Mt. St. Helens

1. On Sunday, May 18, 1980, the largest volcanic eruption to occur in North American historic times transformed a picturesque volcano into a decapitated remnant. On this date in southwestern Washington State, Mount St. Helens erupted with tremendous force. • What happened?? • Approximately 1 km3 of ash erupted. • Summit decreased by 1,350 feet. • Claimed 59 lives • Ash propelled 11 miles into the atmosphere. • Ash covered surrounding areas of Yakima, Tri-cities, and • northern Oregon for 3 days – Noon felt like night. • Feb. 1981- highest birth rate in Portland and surrounding areas –TRUE FACT • Advice from the authorities: • If there is another major eruption, put your head between your • legs and kiss your ash goodbye! Before After Mt. St. Helens

2. The “buzzword” is VISCOSITY What is viscosity? Viscosity = how well a material flows more viscous – flows very slowly (high viscosity) less viscous – flows quickly (low viscosity) Does glass have viscosity? 5

3. Why do volcanoes have different eruptive styles??? • high viscosity • high SiO2 • felsic • “pasty” • explosive • Factors influencing eruptions • dependant on the magma’s viscosity • high viscosity –”pasty” explosive • low viscosity –”fluid” flows easily • Factors influencing viscosity • Temperature of magma • T viscosity = fluid flow • T viscosity = pasty flow A • low viscosity • low SiO2 • mafic • “fluid” • non-explosive • Chemical composition • SiO2 content (high or low) mafic composition: (50% SiO2)=“fluid” flow intermediate comp.: (60% SiO2) felsic composition: (70% SiO2) =“pasty” flow B

4. Dissolved gasses – influencing the movement of magma (volatiles – water, CO2, SO2….) Silica content and volatiles erupt two types of materials: Gas charged expands 100 times its volume Gas charged lava expands 100 times its volume lava fountains very explosive lava flows fluidly Volatiles migrate upwards with difficulty volatiles easily migrate upward magma low in SiO2 magma high in SiO2

5. sulfur dioxide SO2 > 1% 5% Carbon dioxide water vapor 15% 70% 5% volatiles • Dissolved gasses (volatiles) • 1-6% of total magma wt. • contributes to atmosphere Magma chamber

6. Types of Basaltic Lava Flows (low silica (SiO2) content) Pahoehoe Aa • very fluid, thin, • broad sheets • flows 10-300 km/hr • (30-900 ft/hr) • high volatile gas content • smooth “skin,” ropey • type flow • very “pasty,” sticky, • thick, cool flows • flows 5-50 m/hr • (15-150 ft/hr) • low volatile gas content • rough, blocky, sharp, • angular type flow

7. Pyroclastic materials Bombs Ash Volcanic Bombs Nuee-Ardente Lahars Lahars mud flows Ash • Ryholitic magmas • high silica • very explosive • thick, pasty • high viscosity • pyroclastic ejections Nuee-Ardente

8. Four (4) types of Volcanoes • Volcano type is dependant on • SiO2 content. shield composite (stratovolcano) cinder cone Explain the differences. plugged dome

9. Shield Volcano - Hawaii Broad, low angle flanks • Shield Volcanoes • Hawaiian Islands, Iceland, Galapagos Islands • commonly rise from the deep ocean floor • formed by the accumulation of fluid basaltic flows • low silica content (basaltic composition) • low viscosity • less than 1% pyroclastic debris • non-explosive eruptions • pahoehoe flows • aa flows 15

10. Stratovolcano • Composite Cones (stratovolcanoes, stratacomposite) • Western U.S. coast, Western South American coast, Japan • typically form in the ocean along continent convergent boundaries • found along the ring of fire Steep high angle flanks • Formed from layering deposits of • ash, lava, and pyroclastic flows • High silica content (70%)- (Rhyolitic • composition) • high viscosity flows • Abundant pyroclastic activity • deadly airborne debris • Explosive eruptions – very hazardous

11. Cinder Cones • Exist all over the Earth’s surface • Typically, located in volcanic fields (Flagstaff AZ-600+) • Formed by gas rich basaltic flows (low viscosity, low silica) • producing small sized material. Common • rock scoria and volcanic glass • Single eruptive episode lasting • a short time • Composed of scoria and loose • pyroclastic material • Commonly forms along the • flanks of preexisting volcanoes • Very high, steep angle flanks • 30-40 degrees • average 100 – 1000 feet

12. Cinder Cones

13. Lava Dome (plugged dome volcano) • results from viscous lava extruding into the crater. • high degassing of extrusion  “pasty – sticky” lava • most preserved domes are chemically high SiO2 content • commonly rhyolitic or dacitic in composition Rhyolitic / Dacitic lava plug High SiO2

14. Plugged Dome Volcano Plugged Dome

15. Proclaimed has a National • Monument in May, 1907 • Eruption of Lassen Peak • May, 1914 • Subsequent eruptions between • 1914 – 1921 • Lassen Volcanic National Park • established August 9, 1916

16. The Ring of Fire Cascade Mt. Range Stratovolcanoes 17

17. Baker Ocean to Continent Convergence Rainier Pacific Plate St. Helens Adams Oceanic plate is subducted beneath continental plate. Melting plate ascends upward mixing with continental material. Hood Jefferson Three Sisters Newberry Volcano Crater Lake North American Plate • High SiO2 – High viscosity • explosive volcanoes • “pasty” lava flows • composite type volcanoes • andesite/rhyolite rocks McLaughlin Medicine Lake Volcano Shasta Lassen Peak

18. Common Volcanic Rock Types Lassen Peak National Park Black Dacite Light Dacite Andesite Banded Pumice

19. Dacite eruption from 1915 • 63% to 68% SiO2 • plagioclase feldspar, amphibole, pyroxene • erupted around 800-1000 0C $0.25 plagioclase feldspar

20. (Mount Tehama) volcanic base of Mount Tehama • Mount Tehama caldera (Brokeoff Volcano) • exploded and collapsed during Late • Pleistocene leaving volcanic remnants • “Huge” composite volcano • 3 ½ mile wide volcanic caldera • Lassen Peak formed on the northern • flank of Mount Tehama

21. Projected cross-section of Mount Tehama Mount Diller Mount Brokeoff Estimated 11,000 feet +

23. Sulfur Works Bumpass Hell Hot Springs • Hot springs, fumaroles, strong rotten egg odor indicates the • presence of hydrogen sulphide (H2S) • Represents the center of Brokeoff cone (Mount Tehama) • Results of hydrothermal alteration: • hard gray-green andesite lava  bright colored clays • other volcanic rocks  reduced to red iron oxides • presence of sulphuric acid (H2SO4)  rapidly • reducing volcanic rock to clays • High water acidity  pure opal (not gem quality)

24. Bumpass Hell – Named for a cowboy that worked in the area in the 1860’s (Kendal Bumpass) Scalded his feet and when ask where he was, he replied ------ IN HELL

27. Devil’s Kitchen • Strongly acidic resulting in holes • and pits eaten in bedrock • Sulfur Works, Bumpass Hell, Devil’s • Kitchen associated with a north-west • trending fault system • Less civilized than Bumpass Hell and • 6000 feet elevation • Relatively small hike from Bumpass • Hell with a 440 feet elevation gain • Stepping back into the Mesozoic!

29. Lassen Peak Statistics: • Stands 10,457 feet to summit • Located at northern end of the Sacramento valley • Landmark for immigrants entering the valley • (around the 1800’s) • Named after Peter Lassen, Danish blacksmith • (1830) • Peter Lassen guided parties of immigrants into • California using Lassen Peak, but frequently • got lost ---- as the story goes.

30. Lassen Peak Geologic History • Radiometric dating shows the • formation of Lassen Peak around • 31,000 years ago --- along the • northern flank of Mount Tehama • Streams of dacitic lava flows moved • to the north reaching 1500 ft covering • 20 mi2 • 25,000 -31,000 years, Lassen grows • rapidly reaching 1800 ft in a few • years --- becoming the largest • plugged dome type volcano • 18,000 – 25,000 years, Lassen Peak • significantly altered by glaciation • 30 additional smaller steep-sided • dacitic domes form (Bumpass Mt, • Helen Ridge, Eagle Peak, Valcan’s • Castle, Reading Peak

31. Lassen Peak Geologic History • 300 – 1100 years, several dacitic • pumice domes form with abundant • avalanches producing topography • similar to the Chaos Crags deposit. • This resulted in the Chaos Jumbles • deposit and the damming of • Manzanita Lake • Mid 18th century, formation of Cinder • Cone (NE-section of the park). Ash • falling on the streams of lava resulted • the formation of Painted Dunes. • A flow of quartz-studded basaltic lava • flows from Cinder Cone damming Butte • and Snag Lakes avalanche debris

32. May 22, 1915 1914 -1921 Lassen Peak Activity “The Great Explosion” Eruptions seen as far as 150 miles away Explosions recurred at irregular intervals on Lassen Peak for most of 1914. Later, on May 19, 1915, a mass of lava rose in the summit crater and spilled 1,000 feet (300 m) down the western side of the volcano.Extensive lahars (mudflows) were created on the northeastern side as snow banks were melted. Subsequent eruption 1915 A great explosion blasted out a new crater three days later on May 22, 1915. A volcanic cloud rose 40,000 feet (12,000 m) along with flowing lava creating various dams and lakes currently observed today. Volcanic activity declined, finally ending in 1921.

35. Lassen Peak Cinder cones producing Eagle Peak and Bumpass Mountain. Photo taken in the southwest direction. Smaller cinder cones compared to the plugged dome Lassen Peak Eagle Peak Bumpass Mtn Lassen Peak

36. Chaos Crags (older) Avalanche debris produced on the north side of Lassen Peak between 300-1100 years Sept 2006 Chaos Jumbles (younger) Oct, 1930

37. The Painted Dunes are composed of oxidized cinders lying over the Fantastic Lava Beds. In the distance, the concave flank (right side) of Lassen Peak is where the lahars blew down the peak in 1915 to create the Devastated Area. Painted Dunes

38. shield volcano Avalanche deposits Painted Dunes Plugged Dome Composite cone (Mt Tehama)