Lecture 6:    Plate Tectonics and the Dynamic Earth, Part II
Focus Question: Why do the lithospheric plates move, and how do we know?
Reading:  Chapter 3, p. 73-96

This is a text version of the notes presented in class.

Feel free to email the instructor (knamjesn@kent.edu) with any
comments about the format and ease of use of these online notes.

1. On Tuesday, we considered the different activities going on in the mid-ocean ridges and in the trenches, and the convection cells in the asthenosphere that drive those activities.  Today, we will look at the kinds of motion we see along plate boundaries, and how it is we know that plate motion has occurred through time.  In brief, we will talk today about
            a. plate boundaries-3 types:
                 divergent (pulling apart)
                  convergent (collision)
                   transform faults (side to side motion)
            b. the magnetic patterns on the ocean floor
            c. the past motion of the lithospheric plates

2.  Divergent boundaries (pulling apart) is seen in the oceans at the mid-ocean ridges and occasionally on the continents.
            a.  mid-ocean ridge system, new ocean floor is being made
            b.  the East African Rift Valley, a continent splits apart

3.  Convergent boundaries (collision) is seen in the oceans at the sites of trenches.  The geologic term for these trenches & the mountains they produce is a subduction zone.

            a. When an ocean lithospheric plate collides with another ocean lithospheric plate, the result is a trench and a volcanic island arc (e.g., Japan, Bering Strait, Philippines)

            b. When an ocean lithospheric plate collides with a continental lithospheric plate, the result is a trench and a volcanic mountain chain (e.g., the Andes Mountains, the Cascades)

            c. When two continental lithospheric plates collide, no trench is formed.  Instead, an immense mountain range develops (e.g., the Himalayas)

4.  Transform faults are plate boundaries where 2 plates are sliding past one another in opposite directions.  Shallow earthquakes occur here.  We see these in the fracture zones that cross the mid-ocean ridges.  A famous transform fault is the San Andreas Fault that causes such trouble in Southern California.

5.  For centuries, people have made observations about the jigsaw puzzle appearance of the continents.  In the early 20th century, Alfred Wegener, a German meteorologist, proposed a theory known as “Continental Drift”.

a. based on distribution of fossils, plants, ore deposits, shape of continents
            b. Did not have a mechanism, a process, to explain the motion

6  Following WWII, sea floor mapping for defense purposes became a significant source of information.  When detailed maps of the sea floor were released in the early 1960’s, it was apparent that:
            a. the mid-ocean ridge system was global
            b. magnetic patterns occurred in the ocean crust (Normal (North) and Reversed (South) Polarity
            c. those patterns were symmetrically arranged around the ridge system
            d. the ocean crust was youngest on the ridge, and got progressively older away from the ridge on either side.

(This information, by the way, launched the modern understanding of plate motion, called Plate Tectonic Theory)

7. The magnetic memory in the ocean floor tracked the motion of the new sea floor as it moved away from the ridge.  By age-dating the ocean floor , it  was seen that the further away from the ridge one went, the older the rocks became.
a. because age and distance from the ridge were known, speed of the sea floor spreading could be calculated (speed = distance/time)
            b. reconstructions of the past position of the ocean basins and continents
                could be made

8.  Convection cell  motion in the asthenosphere drives lithospheric plate motion.  New ocean floor is made at the mid-ocean ridges and is destroyed in the subduction zones by way of the trenches.  The track of that motion is seen in the magnetic patterns.
        a. We can now see how the ocean basin and the position of the continents has changed through time.
        b. We can now see why we know those changes occur, because we can see new ocean floor made at the ridges, move away from the ridges, and be recycled in the trenches.
        c. Continents are not recycled back into the asthenosphere-they are not dense enough.  SO, Continents are much older than ocean basins.  If you want to look at really old fossils and really old rocks, you have to turn to the continents.

9.  Next lecture:  Deep Sea Vents and Review for Exam I
Focus Question:  What can hydrothermal vents tell us about life in the dark?
Reading:  Chapter 4, p. 97-127 (87-112), Chapter 16, pg 466-467 (pg 407-408)

Note on Preparing for an Exam Review-The important thing is that you do prepare.  Look over your notes-do you have some topics that you are in a fog about?  Ask questions during the review!  Figure that if you have a question, probably 20 other people have the same question, so go ahead and ASK!