America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Location Map - Southern California National Parks and Monuments
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Volcanic Highlights and Features:
[This list is just a sample of
various Devil's Postpile volcanic features or events and is by no means inclusive.]
The Devils Postpile is a fine example of columnar basalt. Until recently, it was thought to have been formed about one million years ago. Current studies suggest that the Devils Postpile was formed less than 100,000 years ago when a cooling lava flow cracked into multisided columns.
As basalt lava erupts from volcanic vents and cools,
it shrinks and cracks. Sometimes vertical columns
form. Well developed columns result from homogeneous
lava cooling at a uniform rate. At Devils
Postpile the rock columns have from three to seven sides.
The top of the postpile shows the columns'
geometry and the polishing and scratch marks of glacial ice.
The postpile's sheer wall face is 60 feet high.
Formation of Devils Postpile began when basalt lava erupted in the valley of the
Middle Fork of the San Joaquin River. As lava flowed from the vent,
it filled the valley near the postpile
to a depth of 400 feet. Radiometric dating of rocks thought to
correspond with this basalt
(a dark gray,
fine grained rock with feldspar crystals) suggests an age of less than 100,000 years.
Surface cracks formed when tensions caused by the shrinkage of the cooling
lava were greater than the
lava's strength. Each crack branched when it reached a critical length.
Together with other cracks it
formed a pattern on the surfaces of the flow. Ideal conditions allowed
surface cracks to deepen and
form long post-like columns.
Some 10,000 years ago a glacier flowed down the Middle Fork of the San Joaquin
River and overrode the fractured mass of lava. The moving ice quarried
away one side of the postpile,
exposing a sheer wall of columns 60 feet high. Many fallen columns lie
fragmented on the talus slope
below.
Excerpts from: USGS/NPS Geology in the Parks Website, August 2001 |
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| Devils Postpile - Volcanic Rocks |
Devils Postpile Volcanic Rocks:1
| Devils Postpile National Monument |
Mammoth Pass:1
Lava that erupted from a vent near Mammoth Pass once was thought to be part of,
and indeed the source of, the Postpile basalt. This was simply because this
lava was erupted from an obviously higher elevation and cascaded into the
Middle Fork valley toward the Postpile. The Mammoth Pass rock, however,
is andesite, a rock with somewhat more silica than basalt, and resulted
from a separate volcanic event. In appearance the andesite lacks the small,
but visible, crystals of feldspar that characterize the Postpile basalt.
Pumice from Mono and Inyo Craters:1
Pumice, a frothy volcanic glass, is a porous lightweight material of
pastel shades that covers the ground at various places in the area.
Its unweathered, loosely compacted nature indicates it was deposited recently.
This pumice was formed in postglacial time when molten rock of high
silica content and abundant dissolved gasses erupted from the
Mono and Inyo Craters northeast of the Monument.
Rainbow Falls:1
At Rainbow Falls the Middle Fork of the San Joaquin River drops 101 feet over a
cliff of volcanic rock that is somewhat different from the rock making up the
Devils Postpile. Although the rock is locally massive or poorly jointed,
as that near the top of the falls, it is generally characterized by nearly horizontal,
thinly-spaced joints giving the rock a platy appearance.
This rock is the rhyodacite of Rainbow Falls.
While there is glacial polish on the top of the massive volcanic rock
and the cliff at the falls is similar in appearance to glacially
sculptured lee slopes, the cliff was not formed by ice.
Ranger Cabin East of Reds Meadow:1
A totally different type of volcanic rock that occurs near the Monument is a
pyroclastic rock or welded-tuff. This tuff formed from volcanic ash
that was hot and plastic when it fell and fused into a generally cohesive mass.
Exposures of this tuff can be seen behind the old ranger cabin east of Reds Meadow.
At this locality the tuff has columnar joints, but the columns are nearly
horizontal rather than vertical. The exact source of the volcanic ash is not known,
but it is probably related to the extensive deposits of the Bishop Tuff,
a welded tuff that erupted from the Long Valley caldera, and can be seen in
road cuts near Tom's Place on Highway 395.
Red Cones:2
The most recent volcanic event in the Devils Postpile area, the only one younger than
that which produced the Postpile flow, built the Red Cones about one and one-half miles
southeast of the Monument. The two basalt cinder cones, with well-preserved summit
craters, illustrate what the eroded pile of cinders near the Upper Soda Springs
Campground might once have looked like. A basalt lava flow that issued from the base of
the southern cone extends down Crater Creek to within one mile of the Middle Fork,
perhaps similar to the way that the Postpile flow might have issued from the ancient
cinder cone that once existed near the Upper Soda Springs Campground. The Red Cones
and their lava flow are so well preserved because they are less than 10,000 years old, and
thus escaped the powerful excavating force of glacial erosion that so drastically modified
the older volcanic rocks.
Reds Meadow:2
Deposition of the tuff near Reds Meadow about 760,000 years ago was
concurrent with extensive volcanic activity that created the Long Valley
caldera and produced the Bishop Tuff.
Soda Springs:1,2
The Soda Springs lie on a San Joaquin River gravel bar north of the postpile.
Gases driven upward from hot areas deep in
the Earth combine with groundwater to produce cold and highly
carbonated mineralized springs. Iron in
the water oxidizes on exposure to air and stains gravel a reddish brown.
These Mineral springs are further
manifestations of the recency of local volcanic
activity. A hot spring issues forth at Reds Meadow, just east of the
Monument, further attesting to the continuing volcanic activity in the area.
Surface Pumice:2
Pumice erupted from the Inyo and Mono Craters and covers the area
as a surface deposit.
The Buttresses:1,2
The oldest volcanic rock in the vicinity of the Monument is believed to
be the basalt now exposed at The Buttresses, although its exact age is
not known.
In appearance this basalt superficially resembles the Postpile basalt,
but the abundant visible crystals are mostly pyroxene rather than feldspar,
a subtle but important difference between the two rock types.
The basalt of The Buttresses was erupted from one or more vents west of the
Monument and flowed eastward into the Middle Fork valley.
Upper Soda Springs Campground and Pumice Flat:1
The Postpile lava reached the surface in the vicinity of the
Upper Soda Springs Campground at the north end of Pumice Flat.
Across the San Joaquin River from the campground, rust-colored
basalt cinders - products of explosive eruptions - rest upon
granite bedrock and are probably the remains of a volcanic cinder cone,
similar to the Red Cones southeast of the Monument.
Two basalt dikes - tabular bodies of solidified lava - can be seen
cutting upward through the cinder pile; when molten, these dikes
may have been feeder vents for lava flows. We do not know how
extensive the lava outpourings were, because most of the basalt
has been removed by combined glacial and stream erosion,
but we can piece together some minimum figures from the scattered
outcrops that remain. Lava filled the valley from side to side
for a distance of at least three miles, from Pumice Flat south
to the Devils Postpile, and in the vicinity of the Postpile the
lava was probably more than 400 feet deep.
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