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First it expanded, now the volcano is shrinking

First it expanded, now the volcano is shrinking


Shrinking from old age?

Since its 1980 eruption, the summit elevation of Mount St. Helens has decreased. A survey in 1982 gave a measurement of 8,365 feet. However, a lidar survey done in 2009 found the maximum elevation to be 8,330 feet, a full 35 feet lower. The difference in elevation is likely due to erosion and loss of rimrock by collapes of the steep crater-walls, according to the U.S. Geological Survey.

Could the volcano have had a Russian name?It’s common knowledge that Capt. George Vancouver named Mount St. Helens in 1792 after The 1st Baron of St. Helens, Alleyne FitzHerbert. But who was FitzHerbert, besides being a friend of Vancouver’s? Well, for one thing, he never saw the mountain named after him. But he was well traveled. He was England’s foreign minister to Russia from 1783 to 1788, during the reign of Russian Empress Catherine the Great. He also was foreign minister to Spain from 1790 to 1794. Notably, a lot of the Alaska’s volcanos have Russian names because Russia-sponsored explorers discovered that area startingin the mid 18th century: Davidof, Vsevidof and Veniaminof are among them.

What about the lava dome, or should it be lava domes?What is known as the lava dome is actually two separate domes: The first was created in successive eruptions starting in October 1980 and continuing periodically through 1986. A second dome, which formed during a continuous eruption from 2004-08, formed just to the south of the first dome and eventually merged with it. The summit of the combined lava dome is at 7,155 feet above sea level — nearly 1,200 feet below the summit — and nearly 900 feet above the 1980 crater floor. It is 3,500 feet in diameter at its base. Its estimated volume, 97 million cubic yards, is enough to cover three lanes of freeway 12 feet deep from Seattle to San Francisco. Two earlier lava domes that formed in June and August 1980 were blasted away by subsequent explosive eruptions.

The ashfall mythIt’s amusing when people say they remember the ashfall in the Kelso-Longview area from the eruption on May 18, 1980. But there was none. All of that ash blew eastward, away from Cowlitz County. It wasn’t until the following Sunday, May 25, 1980, that the county got its first — and only — significant ashfall from the volcano. That eruption occurred during an overnight rainstorm, and the coating of wet ash on power substations caused massive power outages throughout the county.

Was that another eruption?After the mountain blew so catastrophically, it was common for local people during the summers immediately following the eruption to ask whether giant smoke plumes caused by slash burns — giant fires set to clear logging debris and which are now rare — were eruptions on the east. Landscape-scale slash burns now are largely a thing of the past, but you can count on volcanic eruptions being part of Cowlitz County’s future for millennia.

So how many eruptions have there been since 1980?Too many to count, really. The mountain had erupted steam, ash and gas for two months before the big blow on May 18, 1980, a period in which its north flank swelled five feet a day. Following May 18, five explosive eruptions later that year sent clouds of ash and steam eight or nine miles into the sky on May 25, June 12, July 22, Aug. 7, and Oct. 16. Quiet “dome building” eruptions continued through October 1986. A period of continuous dome-building occurred from 2004-08. The last explosive eruption of any significance occurred on March 19, 1982. For the last 12 years, the mountain has been largely quiet except for periodic earthquake swarms, which geologists interpret as fresh molten rock moving into the volcano to refuel it for its next eruption. And no one knows when, or how big, it will be. But of all Cascade Range volcanoes, scientists consider Mount St. Helens by far the most likely to erupt next. It’s also rated the second most dangerous volcano in the nation by the USGS, but only because so many people live downvalley of Mount Rainier, which is ranked most dangerous.

Why is that big stack of dredge spoils near the mouth of the Toutle River nicknamed Harry’s Mountain?No, it is not named after Harry Truman, the 84-year-old lodge owner at Spirit Lake who perished along with his cats on the morning of May 18, 1980. It is named after Harry Claterbos, the Astoria-based contractor who dredged that section of the Cowlitz River under a U.S. Army Corps of Engineers contract following the eruption.

So, just how many volcanoes?Most people know that Mount St. Helens is a volcano on the “Ring of Fire,” a 25,000-mile long, horseshoe-shaped arc that borders the Pacific Ocean. But just how many volcanoes are located along the ring? Mount St. Helens is one of 452 volcanoes on the Ring of Fire, which contains 75% of the world’s active and dormant volcanoes.

Now that’s a lot of debrisThe debris avalanche that toppled off the top and side of Mount St. Helens on May 18, 1980 is considered the largest landslide in recorded history. It was about 16 miles long, contained more than 3 biilion cubic yard of debris, has an average depth of more than 100 feet and is about 600 feet deep at its thickest point. But another landslide off the volcano during its so-called Cougar eruptive stage (28,000 to 18,000 years ago) likely was bigger. That one came off the south side of the volcano. It originated near Butte Camp in the southwest part of the present-day mountain and left a deposit 600 to 900 feet thick and 17 miles long. It temporarily dammed the Lewis River, and when that dam broke it caused flooding downstream as far as the Columbia River and filled the lower Lewis River Valley with volcanic debris at least 200 feet thick, according to the USGS.

What you see todayMost of what you see of Mount St. Helens is younger than 3,000 years old, which makes it younger than the pyramids in Egypt. And although the volcano’s eruption on May 18, 1980 was dubbed “a worst case scenario,” an eruption that occurred about 3,600 years ago was four times larger, measured by how much ash got blown into the sky.

Contact City Editor Andre Stepankowsky at 360-577-2520.


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