Britt Fiscus

Dugway Geodes 

On October tenth and eleventh I went on a field trip with the Snow College Geology FieldStudies class. One of the stops we made on the tenth was to the Dugway Geode Beds in Juab County. We drove a couple of miles into the beds to find a promising pit, then started searching and digging for geodes. It was a very successful trip and we all took home several geodes. Finding these beautiful crystal-filled rocks made me curious about how they form.

I found out that they form in two different ways. They can form in sedimentary rocks when organic matter such as a tree root, rots away and leaves a cavity in the ground. If the cavity is still preserved in the ground after the sediment becomes rock, it has the potential to become a geode. (Baggaley, 2012) To become a geode, water has to find its way into the cavity of the rock to deposit the minerals that are necessary to create crystals. After millions of years of mineral rich ground water running through tiny cracks in the rock, crystals will form in the cavity.

The other way that geodes can form is in volcanic rock. Cooling lava or magma often contains gases that are trapped in bubbles.  After the lava solidifies into a rock the process is the same as sedimentary geodes.  The geodes formed from water – in this case water that was hydrothermal (heated by the magma in the area) and so it contained a lot of dissolved silica.  This silica rich water seeped into holes left by the gases and precipitated quartz.  This is the most common type of crystal found in geodes. They were formed in volcanic rock called rhyolite. According to http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm they were formed in rhyolite that formed 6 to 8 million years ago.  Then, around 32,000 to 14,000 years ago Lake Bonneville covered western Utah. The lake’s activity eroded the rhyolite that the geodes are found in and redeposited it as lake sediment where we find them today.(Ege, UGS.gov)

References

Baggaley, Kate. "Where Do Geodes Come From?" Scienceline. 20 Nov. 2012. ​​Web. 23 Oct. 2014 http://scienceline.org/2012/11/where-do-geodes-come-from/

Ege, Carl. "Dugway Geodes - Utah Geological Survey." Dugway Geodes - Utah ​​Geological Survey. Web. 23 Oct. 2014. http://geology.utah.gov/utahgeo/rockmineral/collecting/rkhd0500.htm