Devils Postpile: lava that pooled, then froze into hexagons

Devils Postpile is 12 to 18 metres of vertical basalt columns, and it is one
of the finest examples of columnar basalt anywhere. Getting it required an
unusual sequence: a volcano, a natural dam, and then a glacier to cut the
result in half and show you the inside.

The lava that could not drain away

About 100,000 years ago a volcano north of Pumice Flat erupted very runny
basaltic lava into the valley of the Middle Fork of the San Joaquin River. It
ran at least 5 km downstream and pooled behind a natural dam — probably a
glacial moraine — to a depth of roughly 122 metres.

That depth is the whole point. A typical basalt flow makes short columns.
These are long because there was that much molten rock sitting still, cooling
slowly, in one place.

Why the columns are hexagons

The top and bottom of the pool chilled fast — air above, cold granite bedrock
below — and as the lava cooled inward it shrank, as almost everything does on
solidifying. Shrinking under restraint puts the rock in tension, and tension
cracks it:

1. A surface crack opens where the tension beats the strength of the lava.
2. At about 25 cm the crack branches into a Y, with roughly 120° between
   the arms — the angle that relieves the most stress.
3. Each new branch splits again at the same critical length. Together the
   cracks make an irregular polygonal net.

To get long, uniform, vertical columns the flow has to be even in thickness,
uniform in chemistry, and flat on both cooling surfaces. Where it is not, the
columns tilt or curve — and at Devils Postpile the extremely curved columns
stand right beside the rare straight ones.

Half the flow is gone. Glaciers removed it, which is why the thick middle is
exposed at all.

What the ice did, and what it left behind

The alpine glacier that came down the Middle Fork **10,000 to 12,000 years
ago** scraped away most of the lava flows here. The vertical face of the
Postpile is its cut.

The top of the pile is worth as much as the face. It shows the polygonal
pattern in cross-section — and it is polished, because fine silt carried in
the ice worked like a scouring pad across the column ends. Pebbles and
boulders in the same ice cut parallel striations and grooves into the basalt.

Two other things around the monument record the direction the ice moved:

• Roches moutonnées — rounded bedrock with one gentle slope and one steep
  one. Ice polishes the upstream face smooth and plucks jointed blocks off the
  downstream side, so the gentle slope always faces the oncoming ice.
• Glacial erratics — boulders carried in and dropped when the ice melted.
  Many around Devils Postpile are metamorphic rock that outcrops only to the
  north.

The other volcanoes in the neighbourhood

• The Buttresses — the oldest volcanic unit here, from vents west of the
  monument.
• Rainbow Falls' cliff — a rhyodacite flow.
• Long Valley Caldera tuff — dense, hard, from the catastrophic eruption
  that made a caldera 16 by 32 km. Exposed behind the old ranger cabin east of
  Reds Meadow.
• Red Cones — a 10,000-year-old basalt flow and its cinder cones, 2.4 km
  to the south-east.

Rainbow Falls, and how a waterfall walks backwards

Rainbow Falls drops 31 metres over volcanic rock, the highest fall on the
Middle Fork, about 4 km from the ranger station. On a sunny summer day there
are rainbows in the mist.

It is also eroding, in the way waterfalls do. Softer rock at the base wears
out faster than the hard layer above it — undercutting — leaving an
alcove. Eventually the unsupported upper layer collapses into boulders at the
foot of the falls, and the waterfall has moved slightly upstream.

What the rocks are

Quaternary extrusive volcanics from 2.6 million years ago to the present, over
Cretaceous intrusive igneous rock from 145.5 to 65.5 million years ago,
ranging from granodiorite to leucogranites — granites rich in potassium
feldspar and aluminium.

The monument was established in 1911 and lies south-east of Yosemite in the
Sierra Nevada, inside the
Cascade–Sierra Mountains physiographic province. No collecting of geologic material is permitted, including
for classroom use; scientific collecting needs an approved permit.

Sources

Written from the National Park Service's Geodiversity Atlas entry for Devils
Postpile National Monument, a work of the United States government in the
public domain. Lava depth, flow length and column formation from Huber and
Eckhardt (2002); the extent of glacial removal from Allen Glazner, University
of North Carolina (2009).