50 Earth's Changing Face-Mid

Earth's Changing Face Written by Celeste Fraser

Introduction "Mount Redoubt had recently erupted. When I arrived in Anchorage, Alaska, three days later, it was raining. I wiped the rain from my face and noticed that my fingers were black. What is that? I wondered. I looked around. A black film covered everything. I realized it was raining volcanic ash!" That story was told by a teacher. Mount Redoubt is a volcano. Volcanoes are places where gases, ash, and lava can spew out from beneath Earth's surface. Volcanoes are forces that create new features on Earth. Other forces, like water and wind, change existing features. Some changes happen quickly. Others take place very slowly. In this book you will read about Earth's changing "face."

Earth's Structure To understand Earth's surface, you have to look deep inside it. Imagine a hard-boiled egg. It has three layers: the eggshell, the white, and the yellow yolk inside. Earth has layers, too. Its layers differ in thickness, temperature, and weight. The Crust Earth's outer layer is called the crust. Unlike an eggshell, its thickness varies. It ranges from about 5 to 40 kilometers (3-30 mi.) The thickest crust is simply land that is higher in elevation. These are the landmasses where people live. We call the largest of these landmasses continents and the smaller ones islands. The thinnest part of the crust is land that is lower in elevation. It forms the floor of all the oceans.

The crust is not smooth like an eggshell but instead is uneven. The different shapes of land are called landforms. Look at the collection of pictures on pages 22 and 23 to see some of Earth's many landforms. We will learn about the forces that created and shaped these landforms in the next section.

The Mantle Deeper inside Earth, temperatures get hotter. This layer is called the mantle. The rock there is in a semi-liquid, rubbery state. The mantle is very thick, making up about 80 percent of the planet. The Core Deeper still, at the very center of Earth, is the core. It consists of two layers—the outer core and the inner core. Although both layers are made of iron and nickel, they are very different. The metals in the outer core are in liquid form, with a temperature of about 2200 degrees Celsius (4000°F). That's nearly as hot as the Sun's surface! The inner core is even hotter—about 5000 degrees Celsius (9000°F). With the rest of Earth's weight pressing down on it, the inner core is squeezed very tightly. It is a ball of solid metal. Wowser! Because lava can be so hot—up to 1250°C (2000°F)—there's nothing people can do to stop it!

Forces That Create Landforms Landforms such as mountains are constantly being created, inch by inch, over millions of years. To understand how and why, we need to understand more about Earth's interior. Moving Magma Moving Crust Like a cracked eggshell, Earth's crust is cracked. The pieces of crust are called plates. The plates move about as fast as your fingernails grow—two to three inches a year! To understand why, picture a pot of thick soup. The hottest soup is at the bottom of the pot, nearest the heat source. The coolest soup is at the surface, farthest from the heat source. If you have ever watched soup boil, you know that it moves. Hot substances rise. Cool substances sink. These temperature changes cause the soup to move in circular motions called convection currents.