We were contacted by a couple who wanted a customized geology tour of the Yosemite region. We spent one day in Yosemite looking at the Geology of Yosemite Valley and Glacier Point. The following day, we went over Tioga Pass where we visited interesting geologic sites on the Sierra’s east side. After checking out the glacial features along the Tioga Pass Road, we visited Panum Crater, the youngest of the Mono Craters, near the shore of Mono Lake. This crater was caused by an explosive eruption about 600 years ago. At the time, gas present in the magma under pressure exploded causing a circular ring of debris. The picture below of North Coulee illustrates what the crater would have looked like after the first phase of eruption.

 
 
 
 

Then the lava, now depleted of gas, slowly continued erupting, building a lava dome in the crater’s center, visible on the right side of the picture on the left.

 
 
 
 
 
 

Much of the lava cooled quickly resulting in a massive dome much of which is made up of obsidian seen in the picture on the right.
 
 
 
 
 
 

Then we went down to Mono Lake’s South Tufa Tower site where we viewed those formations that precipitated out of Mono Lake when the water level was higher where fresh water springs entered the lake from below. Since then the lake level has dropped revealing these towers.

 
 
 
 
 
 
 
 
 
 
 

That night we spent in Mammoth Lakes; the following morning we visited Convict Lake, the Hilton Creel Fault, and “The Big Pumice Cut” where the approximately 800,000 year old Sherwin Glacial Till (deposit), the shrub-cover lower left side of the picture below is covered by the pumice ash-fall from the 760,000 year old explosion from the Long Valley Caldera. The ash is in the upper and right side of the picture below.

 
 
We then continued south to the Owens River Gorge which has more recently incised itself in the 760,000 year old Bishop Tuff that resulted from the Long Valley Caldera eruption .
 
 
 
 
 
 
 
 
 
 

 
 

That afternoon, a storm came in and cut short our explorations, but by the next morning the worst of it had passed, and we had spectacular views of the mountains and other geologic features.
 
 
 
 

 
 
 
 

The “Earthquake Fault” really just an earth fissure, just east of Mammoth Lakes:

 
 
 
 
 
 
 
 

The Inyo Craters, caused by hot magma rising into a layer of groundwater that explosively turned to steam creating these craters.
 
 
 
 
 
 
 
 

Then we drove around June Lake Loop.

In Lee Vining, we found that Tioga Pass was still closed so we headed north and found our way back to the west side of the mountains on Hwy 88.

We got back to mariposa around 11:00 p.m., very tired but also very satisfied that we’d seen as much as we did considering the severity of the storm.