Internal Structure of Earth and Plate Tectonics Explained

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