VOLCANIC TEPHRA STUDIES

Tephra: unconsolidated, fine-grained pyroclastic material (volcanic ash)
Other pyroclastic material: lavas, gas, aerosols (Sulfates, Nitrates, Chlorides)

Mafic flows (Hawaii, Japan) not explosive = local tephras
Felsic eruptions (Cascades, Indonesia) = explosive, widespread tephra, abundant aerosols

Mt. Lassen, Calif., May 1914
Fernandina, Galapagos, 1968
Fuego, Guatemala, 1974
Mount St. Helens, Washington, May 18, 1980
El Chichon, Mexico, 1982
Pinatubo, Philippines, 1991

Katla (Iceland)
Omate (Peru)
Pichincha (Ecuador)
Teon (Banda Sea) - dvi 2500

• originally thought to be one or a few ashs (Glacier Peak)
• now: many sources, many events "PLUMES"

• K/Ar, Ar/Ar
• Fission Track
• Obsidian Hydration

• Radiometric
• Tree Rings
• Thermoluminescence

• Sediments: aquatic and terrestrial
• Ice Cores: acidity, shards
• Tree Rings: narrow or frost-damaged rings

• shards: pieces of volcanic glass
• viewed microscopically have characteristic shapes, vesicles
• Mazama ash has cylindrical shards with stretched bubbles
• More common: flat with round bubbles, ragged edges

• density separation of heavier mineral
• petrographic analysis
  indicators: Cummingtonite (amphibole) Mt St Helens Series
  composition: ternary diagrams of (for example) Ca, K, Fe, Sr

• mineral composition of individual glass shards

• Sorting: Size decreases away from source - small shards hard to identify
  Mineral grains (phenocrists), denser than glass, settle out so mineralogy of ashes changes away from source
• Mixing: Ash layers seldom pure
  • mixed with other mineral grains = ID difficult
  • resedimentation: abundant ash on surface or in sediments
    e.g., in the Columbia Basin Mazama ash blew around for 3000 yr

South Pacific: Tambora, Sunda Strait AD 1815 (dvi 3000)
Central America: Cosequina, Nicaragua AD 1835 (dvi 4000)
Mediterranean: Santorini 1626 BC (dv 1000?), Vesuvius 1785 BC (dv 2500)
Misidentified! in ice core Keenan, 2003
North America:
Aleutian Islands Katmai 1912 (dv 500)
Sierra - Cascade Range Mt St Helens 1500 (dv 800?)
Yellowstone Park, Lava Creek B 620,000 B.P.

LATE PLEISTOCENE TEPHRAS IN THE WESTERN U.S.

Mount Saint Helens: most frequent May 18, 1980 1-2 km3 T 1890's W (450 B.P.) beginning of Little Ice Age Y (e) (3500 B.P.) beginning of Neoglaciation S (14,360 B.P.) Pleistocene/Holocene Glacier Peak: B, G ~ Pleistocene/Holocene boundary Dusty Peak 5000–5080 14C yr B.P. (5500–5900 cal yr B.P.) Glacier Peak B 11,250 14C yr B.P. Glacier Peak G 12,750 14C yr B.P.
Sierra Nevada Inyo Crater 725 ± 60 760±60 Mono Craters 560 ± 20 1200 ± 40 2060 ± 75 3730 ± 60 7270±70 8235±105 9710±80 Mazama: most widespread Holocene ash 30-40 km3, 2 major lobes Mount Mazama 6700±100 (7428-7685 cal B.P.) Mount Mazama 6990±300 (7279-8389 cal B.P.)
LARGEST PLEISTOCENE ERUPTIONS (Sarna-Wojcicki & Davis, 1990) NAME age source km3 Mazama 7 Ka Mt. Mazama OR 35 Rockland 400 Ka Lassen Peak CA 120 Lava Creek 620 Ka Yellowstone Pk WY 1000 Bishop 740 Ka Long Valley CA 500 Tshirege 1.15 Ma Valles Caldera NM 300 Mesa Falls 1.27 Ma Yellowstone Pk WY 280 Otowi 1.47 Ma Valles Caldera NM 300 Huckleberry Ridge 1.97 Ma Yellowstone Pk WY 2500

• The Midwestern Glacial Chronology Illinoian Stage and Land Mammal Chronology relied on correlations of the "Pearlette Volcanic Ash," which was later shown to be several ashes of different ages from Yellowstone Park, WY (Fillmore et al.)
  2.02 +/-0.08 Ma Huckleberry Ridge
  1.27 +/-0.1 Ma Mesa Falls
  0.616+/-0.008 Ma Lava Creek
• Prior to 1955 (first ¹⁴C dating), H.P. Hansen correlated late Quaternary pollen record of the Pacific Northwest based on the assumption that there was just one volcanic ash, from Glacier Peak.