MelanieStories: City of Rocks 7/10-11/2020

"Hey, Melanie! Take us on a tour of the City of Rocks!"

I thought you'd never ask.

Cheech, resting up for the big COR adventure.

This is a long post, but it's not nearly inclusive of everything that COR has to offer. COR is so rich in geologic history, but is also archaeologically significant to the expansion of the United States.

2.5 billion years ago, a big underground magma chamber called a “pluton” was cooling off. Since this magma chamber is in the continental crust, which is mostly silica (quartz and other silicates), the magma chamber is light colored called “felsic” as opposed to a basaltic lava which is dark (more iron and magnesium), or “mafic”. This 2.5 billion year old pluton is now a metamorphic complex called the “Green Creek Complex”. The GCC is called a 'complex' because it's a complex variety of rocks within one huge formation. The GCC is is now composed of granite, gneiss (which a lot of people still call granite...grrrr...), and schists (too many bad jokes here). Before it was a 'complex' it was happily sitting there, cooling down for about 2.22 billion years, waiting for its chance in the sun, when a little heat starting to well up underneath. Next thing it knows, there’s another magma chamber down there! The magma chambers come from hot spots deep in the earth. This second magma chamber, called the “Almo Pluton” starts pushing its way into the first, melting the country rock, metamorphosing the existing granitic pluton, creating some beautiful, glittery schists and gneisses. Now we have a 2.5 billion year old pluton of metamorphosed granite sitting right on top of a 28 million year old pluton.

The 28 million year old Almo pluton granite intruded into the 2.5 billion year old Green Creek Complex, heating the granite up so much that it starts to recrystallize and the light minerals separate from the dark minerals. When rocks go under intense heat and pressure, but don't actually melt, this is called "metamorphosing" the rocks. These granite plutons are intrusive igneous rocks, meaning that they were melted magmas that cooled off underground. While these were cooling, mixing, intruding underground, there were extrusive igneous basalt flows happening above ground, on top of the existing sedimentary layers of an ancient lake bed.
As the plutons (massive magma chambers that cooled underground) were uplifting, the sedimentary layers, along with the basalt layers, eroded away. There are a few basalt remnants scattered around.

Within the granite, especially the older Green Creek complex, are many veins of beautiful white quartz and feldspars. These happen in a couple of ways. When then the rock breaks and water carrying silicon dioxide molecules run through these cracks, depositing beautiful white quartz into the cracks. Most of the 'veins' in the Green Creek Complex are called 'dikes". These pegmatitic dikes happen when the rock is solidifying, and a hotter, more liquid 'squirt' of granite is pushed into the cracks or just squirted into the mostly but not completely cooled chamber. The pegmatites are very large crystals of quartz or feldspar within a granitic matrix. The dikes can look like either straight cracks glued together with tacky glue, or like a squirt of toothpaste (migmatites...I looooove migmatites all squirty and squiggly...).

Thanks to continental expansion (ask me about Nevada, sometime, just don't dare say it's boring...) the granites and other rocks of these formations have risen to the surface to experience...erosion. Some of the features of erosion, are that the wind, water, and salt from the Great Salt Lake are constantly chipping away at these granites and the minerals in the rocks. Some of this results in windows in the granite, featured below. The red staining on the granite is from the erosion processes breaking down the minerals containing iron. The iron will, as iron does when exposed to the elements, create rust! Even though the rocks are mostly white minerals like quartz and feldspar, there are still small amounts of iron and magnesium. As the magma is cooling, the iron and magnesium molecules attach to other molecules swimming around in the hot magma to create minerals like micas (biotite, in this instance). Biotite is easily broken down by weather, releasing the elements and forming rust. The GCC has a higher concentration of iron minerals, so it appears darker and rustier.

I've probably given too much information and not enough. Let me know.

Next, are a bunch of pictures with descriptions.

Breathtaking view! Imagine being the first people to see this gorgeous valley. 200 years ago...even 150 years ago, this valley was full of grasses and food plants. The 250,000 pioneers who traveled through his area depleted the resources here, leaving it hard to live for the Native American Tribes who had called this area home, and who had lived here for thousands of years.

Cheech. He's only got one or two small hikes in him per trip. He loves it, can't resist getting out of the car, but it wears him out. This 1 mile hike was about all he wanted to do this weekend. Granted, it was the hottest day of the year.

Joel, climbing on rocks. Cheech doesn't like that Joel is climbing without him.

I didn't have my guidebook with me at the time, so I wasn't sure what I was supposed to be learning here. Therefore, I didn't get a great picture of the weathering in this shot, however, there are plenty of pictures that show weathering. I'll explain it here, though.

Weathering. Wind, water, and salt from the Great Salt Lake are constantly chipping away at the granite in place, wearing the rocks down into sand. The pebble sized sand from granite is called 'grus'. It's mostly feldspar crystals, because feldspar is easily weathered.

Contact zone between the GCC and the Almo Pluton. It's so distinct! I get so excited seeing things like this.

It's really easy to tell the two rock formations apart. The Green Creek Complex has MUCH larger crystals, and is much more weathered looking. It's not nearly as smooth as the Almo Pluton. Look at those feldspar crystals!

Windows are a result of weathering. It looks like this is GCC on top of Almo.

The line through the rock... it looks to me like a crack when the rock was being moved, and hotter, more liquid felsic (remember that word?) was pushed into the crack, sealing the rock together.