In The Beginning

Can anyone believe the summer is just about over? It seems just yesterday I was driving into Sumpter for the first time. The season has gone so quickly, but we still have a few weeks left. As my time in Sumpter grows to an end, I would like to leave the wonderful people of this park with a small gift. While living here in Sumpter I have been referred to as the Geologist. While I am not finished with school yet, or registered I do hope to become a true geologist in the future.   As a parting gift I have decided to dedicate the last few blogs to the geology of the area.

General Information:

Plate Tectonics is the theory that the surface of the earth rides on the top of large plates. These plates move by going under one another, squishing up against each other, and spreading away from each other.  When one plate goes under another, or subducts, it causes volcanos. The volcanos are formed because the one plate going under the other one melts and causes magma, or lava, to form. Depending on what the plate is melting into and how it melts creates different types of rock.

Island Arcs are formed from subduction. Imagine two tectonic plates that are under the ocean meet together. The plates are going towards one another so one of them has to give up and subduct under the other one. When this happens, the magma comes up and forms a line of volcanos. To humans they may look like a line of random volcanos in the middle of the ocean, but they really form on the line where the two plates meet.


The parent rock, or oldest rock, of the area is called Andesite. Andesite is created from one plate subducting under another one and forming an island arc. As the plate these islands are on continues to move it gets closer and closer to the continent. Finally when it reaches the continent the poor islands on top are between a rock and a hard place. These islands stick up too far to go under the continent so they get squished up on top of it. Once these islands become part of the continent they become what is called Exotic Terranes.

The Andesite of the Sumpter Valley was created in this way. 400-100 million years ago the Andesite of the Sumpter Valley was getting squished up on the continent so we would be able to enjoy it today. If you get a chance to go hiking around Sumpter, pick up an Andesite rock and you will be able to hold a few hundred million years of history.


Graphic of Subduction Zone:
U.S. Department of the Interior | U.S. Geological Survey
Picture of Andesite:

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2 Responses to In The Beginning

  1. Sumpter Chatter says:

    thanks for the great info Sara! We have learned so much from your time spent here this Summer and we do appreciate it a lot! Just think….you have made so many people even wiser than they once were! 🙂 Good job!

  2. sumpterrocks says:

    Great post! Very interesting and informative!

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