Over the years I have driven past this small round valley and thought to myself what made it? My favorite theory was that a meteor had struck the earth here and left this crater. So today I decided to research it and this is what I found out.
At this location you will find Basaltic Phreatomagmatic deposits and a Maar volcanic crater that was formed about 730,000 years ago. So you say to yourself, what is a phreaking magmatic maar crater anyway. I am glad you asked. It is when a volcano forms when it comes into contact with water during it’s creation. At the time this volcano was created, Lake Idaho occupied the entire Treasure Valley. When the Grouch Drain Maar came to life, hot magma rose to the surface and came in contact with the water of the lake and violently exploded, creating the round crater you see before you. To record this find, please e-mail me the answers to the following questions about the crater.
#1 Sometime in the past, one of the walls of this crater eroded and gave way which allows water to drain out of it. Which wall of the crater is missing, (compass direction.)
#2 What color of cinder is showing along the excavation pits on the north wall of the Crater.
Here is a nice description of the geology of this area. Course Syllabus: Volcanic Geology of the Snake River Plain (idahomuseum.org)