Igneous Rocks

On our Igneous Rocks day, we did this igneous fudge activity. It was fun to make fudge, and it tasted good, but we found it really hard to tell the difference between the "intrusive" and the "extrusive" kinds. You could kind of detect a larger crystal size in the pan that cooled in the refrigerator, but it wasn't obvious. If I were doing it again, I would have instead re-done this igneous sugar activity that we did during our volcano unit. The results are much more stunning!  Of course, I think I also need a new candy thermometer (mine is about 30 degrees off, as far as I can tell, which doesn't make fudge-making easy).

We examined the igneous rocks from our rock collection.

We enjoyed this video on how granite is made into countertops.

We are lucky to have easily accessible examples of all three kinds of rocks in our nearby canyons, so using this guide, we spent a day driving around and looking at these examples. The igneous rock we saw was Quartz Monzonite in Little Cottonwood Canyon, which we have always known as "temple granite." Apparently, though it looks a lot like granite and the pioneers who built the Salt Lake Temple called it granite, it is not a TRUE granite. That's because true granite has over 20% quartz, while this rock at the Temple Quarry only contains about 5-20% quartz.


Little Cottonwood Canyon is a great example of a U-shaped valley, which means it was formed by a glacier.

There's an interesting little nature trail by the quarry, with signs that tell about some of the history of the area. Very beautiful.

Such a lovely Fall day!

Down in this rocky bed, it's fun to climb around and see the huge boulders of quartz monzonite!

As you can see, it has a lovely white-and-black-speckled appearance. It does look like granite. I don't know if you could tell them apart by just looking. You can tell it's an intrusive igneous rock, though,  because the mineral crystals are so large. The polished sheets of this rock that are on the outside of the Temple and the Conference Center are so pretty---the crystals gleam in the sun!

Basically this entire mountain is made up of quartz monzonite! It must have been a huge intrusion of magma which then was exposed through weathering and erosion.