51 Earth's Changing Face-High

Earth's Changing Face Written by Celeste Fraser

Introduction "I had to delay my trip to Anchorage, Alaska. Mount Redoubt had just erupted, sending ash high up into the air. Planes were grounded. When I finally arrived, it was raining. I wiped the rain from my face and noticed my fingers were black. What is that? I wondered. I looked at the street, sidewalk, and parked cars and saw that a black film covered them, too. It was raining volcanic ash!" That story was told by a teacher who went to Alaska in 1989. Volcanoes erupt all over the world. A volcano is a place where gases, ash, and lava can spew out from deep beneath Earth's surface. Volcanoes are examples of forces that create new features on Earth. Other forces, like moving water and wind, create new features and alter existing features. Some of these changes happen quickly and violently. Others take place very slowly, over millions of years. 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 with a cracked shell. It has three layers: the eggshell that surrounds it, the white, and the yellow yolk inside. Earth has layers, too. Scientists think that Earth formed about 4.6 billion years ago. As more and more matter gathered to create our planet, the force of gravity pulled on it, and the outer layers pressed down on the lower layers with greater and greater force. This squeezing of matter produced several layers and intense heat beneath the surface. The conditions that formed Earth also cause its outer surface to constantly change.

The Crust Earth's outer layer is called the crust. It completely surrounds the planet, but unlike an eggshell, it differs greatly in thickness. The crust's thickness ranges from about 5 to 40 kilometers (3-30 mi.). The thickest crust is simply higher in elevation. This thicker crust forms the landmasses where people live. We call the largest of these landmasses continents and the smaller landmasses islands. The thinner part of the crust is lower in elevation. It forms the floor of all the oceans.

The crust is not smooth like an eggshell but instead is uneven, made of various features called landforms. The most common landforms are those that rise above the surface, such as mountains and hills. But there are hundreds of different landforms, including plains, valleys, and canyons. Look at the collection of photographs 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, and pressure increases. The layer just below the crust 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. Though both layers are made of the metals iron and nickel, they are very different. The metals in the outer core are liquid and about 2200 degrees Celsius (4000°F)— nearly as hot as the Sun's surface! The inner core is even hotter— about 5000 degrees Celsius (9000°F). It is under such great pressure that despite the extreme heat, it cannot melt. Instead, it is squeezed so intensely that it is a hot ball of solid metal.

Forces That Create Landforms Earth's 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 has lots of cracks. The large sections of crust are called plates. The plates move about as fast as your fingernails grow— two to three inches a year! To better understand how plates move, let's think about a pot of thick soup. The hottest part of the soup is at the bottom, 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. Hotter soup rises. Cooler soup sinks. The mixing of the hot and cooler soup causes it to move in a circular motion called a convection current.