Editing Mount St. Helens (section)

===Castle Creek and Sugar Bowl eruptive periods===
[[File:East Dome Mount St. Helens.jpg|thumb|East Dome on the east flank of Mount St. Helens in 2013.]]
The next eruptive period, the Castle Creek period, began about 400&nbsp;BC, and is characterized by a change in the composition of St.&nbsp;Helens' lava, with the addition of [[olivine]] and [[basalt]].<ref name=Harris1988/>{{rp|page=216}} 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&nbsp;BC that traveled all the way into the Lewis and Kalama river valleys.<ref name=Harris1988/>{{rp|page=216}} Others, such as Cave Basalt (known for its system of [[lava tube]]s), flowed up to {{convert|9|mi|km}} from their vents.<ref name=Harris1988/>{{rp|page=216}} During the first century, mudflows moved {{convert|30|mi|km|-1}} down the Toutle and Kalama river valleys and may have reached the [[Columbia River]]. Another 400&nbsp;years of [[dormancy]] ensued.

The Sugar Bowl eruptive period was short and markedly different from other periods in Mount St.&nbsp;Helens history. It produced the only unequivocal laterally directed blast known from Mount St.&nbsp;Helens before the 1980 eruptions.<ref name="USGS-EruptiveHistory"/> 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.<ref name="USGS-EruptiveHistory"/> East Dome, a small hypersthene-homblende dacite dome on the east slope of the volcano, was likely formed around the Sugar Bowl period.<ref>{{Cite web |date=2017-06-02 |title=The Eruptive History of Mount St. Helens |url=https://volcano.oregonstate.edu/eruptive-history-mount-st-helens |access-date=2023-03-02 |website=Volcano World |language=en}}</ref> Formation of East Dome was preceded by an explosive eruption.<ref>{{Cite web |title=Global Volcanism Program {{!}} Image GVP-02835 |url=https://volcano.si.edu/gallery/ShowImage.cfm?photo=GVP-02835 |access-date=2023-03-02 |website=volcano.si.edu |language=en}}</ref>