Global Plate Tectonics

1. Global Plate Tectonics • The Theory of Plate Tectonics was a revolutionary theory formulated in the 1960s to explain continental drift and the geologic history of the Earth • Basic to the theory is the fact that the Earth’s diameter has remained stable for hundreds of millions of years and the idea that the Earth’s surface is divided into plates

2. Global Plate Tectonics • Plate Tectonics = Sea Floor Spreading + Continental drift • These two concepts came from separate sets of observations • The Plate Tectonic theory: • combined these ideas • answered “all” our questions • provided incredible predictive POWER!

3. Plate Tectonics: The Unifying Theory • Oceans and continents are geologically and morphologically distinct • These geologic differences between rocks are responsible for the differences between continents and ocean basins. Specifically: • The composition of the crust underlying the continents and the ocean • The thickness of the crust underlying the continents and the ocean

4. Geologic Differences Between Continents and Ocean Basins (Cont.) • Continental crust consists of granite • Igneous in origin • Light in color • Mostly oxygen, aluminum, silicon • Oceanic crust consists of basalt • Igneous in origin • Dark in color • Rich in oxygen, silicon, and magnesium • More dense than granite

5. Geologic Differences Between Continents and Ocean Basins (Cont.) • Essentially all seafloor is composed of volcanic basalt • Continents are composed of a variety of rocks • The density of granite • is 2.7 to 2.8 g/cm3 • The density of basalt • is 2.9 g/cm3

6. Plate Tectonics Introduction • But what causes and maintains these differences? • The Plate Tectonic Theory has unified much of geology • not just marine geology; all geology • allows us to understand, explain and predict • Is a new theory: • proposed a few decades ago • generally accepted in the 70’s • We’ll consider continental drift, seafloor spreading, and plate tectonics, and then apply them to understanding the earth’s morphology and dynamics

7. Continental drift • The theory of continental drift was proposed in 1915 by Alfred Wegener to explain the earth’s history. • The hypothesis was based on geologic and paleontological (fossil) evidence. • According to the theory a single continent, Pangaea existed more than 200 million years ago.

8. Continental Drift (Cont.) • Pangaea is the name given to the large landmass • It broke apart 100 to 150 million years ago. • The fragments became the continents. • Wegener’s theory did not consider how the continents drifted apart. • This was his downfall • he envisioned continents plowing through seafloor

9. Pangaea

10. Continental drift (cont.) • There are several lines of evidence for continental drift, including: • 1: “map fit” • when you reposition the continents on a globe the all fit together • This was done by Sir Edward Bullard • the fit is stunning! • LOTS of strange theories to explain this prior to PT

11. Continental Drift • Later map fits were better • This one uses the 1,000m isobath, which is a good metric for where the continent ends. • Note that we aren’t talking about Plate Boundaries (yet).

12. Continental Drift (cont.) • Wegener’s bold new hypothesis was ignored by most for more than 50 years. • Many objected to the idea that granitic crust could plow into basaltic crust. • To understand the concept of continental drift, we can look at the motion of the continents over time in a movie • Drift 1 • Drift 2

13. Continental drift (cont.) • 2: glacial scarring: • glaciers scratch the rocks • these patterns “match” on opposing continents • must have happened before breakup of original continent

14. Glacial Scarring is pretty easy to spot where it has been preserved.

15. Continental Drift • When you look at specific trends, it is clear that the continents were connected during the glaciations • This map was made by Wegener in 1915! • No wonder he believed in CD!

16. Continental drift (cont.) • 3: geologic evidence • Rock type • age • mineralogy • tectonic trends • faults • failed rifts

17. Continental Drift Rock Ages

18. Continental drift (cont.) • 4: Fossils • identical fossils found on continents which are now separate • must have been connected at come point in the distant past: • The “land bridge” hypothesis • can determine age of separation by dating fossils

19. Continental drift (cont.) • 4: Fossils • These fossils were key • they existed on 4 continents which are currently widely separated • indicated a previous link AND changes in climate and / or position

20. Continental Drift • 5) Polar “Wandering” • using either climatologic or magnetic evidence, you can show that RELATIVE positions of poles have changed • part of this is due to real motion of poles but results from different continents disagree, so... • most of it is due to motion (rotation) of continents relative to magnetic poles • If you backtrack continents to their former positions: • the discrepancies go away • you an plot the actual position of the poles through time

21. Sea Floor Spreading • This is a separate body of evidence which was later combined with continental drift to produce the unified concept of Plate tectonics • includes creation and destruction of sea floor • didn't really involve the concept of plates and didn't connect to continental motion.

22. Seafloor Spreading • Vening Meinesz found large negative isostatic gravity anomalies at trenches, and invoked convection as an explanation for such the cause of such downwarps or tectogenes. • Harry Hess, a Naval officer (1962), and Robert Dietz (1961) finally put it all together when sufficient marine geological and geophysical data were gathered. • perfected ideas relating to the transport of crust by convective currents, proposed that new crust is generated at the Mid Ocean Ridges (MOR's).

23. Seafloor Spreading • Evidence: 1) Topography • Ewing and Heezen at Lamont Doherty Geological (now “Earth”) Observatory discovered the continuity of the MOR's' central position. • Deepest “should” be in middle but wasn’t • Trenches at very edges

24. Sea-Floor Spreading (Cont.) • The midocean ridge was discovered in the mid-20th century from soundings of the deep sea-floor • It forms a continuous mountain range girdling the earth • ~60,000 km long • ~3 to 4 km high • Geologically active • Volcanism, • earthquakes, • faulting

25. Sea-Floor Spreading (Cont.) • Although ridge systems resemble mountain chains they are formed in different ways. • Mountain chains form by compression • Ridge systems form by tension

26. Sedimentary RocksSqueezed By Compression • This happens any place the crust is squeezed • the pressure (compression) wrinkles the rocks • the Appalachian Mountains are a good example

27. Sea-Floor Spreading (Cont.) • Ridge systems form by tension • as the crust moves away, new lava fills the gap

28. Seafloor Spreading • 2) Age of deepsea sediments and rocks: • Oldest deep ocean crust is 170 my old • Age of basalt crust older proceeding away from MOR crest. • Derived mostly from sediment data

29. Seafloor Spreading • 3) Nature of ocean crust rocks: • Essentially all seafloor is composed of volcanic basalt • Continents are composed of a variety of rocks • The density of granite • is 2.7 to 2.8 g/cm3 • The density of basalt • is 2.9 g/cm3

30. Seafloor Spreading • 4) Paleomagnetism • Vacquier, Mason, Raff discovered LMA's in NE Pacific in late 1950's (Scripps) TRANS • originally thought to result from alternating bands of sediments and volcanic rocks. • Origin explained by Vine and Matthews of Cambridge in 1963 in terms of seafloor spreading theory. • Symmetry and parallelism to MOR crest. • LMA's mapped over world ocean by Heirtzler, Pitman, Dickson, LePichon, Herron (Lamont)

31. Seafloor Spreading • 5) Seismicity (earthquakes): • occurs in defined patterns, in continuous chains

32. Seafloor Spreading • 6) Volcanoes: • generally co-occur with earthquakes; same patterns

33. Seafloor Spreading • The infamous “Ring of Fire” in the Pacific is an expression of this distribution • both earthquakes and volcanoes