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Geologic Trip, Mammoth
Area Devils Postpile The Devils Postpile is
the most spectacular and best-known example of columnar jointing in
California. The postpile is exposed on the
south side of the steep river valley formed by the Middle Fork of the San
Joaquin River, about three miles west of Mammoth Mountain. The basalt that forms the postpile
came from nearby vents on the valley floor about
100,000 years ago, during the Tahoe glacial episode.
After leaving the vents, the lava flowed down the river valley about 2.5 miles. At that point it encountered an
obstruction, perhaps a glacial moraine, and the lava ponded in
back of the obstruction until it reached a thickness of 400 feet - much
thicker than most basalt flows. The thick mass of lava then cooled very slowly and formed the columns as contraction joints while
cooling. The Middle Fork of the San Joaquin River then cut through and
removed much of the lava flow, leaving these large columns along the south
side of the valley. A short path leads
to the top of the Devils Postpile where the glacier polished the top of the
basalt columns. This photograph shows the top of the Devils
Postpile lava flow. The
tops of the basalt columns have been polished and striated by the Pleistocene
glaciers that flowed down the valley of the
Middle Fork of the San Joaquin
River. The striations indicate the direction of movement of the glacier. Exterior Websites NPS: _____ |
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Columnar
Jointing 1. As the lava cooled,
it shrank and small contraction cracks began to form. 2. After the cracks
extended about ten inches, they branched to form an angle of about 120º. This
provided the maximum stress relief. 3. After each new crack extended another ten inches, it
branched again, forming six-sided columns. |