1. Earthquakes and Volcanoes

2. Forces in the Earth’s Crust
The movement of the Earth’s plates creates enormous forces that squeeze or pull the rock in the crust
These forces are examples of stress
Can change the shape or volume of the rock making up the crust
Stress stores energy in the rock that is released when the rock changes shape or breaks

3. Types of Stress Three different kinds of stress can occur
Tension- pulls on the crust, stretching the rock so that it becomes thinner in the middle
Occurs where two plates are moving apart
Divergent boundaries
Compression- squeezes the rock until it folds or breaks
Happens when two plates push against each other
Convergent boundaries
Shearing- pushes a mass of rock in two opposite direction
Can cause rock to break and slip apart
Transform fault boundaries

4. Faults
A fault is a break in the crust where rock surfaces slip past each other
The rocks on each side of the fault can move up or down or sideways
Most faults occur at plate boundaries where the forces of plate motion push or pull the crust so much that it breaks
Three types: normal fault, reverse fault, and strike-slip

5. Changing Earth’s Surface
The forces of plate movement can change a flat plain into landforms such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus
Anticline- A fold that bends upward into an arch
Syncline- A fold that bends downward to form a valley
Folded mountains- Caused by the compression and folding of the crust over a wide area
Fault-block mountains- Forms when a block of rock moves upward between two normal faults
Plateaus- Often forms when forces within the Earth push up a large, flat block of rock

6. Earthquakes
Earthquakes- Shaking and trembling that results from the sudden movement of part of the Earth’s crust
The most common cause is faulting
When part of the crust is pushed together or pulled apart
Occur when the stress along a fault overcomes the force of friction and releases stored energy
Can also be triggered by volcanic eruptions, collapse of caverns, and meteor impacts

7. Where Earthquakes Occur
Most earthquakes occur at plate boundaries
Focus- Point beneath the Earth’s surface where the rock breaks and moves
Depth of the focus depends on where it occurs
Earthquakes at divergent boundaries are shallower than those that occur at subduction zones
Epicenter- Point on the Earth’s
surface directly above the focus

8. Seismic Waves
Seismic waves- Shock waves produces by earthquakes ( 3 types)
Primary Waves (P Waves)
Secondary Waves (S Waves)
Surface Waves (L Waves)

9. Primary Waves
Primary waves (P waves)- Push–pull seismic waves that can travel through solids, liquids, & gasses
Travels from the focus by
compressing and expanding
the material it passes through
Fastest of the earthquake waves

10. Secondary Waves
Secondary waves (S waves)- Side-to-side moving earthquake waves which can travel through solids but not liquids or gasses
Rock particles move at
right angles to the direction
of the wave
Travels through the interior
from the focus
Slower than P waves, but faster than L waves

11. Surface Waves Surface waves (L waves)- Up-and-down earthquake waves
Move along the Earth’s surface like waves travel in the ocean
Originate at the epicenter
Bend and twist the Earth’s
surface, causing most of the
damage during an earthquake

12. Locating Earthquakes Seismographs- (Instruments
used to detect and measure
seismic waves) are used to
locate earthquakes
Data about each type of seismic wave is taken from the seismograph and plotted on a time-travel graph
The epicenter is located by taking the distances from three different reporting stations and finding the point where they intersect (also called triangulation)
The depth of the focus is determined by measuring the lag time of the L waves (the longer the lag time, the deeper the focus)

13. Measuring an Earthquake
Seismographs are used to measure the strength, or magnitude, of an earthquake
Magnitude is determined by measuring the amplitude (height) of the largest wave recorded by a seismograph
Three commonly used methods of measuring earthquakes are the Mercalli scale, the Richter scale, and the moment magnitude scale

14. The Mercalli Intensity Scale
Measures the intensity of an earthquake
Based on the damage done to different types of structures
Identifies what someone might experience (see, hear, or feel) during the earthquake
Scale ranges from I to XII, where I is hardly felt and XII indicates total destruction

15. The Richter Scale
Used to describe the magnitude or strength of an earthquake
Measures the amount of energy released
Each number on the scale indicates an earthquake that is ten times stronger than the next lower number
A magnitude 5.0 earthquake is ten times stronger than a 4.0 quake
Major earthquakes have magnitudes of 7.0 or higher

16. The Moment Magnitude Scale
Measures the total energy of an earthquake, called the seismic moment
The seismic moment of an earthquake is determined based on three factors
The distance that rock slides along a fault surface after it breaks, called the fault slip
The area of the fault surface that is actually broken by the earthquake
How rigid the rocks are near the broken fault
Seismologists multiply the fault slip, fault area, and rigidity together to determine the actual seismic moment

17. Earthquake Damage
The amount of damage mostly depends on the earthquake’s magnitude and its proximity to populated areas
Other factors that determine the amount of destruction include:
Duration of the quake
Time at which the earthquake occurs
Types of buildings
Material on which structures are built (can produce liquefaction)
Fire caused by broken gas mains
Broken waterlines hampering firefighters
Tsunamis along coastal areas

18. Volcanoes Inside a volcano
Volcano- A weak spot in the Earth’s crust where molten rock and other materials reach the surface
Inside a volcano
Crater- Depression at the
summit of a volcanic cone
Magma chamber- Large
reservoir of magma below
the Earth’s crust
Pipe- Tube that connects the magma chamber to the Earth’s surface
Vent- Opening from which
volcanic material is ejected

19. Volcanism Releases Magma
Magma- Melted rock below Earth’s surface
Magma forms where temperatures are high enough to melt rock
Asthenosphere
Plate boundaries
Magma rises to the surface because it is less dense than the surrounding material
The rate at which magma flows (its viscosity) is determined primarily by its silica content and temperature

20. Two Types of Magma Felsic Magma Mafic Magma Also called granitic magma
High silica content
Viscous or thick
Slow moving
Contains a lot of water
Creates explosive volcanic eruptions
Mafic Magma
Also called basaltic magma
Low silica content
Less viscous or thin
Flows easily
Contains very little water
Produces quiet volcanic eruptions

21. Gases in Magma
Magma contains dissolved gases that are released during an eruption
Gases are primarily water vapor, carbon dioxide, and sulfur
Magmas containing higher amounts of dissolved gases produce more explosive eruptions than those with smaller amounts

22. Temperature of Magma
Magma ranges in temperature from about 1000°C to 1200°C
The hotter the magma, the easier it flows
Hotter magma is less viscous than cooler magma
Hotter magmas trap less gas
Hotter magmas are associated with quieter eruptions

23. Lava Lava- Magma that reaches the surface How lava differs from magma
Two types: AA
Pahoehoe
How lava differs from magma
Composition is slightly different
Some gases have escaped
New material is often added when the magma comes in contact with other rock
Temperature is lower

24. Volcanic Eruptions
Three factors determine the nature of a volcanic eruption
Composition of the magma
Temperature of the magma
Amount of dissolved gases
Different types of eruptions form different types of volcanoes

25. Kinds of Volcanic Eruptions
Geologists classify volcanic eruptions as quiet or explosive
Quiet Eruptions-
Low silica, low viscosity magma that flows easily
Gases bubble out gently
Lava oozes quietly from the vent
Explosive Eruptions-
High Silica, high viscosity magma that clogs the volcano’s vent
Trapped gases build up pressure until they explode
Eruption breaks lava into fragments that quickly cool and harden into pieces of different sizes
Results in a pyroclastic flow of hot gases, ash, cinders and volcanic bombs

26. Three Main Types of Volcanoes
Shield Volcanoes
Cinder Cones
Composite Volcanoes

27. Shield Volcanoes Large, gently sloping dome-shaped volcanic mountains
Made from fluid, basaltic lava (mafic magma)
Produced by quiet
eruptions
Formed at “hot spots”
Example: Mauna Loa
(Hawaiian volcano)

28. Cinder Cones Small, steep-sided volcanoes
Produced by violent, pyroclastic ejections of material from a central vent
Made of cinders and other rock particles (felsic magma)
Usually occur in groups
Found along convergent
boundaries
Example: Paricutin, Mexico

29. Composite Volcanoes Large, steep-sided, cone-shaped volcanic mountains
Built of alternating layers of rock particles (pyroclastic material) and fluid lava
Produced by very violent eruptions
Found along convergent
boundaries
Example: Mt. St. Helens

30. Where Volcanic Activity Occurs
Divergent boundaries- produces rift zone eruptions
Convergent boundary- creates subduction zone eruptions
At hot spots, in the middle of lithospheric plates- produces hot spot eruptions

31. Rift Eruptions
Occur along narrow fractures in the crust (usually along divergent boundaries)
Mid-Atlantic Ridge
East African Ridge
Magma wells up to fill the gap
as the crust splits
Eruptions are typically quiet
Magma is basaltic
Magma contains little gas

32. Subduction Boundary Eruptions
Occur at convergent boundaries where one plate is driven below another
Magma tends to be thick (viscous) and contain large amounts of dissolved gas
Eruptions are usually explosive
Form steep-sided volcanoes (cinder cones or composite)
Most volcanoes occur at subduction boundaries along the edge of the Pacific Ocean (the Ring of Fire)

33. Hot Spots
Areas of volcanic activity which occur in the middle of plates (also called intraplate volcanism)
Form volcanoes with broad, gently sloping sides (shield volcanoes)
Magma is thin and flows easily, similar to that of rift eruptions
Produces quiet eruptions
Thought to be caused by hot
plumes of magma rising from
deep within the Earth
Example: The Hawaiian Islands

34. Life Cycle of a Volcano
The terms active, dormant, and extinct are used to describe a volcano’s stage of activity
Active- A volcano that is erupting or has shown signs that it may erupt in the near future
Dormant- A volcano that is not currently active, but may become active in the near future
Extinct- A volcano that is no longer active and is unlikely to erupt again