Mount St. Helens
| Mount St Helens | |
|---|---|
 3000 ft (1 km) steam plume on May 19, 1982 |
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| Elevation: | 8,364 ft (2,550 m) |
| Location: | Washington, USA |
| Range: | Cascades |
| Coordinates: | 46°11′28″N, 122°11′39″W |
| Topo map: | USGS Mount St. Helens |
| Type: | Active stratovolcano |
| Age of rock: | < 40000 yr |
| Last eruption: | 2005 |
| First ascent: | 1853 by Thomas J. Dryer |
| Easiest route: | hike |
Mount St. Helens is an active stratovolcano in Skamania County, Washington, in the Pacific Northwest region of the United States. It is located 96 miles (154 km) south of Seattle and 53 miles (85 km) northeast of Portland, Oregon. The mountain is part of the Cascade Range and was initially known as Louwala-Clough which means "smoking or fire mountain" in the language of the local native Americans, the Klickitats. It was named for British diplomat Lord St Helens who was a friend of George Vancouver, an explorer who made a survey of the area in the late 18th century. This volcano is well known for its ash explosions and pyroclastic flows.
It is most famous for the catastrophic eruption on May 18, 1980. That eruption was the most deadly and economically destructive volcanic eruption in the history of the United States. Fifty-seven people were killed and 250 homes, 47 bridges, 15 miles (24 km) of railways and 185 miles (300 km) of highway were destroyed. The eruption caused a massive debris avalanche, reducing its summit from 9,677 feet (2,950 m) to 8,364 feet (2,550 m) in elevation and replacing it with a mile-wide (1.5 km-wide) horseshoe-shaped crater (see geology section or 1980 eruption of Mount St. Helens for more detail). The debris avalanche from the 1980 eruption was up to 0.7 cubic miles (2.3 km3) in volume, making it the largest in recorded history. However, the scale of it still pales in comparison to far larger debris avalanches that have occurred in the geological past elsewhere on Earth.
Like most other volcanoes in the Cascade Range, St. Helens is a great cone of rubble consisting of lava rock interlayered with ash, pumice and other deposits. Mount St. Helens 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; another formed Goat Rocks dome on the northern flank. These were destroyed in St. Helens' 1980 eruption.
Mount St. Helens is a part of the Pacific Ring of Fire which includes over 160 active volcanoes.
Geographic setting and description
Mount St. Helens is 34 miles (55 km) almost due west of Mount Adams, which is in the eastern part of the Cascade Range. These "sister and brother" volcanic mountains are each about 50 miles (80 km) from Mount Rainier, the giant of Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is about 60 miles (95 km) southeast of Mount St. Helens.
Mount St. Helens is geologically young compared to the other major Cascade volcanoes. It was formed only within the last 40,000 years, and the pre-1980 summit cone started to grow only about 2200 years ago. The volcano is also known to have been the most active in the Cascades within the last 10,000 years.
Even before its loss of height, Mount St. Helens was not one of the highest peaks in the Cascade Range. Its summit altitude made it only the fifth highest peak in Washington. It stood out prominently, however, from surrounding hills because of the symmetry and the extensive snow and ice-cover of the pre-1980 summit cone, earning it the nickname, "Fujiyama of America" or "Mount Fuji of America". The peak rose more than 5,000 feet (1500 m) above its base, where the lower flanks merge with adjacent ridges. The mountain is about 6 miles (9.5 km) across at its base which is at an altitude of about 4,400 feet (1340 m) on the northeastern side and about 4,000 feet (1220 m) elsewhere. At the pre-eruption timberline (upper limit of trees) the width of the cone was about 4 miles (6.4 km).
Streams that head on the volcano enter three main river systems — the Toutle River on the north and north-west, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow that dump an average of about 140 inches (3.6 m) of water on Mount St. Helens a year, according to National Weather Service data. 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 the best access routes to the mountain run through Cowlitz County, Washington on the west. Washington State Route 504, locally known as the Spirit Lake Memorial Highway, connects with the heavily traveled Interstate 5 at Exit 49, about 34 miles (55 km) to the west of the mountain. That major north-south highway skirts the low-lying cities of Castle Rock, Longview and Kelso along the Cowlitz River and passes through Vancouver, Washington- Portland, Oregon, metropolitan area less than 50 miles (80 km) to the southwest. The community nearest the volcano is Cougar, Washington which is in the Lewis River valley about 11 miles (18 km) south-southwest of the peak. Gifford Pinchot National Forest surrounds Mount St. Helens, but some land owned by Washington is in private hands.
Human history
The first recorded sighting of Mount St. Helens by Europeans was by Royal Navy Commander George Vancouver and the officers of HMS Discovery on May 19, 1792, while they were surveying the northern Pacific Ocean coast from 1792 to 1794. Vancouver named the mountain for British diplomat Alleyne Fitzherbert, 1st Baron St Helens on October 20, 1792, as it came into view when the Discovery passed into the mouth of the Columbia River.
Years later, the mountain was visited by its first major eruption after explorers, traders, and missionaries heard reports of an erupting volcano in the area. Much later geologists and historians determined that the eruption took place in 1800 and was the start of the 57 year long Goat Rocks Eruptive Period (see geology section below). Alarmed by the "dry snow", the Nespelim of northeastern Washington spent a great deal of time in prayer and dance instead of collecting food for winter and therefore had a hard winter.
In late 1805 and early 1806 members of the Lewis and Clark Expedition spotted St. Helens from the Columbia River but did not report any eruption in progress or recent evidence of one. They did report the presence of quicksand and clogged channel conditions at the mouth of the Sandy River near Portland, suggesting an eruption by Mount Hood sometime in the previous decades.
The first authenticated eyewitness report of a St. Helens eruption was made in March 1835 by Dr. Meredith Gairdner, then working for the Hudson's Bay Company stationed at Fort Vancouver (the first geologist apparently viewed the volcano 6 years later). He sent an account to the Edinburgh New Philosophical Journal, which published his letter in January 1836. James Dwight Dana of Yale University, while sailing with the Charles Wilkes U.S. Exploring Expedition, saw the peak (then quiescent) from off the mouth of the Columbia River in 1841. Another member of the expedition later described "cellular basaltic lavas" at the mountain's base.
In either late fall or early winter 1842 the so-called "Great Eruption" was seen by settlers and missionaries in the area. Large ash clouds were reported for this small volume outburst and mild explosions followed for 15 years. All these eruptions were likely phreatic ( steam explosions). The Reverend Josiah Parrish in Champoeg, Oregon witnessed Mount St. Helens in eruption on November 22, 1842. Ash from this eruption may have reached The Dalles, Oregon 48 miles (80 km) southeast of the volcano.
Future California governor Peter H. Burnett in October 1843 recounted a story of a Native American man who badly burned his foot and leg in either lava or hot ash while hunting for deer. The story went that the injured man sought treatment at Fort Vancouver but the contemporary fort commissary steward, Napolean McGilvery, disclaimed knowledge of the incident. British lieutenant Henry J. Warre sketched the eruption in 1845 and two years later Canadian painter Paul Kane created watercolors of the gently smoking mountain. Warre's work showed erupting material from a vent about a third of the way down from the summit on the mountain's west or northwest side (possibly at Goat Rocks), while one of Kane's field sketches shows smoke emanating from about the same location.
On April 17, 1857 the Republican, a Steilacoom, Washington newspaper, reported that "Mount St. Helens, or some other mount to the southward, is seen . . . to be in a state of eruption". The lack of a significant ash layer associated with this event indicates that it was a small eruption, which may have been nothing more than billowing clouds of steam and dust. This was the first reported activity from the volcano since 1854 and the last until 1980.
During the lead-up to the 1980 Mount St. Helens eruption, 84 year old innkeeper Harry Truman, who had lived near the mountain for over 50 years, became nationally famous when he decided not to evacuate before the impending eruption, despite repeated pleas by local authorities. His body was never found after the May 18, 1980 eruption, which left a huge crater open to the north (see geology section below). In total, 57 people were killed or never found. Had the eruption occurred one day later, when loggers would have been at work, rather than on a Sunday, the death toll would almost certainly have been much higher.
U.S. President Jimmy Carter surveyed the damage and stated it looked more desolate than a moonscape. A film crew, led by Seattle filmmaker Otto Seiber, was dropped by helicopter on St. Helens on May 23 to document the destruction. Their compasses, however, spun in circles and they quickly became lost. A second eruption occurred on May 25, but the crew survived and were rescued two days following the second eruption by National Guard helicopter pilots. Their film became a popular and top-selling documentary "The Eruption of Mount St. Helens."
In 1982, President Ronald Reagan and the U.S. Congress established the Mount St. Helens National Volcanic Monument, a 110,000 acre (445 km2) area around the mountain and within the Gifford Pinchot National Forest.
Following the 1980 eruption, the area was left to gradually return to its natural state preceding the devastation.
Geologic history
According to geological evidence, St. Helens started growth in the Pleistocene 37,600 years ago with dacite and andesite eruptions of pumice and ash. 36,000 years ago a large mudflow cascaded down the volcano (mudflows were very significant forces in all of St. Helens' eruptive cycles). Parts of this ancestral cone were fragmented and transported by glaciers 14,000 to 18,000 years ago during the last ice age. Mt. St. Helens is known to have erupted in four major stages, the present having begun around 2500 BC after 6000 years of dormancy. The four stages were interspersed with very long periods of dormancy or low activity levels lasting for up to a few thousand years. Repeated eruptions of pyroclastic flows, pumice, and ash followed until about 8500 BC when the volcano went dormant for roughly 6000 years. The early eruptive stages of the volcano 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 period since about 2500 BC is called the "Spirit Lake Stage".
Smith Creek and Pine Creek Eruptive Periods
Starting around 2500 BC, the Smith Creek Eruptive Period began with eruptions of large amounts of ash and yellowish-brown pumice covered thousands of square miles. An eruption in 1900 BC was the largest known eruption from Mt. St. Helens during the Holocene, judging by the volume of one of the tephra layers from that eruptive period. This eruptive cycle lasted until about 1600 BC and left 18 inch (46 cm) deep deposits of material 50 miles (80 km) distant in what is now Mt. Rainier National Park and trace amounts have been found as far northeast as Banff National Park in Alberta and as far southeast as eastern Oregon. All told there may have been up to 2.5 cubic miles (10 km3) of material ejected in this cycle.
At the beginning of the Pine Creek Eruptive Period, St. Helens came alive again around 1200 BC after 400 years of dormancy. This cycle, which lasted until about 800 BC, is 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 (65 km) of the Lewis River valley sometime between 1000 BC to 500 BC.
Castle Creek and Sugar Bowl Eruptive Periods
The next eruptive cycle, the Castle Creek Eruptive Period, began roughly around 400 BC and is characterized by a change in composition of St. Helens' lava, which diversified by adding olivine and basalt to the mix. It was during the Castle Creek Period that the pre-1980 summit cone started to form. Also different was the presence of significant lava flows in addition to the previously much more common fragmented and pulverized lavas and rocks ( tephra). Large lava flows of andesite and basalt covered parts of the mountain, including one around the year 100 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 8 to 9 miles (13 to 15 km) from their vents. Also around the 1st century, mudflows moved 30 miles (50 km) down Toutle and Kalama river valleys and may have reached the Columbia River. Another 400 or so years of dormancy ensued.
Sometime around the year 400, the Sugar Bowl Eruptive Period began with small quantities of ash and lava erupted from St. Helens' north flank. This period ended with the emplacement of dacite domes, including Sugar Bowl around the year 800.
Kalama and Goat Rocks Eruptive Periods
Roughly 700 years of dormancy was broken about the year 1480 when large amounts of pale gray dacite pumice and ash started to erupt in the Kalama eruptive cycle. 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. At least seven different beds were laid down in the most voluminous eruptive cycle for 3000 years. Ash and pumice piled to a thickness of three feet (1 m) six miles (9.5 km) northeast from the volcano and two inches (5 cm) deep 50 miles (80 km) away in the same direction. Large pyroclastic flows and mudflows subsequently rushed down St. Helens' west flanks and into the Kalama River drainage system. The source for at least some of these debris flows may have come from the explosion of a dacite dome close to or at the summit.
The next phase of this 150 year long cycle 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 of ash. After that, blocky andesite lava 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. This cycle ended with the emplacement of a large dacite dome at the volcano's summit. The several hundred foot high dome 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 southeast crater wall. St. Helens reached its greatest height and highly symmetrical form by the time the Kalama eruptive cycle ended on or around 1647. 150 years of quiet returned to the volcano.
The 57 year long Goat Rocks Eruptive Period started in 1800 and is the first cycle for which oral and written records exist. As with the Kalama cycle, the sequence of events started with an explosion of dacite tephra followed by an andesite lava flow and then culminated with the emplacement of a dacite dome. The 1800 eruption probably rivalled the May 18, 1980 eruption in size, although unlike the 1980 eruption, 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 small eruptions between 1831 to 1857 of ash reported as well, including a fairly large one in 1842. The vent apparently was at or near Goat Rocks on the northeast flank.
The 1980 eruption
- Main article: 1980 eruption of Mount St. Helens
Mount St. Helens woke up on March 20, 1980, with a Richter magnitude 5.1 earthquake. Steam venting started on March 27. By the end of April, the north side of the mountain started to bulge.
With little warning, a second Richter magnitude 5.1 earthquake triggered a massive collapse of the north face of the mountain on May 18. This was the largest known debris avalanche in recorded history. The magma inside of St. Helens burst forth into a large-scale pyroclastic flow which flattened vegetation and buildings in an area of over 230 square miles (600 km2). This eruption was a 5 on the Volcanic Explosivity Index scale.
For more than nine hours, a vigorous plume of ash erupted, eventually reaching 12 to 15 miles (20 to 25 km) above sea level. The plume moved eastward at an average speed of 60 miles per hour (95 km/h), with ash reaching Idaho by noon.
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 River and Cowlitz River, destroying bridges and lumber camps. A total of 3.9 million cubic yards (3.0 million m3) of material was transported by the lahars.
By around 5:30 p.m. on May 18 the vertical ash column declined in stature and less severe outbursts continued through the night and following several days. In all, St. Helens released an amount of energy equivalent to 27,000 Hiroshima-sized atomic bombs and ejected more than a cubic kilometer of material. The removal of the north side of the mountain reduced St. Helens' height by about 1300 feet (400 m) and left a 1 to 2 mile (1.6 to 3.2 km) wide and 0.5 mile (800 m) deep crater with its north end open in a huge breach. Fifty-seven people were killed along with 1500 elk, 5000 deer, and an estimated 11 million fish. In addition, 200 homes, 47 bridges, and 185 miles (300 km) of highway were destroyed.
Post-1980 activity
Between 1980 and 1986, activity continued on St. Helens, with a new lava dome forming in the crater. Numerous small explosions and dome-building eruptions occurred during this time.
Beginning with the winter snows of 1980-1981, a still unnamed horse shoe-shaped glacier began to evolve in the shadow of the crater. As of 2004, it covers about 0.36 square mile (0.93 km2). Until the beginning of the volcanic activity of 2004, it was considered the only glacier in the lower 48 states of the U.S. that is growing.
Between 1989 and 1991, a series of seismic events occurred, sometimes accompanied by small explosions from the dome. Later, in 1995, 1998, and 2001, earthquake swarms were recorded beneath the crater, though without explosive activity.
2004-present activity
Mount St. Helens became active again in autumn 2004, indicated initially by hundreds and then thousands of localized earthquakes, and followed by several significant emissions of steam and ash.
Magma reached the surface of the volcano around October 11, 2004, resulting in the building of a new lava dome on the existing dome's south side.
Included in the new dome was a feature dubbed the 'whaleback' (named such due to its close resemblance to the back of a whale), which was a long shaft of solidified magma being exuded by pressure of magma underneath it. This interesting feature was very hot but fragile. The edges of it began crumbling rapidly, forming loose material around the new dome. Currently, the whaleback is still growing but crumbling nearly as rapidly as it is growing.
On February 1, 2005, the new lava dome on Mount Saint Helens measured 7,642 feet (2,329 m) in elevation. This brought its elevation to 1,363 feet (415 m) above the 1980 crater floor, approximately 2,000 feet (610 meters) above the surface of the crater glacier, and 721 feet (220 meters) below the highest point of the volcano. The 'whaleback' feature measured approximately 1,550 feet (472 m) in length and 500 feet (152 m) in width. The diameter of the new dome was about 1,700 feet (518 m) at this time and it then contained about 50 million cubic yards (38.5 million m3) of material. The total amount of glacier lost by this date was estimated to be between 5 and 10 percent, but the flow of water out from the crater had changed by almost nothing as the porous nature of the floor of the crater caused the water to be absorbed like a giant sponge.
Mount St. Helens had major activity again on March 8, 2005 when a 36,000-foot plume of steam and ash emerged, which was visible from Seattle, Washington and rained ash on Yakima, Washington. This activity was not considered a large eruption, merely a minor release of pressure consistent with the nature of ongoing dome building. The release was accompanied by a 2.5 earthquake.
As of May 5, 2005, the highest point on the new dome was 7,675 feet (2339 m), 688 feet (210 m) below the highest point of the volcano. Growth of the new dome continues steadily and has not abated, and small earthquakes continue to be observed every few minutes. The 'whaleback' feature is disintegrating steadily but continues to be extruded as solidified lava pushes upward from underneath it. If the growth of the new dome continues at its current pace, the new dome could replace the amount of material lost in the 1980 eruption (estimated at 3.7 billion cubic yards or 2.85 km3) within the next 40-50 years.
The Mount St. Helens VolcanoCam [1] located at Johnston Ridge is able to view the new dome especially at night when the glow of new magma is visible via the camera's infrared capabilities. While geologists warn that an eruption similar to the May 1980 eruption is still possible, the chances are low.
On July 2, 2005, the tip of the whaleback feature broke off, causing a rockfall that sent ash and dust several hundred meters into the air. [2]
