Layering of Volcanic Rocks in Yellowstone Reveals Eruption Sequence Repetition
Decades of geologic work in Yellowstone has led scientists to understanding how and when eruptions took place from the Plateau. Stratigraphic layers of volcanic rocks can be grouped into three sequences. The major ash-flow tuffs that erupted explosively at the climax of each cycle are the primary rock units of Yellowstone in areas beyond the caldera complex. The table below is a rough stratigraphy depicting the work of geologists who identified, described, and mapped the different volcanic rock deposits in the Yellowstone area and determined the order in which they erupted during each of the three cycles of activity. The units are shown in stratigraphic sequence (the youngest at the top of the table, oldest at the bottom). Table is from Christiansen (2001).
|Volcanic Cycle||Precaldera Rhyolite||Caldera-forming ash-flow tuff||Postcaldera rhyolite||Contemporaneous plateau-marginal basalts1|
|Third||Plateau Rhyolite2||Basalts of Snake River Group
Madison River Basalt
Basalt of Geode Creek
Swan Lake Flat Basalt
Basalt of Mariposa Lake
|Mount Jackson Rhyolite
Lewis Canyon Rhyolite
|Undine Falls Basalt
Basalt of Warm River
Basalt of Shotgun Valley
|Second||Island Park Rhyolite||Basalt of the Narrows|
|Big Bend Ridge Rhyolite3|
|First||Big Bend Ridge Rhyolite 3|
|Rhyolite of Snake River Butte||Junction Butte Basalt|
2 The Plateau Rhyolite comprises more than 20 individual flows ranging in age from 70,000 to 160,000 years. Flows of the contemporaneous basalts (Madison River basalt, etc.) range in age from > 110,000 to < 640,000 years ago.
3 Part of the Big Bend Ridge Rhyolite comprises postcaldera flows of the first cycle and part comprises precaldera flows of the second cycle.