Wednesday, September 30, 2009

6th Grade-FORCES SHAPING THE EARTH (Chapter 1, Section 3)

The earth has three layers. They are the core, mantle, and crust.
The earth's core is its center. It is made of hot iron mixed with other metals and rock. The core has two parts. The very center is solid. The outer layer is hot liquid metal. Around the core is the mantle. This is a layer of rock. It is 1,800 miles thick.

The mantle has two parts. The inside part is solid rock. The outside part.sometimes melts. This melted rock is called magma.When a volcano explodes, magma flows to the earth's surface. The top layer of the earth is the crust. It is thinner than the outer layers. It is about 31 to 62 miles deep. The ocean floors are part of the crust. The crust is thinner there. The crust also includes the continents. These are seven huge land areas. The crust is thicker below these land areas.

Plate tectonics is a theory about the earth. It states that the crust is not a solid shell. Instead, it is made up of plates. These plates float on the mantle's liquid rock. They often move in different directions. Oceans and continents sit on these giant plates. Millions of years ago the continents use to fit together but they moved apart. The plates are still moving, they move a few inches a year. Sometimes plates pull apart. Sometimes they push together. Two continental plates smashing together makes mountains.

A continental plate is thicker than an ocean plate. When these two kinds of plates hit, the continental plate will slide over the ocean plate. The edge of the lower plate melts. The liquid rock may erupt in a volcano. The two sliding plates may also cause the earth's crust to move suddenly. This is an earthquake. Earthquakes can destroy buildings. Earthquakes under the ocean can cause huge waves called tsuamis. These waves can flood towns next to the ocean.

Sometimes two plates do not hit head-on. They rub their sides together as they move different ways. This causes faults.These are cracks in the earth's crust. Earthquakes can happen near faults. Forces inside the earth cause volcanoes and earthquakes. These change the earth's landforms. Forces on the earth's surface keep changing these landforms. Weathering is the process of breaking rocks into smaller and smaller pieces. Huge rocks become gravel . Gravel becomes sand. Sand becomes soil.

Water and frost cause this to happen. Water drips into cracks in rocks and freezes. Ice gets bigger as it freezes. As the ice gets bigger in the crack, it splits the rock. `Chemicals and plants also cause weathering. Chemicals in dirty air mix with rain. The rain falls to the earth. The chemicals eat away the rocks. Plant seeds fall into the cracks. The plants spread their roots. In time, the roots cause huge rocks to break apart. 

Erosion is the process of wearing away or moving weathered material. Water, wind, and ice cause erosion. They carry away rocks and soil. Rain picks up sand and dirt as it runs downhill. Rivers pick up sand and soil along their banks. 
Wind also blows soil and sand to there places. Sand in the wind works like sandpaper. It hits rocks and rubs them smooth. Ice is the third cause of erosion. Glaciers are giant sheets of ice. They form high in mountains. As they move, the change the land. They carry rocks down the mountains. The rocks are like sandpaper, too. They grind everything below them as they move. In time, the weight of the ice cuts valleys at the mountains' base.


1. How many layers does the earth have? Name them.
2. What is Plate tectonics?
3. What do oceans and continents sit on?
4. What happened to the continents over the years?
5. What causes an earthquake?
6. What tsunamis?What happens when these occur?
7. What causes faults?
8. What changes the earth's landforms?
9. What is weathering?
10. Describe one thing that occurs in the weathering process?
10. What is erosion?
11. What are glaciers made up of?
12. What do they(glaciers) form?