WHAT ARE EARTHQUAKES In an earthquake huge masses of rock move beneath Earth’s surface and cause the ground to shake Earthquakes also called temblors are happening all the time around the world Often they are too small for people to feel A powerful earthquake hit eastern Alaska in 2002 When the ground shakes roads and buildings can be damaged Most earthquakes are caused by changes in Earth’s outermost shell or crust The crust is made up of about a dozen dozen masses of rock called plates In different places plates move 4
THINK ABOUT IT Earthquakes can be frightening Why do you think people used to wonder if the earth shaking was caused by gods or monsters core 4 300 miles apart collide or slide past each other Over time movement causes pressure to increase When pressure becomes too great the rock masses suddenly shift along a crack in the crust called a fault The shifting rocks release energy in the form of shock waves These waves spread through the rock in all directions causing an earthquake In the most powerful quakes people thousands of miles away can feel the ground shake mantle 1 800 miles crust 3 19 miles 2014 Encyclopædia Britannica Inc Earth has three main layers a thin rocky crust a thick rocky mantle and a metallic core
PLATE TECTONICS The theory or idea of plate tectonics says that Earth’s outer layer is made up of large moving pieces called plates All of Earth’s land and water sit on these plates The plates are made of solid rock Under the plates is a weaker layer of partially melted rock The plates are constantly moving over this weaker layer Most plates move about 2 to 4 inches 5 to 10 centi meters per year The plates that make up Earth’s crust are different different shapes and sizes The map shows the direction of their movement 6
COMPARE AND CONTRAST In a pot of boiling vegetable soup the individual vegetables move around They rise and sink and break apart How is a pot of soup similar to and different from Earth's crust Transform Divergent PLATE MOVEMENT As the plates move they interact at their boundaries in different ways At one type of boundary the plates slide alongside each other At another another type the plates crash into each other In this case the edge of one plate may slide under another and be destroyed Or the two edges of the plates may rise up and form mountains mountains At the third type of boundary the plates move apart from each other When they do the melted rock beneath the plates begins to In plate movement plates can slide rise up This melted rock cools as past each other move away from each it rises and forms new crust other or move toward each other Convergent
