Posted by: Alan Pitts | January 28, 2011

Hell’s Half Acre

After a very lengthy hiatus from science blogging and general academia,  I’m getting back into the “flow” of things with some fresh material.  And when I say fresh I mean 4,000 years old “fresh”, and some of the youngest rocks I’ve seen in the field.

Today’s post comes from Hell’s Half Acre, Idaho which I accidentally stumbled upon during my cross-country road adventure last July.   An incredible roadside find North of Pocatello at a rest stop with a paved path leading through a small section of the 222 square mile lava flow.

Here is a map of the area from the Bureau of Land Management

The larger National Natural Landmark  and Wilderness Study Area indicated on the map were not accessible from the rest area off of RT 15 so I had to settle for a quick stroll on the paved path that leads through the lava flow.

Here is an aerial view from google earth:

In this view you can see how different the volcanic landscape looks from the surrounding agriculture.

And here is another picture taken from the crew aboard the International Space Station and featured in NASA’s Earth Science Observatory Earth Science Picture of the Day on November 17 2008.

The light brown area surrounded by black rock is a Kipuka. A Hawaiian term, for opening and is a small area of the preexisting rock that was completely encapsulated by lava, but not covered.  The ground inside this opening has been converted into agricultural space and looks like a fertile little island in a sea of unwelcoming basalt.

A hellish landscape indeed, everywhere I walked I found large jagged cracks, caves and casasms.

Towards the middle left of the photo there is a bowl-shaped depression that was formed by the collapse of the thin unstable crust .

I was able to find lots of great primary structures.  Like this ropey Pahoehoe lava which forms in more fluid lavas as the outside cools much faster than the inside and creates this “ropey” characteristic.

More Pahoehoe

And this more “blocky” textured A’a lava which moves slower and leaves oddly-shapen and irregular blocky mounds.

And here is me holding a large piece that looks very columnar, and is loaded with vesicles.

And here might be all them in one. I see ropey, blocky, columnar jointing, and if you were sitting as close as I was, you would see tons of vesicles.

The lava flow at hells half acre is the easternmost and one of the youngest units in the Snake River group, which is dominated by volcanic rocks ranging from 4 million to 4 thousand years old.  The older ryholitic units are associated with the migration of the Yellowstone super-volcano hot spot, and the younger tholeiitic flood basalts like these at HHA are associated with Basin and Range extension and crustal thinning.

It should be noted that the hellishness of the place might possibly extend beyond the area  mentioned above. As not 15 miles down the road from HHA I was pulled over by a couple of hard nosed Idaho state troopers who, judging by the aggressive angle of their hat brim, weren’t at all into earth science.   After a brief and unsuccessful attempt to relate to the officers by explaining a bit about their regional geology, my research crew and I were thoroughly shook down, brow beat, accused of speeding, rock thievery and much worse. But after a lot of bad noise we were allowed to go

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Responses

  1. Nice writeup about Hell’s Half Acre, a place I’m very familiar with. In Idaho, some kipukas are used as research areas. When they become isolated by the surrounding lava, it essentially makes it impossible to get to by people or large animals. Researchers use kipukas as a proxy for a “natural” habitat of the sagebrush steppe community – a habitat that is increasingly threatened in the mountain west.

    Next time you make it to Idaho, be sure to stop off at Craters of the Moon. Lavas fresher and younger than Hell’s Half Acre. And definitely don’t take rocks from here… the National Park Service considers it “stealing from the federal government”!

    • Thanks for the feedback Tiffany. Yeah, I was really amazed by the Snake River Plain and wish I could have stayed longer. Craters of the Moon is definitely on the list for my next trip, where I will be practicing a strict “catch and release” policy on rocks.

      From the aerial views I could see how the kipukas were isolated and almost seemed “trapped” to me. I hadn’t really considered how they are also protected from the outside world by a nearly impassable natural boundary, nor was I aware that they were used as research areas for natural habitat. That’s really interesting, thanks a lot for sharing !

  2. Welcome back sobrino, a great article.

    • Thanks Uncle Ray. Feels good to be back


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