56 What's Inside Planet Earth?-High

What's Inside Planet Earth? FOCUS Question What is below Earth's surface, and how do we know?

What's Down There? What might you experience if you could journey to Earth's center. You may have watched a science fiction movie or read a fiction book about taking such a voyage. Nobody has actually gone there, however, and it's likely impossible. That's why, until recently, Earth's interior remained a mystery. Scientists could only speculate about what lies deep below Earth's surface. Earth is composed of three main layers. The crust is the outermost and thinnest layer. The middle layer, which lies beneath the crust, is the mantle. The core is the innermost layer. This layer is divided into a liquid outer core and a solid inner core. But how do we know this? Let's find out how scientists solved the mystery.

Interior Sleuths Solving the mystery of Earth's interior wasn't easy for geologists— scientists who study Earth. People can't see through solid rock or dig to Earth's center. The deepest hole ever drilled only reaches about 12 kilometers (7.5 mi.). Instead, geologists use two kinds of information to study Earth's interior. They use direct evidence and indirect evidence. Observations you can make yourself are direct evidence. Scientists observe rocks and lava on Earth's surface. They study rocks from mines and holes drilled below the surface. Indirect evidence is information you use to infer an answer when you can't make observations yourself. It includes recordings of earthquake waves called seismic (SIZE-mik) waves. By studying these waves, geologists can infer what lies thousands of kilometers below the surface.

Seismic Waves During an earthquake, seismic waves move outward throughout Earth. Instruments around the world record these seismic waves. The waves include P waves, which can travel through solids and liquids, and S waves, which can travel through solids but not liquids. When seismic waves move from one material into a different material, their speed and direction change. Either their path bends or the new material blocks them. If Earth were made up of a single layer of the same material, the seismic waves would pass straight through the planet. Instead, however, seismic waves bend or stop. Using this indirect evidence, scientists inferred that Earth consists of different layers. These layers must be made up of different materials.

The Crust Earth's crust is the layer we call home. It's thousands of degrees cooler than the other layers. Geologists know more about it than the mantle or core because they can collect direct evidence. The crust is broken into pieces, called tectonic plates, which slowly move on Earth's surface. The movement and interaction of these plates create and change landforms. Plate movements create mountains, volcanoes, trenches, and mid-ocean ridges. The crust varies in thickness and is made up of different rock types. The continental crust makes up the continents and consists mostly of a type of rock called granite. The oceanic crust is under the oceans and consists mostly of basalt.

The Mantle Both P and S waves pass through the crust and mantle. Scientists infer that this likely means both layers are solid. Seismic waves also travel faster in the mantle, meaning that the mantle is denser than the crust. To learn more about the mantle (see page 5), scientists study volcanoes. During an eruption, mantle rock is forced up to the surface. This rock reveals that the mantle is made mostly of rock called peridotite (puh-RI-duh-tite). Although the mantle is mostly solid, it moves slowly over time. The tectonic plates in the crust sit on top of the mantle. Heat within the mantle moves by way of convection currents. These currents help move the plates and therefore can change landforms on Earth's surface.