Have you ever heard the word geology? Do you know what it means? Geology is the study of the earth and what it is made of. Geologists study rocks, minerals, land, oceans, and even what's below the surface of the earth! They study how these things change over time.
Did you know rocks can change? Weather and other natural forces can wear them down into smaller or different-shaped ones. Rocks can also be completely transformed into new kinds of rocks! These changes are part of the rock cycle. In the rock cycle, rocks get melted down, morphed, or compacted into new rocks. Rocks don't go to waste in nature. They get recycled over and over again! Take a look at this interactive rock cycle animation to learn more, or check out this diagram of the rock cycle.
There are many ways rocks can change. Wind and rain weaken and wear them down. This is called erosion or weathering. Sometimes water gets into small cracks in rocks and then when it freezes, the water expands (takes up more space), making the crack larger. After freezing and thawing a few times, the expanding water that turns to ice can break rocks apart! Roots from plants growing in cracks or holes in rocks can also break them apart.
Smaller pieces of rocks can be moved around more easily by water (such as streams and rivers) or by falling away from the side of a mountain of cliff. Over a long period of time, the rocks continue to break into smaller and smaller pieces. Eventually, some rocks break down so small that they become sand or dirt. After a while, layers of sand or soil and small bits of rock are piled up on top of each other at the bottom of lakes and oceans. This is called sediment. Sediment can come from any type of rock. The weight of all the layers and the water on top of them compresses the layers at the bottom and form a new kind of rock, called sedimentary rocks. Sometimes plants and animals get trapped between the layers as they are turning to rock and leave fossils in between the layers of rock! Limestone is a common sedimentary rock that often has fossils in it.
Some rocks are formed deep below the surface of the earth and others are formed above the surface. It's very hot deep inside the earth - so hot that part of the earth's center is liquid, melted rock, called magma. Magma is just melted rock. It comes from sedimentary, metamorphic, or igneous rocks that have been pushed down far enough into the earth to get really hot and melt. When magma is pushed up to the surface of the earth through a volcano, it becomes lava. When lava cools, it becomes igneous rock.
Another kind of rock is formed when either igneous or sedimentary rocks are pushed down into the earth. Once they're in there, they get lots of heat and lots of pressure. Eventually, all the heat and pressure completely changes them into a new type of rock, called metamorphic rock. Metamorphic means "change in form" and that's exactly what happens when a rock becomes metamorphic. This process is sort of like when you bake cookies - a big change happens while the dough is in the oven, and something very different comes out at the end! Remember that sedimentary rock we talked about, called limestone? When it becomes metamorphic, it turns into marble! Metamorphic rocks are usually very hard and sometimes have a striped or banded look (because all of the pressure makes the crystals line up flat). They also tend to have a flaky or grainy texture when broken and will often break apart in sheets.
Minerals are solid substances that are formed from elements found naturally in the earth. Elements are pure and cannot be broken down into other substances. Minerals are usually formed from two or more elements joined together, but some minerals are made of just one element. An example is the diamond, which is made from the element carbon. Minerals have a crystal structure. You can learn more about crystals here. Scientists have discovered over 2000 types of minerals on earth.
Rocks are mixtures of different minerals. All rocks are made up of two or more minerals, just like minerals are made from two or more elements. Minerals are what give rocks their rough texture.
Why do minerals often have larger crystals than rocks that contain the same minerals? When a mineral grows on its own without combining with any other substance, it has more space to grow, so larger crystals can form. When minerals grow as part of a rock, particularly an igneous or sedimentary rock, the crystals don't have as much space to grow. They get crowded out by other minerals and by being compacted down by soil, water, or other causes.
These kinds of rocks can look a lot different from each other. Pumice has lots of holes and is very lightweight - in fact, it can float in water! The holes were caused by gas that was trapped in the melted rock before it cooled. Obsidian is heavier, all black in color, and very smooth. In fact, because the minerals in obsidian cooled very quickly, it doesn't have any crystals and is actually a type of glass! (The photo to the right shows lava cooling to form obsidian.) Granite is a type of igneous rock that forms below the earth's surface. It has a course, grainy texture with lots of crystals because it cooled slowly. You can find pictures and learn more about several kinds of igneous rocks here.
Sandstone, limestone, and shale are very common sedimentary rocks. You can usually see the layers of sediment that formed the rock. Sometimes there are fossils between the layers of sedimentary rocks, which are indentations left by plants or animals that got trapped as the rock was forming. Minerals in between the layers act like cement to keep the layers together. Some sedimentary rocks can be broken apart pretty easily, though. Grains of sand can be rubbed or scraped off of sandstone, for example.
Baking layered bar cookies is a fun way to show how sedimentary rocks form. (Click on "Sedimentary Rock Snacks")
Metamorphic means "change in form" and all rocks can become metamorphic at some point. They are very hard and have a "grainy" texture. Some examples are schist, slate, and gneiss (say: NICE). One example of a rock that can change into another rock is limestone (a sedimentary rock). It can change into a metamorphic rock known as marble when the conditions are right. The crystals of the minerals inside the limestone become larger when the temperature and pressure around the rock increases. Another example is carbon, which turns to diamonds under the right conditions! Some metamorphic rocks have lines or stripes on them - the mineral crystals inside the rock were under so much pressure while the rock was changing that they didn't have anywhere else to move, so they all lined up nice and flat.
For science projects related to this topic, click here.