Geology of Mount St. Helens National Volcanic Monument
Mount St. Helens is a stratovolcano, a steep-sided volcano located in the Pacific Northwest region of the United States in the state of Washington.
Sitting about 97 miles south of Seattle and 52 miles northeast of Portland, Oregon, Mount St. Helens is the most active volcano within the Cascade Range and has the highest probability out of all U.S. volcanoes other than Hawaii and Alaska to erupt in the future. During the past few thousand years Mount St. Helens reached its pre-1980 elevation of 2,950 m (9,677 ft) making it the fifth highest peak in Washington at the time and giving it the nickname of “Mount Fuji of America.”
The Cascade Range, where Mount St. Helens resides, is a perfect example of a fundamental concept in geology known as a subduction zone, a place where oceanic crust and continental crust collide. Here, the Juan de Fuca (oceanic) plate dives beneath the North American (continental) Plate. Oceanic crust is more dense than continental crust, so as the Juan de Fuca plate collides with the North American Plate, it is forced downward, deeper within the Earth where temperatures are higher. The ocean crust partially melts at depth and also releases less dense materials (water and gases). The less dense material rises, melting and absorbing surrounding rock as it bubbles upward to form magma chambers. These chambers behave similarity to a soda can, staying dormant most of the time unless a sudden disruption occurs. Can you guess what may disturb the balance in the chamber and set off a volcanic eruption? Earthquakes! Just as a sudden and violent shake of a soda can will cause the liquid to escape quickly when opening, volcanoes will react to this quick change in motion and pressure by erupting onto Earth’s surface.
The 275,000 year old geologic history of Mount St. Helens has displayed both relatively quiet outpourings of lava and violent explosive eruptions of volcanic ash and rock fragments, known as tephra. Volcanologists have separated the eruption history of this volcano into four main stages, each followed by a dormant, nonexplosive period.
Ape Canyon Stage: This stage spans from 275,000 to 35,000 years ago and had two major lava dome eruption events. Evidence can be found in rocks as far as eastern Washington, many of which were altered by hydrothermal (hot water) activity, indicating explosive eruptions. This stage was followed by a dormant interval from 35,000 to 28,000 years ago.
Cougar Stage: The Cougar Stage was one of the most explosive periods for Mt. St. Helens, taking place from 28,000 to 18,000 years ago. The explosions varied to form lava flows and domes, large ash ejections, pyroclastic flows, a debris avalanche and lahars. A debris avalanche is a mass of rock, soil and snow that runs down the side of a volcano to the valley floor, traveling several kilometers from the source, and leaving a horseshoe-shaped crater.. The debris avalanche was the most catastrophic event of the Cougar Stage, leaving a massive deposit behind.
Swift Creek Stage: Swift Creek volcanism occurred between 16,000 to 12,800 years ago. During this relatively short stage multiple domes grew on the volcano, reaching an altitude 2,100 m (7,000 ft). Several of the unstable domes collapsed throughout the volcano-building phases, creating fan-like deposits made of pyroclastic flows and lahars. This growth period was followed by another dormant interval spanning from 12,800 to 3,900 years ago.
Spirit Lake Stage: The Spirit Lake Stage started about 3,900 years ago and continues today. This stage mainly consists of volcanic dome-building events. The deposits are well-preserved, allowing scientists to collect more data than what is available from previous stages. The Spirit Lake Stage can be further broken down into six eruptive periods.
Smith Creek Eruptive Period (3.9 to 3.3 ka): Although the shape of the volcano did not significantly change during the Smith Creek period, there were two violently explosive eruptions. One eruption was about four times larger than the familiar 1980 eruption, making it the most voluminous eruption over the volcano’s history. The other major eruption sourced from an extruded lava dome, and sent lahars as far south as the Columbia River.
Pine Creek Eruptive Period (2.9 to 2.5 ka): This period consisted of tephra ejections, pyroclastic flows, dacite domes, and small avalanches which later formed debris fans. Scientists estimate Mt. St. Helen’s maximum elevation to be about 2,100 m (7,000 ft) toward the end of this eruptive period.
Castle Creek Eruptive Period (2.025 to 1.7 ka): The Castle Creek Eruptive period consisted of dacite domes with tephra, pyroclastic flows, lava flows and lava domes. Recent studies have also shown record of three basaltic eruptions, indicating that the chemical composition had changed, a common occurrence in stratovolcanoes. This period built the cluster of domes into composite volcano, ending with a summit elevation of about 2,450 m (8,000 ft).
Sugar Bowl Eruptive Period (C.E. 850 to 900): The Sugar Bowl period was a short dome-building period, with the largest eruption being about 1/10th the size of the 1980 eruption. This eruptive period did not drastically alter the shape of the volcano.
Kalama Eruptive Period (C.E. 1479 to 1720): The early Kalama Eruptive period began with two large explosive eruptions, taking place within relatively a short time period between the two. This unique pairing of eruptions is rare among worldwide volcanic studies. A large andesitic eruption took place during the mid-Kalama period, sending pyroclastic flows and hot lahars from the volcano. The late Kalama phase saw the rise of Summit Dome, a dacitic dome that grew over a 100-year period and eventually reaching Mt. St. Helen’s pre-1980 form.
Goat Rocks Eruptive Period (A.D. 1800– 1857): This period mainly consisted of smaller eruptions producing ash, tephra, and lava flows. Toward the end of this eruptive phase, the minor eruptions are believed to have been steam-driven, without magma rising significantly to the surface. This stage set the final building blocks before the 1980 eruption.
Modern Eruptive Period: On March 16, 1980, Mt. St. Helens began experiencing earthquake activity. On March 27th, 1980, after several hundred earthquakes, the volcano erupted for the first time in over 100 years. The initial steam blast created a 60-75-m (200- to 250-ft) wide crater, which grew to about 400 m (1,300 ft) in diameter within one week. Earthquakes became more and more frequent, with over 10,000 quakes occurring by May 17th. By this time, the seismic movement had shifted enough land mass to create a bulge or swelling region that grew at a consistent rate of about 2 m (6.5 ft) per day. This drastic deformation, also known as a cryptodome, indicated that magma was bulging from below and waiting to erupt onto the surface.
On May 18th, 1980, without immediate warning, a 5.1 magnitude earthquake shook the volcano as its bulge at the northern flank slid away. This landslide is now the largest debris avalanche in recorded history, and is about the size of a million Olympic swimming pools. Because the cryptodome was a highly-pressurized, high-temperature body of magma, its removal during the landslide caused a massive depressurization of the volcano’s magmatic system, like opening a soda can after it has been shaken. A lateral blast removed the upper 300 m (nearly 1,000 ft) of the cone, sending hot material at least 480 km/hr (300 mi/hr) from the crater and flattening the dense forest as it traveled. Within 15 minutes, an eruption cloud of tephra filled the sky at a height of more than 24 km (15 mi or 80,000 ft).
The major loss in pressure resulted in the onset of a 9-hour long Plinian eruption. The new, northward-opening amphitheater shape was revealed shortly after the eruption ended, disappointing many locals that its “perfect shape” was gone. Within a day of the eruption, 520 million tons of ash was distributed eastward across the United States, and the ash cloud circled the globe over the next 15 days.
The summit elevation dropped to 2,539 m (8,330 ft) due to the collapse of the crater walls. Chemical analysis of the eruptive products shows that the complexity of the magmatic system has increased as the volcano has matured. Scientists have also installed updated GPS devices, seismometers, gas meters, and cameras to increase precision and accuracy of research analysis and continuous monitoring.
