VOLCANIC TEPHRA STUDIES

Kinds of eruptions:
Krakatau, Java, 1883


Twentieth Century eruptions: Historic Eruption centers: Tephra used since 1940s as TIME STRATIGRAPHIC MARKERS in Western U.S.

DATING: DIRECT
DATING: BRACKETING
Records of Quaternary Tephra
IDENTIFICATION: Shard Morphology Bulk Chemistry - Mineral Inclusions
Shard Morphology Electron Microprobe Problems in Identification:
Sources of late-Quaternary tephras:

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
      50005080 14C yr B.P.
      (55005900 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


Problems with using tephra for correlations (assumes complete record of all ashes)

Volcanic Tephra Readings

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