1. Earthquakes and Volcanoes
Section 1
Earthquakes

2. Earthquakes—large vibrations that move through rock or other Earth materials
Elastic rebound—when rocks strain and then break, the broken pieces snap back.
Rocks deform slowly over long periods of time
Potential energy builds up in them
Energy is released suddenly when rocks break and move.
The movement causes vibrations that move through Earth.

3. Fault—the surface of a break in rock
Normal fault—caused by tension forces, rock above the fault moves down compared to rock below the fault
Reverse fault—caused by compression forces, rock above the fault moves upward compared to rock below the fault
Strike-slip fault—caused by shear forces, rock on either side of the fault moves past one another in opposite directions

4. Seismic waves—when strained rock’s potential energy is released, it moves outward from the fault in seismic waves
Focus—the point inside Earth where movement along a fault first occurs and energy is released
Epicenter—the point on Earth’s surface located directly above the focus

5. Seismic waves start at the focus and travel away in all directions
Primary waves—cause rock to move back and forth in the same direction the waves are moving
Secondary waves—cause rock to vibrate at right angles to the direction the waves are moving
Surface waves—slowest, largest, most destructive waves

6. Measuring earthquakes
Seismograph—instrument that records and earthquake’s vibrations
If seismic wave arrival times are recorded from three stations, the epicenter can be determined
Richter scale—measures an earthquake’s size, or magnitude, based on the heights of lines representing the amount of energy released through seismic waves recorded on a seismograph

7. Earthquake damage
Modified Mercalli intensity scale—measures an earthquake’s intensity based on the amount of structural and geologic damage
Most earthquake damage is caused by surface waves
Tsunamis—when an earthquake occurs on the ocean floor, the sudden movement pushes against the water and creates powerful waves that can travel thousands of kilometers

8. Predicting earthquakes
Seismic-safe structures are able to stand up against an earthquake’s vibrations.
Many high-rise buildings stand on huge steel and rubber springs
Underground water and gas pipes are replaced with pipes that will bend.
Highways have cement pillars with spiral reinforcing rods around them
Predicting earthquakes
Long-range forecasts predict whether an earthquake is likely to occur in a given area within 30 to 100 years.

9. Earthquakes and Volcanoes
Section 2
Volcanoes

10. Volcanoes—cone shaped hills or mountains formed by magma eruptions
When magma flows onto Earth’s surface through a vent, it is called lava
Tephra—bits of rock or solidified lava dropped from the air after an explosive eruption
Some volcanoes form where Earth’s plates collide
One plate subducts, or is forced underneath, the other
Part of the plate that is forced underneath melts, forming magma chambers

11. How forceful an eruption is depends on the composition of the magma
Avalanches of hot, glowing molten rock that flow in cushions of hot gases down a side of a volcano are called pyroclastic flows
How forceful an eruption is depends on the composition of the magma
More silica makes magma thicker and more resistant to flow
Water vapor trapped in the magma becomes steam and creates pressure
The type of lava and gases contained in the lava determine the type of volcano that forms

12. Four types of volcanoes
Shield volcanoes—basaltic lava, which flows easily
Forms a broad volcano with gently sloping sides
Largest type of volcano
Form where Earth’s plates are separating and magma is forced upward between plates
Cinder cone volcanoes—high gas content in the magma
Explosive, but short-lived, eruptions
Form a small cone of volcanic material from tephra

13. Composite volcanoes—made of alternating layers of lava and tephra
Steep-sided mountains
Form where Earth’s plates are colliding and being forced underneath each other, or subduction zones
Fissure eruptions—magma that is very fluid
Oozes from cracks in Earth’s surface
Magma flows freely across the land, as flood basalts
Most of Earth’s crust beneath the ocean is flood basalts

14. Earthquakes and Volcanoes
Section 3
Earthquakes, Volcanoes, and Plate Tectonics

15. Earth’s crust is broken into plates that move around.
Most volcanoes form where plates are colliding or moving apart.
Divergent plate boundaries—where move apart
Long cracks called rifts form between them
Fissure eruptions are common where plates separate
Convergent plate boundaries—where plates collide and denser plates subduct, or are forced underneath less dense plates
Most earthquakes occur where plates are colliding or moving apart.