Figure 27.1 Comparing the amount of ocean floor produced to the time it took to produce it gives an estimate of ocean spreading or growth rate.

1. spreading rates
   • North Atlantic ==> ca. 1-2 cm/yr

   • South Atlantic ==> ca. 1.5-2.5 cm/yr

   • East Pacific ====> ca. 3-8 cm/yr

2. lithosphere ==> more than 20 plates

   
   
   Figure 27.2 Tectonic plates of the lithosphere.
   
   
   

3. plate boundaries

   
   
   Figure 27.3 Types of plate margins.
   
   
   

   • divergent

     
     
     Figure 27.4 A) Rising magma upwarps the crust, causing numerous cracks in the rigid lithosphere. B) As the crust is pulled apart, large slabs of rock sink, generating a rift zone. C) Further spreading generates a narrow sea. D) Eventually, an expansive ocean basin and ridge system are created (from The Earth - An Introduction to Physical Geology (2nd ed.), by Tarbuck & Lutgens, Merrill Publishing Co., 1984).
     
     
     

   • convergent

     
     
     Figure 27.5 Zones of plate convergence. A) Oceanic-continental. B) Oceanic-oceanic. C) Continental-continental. (from The Earth - An Introduction to Physical Geology (2nd ed.), by Tarbuck & Lutgens, Merrill Publishing Co., 1984).
     
     
     
     
     Figure 27.6 Distribution of the world's major oceanic trenches (from The Earth - An Introduction to Physical Geology (2nd ed.), by Tarbuck & Lutgens, Merrill Publishing Co., 1984).
     
     
     

   • transform

     
     
     Figure 27.7 The role of transform faults in permitting relative motion between adjacent plates. The Mendocino escarpment permits sea floor generated at the Juan de Fuca ridge to move southeastward past the Pacific plate (from The Earth - An Introduction to Physical Geology (2nd ed.), by Tarbuck & Lutgens, Merrill Publishing Co., 1984).
     
     
     
     
     
     Figure 27.8 Transform faults in mid-oceanic ridges. If the faults that were offsetting the mid-oceanic ridges were strike-slip faults, blocks would be moving in opposite directions, as shown in A. The preferred explanation, shown in B, is that the ridges are transform faults resulting from spreading from the ridge axes. Thus opposite movement (and earthquake foci) occurs only between the offset ends (from The Changing Earth - Introduction to Geology (2nd ed.), by Mears, Jr., D. Van Nostrand Co., 1977).
     
     
     

Figure 27.9 Various hypotheses have been proposed to explain the forces that drive the plate tectonic mechanism. Since none of them are universally accepted, they only pose worthwhile problems, and give a direction for ongoing and future scientific investigations (from The Changing Earth - Introduction to Geology (2nd ed.), by Mears, Jr., D. Van Nostrand Co., 1977).




Figure 27.10 Possible convection in the mantle.




Figure 27.11 The tectonic system.

Figure 27.12 Break up of Pangea from 200 Ma to present.




Figure 27.13 Earth's seismicity related to plate margins. Color codes for foci depths include white = shallow, tan = intermediate, and red = deep.




Figure 27.14 Major surface features of the Earth.