‘Solid as a rock.’ Have you heard someone say that before? Rocks have a reputation for being solid, hard, and indestructible. Rocks line river beds and jut above the landscape as mountain peaks; they are fun to collect and sometimes are very beautiful. Each rock is different – some are smooth and round, some are sharp and dangerous. They come in all colors: pink, green, orange, white, red. They are everywhere, and we take their presence for granted and assume that they are unchangeable.
But rocks are not unchangeable! Just like the water cycle, rocks undergo changes of form in a rock cycle. A metamorphic rock can become an igneous rock, or a sedimentary rock can become a metamorphic one. Unlike the water cycle, you can’t see the process happening on a day-to-day basis. Rocks change very slowly under normal conditions, but sometimes catastrophic events like a volcanic eruption or a flood can speed up the process. So what are the three types of rocks, and how do they change into each other? Keep reading to find out!
Three types of rock:
Igneous rocks are formed when hot magma (melted rock) is rapidly cooled, either by hitting underground air pockets or by flowing from the mouth of a volcano as lava. Granite, obsidian, and pumice are all common examples of igneous rocks. Pumice is a very porous rock, because when the lava cooled, pockets of air were trapped inside. Because of all those air pockets, pumice can actually float!
Sedimentary rocks are formed by layers of sediment (dirt, rock particles, etc.) being mixed and compressed together for extended periods of time. Common examples of these rocks are limestone, sandstone, and shale. Sedimentary rocks often have lots of fossils in them because plants and animals get buried in the layers of sediment and turned into stone.
Metamorphic rocks are a combination of rock types, compressed together by high pressure and high heat. They usually have a more hard, grainy texture than the other two types. Schist, slate, and gneiss (pronounced like ‘nice’) are metamorphic rocks.
These rocks change over hundreds of years in the six steps of the rock cycle:
- Weathering & Erosion. Igneous, sedimentary, and metamorphic rocks on the surface of the earth are constantly being broken down by wind and water. Wind carrying sand wears particles off rock like sandpaper. Rushing river water and crashing surf rub off all the rough edges of rocks, leaving smooth river rocks or pebbles behind. Water seeps into the cracks in mountain rocks, then freezes, causing the rocks to break open. The result of all this: large rocks are worn down to small particles. When the particles are broken off a rock and stay in the same area, it is called weathering. When the particles are carried somewhere else, it is called erosion.
- Transportation. Eroded rock particles are carried away by wind or by rain, streams, rivers, and oceans.
- Deposition. As rivers get deeper or flow into the ocean, their current slows down, and the rock particles (mixed with soil) sink and become a layer of sediment. Often the sediment builds up faster than it can be washed away, creating little islands and forcing the river to break up into many channels in a delta.
- Compaction & Cementation. As the layers of sediment stack up (above water or below), the weight and pressure compacts the bottom layers. (Try making a stack of catalogs and watch how the bottom one gets squished as you add more on top – this is the same idea as the compaction of layers of sediment.) Dissolved minerals fill in the small gaps between particles and then solidify, acting as cement. After years of compaction and cementation, the sediment turns into sedimentary rock.
- Metamorphism. Over very long periods of time, sedimentary or igneous rocks end up buried deep underground, usually because of the movement of tectonic plates. While underground, these rocks are exposed to high heat and pressure, which changes them into metamorphic rock. This tends to happen where tectonic plates come together: the pressure of the plates squish the rock that is heated from hot magma below. (Tectonic plates are large sections of the earth’s crust that move separately from each other. Their movement often results in earthquakes.)
- Rock Melting. Can you imagine ‘rock hard’ rocks melting? That’s what they do in the depths of the earth! Metamorphic rocks underground melt to become magma. When a volcano erupts, magma flows out of it. (When magma is on the earth’s surface, it is called lava.) As the lava cools it hardens and becomes igneous rock. As soon as new igneous rock is formed, the processes of weathering and erosion begin, starting the whole cycle over again!
See if you can find sedimentary, igneous and metamorphic rocks where you live. As you study them, think about how they have undergone many slow changes to become what they are. Draw a picture of the rocks you find and then draw a diagram of the whole rock cycle. Keep reading to see how you can experience the rock cycle process for yourself!
Wind and water and blowing sand can, over time, rub away the rough edges of rocks, leaving smooth stones. In nature this is called ‘weathering.’ But weathering can happen at home, too! Many people use a rock tumbler to shape and polish the rocks they collect. A rock tumbler mixes the stones with several types of grit to rub away the roughness, just like rocks ‘tumbled’ with sand in an ocean or river. The results are smooth, shiny rocks. When people tumble gemstones (usually semi-precious ones they find), they can end up with beautiful colors and can even use the polished stones to make their own jewelry.
A gem or a gemstone is any mineral that can be cut and polished for jewelry or other decoration. The most precious gems are chosen for their beauty, rarity, and durability. Semi-precious gems usually have one or two of these characteristics, but fall short in some area. Fluorite, for instance, is very beautiful but it is too soft and will scratch easily. Agate, quartz, and amethyst are other examples of semi-precious gems.
Diamonds are gemstones that are considered very precious, and for good reason. Though they are made of carbon, one of the most common elements, diamonds are generally regarded as some of the most beautiful gemstones. They are relatively rare, because much diamond is not of jewelry quality. Diamond mines usually have 1 part diamond to 40 million parts other rock, but a diamond high-quality enough to be in an engagement ring is the product of the removal and processing of 200 to 400 million times its volume of rock! The diamond’s strongest point, however, is its durability. It is the hardest substance found in nature, four times harder than the next hardest natural substance, corundum (sapphire and ruby). The grit in a regular rock tumbler wouldn’t have much effect on a diamond! It also has the highest melting point, and conducts heat five times better than the second best element, silver.
Starburst Rock Cycle
Have you ever made a rock collection? Part of the fun is gathering as many different rocks as you can find. But even though rocks come in many different colors, shapes, and sizes, they all fit into one of three categories based on how they were formed. Learn about the rock cycle, the three main types of rocks, and what happens when rocks get so hot that they melt!
What You Need:
- Starburst candy (assorted colors)
- Adult’s help
- Heat source (like a toaster oven, hot plate, blow dryer, etc.)
- Wax paper cut into a 6-8” square
- Aluminum foil cut into a 8-10” square, or a weighing boat
- Rock cycle chart
What You Do:
1. Unwrap four different-colored Starburst candies. Have an adult use the scissors to help you cut each piece of candy into 9-12 pieces.
2. Pile up the the pieces and mix them around.
3. Next, rub your palms together back and forth quickly for several seconds. Then pick up the pile of candy and push the pieces together to form a ball. Your ball should look lumpy with the candy pieces visible.
4. Next, have an adult help you use your heat source to soften the lumpy ball of candy. Once it’s slightly heated but not too hot to handle, place it in the middle of your square of wax paper and fold the paper in half over it.
5. Once candy lump is inside the wax paper, it’s time to apply pressure. Some ideas include rolling it with a rolling pin, placing a pile of heavy books on top of it, stepping on it, or whatever else you can think of! After you’ve mashed it well, remove the candy blob from the wax paper and fold it up. Then put it back in the wax paper and repeat the process of applying pressure, this time trying a new technique, if you want. When you remove the Starburst from the wax paper now, it should be soft and pliable, so you can easily roll it into a ball.
6. If using foil, fold each side up to fashion a dish and place the candy blob inside.
7. Have an adult help you use your heat source to apply enough heat to completely melt the lump of candy. You’ll know you’ve heated it sufficiently when the candy is liquified. (Do not put aluminum in a microwave!).
8. Have an adult use tongs to remove the foil container from the heat source and place it somewhere out of reach to allow it to cool.
9. Once it’s cool enough to handle, carefully peel the candy from the foil. How is it different from the candy you started with?
The three main types of rock (sedimentary, metamorphic, and igneous) are distinguished based on how they’re formed. The entire process by which rocks are formed is called the rock cycle, and like a circle, it has no beginning or end.
In this rock cycle project, we simulated the formation of sedimentary rocks by pressing the pieces of Starburst into a lump. We simulated the formation of metamorphic rocks by adding heat and pressure in steps 4 and 5. The final three steps of the project show how igneous rocks are formed.
Since the rock cycle is continuous, do you think you could use your igneous “rock” from the final steps and start the whole project over? Give it a try!
For more fun studying and experimenting with geology, check out these science projects: