Become a backyard geologist—in one week

Earth is teeming with millions of different plant and animal species but contains just three categories of rock. 

To learn what makes these three rock categories distinct, look to plate tectonics.  

Earth’s crust is broken into shifting plates, which move at mere millimeters a year above the hot mantle, causing earthquakes (most of which are too small to be felt). As tectonic plates collide, sink, or pull apart, they create rocks in three main ways: when magma cools, when intense heat and pressure transform existing rock, and when erosion breaks down rock into sediment that later hardens into new stone. 

In the following days, we will examine the three different types of rocks and how they are formed, what they tell us about the earth, and where you can find them. We’ll also take a closer look at some of the specific rocks from each category, so you know what to look for outside. And on Friday, we’ll take a special look at fossils.  

Let’s first dig into how rocks form.  

In places like Death Valley National Park, all three types of rock exist together. In other regions, like the Pacific Northwest’s Columbia Plateau, you can walk for days in a single direction and find little more than igneous rocks. 

The average person wandering about their yard or local hills is most likely to encounter sedimentary rocks, which cover about 75 percent of the Earth’s exposed land surface. But farther below the surface, some 90 to 95 percent of the top 10 miles of Earth’s crust is composed of igneous and metamorphic rocks.  

Want to experience more wonder? National Geographic’s Wonder List features playful prompts and activities that turn everyday moments into wonder-filled discoveries—for families, anywhere, every day.