Mount St. Helens - Summary of Volcanic History with Road Guide Information |
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| Eruptive stages and periods are separated by dormant intervals for which no unequivocal volcanic products have been recognized. Years before 1980 are based on tree-ring dates (Yamaguchi, 1983), Carbon14 ages, and (or) historical records. A.D. dates during the Goat Rocks and Kalama eruptive periods from Yamaguchi (1983, 1985). All tephras consist of multiple layers not subdivided further here; only the thickest layers of W, B, and Y tephras are distinguished. Locations are of outcrops that illustrate the various deposits during each eruptive period. After Crandell and others (1981), Mullineaux and Crandell (1981), and Crandell (1988) | ||||
Eruptive stages and periods |
Eruptive products |
Road Guide Location
|
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|
Spirit Lake Eruptive Stage |
Modern eruptive period |
1980-88 (this writing, 1990) | Avalanche |
Leg I, Stop 22
Leg II, Stop 11 Leg II, Stop 13 |
| Lateral blast |
Leg I, Stop 11
Leg I, Stop 12 Leg I, Stop 13 Leg I, Stop 14 Leg I, Stop 15 Leg I, Stop 17 Leg I, Stop 20 |
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| Blast deposit1 |
Leg II, Stop 12
Leg II, Stop 13 |
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| Lahar |
Leg I, Stop 3
Leg I, Stop 4 Leg I, Stop 5 Leg I, Stop 6 Leg I, Stop 8 |
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| Pyroclastic flow |
Leg I, Stop 20
Leg I, Stop 22 |
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| May 18, 1980 Tephra |
Leg I, Stop 11
Leg I, Stop 12 Leg I, Stop 17 |
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| Domes (dacitic) | Crater | |||
|
Goat Rocks eruptive period |
150-100 years B.P. | Dome (dacitic) | (Destroyed May 18, 1980) | |
| Lava flows (andesitic) | North flank of volcano | |||
| A.D. 1800 | T tephra (dacitic) |
Leg I, Stop 15
Leg I, Stop 16 Leg I, Stop 17 Leg I, Stop 30 |
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|
Kalama eruptive period |
500-338 years B.P. | Pyroclastic flows (dacitic, andesitic) |
Leg I-C, Stop 22
Leg I, Stop 23 |
|
| A.D. 1647? | Summit dome (dacitic) | (Destroyed May 18, 1980) | ||
| Lahars from summit |
Muddy River fan
(Leg I-B, Stop 5) |
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| Lava flows (andesitic) | Southeast flank | |||
| X tephra (andesitic) |
Leg I, Stop 4
Leg I, Stop 6 Leg I, Stop 16 Leg I, Stop 17 |
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| A.D. 1482 | We tephra |
Leg I, Stop 4
Leg I, Stop 6 |
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| A.D. 1480 | Wn tephra |
Leg I, Stop 16
Leg I, Stop 17 Leg I, Stop 20 Leg I, Stop 30 |
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|
Sugar Bowl eruptive period |
1,200 years B.P. | Dome (dacitic) | Northeast flank of volcano | |
| Pyroclastic flows (dacitic) | Northeast flank of volcano | |||
| Deposits of lateral blast (dacitic) |
Leg I, Stop 17
Leg I, Stop 20 |
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|
Castle Creek eruptive period |
2,500-1,600 years B.P. | Bu tephra (basaltic) |
Leg I, Stop 4
Leg I, Stop 16 Leg I, Stop 17 |
|
| Bi tephra (dacitic) | ||||
| Bo tephra (andesitic) | ||||
| Bh tephra (andesitic) | ||||
| Lava flows (basaltic) | ||||
| Pyroclastic flows (dacitic, andesitic) | ||||
|
Pine Creek eruptive period |
3,000-2,500 years B.P. | P tephra (dacitic) |
Leg I, Stop 4
Leg I, Stop 16 Leg I, Stop 17 |
|
| Pyroclastic flows (dacitic) | Canyon of Pine Creek | |||
| Domes (dacitic) | Crater | |||
| Lahars |
Leg I, Stop 9
Leg II, Stop 4 |
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|
Smith Creek eruptive period |
4,000-2,200 years B.P. | Ye tephra (dacitic) | Leg I, Stop 4 | |
| Pyroclastic flows (dacitic) | ||||
| Yn tephra (dacitic) |
Leg I, Stop 16
Leg I, Stop 17 |
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|
Swift Creek
eruptive stage |
13,700-9,200 years B.P. | J tephra (dacitic) |
Leg I, Stop 4
Leg I-C, Stop 23 |
|
| Pyroclastic flows (dacitic) | West Fork Swift Creek | |||
| Lahars | Leg I-C, Stop 26 | |||
| Domes (dacitic) | ||||
| S tephra (dacitic) | Leg I, Stop 4 | |||
| Fraser Glaciation (ended approximately 13,000 years B.P. | ||||
|
Cougar
eruptive stage |
20,400-19,200 years B.P. | K tephra (dacitic) | Leg I, Stop 1 | |
| Pyroclastic flows (dacitic) |
Leg I, Stop 7
Leg I, Stop 25 |
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| Domes (dacitic) | ||||
| Avalanche | Leg I-C, Stop 27 | |||
| Lava flows (andesitic) | West Fork Swift Creek | |||
| M tephra (dacitic) | Leg I, Stop 1 | |||
|
Ape Canyon
eruptive stage |
approximately 40,000?-25,000 years B.P. | C tephra (dacitic) | Leg I, Stop 6 | |
| Pyroclastic flows (dacitic) | ||||
| Lahars |
Leg I
(Lewis River) |
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| 1 The term "blast deposit" is here used to designate a deposit resulting from any process initiated by the lateral explosion. Precise transport and deposition mechanisms are controversial (Walker and McBroome, 1983; Hoblitt and Miller, 1984; Waitt, 1984a; Walker and Morgan, 1984) | ||||
| -- Michael P. Doukas, 1990, Road Guide to Volcanic Deposits of Mount St. Helens and Vicinity, Washington: U.S. Geological Survey Bulletin 1859, 53p. | ||||
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