Yn tephra

{{Infobox eruption

| name = Yn tephra eruption

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| date = 1860 BCE (?){{cite web|url=https://volcano.si.edu/volcano.cfm?vn=321050&vtab=Eruptions|title=St. Helens|work=Global Volcanism Program|publisher=Smithsonian Institution|access-date=2020-01-28}}

| volcano = Mount St. Helens

| type = Plinian

| location = Washington, United States

| coordinates = {{coord|46.1912000|N|122.1944000|W|type:mountain_region:US-WA_scale:100000_source:gnis|format=dms|display=inline,title}}

| VEI = 6

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The Yn tephra is a geologically recent tephra deposit that covers portions of the U.S. state of Washington and the Canadian provinces of British Columbia and Alberta. It was created by the largest known volcanic eruption from Mount St. Helens, having taken place in possibly 1860 BCE as part of the Smith Creek eruptive period. The tephra consists of pumiceous dacite.{{cite book|last1=Pallister|first1=John S.|last2=Clynne|first2=Michael A.|last3=Wright|first3=Heather M.|last4=Van Eaton|first4=Alexa R.|last5=Vallance|first5=James W.|last6=Sherrod|first6=David R.|last7=Kokelaar|first7=B. Peter|title=Field-Trip Guide to Mount St. Helens, Washington—An Overview of the Eruptive History and Petrology, Tephra Deposits, 1980 Pyroclastic Density Current Deposits, and the Crater|chapter=Eruptive History|page=11|publisher=United States Department of the Interior|year=2017|issn=2328-0328}}