A Tale of Two Volcanoes

America´s most prominent volcanoes-Mount St. Helens in Washington State and Kilauea in Hawaii-might, at first glance, seem similar: Both attract tourists, and both erupted in the past 25 years. But their destructive powers come from quite different volcanic origins.

The erupting vents beneath Kilauea´s surface carve an elaborate lava-tube system into the Hawaiian countryside. The thin basalt lava from the volcano´s vents flows long distances under the cooled surface crust to produce the gradual slopes characteristic of shield volcanoes. The steady lava flow from 22 consecutive years of activity creates an ever expanding network of pathways-and hazards for nearby workers-across the flattened dome of the 4,190-foot volcano. Luckily, the lava from Kilauea´s constant bursts falls almost entirely on its eastern side, descending into the Pacific Ocean rather than on Hawaiian towns.

Set off by a 5.1-magnitude earthquake, Mount St. Helens´s violent eruption on May 18, 1980, launched landslides, lava and smoke. The landslide left an opening through which 540 million tons of bottled-up ash exploded across 22,000 square miles east of the volcano. Flowing lava quickly followed, destroying almost all the wildlife in the 230-square-mile forest surrounding the 8,300-plus-foot summit. Typical of a stratovolcano, Mount St. Helens´s thick magma walls trap expanding volcanic gases within the central vent, creating enormous internal pressure. When the stiff magma is disrupted by seismic activity or can no longer contain the gases, the volcano can suddenly explode. Despite recent rumblings, Mount St. Helens hasn´t celebrated the 25th anniversary of its famed eruption with a reprise of the powerful fireworks-yet.

Check out pictures of these two Earth-made nature destroyers in action.

MT. ST. HELENS

The eruption melted the mountain´s snowcap; when this meltwater mixed with ash and dirt, it formed destructively voluminous mudflows. One such â€lahar†reached the Columbia River, 70 miles away, and blocked its shipping channel with millions of cubic yards of sediment.Photo courtesy of USGS

KILAUEA

Lava breaks through the cooling crust to continue the formation of a new tributary. Most shield volcanoes, including Kilauea, erupt low-silicon basalt, which results in a more fluid lava.Photo courtesy of USGS

MT. ST. HELENS

AfterPhoto courtesy of USGS

KILAUEA

Lava tubes, created by recurring 10 -to 100-yard lava flows that build up walls and eventually form a roof over the flow, act much like a river delta, allowing the molten rock to quickly reach the ocean.Photo courtesy of USGS

MT. ST. HELENS

Before Fir and mountain hemlock trees covered the base of the volcano. The blast flattened 230 square miles of forest. Shown here, before and after images seen from Johnston Ridge.Photo courtesy of USGS

MT. ST. HELENS

Photo courtesy of USGS

MT. ST. HELENS

The ash from the May 18, 1980, eruption of Mount St. Helens traveled at least 12 miles high and eventually moved eastward at 60 mph, layering eastern Washington and Idaho.Photo courtesy of USGS

KILAUEA

Lava plunging into the Pacific at dusk.Photo courtesy of USGS

MT. ST. HELENS

Photo courtesy of USGS

KILAUEA

Lava enters the ocean surface in a whirl of steam from evaporating water. Only cooled lava crust remains behind on the ridge to support later eruptions.Photo courtesy of USGS

MT. ST. HELENS

On March 8 of this year, a small explosion rocked Mount St. Helens. Airplane pilots verified that the resultant smoke and ash reached an altitude of 36,000 feet.Photo courtesy of USGS

KILAUEA

Nothing holds back the molten rock´s fiery trip to the sea on Kilauea´s active eastern rift-not fences, forests or highways.Photo courtesty of USGS