Mt. St. Helens

Mt. St. Helens
Mt. St. Helens in 1982
Type	stratovolcano
Elevation	8,365 ft (2,550 m)
Volcanic Region	Cascades
Current Status	NORMAL
Last Eruption	2008

Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It is 96 miles (154 km) south of Seattle, Washington and 50 miles (80 km) northeast of Portland, Oregon. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey of the area in the late 18th century. The volcano is located in the Cascade Range and is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.

Mount St. Helens is most notorious for its catastrophic eruption on May 18, 1980, at 8:32 am PDT, the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24 km) of railways, and 185 miles (298 km) of highway were destroyed. A massive debris avalanche triggered by an earthquake measuring 5.1 on the Richter scale, caused an eruption, reducing the elevation of the mountain's summit from 9,677 ft (2,950 m) to 8,365 ft (2,550 m) and replacing it with a 1 mile (1.6 km) wide horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.9 km3) in volume. The Mount St. Helens National Volcanic Monument was created to preserve the volcano and allow for its aftermath to be scientifically studied.

As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit, and off its northern flank sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.

Geographic setting and description

Mount St. Helens is 45 miles (72 km) west of Mount Adams, in the western part of the Cascade Range. These "sister and brother" volcanic mountains are approximately 50 miles (80 km) from Mount Rainier, the highest of Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is 60 miles (100 km) southeast of Mount St. Helens.

Mount St. Helens is geologically young compared with the other major Cascade volcanoes. It formed only within the past 40,000 years, and the pre-1980 summit cone began rising about 2,200 years ago. The volcano is considered the most active in the Cascades within the Holocene epoch (the last 10,000 or so years). Prior to the 1980 eruption, Mount St. Helens was the fifth-highest peak in Washington. It stood out prominently from surrounding hills because of the symmetry and extensive snow and ice cover of the pre-1980 summit cone, earning it the nickname "Fuji-san of America." The peak rose more than 5,000 feet (1,500 m) above its base, where the lower flanks merge with adjacent ridges. The mountain is 6 miles (9.7 km) across at its base, which is at an altitude of 4,400 feet (1,300 m) on the northeastern side and 4,000 feet (1,200 m) elsewhere. At the pre-eruption tree line, the width of the cone was 4 miles (6.4 km).

Streams that originate on the volcano enter three main river systems: the Toutle River on the north and northwest, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow. The average annual rainfall is 140 inches (3,600 mm), and the snow pack on the mountain's upper slopes can reach 16 feet (4.9 m). The Lewis River is impounded by three dams for hydroelectric power generation. The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, which is directly south of the volcano's peak.

Although Mount St. Helens is in Skamania County, Washington, access routes to the mountain run through Cowlitz County to the west. State Route 504, locally known as the Spirit Lake Memorial Highway, connects with Interstate 5 at Exit 49, 34 miles (55 km) to the west of the mountain. That north–south highway skirts the low-lying cities of Castle Rock, Longview and Kelso along the Cowlitz River, and passes through the Vancouver, Washington–Portland, Oregon metropolitan area less than 50 miles (80 km) to the southwest. The community nearest the volcano is Cougar, Washington, in the Lewis River valley 11 miles (18 km) south-southwest of the peak. Gifford Pinchot National Forest surrounds Mount St. Helens.

Geologic history

Plate tectonics of the Cascade Range

The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage" (around 40–35,000 years ago), the "Cougar Stage" (ca. 20–18,000 years ago), and the "Swift Creek Stage" (roughly 13–8,000 years ago). The modern period, since about 2500 BCE, is called the "Spirit Lake Stage". Collectively, the pre–Spirit Lake stages are known as the "ancestral stages". The ancestral and modern stages differ primarily in the composition of the erupted lavas; ancestral lavas consisted of a characteristic mixture of dacite and andesite, while modern lava is very diverse (ranging from olivine basalt to andesite and dacite). St. Helens started its growth in the Pleistocene 37,600 years ago, during the Ape Canyon stage, with dacite and andesite eruptions of hot pumice and ash. 36,000 years ago a large mudflow cascaded down the volcano; mudflows were significant forces in all of St. Helens' eruptive cycles. The Ape Canyon eruptive period ended around 35,000 years ago and was followed by 17,000 years of relative quiet. Parts of this ancestral cone were fragmented and transported by glaciers 14,000 to 18,000 years ago during the last glacial period of the current ice age. The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years. Pyroclastic flows of hot pumice and ash along with dome growth occurred during this period. Another 5,000 years of dormancy followed, only to be upset by the beginning of the Swift Creek eruptive period, typified by pyroclastic flows, dome growth and blanketing of the countryside with tephra. Swift Creek ended 8,000 years ago.

Smith Creek and Pine Creek eruptive periods

A dormancy of about 4,000 years was broken around 2500 BCE with the start of the Smith Creek eruptive period, when eruptions of large amounts of ash and yellowish-brown pumice covered thousands of square miles. An eruption in 1900 BCE was the largest known eruption from St. Helens during the Holocene epoch, judged by the volume of one of the tephra layers from that period. This eruptive period lasted until about 1600 BCE and left 18 inches (46 cm) deep deposits of material 50 miles (80 km) distant in what is now Mt. Rainier National Park. Trace deposits have been found as far northeast as Banff National Park in Alberta, and as far southeast as eastern Oregon.[19] All told there may have been up to 2.5 cubic miles (10 km3) of material ejected in this cycle. Some 400 years of dormancy followed. St. Helens came alive again around 1200 BCE — the Pine Creek eruptive period.[19] This lasted until about 800 BCE and was characterized by smaller-volume eruptions. Numerous dense, nearly red hot pyroclastic flows sped down St. Helens' flanks and came to rest in nearby valleys. A large mudflow partly filled 40 miles (64 km) of the Lewis River valley sometime between 1000 BCE and 500 BCE.

Castle Creek and Sugar Bowl eruptive periods

The next eruptive period, the Castle Creek period, began about 400 BCE, and is characterized by a change in composition of St. Helens' lava, with the addition of olivine and basalt. The pre-1980 summit cone started to form during the Castle Creek period. Significant lava flows in addition to the previously much more common fragmented and pulverized lavas and rocks (tephra) distinguished this period. Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 BCE that traveled all the way into the Lewis and Kalama river valleys. Others, such as Cave Basalt (known for its system of lava tubes), flowed up to 9 miles (14 km) from their vents. During the first century, mudflows moved 30 miles (50 km) down the Toutle and Kalama river valleys and may have reached the Columbia River. Another 400 years of dormancy ensued.

The Sugar Bowl eruptive period was short and markedly different from other periods in Mount St. Helens history. It produced the only unequivocal laterally directed blast known from Mount St. Helens before the 1980 eruptions. During Sugar Bowl time, the volcano first erupted quietly to produce a dome, then erupted violently at least twice producing a small volume of tephra, directed-blast deposits, pyroclastic flows, and lahars.

Kalama and Goat Rocks eruptive periods

Roughly 700 years of dormancy were broken in about 1480, when large amounts of pale gray dacite pumice and ash started to erupt, beginning the Kalama period. The eruption in 1480 was several times larger than the May 18, 1980, eruption. In 1482, another large eruption rivaling the 1980 eruption in volume is known to have occurred. Ash and pumice piled 6 miles (9.7 km) northeast of the volcano to a thickness of 3 feet (0.9 m); 50 miles (80 km) away, the ash was 2 inches (5 cm) deep. Large pyroclastic flows and mudflows subsequently rushed down St. Helens' west flanks and into the Kalama River drainage system.

This 150-year period next saw the eruption of less silica-rich lava in the form of andesitic ash that formed at least eight alternating light- and dark-colored layers. Blocky andesite lava then flowed from St. Helens' summit crater down the volcano's southeast flank. Later, pyroclastic flows raced down over the andesite lava and into the Kalama River valley. It ended with the emplacement of a dacite dome several hundred feet (~200 m) high at the volcano's summit, which filled and overtopped an explosion crater already at the summit. Large parts of the dome's sides broke away and mantled parts of the volcano's cone with talus. Lateral explosions excavated a notch in the southeast crater wall. St. Helens reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended, about 1647. The volcano remained quiet for the next 150 years.

The 57-year eruptive period that started in 1800 was named after the Goat Rocks dome, and is the first time that both oral and written records exist. Like the Kalama period, the Goat Rocks period started with an explosion of dacite tephra, followed by an andesite lava flow, and culminated with the emplacement of a dacite dome. The 1800 eruption probably rivalled the 1980 eruption in size, although it did not result in massive destruction of the cone. The ash drifted northeast over central and eastern Washington, northern Idaho, and western Montana. There were at least a dozen reported small eruptions of ash from 1831 to 1857, including a fairly large one in 1842. The vent was apparently at or near Goat Rocks on the northeast flank. Goat Rocks dome was the site of the bulge in the 1980 eruption, and it was obliterated in the major eruption event on May 18, 1980 that destroyed the entire north face and top 1,300 feet (400 m) of the mountain.

Modern eruptive period

1980 to 2001 activity

The May 18 eruption. Composite photograph from 35 miles (60 km) west in Toledo, Washington. The ash-cloud stem is 10 miles (16 km) wide, and the mushroom top is 40 miles (64 km) wide and 15 miles (24 km) high. The footprint of the cloud stem is roughly the same as the devastated area north of the mountain where the forest was knocked down and which three decades later is still relatively barren.

On March 20, 1980, Mount St. Helens experienced a magnitude 4.2 earthquake; and, on March 27, steam venting started. By the end of April, the north side of the mountain had started to bulge. On May 18, with little warning, a second earthquake, of magnitude 5.1, triggered a massive collapse of the north face of the mountain. It was the largest known debris avalanche in recorded history. The magma inside of St. Helens burst forth into a large-scale pyroclastic flow that flattened vegetation and buildings over 230 square miles (600 km2). More than 1.5 million metric tons of sulfur dioxide were released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a five (a Plinian eruption).

The collapse of the northern flank of St. Helens mixed with ice, snow, and water to create lahars (volcanic mudflows). The lahars flowed many miles down the Toutle and Cowlitz Rivers, destroying bridges and lumber camps. A total of 3,900,000 cubic yards (3,000,000 m3) of material was transported 17 miles (27 km) south into the Columbia River by the mudflows.

For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 16 miles (20 to 27 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (100 km/h) with ash reaching Idaho by noon. Ashes from the eruption were found collecting on top of cars and roofs next morning, as far as the city of Edmonton in Alberta, Canada.

By about 5:30 p.m. on May 18, the vertical ash column declined in stature, and less severe outbursts continued through the night and for the next several days. The St. Helens May 18 eruption released 24 megatons of thermal energy; it ejected more than 0.67 cubic miles (2.79 km3) of material. The removal of the north side of the mountain reduced St. Helens' height by about 1,300 feet (400 m) and left a crater 1 mile (1.6 km) to 2 miles (3.2 km) wide and 0.5 miles (800 m) deep, with its north end open in a huge breach. The eruption killed 57 people, nearly 7,000 big game animals (deer, elk, and bear), and an estimated 12 million fish from a hatchery. It destroyed or extensively damaged over 200 homes, 185 miles (298 km) of highway and 15 miles (24 km) of railways.

Between 1980 and 1986, activity continued at Mount St. Helens, with a new lava dome forming in the crater. Numerous small explosions and dome-building eruptions occurred. From December 7, 1989, to January 6, 1990, and from November 5, 1990, to February 14, 1991, the mountain erupted with sometimes huge clouds of ash.

2004 to 2008 activity

Main article: 2004–2008 volcanic activity of Mount St. Helens Magma reached the surface of the volcano about October 11, 2004, resulting in the building of a new lava dome on the existing dome's south side. This new dome continued to grow throughout 2005 and into 2006. Several transient features were observed, such as the "whaleback," which comprised long shafts of solidified magma being extruded by the pressure of magma beneath. These features were fragile and broke down soon after they were formed. On July 2, 2005, the tip of the whaleback broke off, causing a rockfall that sent ash and dust several hundred meters into the air.

Mount St. Helens showed significant activity on March 8, 2005, when a 36,000-foot (11,000 m) plume of steam and ash emerged—visible from Seattle. This relatively minor eruption was a release of pressure consistent with ongoing dome building. The release was accompanied by a magnitude 2.5 earthquake. Another feature to emerge from the dome was called the "fin" or "slab." Approximately half the size of a football field, the large, cooled volcanic rock was being forced upward as quickly as 6 ft (2 m) per day. In mid-June 2006, the slab was crumbling in frequent rockfalls, although it was still being extruded. The height of the dome was 7,550 feet (2,300 m), still below the height reached in July 2005 when the whaleback collapsed.

On October 22, 2006, at 3:13 p.m. PST, a magnitude 3.5 earthquake broke loose Spine 7. The collapse and avalanche of the lava dome sent an ash plume 2,000 feet (600 m) over the western rim of the crater; the ash plume then rapidly dissipated. On December 19, 2006, a large white plume of condensing steam was observed, leading some media people to assume there had been a small eruption. However, the Cascades Volcano Observatory of the USGS did not mention any significant ash plume. The volcano was in continuous eruption from October 2004, but this eruption consisted in large part of a gradual extrusion of lava forming a dome in the crater. On January 16, 2008, steam began seeping from a fracture on top of the lava dome. Associated seismic activity was the most noteworthy since 2004. Scientists suspended activities in the crater and the mountain flanks, but the risk of a major eruption was deemed low. By the end of January, the eruption paused; no more lava was being extruded from the lava dome. On July 10, 2008, it was determined that the eruption had ended after more than six months of no volcanic activity.

Current Volcanic Status

Current Status:

Last Eruption: 2008

Current Activity Information: The Mt. St. Helens volcano is currently undergoing a state of slumber. No unusual activity is present.