While walking through the Sumpter Valley, rock lovers, and non-rock lovers alike have the wonderful opportunity to see lots of beautiful rocks strewn across the landscape. In my experience you may not find something to be beautiful until you know more about it. This may apply to anything from people to rocks. This thought stimulated my desire to share information about the beautiful rocks we see in the valley.

From previous blogs we now know about the rock called Andesite.  We could call ourselves experts on the development of this rock, or we could simply find enjoyment by the fact that we now know how it was made. However you choose to use this information is your choice.  I would like to continue our discussion about the geological history of the area by talking about the most economical impacting geological event in the area.

General Information:

Country Rock for the area is Andesite. When someone mentions country rock they do not mean rock that is from the south and has an accent with a drawl. Country rock is simply the rock that was there before any other rocks came into the area.

An Intrusion is when liquid rock (magma) pushes its way into the country rock. You can think of it like an intruder. You would consider someone coming into your house without permission, or someone that does not belong there to be an intruder. The same concept can be applied to rocks. The intruder is a type of rock that pushes its way into the country rock or rock that was there before.


Once the Andesite had made its self-nice and comfortable, an intruder came. This intruder was granite in its magma form. As granite intrudes into the country rock it starts partially melting and including the country rock in the magma body. The more minerals and chemicals the magma body includes the more enriched the magma becomes.

When people think of magma most people don’t consider the water associated with it.  There is actually a lot of water in any magma; some types of magma have more water than others.  The water in magma is not something that we would consider to be water on the surface. This water is under such pressure, heat, and is full of so much material it cannot stay in a liquid state when it reaches the surface. It may be hard to imagine, but the only thing that is keeping the water that is in magma, in its liquid state is pressure.  When a volcano erupts and all the pressure that was keeping the water in its liquid state is released, the water becomes stem instantly.

As the granite intrudes into the country rock it breaks and cracks the country rock. The water from the magma then squirts into the cracks caused by the inclusion. As the water gets further and further away from the magma it starts to cool down. As the water cools it starts to drop some of the minerals it was carrying.  One of the minerals the granitic magma body carries is quartz. Another material the granitic magma body of this area carried was gold. This is why we had veins of gold in the Sumpter Valley area.

Fun Activities:

You may have heard that chemistry is just like cooking. The same comparison could be made about geology. There are many times that I will be cooking and get excited about how something is cooking because it is exactly how geology works. If you want to spice up your cooking experience I have two activities you should try next time you have the opportunity (for children, please ask your parents’ permission before trying this yourself).

Pressurized water:          The next time you have a pot of water boiling for pasta, cover it with a lid (hopefully a clear lid). You will notice water build-up on the inside of the lid, but there will not be any steam between the lid and the water. Take the lid off and notice all the steam that escapes.  You were not able to see this steam before because it was in a liquid state. The water stays in a liquid state until you release the pressure on the container allowing it to turn to vapor.

Mineral drops:                  Take a pan of water and heat it just below boiling temperatures. Add salt, or sugar until you can’t add any more. Take the saturated water off the heat and allow it to cool. Once it has cooled to room temperature you may need to add just a pinch more of the salt, or sugar before you can notice a difference. Once the water has cooled you will notice all the salt or sugar that fell out of the water. You will not be able to mix any more of the salt or sugar into the water. This is because water can hold more material the hotter it is.

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1 Response to Intrusion

  1. Sumpter Chatter says:

    wow…..Sara….it’s too bad you are leaving soon….any more time here and we would all be much smarter people! 🙂 Thanks for all the interesting info..I’m going to ask my wife if I can try one of your experiments…I’ll let you know how it goes! By the way….the Bandits didn’t take all your gold did they? 🙂

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