Friday, November 25, 2016

Geology of the National Parks Through Pictures - Mojave National Preserve

My next post about the Geology of the National Parks Through Pictures is...

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website

During our tour of the deserts of the Southwest, we decided to drive through Mojave National Preserve on our way from Joshua Tree National Park to Death Valley National Park. Unfortunately the route we needed to take through the park took us across the western part of the park, where a lot of the most interesting things seemed like they were further to the east. Will have to hit this one back up sometime in the future.

 Southern entrance sign.

 A view of the Mojave Desert with some granite outcrops. These mountains are actually dubbed the Granite Mountains. Formed initially as magma bodies from the Farallon Plate subduction zone off the west coast of North America, these granitoid rocks were from a pluton (large solidified magma body) that are Jurassic and Cretaceous in age. There are multiple intrusive events that kept adding magma to the system that slowly cooled over time. These intrusive events are thought to have occurred 170 million years ago (mya), then 150 mya, then 100 mya, and finally 75 mya, towards the end of the Cretaceous.

There was a huge series of sand dunes off to the west of our travels (the Kelso Dunes) that looked pretty cool.  These dunes were created by the southeast winds blowing fine-grained residual sand from the Mojave River sink. The color of the dunes is created by the many rose quartz grains. 

 The Kelso Depot Visitors Center. This was once a major train station in the area. Love the look of the place.

 Although we had to stay towards the western part of the park, we were able to travel by the Cinder Cone Lava Beds, seen in the distance.

 Closer up view of the lava flow and one of the cinder cones. In total their are 32 cinder cones in this group that started erupting 7.6 million years ago and last erupted 10,000 years ago. Unlike the Granite Mountains above, these volcanoes were produced after the subduction of the Farallon Plate beneath North America was complete. After the plate fully subducted beneath North America, the North American plate started to expand due to the lack of pressure squeezing the plate, producing the Basin and Range region of the western US. However, that extension also thinned the crust throughout the western US producing volcanoes not only here but throughout western US including Arizona, Utah, and Idaho. 

 Adjacent to where the lava flow runs right up next to the road. The cinder cones and lava are basaltic, meaning that they have a low silica content and a high temperature when erupted. This also means that they have a low viscosity, meaning it can flow for long distances. The cinder cones themselves are made up of a rock called scoria, which is basaltic rock full of air holes. As the cinder cones erupt, they spout out the basaltic lava that quickly crystallizes, forming these small blocks of scoria known as cinders. Over time the cinder blocks pile up around the vent creating a cone.

 The edge of the lava flow, which had been cut and eroded adjacent to the road. You can see along the contact with the dirt that the lava had turned a light brown color where it fused with the soil.

A piece of fallen lava flow, where you can clearly see the part that was in contact with the soil and fused into it.


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