loa is not erupting. During the week ending April 20, 6 earthquakes were recorded beneath the summit area. Four were deep and long-period in nature. Inflation has slowed beneath the summit over the last few weeks.