Facts of the Matter
Facts of the Matter
A volcano might be down
but never count it outMauna Loa is acting up again! Volcanologists have measured the minuscule swelling of the summit that typically precedes an eruption, indicating that magma is accumulating inside it. A few miles to the northeast, Kilauea continues to pump lava out of the Pu'u O'o vent, providing drive-up entertainment for curious volcanophiles.
These are among the world's most active volcanoes. Even though Mauna Loa hasn't erupted in nearly 20 years, it's not even a millisecond by comparison in the life of this volcanic giant. Mauna Loa and Kilauea are Hawaii's only active volcanoes. Mauna Kea, just across the saddle to the north, has no written account of an eruption. Geologic evidence suggests that the volcano last erupted around 1650, and there are accounts of eruptions in traditional Hawaiian folklore. Mauna Kea is said to be dormant.
Volcanologists use the terms "extinct," "dormant" or "active" to classify volcanoes. An active volcano is one that has been active recently. A dormant volcano has erupted in historical times but not recently. An extinct volcano has not erupted historically. These are good working definitions, but the words "recent" and "historic" are relative terms. The classifications are not, and are not intended to be certain predictors of future volcanic activity.
Elsewhere on the Big Island, Mount Hualalai coughed up two lava flows in 1803 but has not been heard from since. An eruption on Maui that formed La Perouse Bay, near Makena, is thought to have occurred around 1790. The date is not certain, but there are historical accounts and other evidence that place it around the end of the 18th century. These two volcanoes are considered to be dormant like Mauna Kea. Across the isthmus from Haleakala, the West Maui volcano is deemed extinct.
Oahu's most recent eruption was around 10,000 years ago, a small lava flow that formed the peninsula at Makapuu. This and the other most recent volcanic features of Oahu occurred long before there were people living in the islands. Overall they span nearly a million years, but they are all extinct. Diamond Head, for example, erupted through a coral reef offshore close to half a million years ago. The Koolau and Waianae volcanoes, the two primary volcanoes that built the island, erupted from 2 to 5 million years ago. All of Oahu's volcanoes are considered to be extinct. The same goes for Kauai, being older yet.
It is important to remember that these classifications are defined in human terms, whereas nature operates on its own terms. Volcanoes erupt when they are ready, regardless of what names we use to classify their behavior.
Mount Tarawera, on the east coast of New Zealand's North Island, is a good anecdotal example. Until their destruction in 1886, there were hot springs and a tourist attraction called the Pink and White Terraces. There was little evidence that the mountain was volcanic, and there had been no volcanic activity even in Maori legend. It may seem strange to think that no one knew it was a volcano, but the geysers of Yellowstone illustrate that there are not necessarily volcanoes in the vicinity of hot springs.
On a quiet morning in June 1886, Tarawera began an eruption that is still regarded by many as the greatest disaster in New Zealand history. With barely a few rumblings the night before, the eruption began suddenly and without warning. It continued to erupt for four hours as it ejected a huge volume of lava, quite a bit of ash, lots of smoke and made quite a bit of noise. The eruption stopped as abruptly as it began, and Tarawera hasn't been heard from since.
Tarawera's behavior is not out of the ordinary. No volcano erupts at regular intervals like Old Faithful, and there is no way to predict when any particular volcano will erupt. Geologists can detect signs that an eruption might happen, such as small earthquakes and the swelling of the volcano, but not much more.
So when we label a volcano as active, dormant or extinct, it is just that -- a label! It is a convenient way of classifying or organizing similarities among volcanoes, and nothing more. For example, there is nothing that prevents Diamond Head from erupting again merely because we label it extinct. Likewise we can not say for certain that Kilauea will not stop erupting for good tomorrow.
Hawaiian Volcano Observatory
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu