Editing Mount St. Helens (section)

===Kalama and Goat Rocks eruptive periods===
[[File:1890 Clohessy and Strengele engraving of Mount St Helens (cropped).jpg|thumb|upright=1.15|alt=Painting of a rolling landscape with a conical mountain in background.|The symmetrical appearance of St.&nbsp;Helens prior to the 1980 eruption earned it the nickname "[[Mount Fuji]] of America". The once-familiar shape was formed out of the Kalama and Goat Rocks eruptive periods.]]

Roughly 700&nbsp;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 1480 eruption was several times larger than that of May 18, 1980.<ref name="USGS-EruptiveHistory">{{cite report |url=https://pubs.er.usgs.gov/publication/pp1563 |title=Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington |website=United States Geological Survey |id=Professional Paper 1563|first=Donal R.|last=Mullineaux|year=1996 |access-date=2006-11-15}}</ref> In 1482, another large eruption rivaling the 1980 eruption in volume is known to have occurred.<ref name="USGS-EruptiveHistory"/> Ash and pumice piled {{convert|6|mi|km}} northeast of the volcano to a thickness of {{convert|3|ft|m|1}}; {{convert|50|mi|km|-1}} away, the ash was {{convert|2|in|cm|0}} deep. Large pyroclastic flows and mudflows subsequently rushed down St.&nbsp;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.<ref name=Harris1988/>{{rp|page=216}} Blocky andesite lava then flowed from St.&nbsp;Helens' summit crater down the volcano's southeast flank.<ref name=Harris1988/>{{rp|page=216}} 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&nbsp;m) high at the volcano's summit, which filled and overtopped an explosion crater already at the summit.<ref name=Harris1988/>{{rp|page=217}} Large parts of the dome's sides broke away and mantled parts of the volcano's cone with [[Scree|talus]]. Lateral explosions excavated a notch in the southeast crater wall. St.&nbsp;Helens reached its greatest height and achieved its highly symmetrical form by the time the Kalama eruptive cycle ended, in about 1647.<ref name=Harris1988/>{{rp|page=217}} The volcano remained quiet for the next 150&nbsp;years.

The 57-year eruptive period that started in 1800 was named after the Goat Rocks dome and is the first period for which both oral and written records exist.<ref name=Harris1988/>{{rp|page=217}} As with 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 rivaled 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 (state)|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 1831 eruption is likely what tinted the sun bluish-green in [[Southampton County, Virginia]] on the afternoon of August 13—which [[Nat Turner]] interpreted as a final signal to launch [[Nat Turner's slave rebellion|the United States' largest slave rebellion]].<ref>{{cite book |last1=Breen |first1=Patrick H. |title=Nat Turner's revolt: rebellion and response in Southampton County, Virginia |date=2005 |url=https://esploro.libs.uga.edu/esploro/outputs/doctoral/Nat-Turners-revolt-rebellion-and-response-in-Southampton-County-Virginia/9949332806802959 |access-date=21 November 2021}}</ref>) The vent was apparently at or near Goat Rocks on the northeast flank.<ref name=Harris1988/>{{rp|page=217}} Goat Rocks dome was near 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 {{convert|1300|ft|m}} of the mountain.