1. Earth Structure and Plate Tectonics

2. Key Concepts
Earth’s interior is layered, and the layers are arranged by density
Continents rise above sea level because they float on a dense, deformable layer beneath them
The brittle surface of Earth is fractured into about a dozen tile like “plates”
Table 2-1 Principles of the Cell Theory

3. Key Concepts (cont’d.)
Movement of the subterranean material on which these plates float moves them relative to one another
Continents and oceans are formed and destroyed where the plates collide, flex, and sink
Compelling evidence for plate movement is recorded in magnetic fields within the ocean floor

4. The Theory of Continental Drift
Theory proposed by Alfred Wegener
Pangaea – all continents in one landmass
Fossil evidence across continents
Proposed a centrifugal force mechanism
Dismissed as a “crank”

5. Continents That Were Once Joined Formed Continuous Chains
Figure 3.3 Mountain ranges in Scandinavia, Scotland, and North America are now separated by the Atlantic
but are remarkably similar in age and composition. Fossils of the reptile Mesosaurus were found in Argentina
and Africa, but nowhere else. The seed fern Glossopteris was found in all the southern landmasses. If the
continents were once joined, as shown here, these mountain ranges and fossil bands would have formed
continuous chains.

6. 3.2 Earth’s Interior Is Layered
Density stratified, with denser material towards the center
Density: mass per unit volume
Figure 3.5 A cross section through Earth showing the internal layers

7. 3.3 The Study of Earthquakes Provides Evidence for Layering
Earthquakes generate seismic waves
Surface waves – travel along Earth’s surface
Body waves – travel through Earth
P wave – compressional wave
S wave – shear wave
Seismograph – detects and records seismic waves

8. Earthquake Wave Shadow Zones Confirmed the Presence of Earth’s Core
Predicted by Richard Oldham
S waves cannot pass through liquids
P waves are refracted at density boundaries
Data from an earthquake, a “natural experiment,” confirmed theories of Earth’s layering

9. How Earthquakes Contributed to Our Model of the Layered Earth
Figure 3.7 How earthquakes contributed to our model of the layered Earth.

10. 3.4 Earth’s Inner Structure Was Gradually Revealed
Earth’s internal layers are based on chemical composition
Crusts, mantle, and cores
Earth’s internal layers are also based on physical properties
Lithosphere
Asthenosphere
Lower mantle
Core

11. Radioactive Decay and Isostatic Equilibrium
Earth’s interior is heated by radioactive decay
Conduction
Convection
Isostatic equilibrium supports continents above sea level
Buoyancy – displacement of fluid as an object floats

12. Mountains Continental crust Low-density Mantle mountain root Transport
Erosion
Deposition
Subsidence
Uplift
Figure 3.11 Erosion and isostatic readjustment
can cause continental crust to become thinner in
mountainous regions. As mountains are eroded
over time (a–c), isostatic uplift causes their roots
to rise. (The same thing happens when a ship is
unloaded or an iceberg melts.) Further erosion
exposes rocks that were once embedded deep
within the peaks, sometimes exposing once-buried
structures like Half-Dome in Yosemite Valley (d).
Subsidence
Stepped Art
Figure 3-10 p79

13. Erosion and Isostatic Readjustment in Continental Crust
Figure 3.11 Erosion and isostatic readjustment
can cause continental crust to become thinner in
mountainous regions. As mountains are eroded
over time (a–c), isostatic uplift causes their roots
to rise. (The same thing happens when a ship is
unloaded or an iceberg melts.) Further erosion
exposes rocks that were once embedded deep
within the peaks, sometimes exposing once-buried
structures like Half-Dome in Yosemite Valley (d).

14. 3.5 The New Understanding of Earth Evolved Slowly
The age of Earth has been controversial
James Hutton – uniformitarianism
Controversy with biblical school of thought – catastrophism
Darwin and Wallace – biological evolution
Evidence from exploration has convinced most researchers that Earth is of great age

15. 3.6 Wegener’s Idea Is Transformed
Pacific Ring of Fire
Tectonic activity surrounding Pacific Ocean
Oceanic crust dated and found to be young compared to the age of Earth
Echo sounding used to reveal seafloor topography

16. 3.7 The Breakthrough: From Seafloor Spreading to Plate Tectonics
Seafloor spreading – new hypothesis
Mid-ocean ridges are spreading centers
Subduction zones – areas where oceanic crust plunges down into the mantle
Plate tectonic theory – John Tuzo Wilson
Lithospheric plates “float” on asthenosphere
Plate movement
Form at mid-ocean ridges
Pulled downward into the mantle by leading edge

17. The Tectonic System Is Powered by Heat
Figure 3.14 The tectonic system is powered by heat. Some parts of the mantle are warmer than others, and convection
currents form when warm mantle material rises and cool material falls. Above the mantle floats the cool,
rigid lithosphere, which is fragmented into plates. Plate movement is powered by gravity: The plates slide down the
ridges at the places of their formation; their dense, cool leading edges
are pulled back into the mantle. Plates may move away from
one another (along the ocean ridges), toward one
another (at subduction zones or areas of mountain
building), or past one another (as at California’s
San Andreas Fault). Smaller localized convection
currents form cylindrical plumes
that rise to the surface to form hot
spots (like the Hawai’ian Islands).
Note that the whole mantle appears
to be involved in thermal
convection currents.

18. 3.8 Plates Interact at Plate Boundaries
Divergent plate boundaries
Plates moving apart
Rift valley forms as crust is pulled/pushed apart
Figure 3.18 A model for the formation of a new plate
boundary: the breakup of Pangaea and the formation of the
Atlantic.

19. Convergent and Transform Plate Boundaries
Convergent plate boundaries
Plates coming together
Ocean to ocean convergence
Ocean to continent convergence
Continent to continent convergence
Transform plate boundaries
Plates shear laterally past one another

20. 3.9 A Summary of Plate Interactions
Divergences
Divergent oceanic crust (Mid-Atlantic)
Divergent continental crust (rift valleys)
Convergences
Oceanic-continental (South America)
Oceanic-oceanic (northern Pacific)
Continental-continental (Himalayas)

21. 3.10 The Confirmation of Plate Tectonics
The history of plate movement is captured in residual magnetic fields
Paleomagnetism – recording of Earth’s past magnetic field
Iron-bearing minerals in ocean-floor basalts align with the magnetic field and remain when rock solidifies
Magnetometer – records residual magnetism

22. The Age of the Ocean Floors
Figure 3.29 The age of the ocean floors. The colors represent an expression of seafloor spreading over the last
200 million years as revealed by paleomagnetic patterns. Note especially the relative symmetry of the Atlantic
basin in contrast with the asymmetrical Pacific, where the spreading center is located close to the eastern
margin and intersects the coast of California.

23. Plumes, Hot Spots, and Terranes Provide Evidence of Plate Tectonics
Plate movement above mantle plumes and hot spots
Mantle plumes and hot spots remain stationary under moving plates
Sediment age and distribution, oceanic ridges, and terranes
Terranes – small pieces of different crust that gets sutured onto a continent at a subduction zone

24. Terrane Formation
Figure 3.32 Terrane formation. Oceanic plateaus usually composed
of relatively low-density rock are not subducted into the trench with
the oceanic plate. Instead, they are “scraped off,” causing uplifting
and mountain building as they strike a continent (a–d). Though rare,
assemblages of subducting oceanic lithosphere can also be scraped off
(obducted) onto the edges of continents. Rich ore deposits are sometimes
found in them.

25. 3.11 Scientists Still Have Much to Learn about the Tectonic Process
Many questions remain unanswered
Why should long lines of asthenosphere be any warmer than adjacent areas?
Why do mantle plumes form? What causes a superplume? How long do they last?
How far do most plates descend?
(Recent evidence suggests that much of the material spans the entire mantle, reaching the edge of the outer core)

26. Chapter In Perspective
Earth is composed of concentric spherical layers that are density sorted
Layers are confirmed based on seismic evidence
Plate tectonics suggests Earth’s surface is made up of lithospheric plates that interact in different ways at plate boundaries

27. Some Questions from Students
What’s the potential for serious loss of life and property damage due to tectonic plate movement?
Is plate movement a new feature of Earth?
Relatively great. About 40% of the world’s largest cities lie
within 160 kilometers (100 miles) of a plate boundary. By the
year 2015, about 350 million people will be living in high-risk
areas. About 80% of those at risk live in developing nations,
where seismic safety is not a high priority in building design.
Another 210 million people are also threatened by active volcanoes;
most live along the subduction zones of the Pacific Ring
of Fire, where 75% of Earth’s 850 active volcanoes are located.
Since 1600, there have been approximately 262,000 deaths from
volcanic eruptions, about 76,000 in the last century.
No. Multiple lines of evidence suggest we may be in the middle
of the sixth or seventh major tectonic cycle since tectonic movement
began about 3 billion years ago. Megacontinents like Pangaea
appear to have formed, split, moved, and rejoined many
times since Earth’s crust solidified.