History of Mt. St. Helens

Mount St. Helens has had many previous eruptive periods and the mountain we see today was only formed a mere 4,000 years ago. The eruptive periods of Mt. St. Helens can be divided into several categories, many of these periods span several thousand years and contain several eruptions of the volcano, these periods have been grouped together based on the nature of the eruption and volcanic deposits. The past 13,000 years of Mount St. Helens' activity can be accurately traced by analysis of these deposits since a stratified record exists, composed of various volcanic deposits. The older eruptive history is less clear due to erosion of the stratified soil record.

The first record of Mount St. Helens occurs in the Ape Canyon Eruptive Period, during this period which may have lasted over 8,000 years and was comprised of pumice fall, tephra and pyroclastic flows, and one pumice bearing lahar. This first eruptive period was followed by a dormant interval of approximately 30,000 to 20,000 years.

The second eruptive period, known as the Cougar Eruptive Period began about 20,000 years ago This period was comprised of several volcanic events including at least one pumice containing pyroclastic flow, which moved south ward, two sets of air-fall tephra followed this pyroclastic flow. After a short period of dormancy, they're where at least two more pyroclastic flows moved south/southeast. There is very little evidence of what occurred from 18,000 and 13,000 years ago.

The Swift creek eruptive period was Mt. St. Helens third eruptive period. It was mostly comprised of explosive eruptions which produced pyroclastic flows, as well as air fall, debris. Some of this air fall reached as far east as central Washington. After the explosive eruptions, many smaller eruptions ensued, most likely forming a lava dome, and approximately 8,000 years ago a series of large explosions which produced massive amount of tephra, ending the Swift Creek Eruptive Period.

The fourth eruptive period of Mt. St. Helens was the Smith Creek Eruptive Period. Beginning with multiple explosive eruptions the Smith creek eruptive period lasted for 700 years. This time period was filled with volcanic activity, which produced the largest tephra deposit in the past 4,000 years, remnants of this deposit have been found as far north as Canada. Additionally a lake in a similar location to that of Spirit lake was formed during some of the Lahars and pyroclastic flows, which surged down the North Fork of the Toutle River during the Smith Creek eruptive period.

After a short 100-year period of dormancy, the fifth eruptive period of Mt. St. Helens began; this period is known as the Pine Creek eruptive period. Although only a short period of dormancy passed between the Smith and Pine creek eruptive periods there was a distinct difference in volcanic deposits. This difference is mainly in the fact that eruptive products in the Pine Creek eruptive period contained Iron-Magnesium substances. The Pine Creek eruptive period also gave rise to silver lake, which was formed from the lahars and deposits of several small eruptions, which were characteristic of the 500-year pine creek eruptive period.

Castle creek eruptive period had a significant change in eruptive behavior and variety of rock types. During the castle Creek eruptive period both andesite, basalt and dactite were erupted in alternating succession, of andesite, dacite, basalt, andesite, dacite, basalt. Along with these rock formations soil stratification shows that pyroclastic flows occurred about 2,000 to 2,000 years ago during the Castle creek eruptive period. The Castle Creek eruptive period is very important since it was the beginning of the mountain, which we see today, this was also the approximate time period for the formation of the Ape Cave.

More of the mountain was formed, during the Sugar Bowl eruptive period about 1,200 years ago. During this period the Sugar Bowl, a dome of dactite formed at the north base of the mountain. During the formation of this dome a large lateral blast carried, rock fragments north, covering the ground with up to 50cm of rock and ash in places.

The Majority of Mount St. Helens as it appeared before the May 18, 1980 eruption was formed during the eighth eruptive period of Mt. St. Helens. This period is known as the Kalama eruptive period and is thought to have stopped about 100 years ago. The Kalama eruptive period began with the eruption of a large amount of dactite pumice; this pumice was deposited in Northeastern Washington. This eruption was followed by pyroclastic flows down the south side of the mountain. Ensuing eruptions of the Mt. St. Helens caused several other pyroclastic flows, which were followed by the extrusion of the dacite dome which existed until the May 18, 1980.

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