1. The Earth’s Crust
© Lisa Michalek

2. Earthquakes
Any vibrating, shaking, or rapid motion of the Earth’s crust
Most occur when stress builds along a zone of weakness or a break in the rock known as a fault
When the crust shifts, energy is released
The energy radiates in all directions through vibrations

3. Earthquakes
The place underground where the break occurs is the focus of the earthquake
The epicenter is the location at the Earth’s surface just above the focus

4. Measuring Earthquakes
Magnitude (Richter Scale)
Seismographs are the most reliable measures of earthquakes
Each increase in one unit of magnitude means a ten-fold increase in shaking
Intensity (Mercalli Scale)
Based upon the reports of people who experienced the earthquake and observed the destruction

5. Mercalli vs. Richter Scale

6. Seismic Waves
P-Waves
Primary (they arrive first), Pressure, or Push-Pull
Material expands and contracts and particles move back and forth in the path of the wave.
Sound waves that travel through solids, liquids or gases.

7. Seismic Waves S-Waves Secondary (arrive later), Shear, or Side-to-side
Material shears out of shape and snaps back
Travels only through solids

8. Seismic Waves Surface Waves Travel along the earth's surface
The slowest waves but the ones that damage in large earthquakes

9. An Earthquake’s Epicenter
Seismologists
Scientists who study earthquakes
Use the difference in the speeds of P and S-Waves from three seismic recording stations to locate the epicenter

10. Earthquake Origin Time
To find the origin time, a seismologist needs to know the arrival and travel time of the P-waves

11. The Earth’s Layers The Crust Varies from 5-60 KM
In most places, a thin layer of sedimentary rocks covers the mostly granite-like rocks of the continental crust
The oceanic crust, under layers of marine sediments, is composed of darker and denser rocks similar to basalt

12. The Earth’s Layers The Mantle Extends to a depth of about 2900 KM
Earthquake waves travel faster in the mantle than they do in the crust
Composed mostly of dense, the dark mafic minerals olivine and pyroxene

13. The Earth’s Layers The Core Composed of iron and nickel
Outer Core is thought to be liquid because S-waves are unable to pass through the outer core
The Inner Core seems to be solid

14. The Earth’s Layers

15. Earthquake Shadow Zones
When a major earthquake occurs, both P-waves and S-waves are received over most of the earth
The opposite side of the earth will receive P-waves but no S-waves
S-waves cannot penetrate the liquid outer core
There is also a region where neither P-waves or S-waves are received
Refraction (bending) of the waves at the mantle-core boundary causes this ring-shaped region known as the shadow zone

16. Earthquake P-wave Shadow Zones

17. Earthquake P-wave Shadow Zones
A P-wave traveling through the outer core is labeled K
A bounce off the core is labeled c
A P-wave in the inner core is I
S-waves do not pass through the core, because the outer core is fluid

18. The “Ring of Fire”
A large number of the world’s volcanoes and seismic events occur around the edges of the Pacific Ocean
Japan, the western coast of the United States are on the Ring of Fire
These areas are damaged frequently by earthquakes and volcanoes

19. Seismic Hazards
Earthquakes can cause damage by shaking, movement of the crust, or large waves in oceans, called tsunamis
Seismic Risk Level Maps for the U.S.
Probability of damage in 100 years Blue = none green = minor yellow = moderate red = major

20. Volcanic Hazards
When volcanoes erupt they may spew hot lava, hot ash, and/or toxic gases
The lava and ash can bury cities, and the toxic fumes can suffocate people
Volcanoes can also provide fertile soil that is composed of weathered volcanic material

21. Continental Drift
In 1912, Alfred Wegener, proposed that in the distant past, Earth’s continents were all joined as a single landmass
He said that the continents have separated and collided as they have moved over Earth’s surface for millions of years

22. Continental Drift
Wegener proposed that if the land areas were brought back together, the move would line up ancient mountain ranges, similar continental rock formations and evidence of ancient glaciers
There are even similar fossils on both sides of the Atlantic that would be brought back together by the re-assembly of Pangaea

23. Continental Drift

24. Evidence from the Oceans
In the 1950’s both fossils and the analysis of radioactive material showed the age of the oceanic crust increases with distance from the mid-ocean ridges
Some oceans were growing wider from the middle
Scientists also used magnetic measurements of the oceanic crust

25. Plate Tectonics
The surface of Earth is composed of about a dozen major rigid, moving crustal plates and several smaller plates
These plates contain areas of light continental rock and dense oceanic bottoms

26. Plate Boundaries The lines along which plates meet and interact
Convergent boundaries occur from converging plates
As a result of this collision mountains rise as the crust crumples

27. Plate Boundaries
Subduction occurs when a dense oceanic plate dives beneath a lighter continental plate
Subduction forms ocean trenches that are linear fractures and are the deepest parts of the oceans

28. Plate Boundaries
When a plate slides past another plate, they meet at a transform boundary
San Andreas Fault in CA

29. Plate Boundaries
A divergent boundary (rift) is found at the mid-ocean ridges where upwelling material creates new crust that moves away from the ridge in both directions

30. What Moves the Plates?
Convection currents within Earth enable heat to escape from the Earth’s interior
These currents create and expand the ocean bottoms, and they carry the continents as “rafts” of lighter rock

31. Hot Spots
In several places on earth, hot plumes of magma pierce the crust
As a crustal plate moves over this source of magma, volcanoes form at the hot spot
This movement of a plate leads to the formation of a chain of volcanoes of differing ages
One example is the Hawaiian Islands