Earthquakes & Volcanoes
Earthquakes• Most Earthquakes (95%) occur along PLATE
Why do they occur?
Ring of Fire• 80% of the world’s earthquakes occur in a ring
that borders the pacific ocean
• This ring is called “the Ring of Fire”
How they begin...
• An Earthquake starts at a location inside the Earth called a “focus” (foci)
Release of energy starts here...
EPICENTRE• The “epicentre” is the point on the EARTH’s
SURFACE directly above the focus.
• It is not the center where the Earthquake starts!
DEPTHS of EARTHQUAKES...CLASSIFICATION DEPTH OF FOCUS
Shallow Focus 0 – 70 km
Intermediate Focus 70 – 300 km
Deep Focus > 300 km
Most Earthquakes have focus less than 100 km deep
About 3% of quakes
SEISMIC WAVES• Seismic waves are the vibrations (waves) of
energy released by a quake!
THERE ARE THREE TYPES OF WAVESBODY
BODY WAVES (P and S)• P WAVES (P= Primary)
- Fastest (6 km/s)
- Causes ground to squeeze and stretch like a spring!
- Can travel though solids, liquids, and gases
BODY WAVES (P and S)• S WAVES (S= Secondary)
- Slower (3.5 km/s)
- Causes ground to SHAKE
- Can travel though solids (not liquids)
These waves are usually larger...do the most damage!
Travelling Through Layers
P and S Waves
S No S Waves (liq)
SURFACE WAVES (L)• L WAVES (Surface waves)
- Travel along Earth’s surface (like pond ripples)
- Causes ground to move in “rolling motion”
• “Seismometers”Devices that measure the amount of ground motion
Produces a “seismogram”
Tell us... How long it lasted, how fast waves travelled, amount of ground shaking, magnitude...
Ancient Chinese Seismometers
MAGNITUDE•Magnitude = a NUMBER that
rates the strength (energy)
With every magnitude increase of 1 on the scale.... the seismic wave is 10 x higher!
Magnitude Earthquake Effects Estimated NumberEach Year
2.5 or lessUsually not felt, but can be recorded by seismograph.
2.5 to 5.4Often felt, but only causes minor damage.
5.5 to 6.0Slight damage to buildings and other structures.
6.1 to 6.9May cause a lot of damage in very populated areas.
7.0 to 7.9Major earthquake. Serious damage.
8.0 or greaterGreat earthquake. Can totally destroy communities near the epicenter.
One every 5 to 10 years
Earthquake Magnitude Scale
7.8 magnitude (China)
VOLCANOES• We will look at 3 types of volcanoes
(1) Composite Volcanoes(2) Shield Volcanoes(3) Rift Eruptions
(1) Composite Volcanoes• Cone-shaped
volcanoes (layers build up)
• Usually found near “subduction” zones (converging plates)
• Ex: Mt St. Helen’s
Mt St Helen’s, Washington
(2) Shield Volcanoes• These are the LARGEST
volcanoes• DO NOT occur at plate
boundaries...• They form over HOT SPOTS• Usually occur in ocean basins
(weak spot in lithosphere...magma breaks through)
Shaped like a “soldier’s shield” lying down
(3) Rift Eruptions• Occur at “rifts” and “ridges”
(divergent plates)• “Fountains of lava” • Not very destructive or violent
Common along the Mid-Atlantic Ridge