Football Field-Size 'Fin' Rises in Volcano

May 4, 2006 — -- Something amazing is happening on Mount St. Helens.

As the winter clouds that hide the volcano from view for much of the year clear away, scientists have caught their first glimpse of a huge new mountain growing inside the crater.

Scientists call it the "fin," because the 300-foot tall slab of magma and rock stands straight up, and looks remarkably like a fin from one angle. The huge structure is growing inside the volcano's crater, which was formed when Mount St. Helens blew its top during the infamous 1980 eruption.

The volcano shook back to life in September 2004, but in very different fashion from the 1980 blast. The 2004 event marked the beginning of a slow but steady eruption that continues to this day.

Inside the mile-wide crater, the giant new fin is growing at a rate of four to five feet each day. Right now it's about the size of a football field standing on end.

"Of course, the fin gets to about the height of a football field or so and then it starts to get unstable and we get rock falls off the top of it," geologist Dan Dzurisin of the U.S. Geological Survey said.

Those rock falls help build out the bottom of the new formation. Eventually, if the eruption continues, geologists believe the new magma will completely fill in the crater -- a space the size of an entire town.

The fin, which is growing now, is just the latest in a series of at least seven distinct structures that have grown, then disintegrated inside the crater over the last year and a half.

Despite the fact that Mount St. Helens is one of the most closely monitored volcanoes in the world, there is much geologists still don't know. Instruments planted around the crater tell scientists the size and frequency of the daily eruptions, which have become akin to the mountain's heartbeat in recent years. But even so, those who monitor Mount St. Helens every day cannot say when the current eruption will stop -- or whether it will grow more dangerous or if it will erupt in violent, explosive fashion.

There is some concern that while the fin is growing straight up, fueled by rocky magma from within the mountain, more energy is also pushing outward.

The crater's dome is pushing outward at a rate of about one meter a day. A quarter century ago it was just such an outward bulge that eventually blew -- not up but outward, killing 57 people.

Comparisons to the last great eruption are difficult because at least for now, the mountain erupts in a relatively slow, steady pattern.

Now that clearer weather has returned to the mountain, scientists are eager for better real-time photography of what is happening.

"It's been hard to get views," Dzurisin said. "It's been hard to keep track of what's going on. Now the weather's improving, and we have an opportunity to go up there and study in a lot more detail what's going on."

Visitors will have a better chance too. The nearest overview, the Johnston Ridge Observatory, reopens to the general public on Friday.