© Shell Education #50160—Leveled Texts for Science: Earth & Space Science

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Structure of the EarthHave you ever wondered how Earth is put together? Most people live their lives every

day without thinking about the planet under their feet. Some people do: they wonder about Earth; they investigate it, they theorize about it, and they explore it. These people are geologists, and they study Earth and how it is structured. They have discovered a number of interesting things.

For Eggsample...

Imagine Earth as a hard-boiled egg: an egg has a shell and Earth has a crust; an egg has liquid under its shell and Earth has hot magma under its crust. If Earth were an egg, it would be a 6,400-kilometer (4,000-mile) trip from its shell (the crust) down to its center!

We live on Earth’s crust, the cooled and hardened outer shell of the planet. All of the continents of Earth are a part of the crust, and so is the ocean floor.

Just like a cracked eggshell, Earth’s crust is cracked into multiple pieces. If you look at the edges of the continents, you might notice that they look like cracks on an eggshell.

Earth also has other layers beneath the crust. They are the mantle and the outer and inner cores. These layers are made of magma, or molten rock, and their temperatures range from hundreds to thousands of degrees Fahrenheit.

crust

mantle

outer core

inner core

#50160—Leveled Texts for Science: Earth & Space Science © Shell Education

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Recycling Crust

Molten magma rises to the surface through cracks in Earth’s crust; when it cools, it creates new crust. That implies that there is more crust on Earth’s surface today than there was millions of years ago. However, that couldn’t be right, so geologists had a theory to explain the phenomenon.

If Earth oozed molten magma in one place, then it must reabsorb crust somewhere else. Sure enough, studies reveal that the Atlantic Ocean floor is expanding and the Pacific Ocean floor is shrinking. New crust is made in the Atlantic Ocean, and is destroyed in the Pacific.

Geologists found that the Pacific Ocean floor dives down into deep trenches under continents. These trenches are called subduction zones. The expanding and shrinking ocean floors are an example of how Earth is really a recycler. Rocks are created and later recycled. Proof of recycling rocks comes from mapping earthquakes and volcanoes. Most of them are found near undersea ridges and subduction zones.

Comprehension QuestionDescribe how material is recycled through the different layers of the Earth.