Columnar Joints in Welded Tuff EarthCache
Columnar Joints in Welded Tuff
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This cache is located in Fremont Indian State Park in southern Utah. Take Exit 17 from I-70 and stay on the frontage road. You can make all observations from the road, although you may want to walk a few feet off the road to examine the rocks.
Follow the frontage road to the cache coordinates. You will pass the first-class visitor center and museum. Stop here later to learn about the Fremont Indians who inhabited this area. If you have the time, you can make some short walks to observe the petroglyphs that they left for us to ponder their meaning.
Stop at the pullout nearest the coordinates and observe the columnar jointing in the cliff.
Between 30 million and 19 million years ago the area where you are standing was part of the huge Marysvale volcanic field. This rock is called the Joe Lott tuff, named after a settler who built a cabin nearby in the 1870s. The tuff was formed when hot ash erupted from Mount Belknap about 12 miles southeast of the park. Over 100 cubic miles of ash spewed from this crater. Ponder how much larger this eruption was than the well-televised 1980 eruption from Mount St. Helens which spewed a mere 0.15 cubic miles of debris and magma from its crater.
This particular type of tuff is an ash flow tuff called an ignimbrite. It formed when magmas rich in gases erupted with explosive force, sending out huge hot clouds of ash and rock which spread laterally at high speed across the landscape, filling preexisting valleys. Thick flows such as this one were compressed by their weight and the retained heat welded the ash into the compact rock before you. This flow is 200 feet thick. That means its interior was well insulated and would have cooled more slowly than a thinner flow. This allowed cooling cracks to form and shape the columnar joints in this formation. With time, erosion would have widened the joints to their present size. You may have seen similar looking columns at Devils Postpile or Devils Tower. These columns formed in basalt as opposed to tuff. But the processes of formation are the same.
To claim this cache you must answer the following questions. E-mail me the answers; do not post them. Photos are nice, but optional.
1. Walk across the road and examine the columns. How many sides do the columns have? Do all of the columns have the same number of sides?
2. Close examination of the rocks reveals the presence of flattened fragments. Why are they flat?
3. Columnar joints generally form perpendicular to the plane of the cooling surface. What conclusion can you make about the plane of the Joe Lott cooling surface?
Reference: R. Orndorff, R. Wieder & D. Futey, Geology Underfoot in Southern Utah, Mountain Press Publishing Company, 2006.
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