The Rock Cycle - Neither music nor tranportation.

The truth is that if I hadn't gone to post-secondary in Biology it could have been in Geology - the study of rocks. I've been told on occasion that I had rocks in my head, but they couldn't have guessed that it was true. I have always been fascinated by rocks. The word "rock" hardly does justice to the concept. A rock isn't just some hard thing you stand on, skip across the water, or use as a building material. Rocks form the bulk of the Earth's crust, a layer that makes up the surface of the planet we call home. Ranging from 20 km to 120 km thick, it is comprised of over 2000 minerals that go into making it up.

So, what's the difference between rocks and minerals? A mineral is a pure substance, composed of only one kind of element or type of molecule. Silicon dioxide (made of 1 atom of silicon and 2 atoms of oxygen) is the molecule that makes the mineral quartz. Add some feldspar, biotite, and mica to that and you have basalt. Let molten basalt (a type of lava) cool for a VERY long time and you get granite. Take that granite and allow weathering and erosion work on it over many thousands of years and you end up with sand. If the sand is treated with enormous heat and pressure sandstone is produced. Apply even more heat and pressure and quartzite appears.

Those three rocks, granite, sandstone, and quartzite, represent the three different kinds of rocks we find in the Earth's crust. Granite is an igneous rock. It originates many kilometers deep underground where heat and pressure have completely liquified it. As it comes to the surface it cools. The speed at which it cools determines the size of the crystals in it and the presence of gas bubbles. You can take a dozen samples of lava from the same source and cool it in different ways and end up with a dozen different types of rock, all composed of the same minerals.

Sandstone is a type of sedimentary rock. This forms after weathering and erosion have acted on some rock containing quartz. Water and wind move those resulting tiny particles to a place where they deposit into thick mounds. Through several events taking millions of years, these grains of sand coalesce into a granular rock we call sandstone. Arizona is loaded with the stuff. It is a modestly hard rock but friable and easily broken down.

Through a process called subduction, the surface of the Earth can be forced deep beneath the crust where tremendous heat and pressure act upon it. Sandstone will turn into quartzite. It is now a metamorphic rock and is much harder than its original form. If brought to the surface this new rock can be used as countertops, floor tiles, and paving stones. Shale and slate form a similar relationship; shale (a sedimentary rock) breaks apart in layers easily and is very soft. Slate (a metamorphic rock) is much harder and difficult to break. Layers of slate are what the old original chalkboards use to be made of.

If the process of subduction continues with even more heating and compression the quartzite becomes molten. This time though there is something different from the basalt parent that started it all off. The molten rock is composed of mostly quartz and forms a thick, viscous lava. If it comes to the surface now and cools quickly obsidian is formed. Also called volcanic glass, it was a prized commodity by natives because of the sharp edges it could produce. If you ever get a chance to down to the Newberry National Monument in central Oregon you can see some of this for yourself.

The rock cycle is this amazing system allowing the minerals making up the Earth's crust to rotate through different forms over time. Mountain forming, vulcanism, plate tectonics, and all the forces of weathering and erosion work together to produce everything that makes up the surface of our planet. Somehow, the word "rock" just doesn't do all this the justice it deserves.

Thanks for reading.     Ericspix     Eric Svendsen