1. WHAT CAUSES EARTHQUAKES AND VOLCANOES Chapter 8 Section 4

2. Earthquakes  Plate movement occurs slowly so you can’t see or feel it.  The rocks jagged edges catch and pressure builds up  If the pressure is too strong then an earthquake happens

3. Earthquakes  The point underground of an earthquake is the FOCUS  The point directly above the focus on Earth’s surface is the EPICENTER  The energy of an earthquake is carried in waves  The waves can move up or down  The waves can move back and forth  The waves can roll or move in a circular fashion

4. Earthquake magnitude  The greatest damage occurs near the focus and epicenter  As the waves move out- they lose energy  Almost all the largest earthquakes have occurred in California and Alaska  The boundary between the Pacific Plate and the North American plate

5. Richter Scale  The magnitude of an earthquake is measured on the Richter scale  Each increase of 1 on the Richter scale means a 31 times more energy is released

6. Volcanoes  A volcano- an opening in the surface of one of Earth’s plates through which magma rises.  Ash cloud- tiny bits of rock spew out the opening  Crater- steep-sided depression at the top of a volcano  Caldera-crater that is at least 1.6 km wide

7. Volcanoes  Most often occur when one plate sinks beneath another  Most volcanoes occur on the ocean floor

8. Detecting an Earthquake or volcano  Seismometers- detect shaking or tremors in Earth’s crust Magma rising or Earth’s plates moving  Tiltmeter- detects changes in the slope of the land  Seismograph- records the movement or tremors at faults