#devil's postpile

The PCT passes through the west side of Devil’s Postpile National Monument near Mammoth Mountain.  The Monument preserves this curious area of columnar basalt with its distinctive geometric shapes.  Geologically, this phenomenon occurred because this area was a lava lake some 400 feet deep that gradually cooled and solidified.  Contraction stresses developed because the cool solid form of basalt has a lesser volume than the hot liquid form. The stresses led to cracks that eventually separated the columns. The unusual depth of the lava lake resulted in the long, fascinating columns.

Too often the major attraction for hikers, especially thru hikers, in this area is Reds Meadows/Mammoth for food, drink, showers, and civilization and this amazing feature gets quickly passed by.  It is well worth lingering a while . . .

The Devil's postpile.

These columnar basalts near Mammoth Lakes in California are comparatively recent, about 100,000 years old, but are part of a long lived sequence of volcanic events resulting from subduction beneath the west coast of north America. The Sierra Nevada was uplifted and metamorphosed about 140 million years ago by the subduction, that melted the crust and formed the granites of the mountain range.

Volcanism restarted around 30 million years ago, and its remnants litter the landscape of the west of the continent as the second incarnation of the Sierras was uplifted, and their rise accelerated about 2 million years ago. The mountains were eroded by glaciers during the sequence of ice ages that began around that time and ended around 10,000 years back, creating a series of U shaped valleys. The interacting forces of volcanism, uplift and glacial erosion created the range we know today.

The Postpile began as a 120 meter deep basaltic lava lake that spewed from a vent some km northwards of the monument and it flowed down the river valley until it was dammed, probably by a glacial moraine. As it cooled and crystallised the rock shrank and cracked due to internal stress into the hexagonal forms that form this cliff. Due to its great thickness, the lake cooled slowly and evenly, resulting in the long (up to 20 metres) and symmetrical columns. The original rock formation was much taller, further glaciations have excavated the rock and planed off the top leaving striations caused by rocks embedded in the ice, while freeze thaw erosion has split off many columns, visible in the talus slope below.

It was declared a national monument in 1911after lobbying by John Muir when proposals were made to blast it in order to install a hydroelectric dam on the San Joaquin river.

Loz

Image credit: Peretz Partensky.

Devil’s Postpile

Columnar joints form by thermal contraction: solids expand when they are heated and cool when they contract. Solid rock doesn’t easily change its volume, so if a rock cools a lot, the shrinkage can cause it to fracture.

Lava flows cool a lot. They start off over a thousand degrees centigrade and cool to the Earth’s surface temperature. If the rock cools at the right rate and cannot relieve stress, it will fracture and form columns. The natural shape of those columns is hexagonal, but perfect hexagonal columns are rare since rocks don’t cool perfectly – crystals inside the lava create boundaries that fracture things into different shapes. If you’re ever walking on columnar joints, do watch for whether or not they are hexagonal – very few actually are.

Columnar joints form perpendicular to the cooling direction. A simple lava flow that is deposited on the planet’s surface will cool from the bottom and the top, forming vertical columns.

Devils Postpile is a neat set of columnar joints. It formed from a lava flow in the mountains of California during the Pleistocene, a time when the mountains grew large valley glaciers. This lava flow moved downslope until it hit something solid in the way – in this case a glacier. That block of ice caused this lava flow to cool partially from the side, so the columns here twist.

-JBB

Devil’s Postpile National Monument, California

Devil’s Postpile

Gold-hunting prospectors and restless pioneers were the first white men to make note of this 900-foot-long (274m) colonnade of basalt rising 200ft (61m) out of the eastern California forest. To them, the sight was otherworldly; they called it Devils Postpile. Early naturalists wondered at its origin, and so do visitors today as they arrive at the 798-acre preserve from U.S. 395 along the base of the Sierra Nevada.

Geologists say that this is the result of a volcanic fissure that spewed lava here perhaps 900,000 years ago. As the deep river of liquid rock cooled, it cracked, leaving splinters 2ft thick and 40 to 60ft high. The columns vary in shape, some have four sides, others as many as seven. Later, glaciers planed away the top of this freak formation and quarried out a vertical face which exposed the remaining posts.

Other columnar jointed formations have been found around the world including Giant’s Causeway in Northern Ireland, Porto Santo Island, Organ Pipes in Namibia, Heiyuhe Columnar Joints in China and many other places.

~ JM

Image Credit: Devil’s Postpile. Accessed from http://bit.ly/2etrNtM on 21/10/16