◈

 
found an odd rhyolite in my backpack and managed to successfully identify it AND figure out what formation i got it from (mitchell mesa rhyolite from out in the chihuahua desert of west texas). it’s really cool!! you can see that the vesicles and amygdules (filled vesicles) are stretched in one direction, indicating that the lava flowed in that direction. you can also see some linear features along flow direction! the size of the amygdules is impressive—rhyolite forms from fast-cooling lava, so it’s not super common to get significant mineral growth during cooling (although the lack of euhedral crystals indicates that the quartz formed pretty quickly). the presence of vesicles and so much quartz indicates that the lava was very silica-rich, which makes it “sticky” and more explosive during eruption; the vesicles are evidence of fast-escaping gases such as co2, known as volatiles. the blue hue is due to an abundance of sodium in the amphiboles present in the rock! rhyolites tend to be pink-ish, so finding a sodic rhyolite is always fun. this has been in my backpack since january… such are the dangers of using your school backpack as a field backpack. i’ve been struggling to fit my calculator into my bag all semester and now i finally know why! (my bag is also chock full of shale crumbs from my research outcrop… all my notebooks are dirty and textbooks do not fare well in there)