Read: When Earth’s Crust Moves

Earth’s outer shell is broken into enormous slabs called tectonic plates. These plates move slowly over time. At some plate boundaries, they push together, pull apart, or slide past one another. The movement can cause hazards that change the land and affect people. An earthquake happens when rocks suddenly move along a break in Earth’s crust. The movement sends waves of energy through the ground, so the surface shakes. Shaking can damage roads and buildings, and it may trigger landslides. Scientists describe an earthquake’s size using its magnitude. Larger-magnitude earthquakes generally release more energy. A volcano forms where melted rock can rise toward the surface. While it is underground, melted rock is called magma. Once it reaches the surface, it is called lava. An eruption may send lava, ash, and gases into the air. Lava can burn or bury nearby land. Ash can travel far from a volcano and cause problems for breathing, travel, or crops. A tsunami is a series of powerful ocean waves. Some begin when a sudden shift of the seafloor pushes a large amount of water. A tsunami can cross the ocean and flood coastlines when it reaches shallow water. Not every earthquake causes a tsunami. People cannot prevent these hazards, but they can reduce some of the danger. Scientists monitor earthquakes and volcanoes and share warnings when possible. Communities can practice emergency plans, follow official alerts, and build structures designed to better withstand shaking.