Geology And Geophysics

Comparing Volcanoes Mt Lassen and Mt St Helens

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"Comparing Volcanoes Mt Lassen and Mt St Helens"
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Lassen Peak and Mount Saint Helens are the only two volcanoes in the Cascade Mountains to have erupted in the twentieth century. A range that runs from Canada to northern California, it's part of the Ring of Fire. Both volcanoes arose from magma, molten rock, created as the Juan de Fuca plate sank under the North American plate in the Cascadia subduction zone. Both volcanoes are nationally protected areas, and each is a natural treasurehouse.

In 1979, Mount St. Helens was a graceful cone, a volcano about 50 miles northeast of the populous Portland area. Smoke and steam had occasionally risen from the mountain for centuries. In 1980 though, it suddenly lost its northern face in a massive avalanche, followed by blasts spewing ash that would fall as far away as Minnesota. The eruption was expected, but 57 people still died. It was the worst volcanic disaster in United States history. Mount Saint Helens National Volcanic Monument commemorates the dead and is dedicated to the study of the post-eruption terrain.

Mount Lassen, about 400 miles south, is a chunk of dacite rock, a rough lava plug always covered in snow. It stands 2,000 feet above the surrounding terrain and catches the weather, so it gets more snow than anywhere else in California. Its half cubic mile of mass probably makes it the largest lava dome on earth, but it is only a shadow of vanished Mt.Tehama, a stratovolcano that stood slightly south of where Lassen stands, and reached 1,000 feet higher. Lassen was a vent on Tehama's north side. The latest eruption of Lassen Peak began in 1914, after 27,000 years of dormancy, and lasted until 1917. The devastation is still visible in Lassen Volcanic National Park.

There are important differences between the two volcanoes. Mount St. Helens is farther north, so its plant community is different. It is located all too near a large population center, while Lassen is relatively isolated. Lassen is a National Park, where the emphasis is on recreation and exploration. St. Helens is a National Monument, open for exploration, but with an emphasis on study and science.


Lassen is at a fairly high elevation, 5,200 feet at the lowest, and 10,457 at the top of Lassen Peak, so it gets plenty of snow. Its roads open late in June, usually, and close in October. Therefore, Lassen doesn't get many visitors, for a national park. Many come to climb Lassen Peak. It's not a technical climb, though it's not a stroll either.

Mt. St. Helens is a valuable teaching resource for the Portland/Vancouver area. Parts of the mountain are still closed, or are open by permit only, so that the regrowth of vegetation and the transformation of the terrain can be studied in an undisturbed state. Nevertheless, people flock to the monument to stroll the trails, explore the caves, or climb to the crater's rim.

Volcanic Activity

There are areas of the Monument that are still considered unsafe. Geologic activity was noticed again at Saint Helens in 2003, with a new dome bulging up in 2004-2008. The activity was quite strong at first, but then slowed, and the mountain now is quiet, for the moment. Lassen Peak, by comparison, seems deeper into dormancy, although of course both mountains are still carefully monitored.

Lassen Park's many geothermal features show that volcanic activity there is waning. Features like hot springs and fumaroles are an indicator of a magma reservoir deep underground. The fumes emitted by such features change the composition of the soil in ways that geologists can trace. Tracing these changes, scientists see that geothermic activity at the park was once more vigorous and widespread than it is now. Therefore, the magma beneath Lassen is probably cooling.

The Sulfur Works, at the throat of ancient Mount Tehama, has shattered fragment of its slopes all around it, giving a sense of how huge the mountain was. Terminal Geyser, in the southeast corner of Lassen, is not really a geyser, but does headline a geothermic area without so many boardwalks or rails to distract from the natural setting.

At Mt. St. Helens, visitors can walk through a lava tube, Ape Cave, created when a tongue of lava crusted over a core of still liquid rock that kept flowing. It was not created by the modern eruption, but by a prehistoric one. Visitors can choose to walk the easy lower cave, or to scramble through the tougher upper one. Another feature is the tree cast trail, the Trail of Two Forests where prehistoric trees left their forms in stone, for visitors to crawl through. There is a walkable lava tube near Lassen too, at Subway Cave.

Each park offers views of huge devastated areas, swept clean by lava flows or covered by rains of pumice, lava blocks, and volcanic ash. Ash emitted by volcanoes is not anything that's been burned, but a finely pulverized mixture of materials like pumice, scoria, and volcanic glass.

Plants and Animals

Both parks have plants and animals typical of the Cascade Range. Lassen contains over 700 species of plants and 250 species of animals with backbones. At the southern end of the range, it touches three life zones, displaying plants found in the Cascades, the Great Basin, and the Sierra Nevada, and animal life is commensurately varied.

Eight species of conifers grow at Lassen now. Snow plants, brilliant red saprophytes, sprout at the feet of the pines between patches of snow. That is at the lowest elevation of the park, next to the main road. Higher, dense Red Fir forest stays cool in summer shade. Higher still, trees must survive 40 foot snows, or slopes scrubbed clean by winter winds. Gnarled trees grow here, Whitebark Pine where it is dry, and Hemlock where it is relatively moist. These trees may grow flat to the slope, in the community called Krummholtz, crooked trees. Above the treeline would seem to be a desert, but in season it is full of belly-flowers, a community of plants that bloom in high summer. Pika, marmots, and other small rodents rush to harvest their seeds. Tortoiseshell Butterflies flock to alpine meadows, and bald eagles, peregrine falcons, and goshawks fly above.

There are meadows at Lassen too, full of lilies, gentians, blue eyed grass, alpine shooting stars, and at the shady borders, violets. On eastern slopes at Lassen are areas of chaparral, manzanita, huckleberry oak, and ceanothus. Dogbane blooms, and there are quail and cottontail hares.

Mount St. Helens has plant communities typical of the Cascades, but utterly disrupted by the eruption. Scientists are following the progress of the plants as they re-colonize devastated areas, and visitors can also watch as a procession of species begin regrowth in lands now apparently barren. Interestingly, plants shielded by a thin coating of mud or tephra sometimes survived the worst effects of the blast.

Burrowing animals on Mt. St. Helens survived in far greater numbers than might have been expected too. After all, trees up to seven miles from the crater were leveled by the blast. In some of the worst areas, pocket gophers survived beneath the ash. When they came up again, they mixed ash, soil, beneficial fungi and seeds, in a beneficial way.

Logs were left to lie in Spirit Lake, and they are mostly still there. It's a strange sight, but even in the jumbled log piles, microscopic life is plentiful.


There is no doubt that Mt. St. Helens is easier to access for most people. It is quite near a major airport and city services. Visiting Lassen is more of an expedition, meant for tourists willing to spend time traveling. Both mountains reward visitors with grandeur and beauty, and with proof of nature's power to destroy and to renew.

More about this author: Janet Grischy

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