This month we are rockin' it as we explore Petrology, the study of rocks. Specifically, petrology is the branch of geology that studies the formation, composition, and classification of rocks. With so many different rocks that there's no agreed upon number of how many there actually are, there is a plethora of information to unearth about them. Join with us to discover why rocks truly rock.
Have you ever looked at a rock and wondered what it was made of or how it was made? If you're anything like me, the answer would be never...at least until now. But now that I've asked the question, I have found the answer! Rocks are basically minerals, broken pieces of crystals and other rocks, and possibly fossilized shells or plants, that are all clumped together. They combine to create the Earth's crust. The primary components of a rock depend on where and how the rock was formed, but something that ties them all together is that every one has one or more mineral in it.
The Building Blocks of Rocks
Petrology deals specifically with sedimentary, igneous, and metamorphic rocks; however, minerals are the building blocks of rocks. As mentioned before, minerals, along with other pieces of rocks and sometimes organic debris, will combine to make rocks. This means that rocks can have countless compositions of different minerals, rocks, and matter. A sedimentary rock in Texas will be made of different stuff than a sedimentary rock found in Florida. Minerals, on the other hand, are made of one element or a specific compound, and have the same make up no matter where they're found. For example, sulfur is a mineral and sulfur is sulfur everywhere. Its composition never changes.
Sometimes, you can easily see and identify the minerals in rocks. One type of rock it's easy to observe the crystals in is a geode. A geode is basically a rock that has a hole in it and develops crystals inside. There are only two types of geodes - igneous and sedimentary. Igneous geodes occur when air gets trapped in the magma, and as it hardens it leaves a space where crystals form. In sedimentary rocks, organic debris that gets caught in the layers can decompose, which leaves a hole. Water will flow into these spaces, bringing different minerals into the rock. Over a lot of time, those minerals develop into crystals. Quartz (above left) is a mineral that is found in geodes as well as many other sedimentary, igneous, and metamorphic rocks - it is the most abundant mineral on Earth.
Rock 'n Roll
Although rocks are made many different minerals, all rocks are broadly classified as one of three different types: sedimentary, igneous, or metamorphic. Rather than classifying rocks by what they're made of, rocks are classified by the process through which they are formed. Rocks form and transform from other pre-existing rocks through these processes, which combine to become the Rock Cycle. Just like the Water Cycle is the movement and transformation of water from one state to the next, the Rock Cycle is the cyclical movement and transformation of rocks from one type to another.
In order for each type of rock to form, the existing materials need the right conditions. Think of it like a recipe. Brownies need specific ingredients in the right amounts to bake at a certain temperature for a set amount of time. If you add some flour and sugar to your recipe, and bake it a bit longer, you've got a cake. Just as tasty as brownies, but not the same thing. Rock formation is similar (though not quite as sweet).
Let's start with sedimentary rocks. Sedimentary rocks need time and pressure to form. If you add intense heat and even more pressure, you have a metamorphic rock. Now, that metamorphic rock can either 1.) experience erosion and weathering which will eventually lead to the formation of another sedimentary rock, or 2.) melt to form magma and then cool, transforming into an igneous rock. That igneous rock then 1.) undergoes heat and pressure, resulting in a metamorphic rock, or 2.) experiences erosion and weathering to eventually form a sedimentary rock. It could also melt and cool again to form another igneous rock. And so the cycle continues!
Let's Break It Down
Sedimentary, igneous, and metamorphic rocks are all major players in the Rock Cycle, so let's break down each of these rock types a little further.
As the name suggests, sedimentary rocks are composed of sedimentation, or pieces of fragmented rocks, minerals, and organic matter, that have been weathered and eroded by water, wind, and ice. The sediment often travels to bodies of water, such as lakes and rivers. As the sediment layers up over time, the pressure builds and the layers experience increased compression which eventually leads to cementation. This results in sedimentary rocks.
Basically, all the small pieces get squished together to make these rocks. This is where you will find evidence of organic material and fossils. Sedimentary rocks are formed from pre-existing sedimentary, igneous, and metamorphic rocks. Most of the rocks on Earth, about 75%, are sedimentary rocks, and are what we find in our area. Sedimentary rocks include limestone, shale, sandstone, and flint.
Contrary to sedimentary rocks, igneous rocks need lots of heat in order to form. Already formed igneous or metamorphic rocks get so hot they melt and become magma. The magma then cools and crystallizes to form igneous rocks. After it reaches the molten stage, the magma can cool, harden, and crystallize either underground or above ground.
If the magma heats and cools inside the earth, it's an intrusive igneous rock. However, if the process takes place externally, or above ground, then the rocks are extrusive igneous rocks. Extrusive rocks are formed when magma reaches the surface (called lava) and cools quickly. Gas will often get caught in the magma during this process, which causes some rocks to have pockets or holes. Pumice, obsidian, and granite are examples of igneous rocks. Igneous rocks make up about 90% of all the rocks in the Earth's crust.
Last but certainly not least is the metamorphic rock. Igneous and sedimentary rocks which experience intense heat and pressure undergo metamorphosis to become metamorphic rocks. Unlike the other two, metamorphic rocks cannot transform back into a metamorphic rock - it becomes either sedimentary or igneous.
Metamorphic rocks are also different from igneous rocks in that even though they experience intense heat, they don't melt. However, the changes that these rocks undergo are so drastic that it can be extremely difficult, or even impossible, to tell what type of rocks they were originally. Examples of metamorphic rocks include marble, schist, and slate.
Don't Take Them For Granite
It's quite possible that rocks are the humblest of things we find in the natural world. They seem so simple, and yet humanity has used rocks in countless ways for countless purposes throughout history. Marble, a metamorphic rock, and granite, an igneous rock, are utilized as building materials, especially for elements like floors, countertops, and accents. The Taj Mahal in India (above) is actually made out of marble.
The igneous rock pumice is abrasive and often used for cleaning (or scraping dead skin off of peoples' feet - ew!). Obsidian, another igneous rock, has been used as a knife or blade due to its sharp edges (below left). Even coal, one of the most popular resources for energy production, and chalk are sedimentary rocks.
In addition, some of the most popular landmarks, whether sculpted by man or natural formations, are made of rock. Mount Rushmore? Sculpted from granite. The Badlands in South Dakota? Formed from different sedimentary rocks (photo above center). The Blarney Stone in Ireland? A chunk of carboniferous limestone. Stonehenge in England? Made of two different types of rocks - sandstone, a sedimentary rock, and bluestone, an igneous rock (photo above right).
With so many different types, sizes, formations, transformations, and uses in our day to day living, it's no wonder that scientists and collectors alike are interested in rocks. Next time you go to a park or take a hike, make sure to check out what's underneath your feet and all around you - you might be surprised about what you can find. Rocks are truly amazing and you should find out for yourself more reasons why rocks rock!
References and Resources
- “Fun Facts for Kids on Animals, Earth, History and More!” DK Find Out!, www.dkfindout.com/us/earth/rocks-and-minerals/.
- “Geodes.” Geology, geology.com/articles/geodes/.
- “Geological Landmarks.” Rock around the World, rockaroundtheworld.weebly.com/geological-landmarks.html.
- “Geololgy, Rocks and Minerals.” Geology - Rocks and Minerals, flexiblelearning.auckland.ac.nz/rocks_minerals/index.html.
- “Igneous Rocks.” National Parks Service, U.S. Department of the Interior, www.nps.gov/subjects/geology/igneous.htm.
- “Minerals.” National Parks Service, U.S. Department of the Interior, www.nps.gov/subjects/geology/minerals.htm.
- National Geographic Society. “The Rock Cycle.” National Geographic Society, 29 Oct. 2019, www.nationalgeographic.org/encyclopedia/rock-cycle/.
- “Rocks.” National Parks Service, U.S. Department of the Interior, www.nps.gov/subjects/geology/rocks.htm.
- “Rocks and Minerals.” National Parks Service, U.S. Department of the Interior, www.nps.gov/subjects/geology/rocks-and-minerals.htm.
- “Rocks and Minerals: Everyday Uses.” Rocks and Minerals: Everyday Uses | Museum of Natural and Cultural History, mnch.uoregon.edu/rocks-and-minerals-everyday-uses.
- “SEDIMENTARY AND METAMORPHIC ROCKS AND AGE DETERMINATION.” Sedimentary and Metamorphic Rocks and Age Determination, courses.missouristate.edu/EMantei/creative/glg110/sed-met_rks.html.
- Staff, Live Science. “Photos: The World's 6 Most Famous Rocks.” LiveScience, Purch, 24 Nov. 2010, www.livescience.com/29844-worlds-most-famous-rocks.html.
- “Volcanic Landforms: Intrusive Igneous.” National Parks Service, U.S. Department of the Interior, www.nps.gov/subjects/geology/intrusive-igneous-landforms.htm.