Eruptive Periods in Context (*1)

Approximate Dates Stages/periods
Approx. Age in Years Before 1980
Volcanic Activity/Effect
Historical Event(s)
1980- Most recent eruption
Laterally directed blast from cryptodome followed by eruption of dacite tephra and pyroclastic flows. Heat unleashed during the eruption melted glacial snow and ice. The melt water combined with rock and mud to form a liquid sandpaper-like mixture called a lahar. Shattered stumps remain, testifying to the abrasive force of the lahar. The flow surged up a 120-foot high section of hill, scouring away the forest in its path. The lahar raced down the Muddy Tiver and Pine Creek drainages into Swift reservoir. A dome developed in a new crater. Man on the moon (1970)

Alaska granted statehood (1959)

WWll (1941)

WWl (1919)


1776-1857 Dormant interval of 123 years.

Goat Rocks eruptive period.

180-123 (*2) Eruptions of daicite, tephra, andesite lava flow, dacite dome.

X Tephra layer: fiery red arteries of andesitic lava pulsed down the southeast side of the volcano. These lava flows can be seen from the lahar viewpiont and are called the worm flows. Pasty, sticky dacite lava oozed out of the volcano, crowning the pre-1980 summit with a lava dome.

Declaration of Independance (1776)
1480-1650 Dormant interval of about 200 years

Kalama (*3) eruptive period

350-500 (*4) Eruptions of daicite, andesite tephra, dacite dome(s) and pyroclastic flows, andesite lava flows.

We Tephra layer: Scorching hot pyroclastic flows tumbled down the volcano. Winds blew mushroom shaped ask plumes to the east. The Kalama, with Goat Rocks and Sugar Bowl periods, built the symmetrical cone shape of Mount St. Helens. Most rocks visible on the surface originated during the Kalama.

Columbus sets sail (1492)
800 Dormant interval of about 650 years

Sugar Bowleruptive period

1150 Eruption of dacite dome. laterally directed blast, pyroclastic flow(s), air-fall tephra.

Wn Tephra Layer: Swirling plumes of ash rushed skyward. Prevailing winds deposited tephra in southeast British Columbia.

Viking explorations
300 BC-250 AD Dormant internal of about 600 years

Castle Creekeruptive period

1700-2200 Eruptions of andesite, dacite, and basalt tephra, andesite and basalt flows, andesite and dacite pyroclastic flows.A variety of material was ejected from the volcano, marking a significant change in the eruptive behavior of Mount St. Helens. These constructive eruptions built most of the modern volcano. Prior to this time, dacitic lava had been the primary lava type. During this period, pasty dacite lava was accompanied by fluid basaltic lava, and andesitic lava (lava with properties between basalt and dacite).

Bh Tehpra Layer: fluid red ribbons of basaltic lava spilled down the volcano and formed Ape Cave.

Bo Tephra Layer: Dacitic pyroclastic flows and rivers of basaltic lava surged down the slopes of the volcano.

Bi Tephra layer: Andesitic lava and pyroclastic flows spilled down the volcano.

Bu Tephra layer: Glowing rivers of basaltic lava flowed from the volcano.

Roman Empire in power

Christ lives

1200 BC-700 BC Apparent dormant internal of about 300 years

Pine Creekeruptive period

3000-2500 Eruptions of dacite tephra, dacite domes, pyroclastic flows. Mount St. Helens awoke from a brief slumber with a series of small explosive eruptions. Searing hot pyroclastic flows tumbled down all sides of the volcano. Airfall tephra from these eruptions lift four distinctive deposits on Mount Rainier!

Tremendous lahars scoured down the volcano into the Lewis and Toutle river valleys. Spirit Lake over topped the natural dam created during the Smith Creek period and formed a huge mudflow that roared down the North Fork of the Toutle River Valle. This mudflow blocked a stream tributary (similar to the formation of spirit Lake) and formed Silver Lake.

Greek civilization begins


2600 BC-1600 BC Apparent dormant internal of about 300 years


Smith Creekeruptive period

4000-3300 Eruptions of dacite, tephra, dacite domes, pyroclastic flows; probably included dormant intervals as long as several centuries. Billowing clouds of tephra rocketed towards the sky during what was likely the largest eruption in the hitory of the volcano. Nearly 2.5 cubic miles of tephra was ejected from the volcano, in contrast to 0.1 cubic miles deposited during the May 18, 1980 eruption.

Yn Tephra layer: Pumice from this eruption has been found in Canada up to 500 miles away from the volcano.

Ye Tephra Layer: Intense heat from the erupting volcano melted snow and Ice. The melt water mixed with rock and mud, forming cement-like slurries called lahars. A lahar plugged the North Fork of the Toutle river, blocking a stream drainage. Spirt Lake formed behind this natural dam. 

Egypt builds pyramids


12000 BC-9000 BC Apparent dormant internal of about 5000 years(?)

Swift Creekeruptive stage

13000-10000 Eruptions of dacite tephra, dacite domes, litihic and pumiceous pyroclastic flows; probably included dormant intervals of at least a few centuries. Repeated powerful explosions hurled ash and pumice into the air. Many superheated rock avalanches called pyroclastic flows raced down the slopes of the volcano.

S Tephra layer: large eruptions of pumice and ash called tephra were blasted into the air. Tephra from this eruption was found in central Washington!

J Tephra Layer: Winds blew towering clouds of tephra to the west. This was the only time that pumice was deposited to the west in any significant amount. 

First migrations across Bering land bridge into North America.


19000 BC-16000 BC Apparent dormant internal of about 5000 years(?)

Cougareruptive stage

21000-18000(?) Eruptions of pumice tephra, one or more dacite domes and lava flows, lithic and pumiceous pyroclastic flows; probably included dormant intervals of at least several centuries. Crater Lake is formed.


Apparent dormant internal of about 15000 years(?)

Ape Canyon eruptive stage

50000-36000(?) Eruptions of tephra and pumiceous pyroclastic flows.

(*1) - Derived from Crandell, Dwight R., Deposits of pre-1980 Pyroclastic Flows and Lahars from Mount St. Helens Volcano, Washington (USGS Professional Paper 1444: U.S. Fovernment Printing Office, Washington D.C., 1987)

(*2) - Years before 1980, based on tree-ring dates and historic records

(*3) - Boldface periods/stages visible at stratigraphic bands study site

(*4) - Years before 1980, based on tree-ring dates and 14-C dates