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Volcano Watch — How big is an eruption?

March 28, 2002

How big is an eruption? This is a short question with a long answer. Volcanologists, like other people, judge the size of something by comparing it to something else. Volcanic eruptions span such a large range in size, style, and duration that comparisons can be hard, especially between volcanoes with different eruptive styles.

Various schemes have been proposed to categorize eruptions, but all schemes involve some measure of eruption magnitude and some measure of eruption intensity. Magnitude measures the amount of lava produced, usually as the volume or mass of lava, while intensity measures how fast the lava is produced.

Here's an example that shows why both magnitude and intensity are necessary for comparisons. Say we have an eruption that has been going for years, producing small quantities of lava more or less continuously. Now consider another eruption that produced the same amount of lava, but in only a few hours. If we considered only the amount of lava produced (or magnitude), these two eruptions would end up in the same pigeonhole. But if we consider the lava production rate (or intensity), the eruption that got the job done in a few hours is far more energetic than the one that took years. These two cases certainly deserve to be in different categories.

The most widely used measure of eruption size is called the Volcanic Explosivity Index, or VEI. VEI incorporates eruption magnitude and intensity into a single number by assuming that they increase and decrease in unison. It measures magnitude as the amount of fragmental lava produced in an eruption. This works pretty well for most explosive eruptions, such as the May 18, 1980, eruption of Mount St. Helens, which produced about one-half billion metric tons of fragmental lava in less than a day. This eruption is assigned a VEI of 5. The VEI scale doesn't work very well for non-explosive Hawaiian eruptions such as Pu`u `O`o. Since the Pu`u `O`o eruption began in early 1983, it has produced, non-explosively, about 6 billion metric tons of lava, or more than 10 times as much lava as did Mount St. Helens. Yet the Pu`u `O`o eruption is assigned a VEI of 1 because it hasn't produced much fragmental lava.

For comparisons that include non-explosive eruptions, it's better to compare magnitude and intensity values separately, rather than combining them into a single VEI value. The Pu`u `O`o eruption has produced 6 billion metric tons over the past 19 years, at an average rate of 10 metric tons per second. Let's turn this into volume to get a better feel for how much this is. Imagine a cube 91 meters (100 yards) long on each side-the length of a football field. We'll call the cubes CFFs, for "cubic football fields." Over the past 19 years, Pu`u `O`o has produced 2800 CFFs. Not impressed? Then consider the June 1950 eruption of Mauna Loa. This eruption produced 1 billion metric tons of lava-480 CFFs-- in 23 days, at an average rate of 500 metric tons per second. Still not impressed? The June 15, 1991 Pinatubo eruption produced 10 billion metric tons of ash-5,000 CFFs--in a few hours, at a rate of about 900,000 metric tons a second. This is almost two Pu`u `O`o eruptions in a few hours! Remarkably, eruptions of much greater magnitude and intensity are known to occur. Fortunately they are infrequent.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week with lava being occasionally seen in the Pu`u `O`o crater. Bright glow persists over the "rootless" shield area, where short flows emanate from overflows of the perched ponds and from leaks at the base of the shields. A surface flow was observed on Pulama pali for the first time since February 23. Surface flows are also on the fan at the base of the pali and extend onto the coastal flats. There are no ocean entries.

Two earthquakes were reported felt during the week ending on March 28. Residents from Volcano to Hilo were awakened by an earthquake at 4:09 a.m. on the morning of March 22. The magnitude-3.4 earthquake was located 18 km (10.8 mi) north of Pahala at a depth of 11.6 km (7 mi.). A magnitude-2.9 earthquake shook residents of Leilani Estates subdivision at 2:56 p.m. on Thursday, March 28. The temblor was located 3 km (1.8 mi) east of Pu`ulena Crater at a depth of 3.7 km (2.2 mi).