1. Internal Structure of Earth and Plate Tectonics
Chapter 2
Internal Structure of Earth and Plate Tectonics

2. Learning Objectives
Understand the basic internal structure and processes of Earth
Know the basic ideas behind and evidence for the theory of plate tectonics
Understand the mechanisms of plate tectonics
Understand the relationship of plate tectonics to natural hazards

3. 2.1 Internal Structure of Earth
Earth is layered and dynamic
Internal structure of Earth
By composition and density
By physical properties
Figure 2.2

4. Structure of Earth Inner core Outer core Solid
1,300 km (808 mi.) in thickness
High temperature
Composed of iron (90 percent by weight) and other elements (sulfur, oxygen, and nickel)
Outer core
Liquid
2,000 km (1,243 mi.) in thickness
Composition similar to inner core
Density (10.7 g/cm3)

5. Structure of Earth Mantle Crust Solid 3,000 km (1,864 km) in thickness
Composed of iron- and magnesium-rich silicate rocks
Density 4.5 g/cm3
Crust
Outer rock layer of Earth
Density 2.8 g/cm3
Moho discontinuity
Separates lighter crustal rocks from more dense mantle

6. Lithosphere Cool, strong outermost layer of Earth Asthenosphere
Below lithosphere
Hot, slowly flowing layer of weak rock

7. Continents versus Ocean Basins
Crust is embedded on top of lithosphere
Ocean crust is less dense than continental crust
Ocean crust is also thinner
Ocean crust is young (< 200 million years old)
Continental crust is older (several billion years old)

8. Convection
Earth’s internal heat causes magma to heat up and become less dense
Less dense magma rises
Cool magma falls back downward
Similar to pan of boiling water
Figure 2.3

9. How We Know About Internal Structure of Earth
Most of our knowledge of Earth’s structure comes from seismology
Study of earthquakes
Earthquakes cause seismic energy to move through Earth
Some waves move through solid, but not liquids
Some waves are reflected
Some waves are refraction
Information on wave movement gives a picture of inside of Earth

10. Figure 2.4

11. What We Have Learned About Earth from Earthquakes
Where magma is generated in the asthenosphere
The existence of slabs of lithosphere that have apparently sunk deep into the mantle
The extreme variability of lithospheric thickness, reflecting its age and history

12. Plate Tectonics
Large-scale geologic processes that deform Earth’s lithosphere
Produce landforms such as ocean basins, continents, and mountains.
Processes are driven by forces within Earth

13. What Is Plate Tectonics?
Lithosphere is broken into pieces
Lithospheric plates
Plates move relative to one another
Plates are created and destroyed

14. Figure 2.5a

15. Location of Earthquakes and Volcanoes Define Plate Boundaries
Boundaries between lithospheric plates are geologically active areas
Plate boundaries are defined by areas of seismic activity
Earthquakes and volcanoes are associated with plate boundaries

16. Figure 2.5b

17. Seafloor Spreading Is the Mechanism for Plate Tectonics
At mid-ocean ridges new crust is added to edges of lithospheric plates
Continents are carried along plates
Crust is destroyed along other plate edges
Subduction zones
Earth remains constant, never growing or shrinking

18. Sinking Plates Generate Earthquakes
Sinking ocean plates are wet and cold
Plates come in contact with hot asthenosphere
Plates melt to generate magma
Magma rises to produce volcanoes
Earthquakes occur along the path of the descending plate

19. Figure 2.6

20. Plate Tectonics Is a Unifying Theory
Explains a variety of phenomena
Convection likely drives plate tectonics
Figure 2.8

21. Table 2.1

22. Figure 2.9

23. Rates of Plate Motion Plate motion is fast (geologically)
Plates move of few centimeters per year
Movement may not be smooth or steady
Plates can displace by several meters during a great earthquake
Figure 2.12

24. Figure 2.11

25. A Detailed Look at Seafloor Spreading
Mid-ocean ridges discovered by Harry H. Hess
Validity of seafloor spreading established by:
Identification and mapping of oceanic ridges
Dating of volcanic rocks on the floor of the ocean
Understanding and mapping of the paleomagnetic history of ocean basins

26. Paleomagnetism Earth’s magnetic field can be represented by dipole
Forces extend from North to South Poles
Caused by convection in the outer core
Magnetic field has permanently magnetized some surface rocks at the time of their formation
Iron-bearing minerals orient themselves parallel to the magnetic field at the critical temperature known as Curie Point
Paleomagnetism is the study of magnetism of such rocks

27. Magnetic Reversals
Volcanic rocks show magnetism in opposite direction as today
Earth’s magnetic field has reversed
Cause is not well known
Reversals are random
Occur on average every few thousand years

28. Figure 2.13

29. Magnetic Stripes Geologists towed magnetometers along ocean floor
Instruments that measure magnetic properties of rocks
When mapped, the ocean floor had stripes
Areas of “regular” and “irregular” magnetic fields
Stripes were parallel to oceanic ridges
Sequences of stripe width patterns matched the sequences established by geologists on land

30. Figure 2.14

31. Seafloor Age
Using the magnetic anomalies, geologists can infer ages for the ocean rocks
Seafloor is no older than 200 million years old
Spreading at the mid-ocean ridges can explain stripe patterns
Rising magma at ridge is extruded
Cooling rocks are normally magnetized
Field is reversed with new rocks that push old rocks away

32. Figure 2.15

33. Figure 2.16

34. Hot Spots
Volcanic centers resulting from hot materials from deep in the mantle
Materials move up through mantle and overlying plates
Found under both oceanic and continental crust
Plates move over hot spots creating a chain of island volcanoes
Seamounts are submarine volcanoes
Example: Hawaiian Island Chain

35. Figure 2.17

36. Plate Tectonics, Continental Shape and Mountain Ranges
Movement of plates is responsible for present shapes and locations of continents
180 million years ago there was the break-up of Pangaea
Supercontinent extending from pole to pole and halfway around Earth
Seafloor spreading 200 million years ago separated Eurasia and North America from southern continents; Eurasian from North America; southern continents from each other
50 Million years ago India crashed into China creating the Himalayas

37. Figure 2.18a, b

38. Figure 2.18c, d

39. Understanding Plate Tectonics Solves Geologic Problems
Reconstruction of Pangaea and recent continental drift clears up:
Fossil data difficult to explain with separated continents
Evidence of glaciation on several continents

40. Figure 2.19

41. Figure 2.20

42. Driving Mechanism Two possible driving mechanisms for plate tectonics
Ridge Push and slab pull
Ridge push is a gravitational push away from crest of mid-ocean ridges
Slab pull occurs when cool, dense ocean plates sinks into the hotter, less dense asthenosphere
Weight of the plate pulls the plate along
Evidence suggests that slab pull is the more important process

43. Figure 2.21

44. Plate Tectonics and Hazards
Divergent plate boundaries (Mid-Atlantic Ridge) exhibit earthquakes and volcanic eruptions
Boundaries that slide past each other (San Andreas Fault) have great earthquake hazards
Convergent plate boundaries where one plate sinks (subduction zones) are home to explosive volcanoes and earthquake hazards
Convergent plate boundaries where continents collide (Himalayas) have high topography and earthquakes

45. Internal Structure of Earth and Plate Tectonics
End
Internal Structure of Earth and Plate Tectonics
Chapter 2