Of Pillars and Postpiles

Columnar Jointing at Mount Rainier
If you live in Oregon or Washington, you should know your basalt. If you can see bare rock, odds are good you’re looking at basalt. It’s the most common rock in the Earth’s crust, and it’s extremely common around the Northwest thanks, for the most part, to a series of massive eruptions between 17 and 14 million years ago.
 

More on those eruptions some other time. What I really want to show you now is one of the features of basalt called columnar jointing. When basaltic lava is erupted, it often forms a crusty, insulating layer on the top of its flow. This insulating layer allows the lava beneath it to cool relatively slowly. The results are some of the more interesting features of basaltic formations: lava tubes, and basalt columns.

CORIBAHere’s an example of basalt columns in The Dalles, Oregon. This outcrop is right off of Highway 197 at the exit from Interstate 84. You can go see it in person, if you like.
 

The columns in this outcrop are comparatively short and stumpy when you consider some of the more grand specimens like The Devil’s Postpile or The Giant’s Causeway, but they illustrate the features really well.  The columns themselves are approximately 15 feet tall. The height of the whole outcrop is maybe 50 feet.

These columns in the rock form because as the molten rock cools, it condenses, with the molecules packing themselves into a smaller and tighter space. This process causes cracks, or joints, to form in the interior of the flow. The more time the lava is given to cool, the better an opportunity it has to form well organized joints. So that’s what you see here: a layer of lava that was insulated from the rapid cooling forces of the air, and therefore had time to form an organized pattern of joints. You can see a similar phenomenon if you ever see muddy ground that has become crusty and brittle in a drought. Often, an intricate pattern of cracks form as a result.

Not all columns form vertically. The joints form perpendicular to the cooling surface, so you will occasionally see columns that jut off in strange directions. This could be because the lava flow pressed against an old valley wall, or it could be part of the radial pattern when basalt cools in lava tubes.

A casual observer might think that the image above shows three different flows, but in fact, this is all one flow. Columns by Pretz on FlickrUsually when columns form, there are two distinct layers: The colonnade and the entablature. These get their names from the Roman architectural style shown to the left. The similarities between my photo above, and this one should be fairly obvious. The columns themselves are the colonnade and the entablature consists of the blocks that the columns are designed to uphold. In the basalt photo, the entablature is the blocky part of the flow at the very top. This part of the flow cooled much more quickly than did the colonnade, so the joints and fractures are poorly defined.

My photo comes with an added bonus, too. The base layer in the picture is of pillow basalts. These indicate that when the lava flow first advanced on this region, there was a body of water here… probably a lake. Pillow basalts form when the lava is quenched quickly in water. You can see a video of that process here.

Basalt columns are often responsible for one of my favorite things about many northwest waterfalls. Since the columns have such defined fractures, water can easily creep through. In the winter, this water freezes, expands, and helps pop the columns loose. The process is called frost wedging, and it happens frequently when waterfalls pour over an entablature and splash back on a colonnade below.

latourell FallsHere is Latourell Falls in the Columbia River Gorge. You can see the water flowing over a large, lichen-coated entablature, then free-falling the rest of the way. Remnants of the colonnade can be seen behind the falls. The cavernous playground where the columns used to be is brought to you courtesy of frost wedging.
 

This forms what we call a plunge waterfall, where the water drops vertically and away from the cliffside. There are many waterfalls like this in Oregon and Washington. If you ever get the itch, you can go play behind this one.

This post is a belated addition to the columnar jointing meme that has been in the geoblogisphere recently. If you want more, visit some of the other posts at Glacial Till, Outside the Interzone, Magma Cum Laude, GeoTripper, Reading the Washington Landscape, and Highly Allochthonous. I also recommend Erik Klemetti’s post on Eruptions Blog that shows us examples of columnar jointing in rock other than basalt.


Photo Credit: Columns by Pretz on Flickr.
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2 thoughts on “Of Pillars and Postpiles

  1. Pingback: Take a Hike: Latourell Falls | Uncovered Earth

  2. Pingback: Take a Hike: Latourell Falls | Cascadia Blog

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