1. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Names and types of major plates
Symbols for plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hotspots
What drives it all?

2. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Names and types of major plates
Image from USGS plate tectonics web site

3. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Symbols for plate margins
Convergent margin
Teeth on upper plate side
Transform margin
Divergent margin

4. Major tectonic plates, plate boundaries, plate motion vectors
IV. Modern Plate Tectonic Theory
Major plates and plate margin
Major tectonic plates, plate boundaries, plate motion vectors
Arrow are plate vectors, numbers are rates of plate motion in centimeters per year

5. IV. Modern Plate Tectonic Theory
Major plates and plate margin
A slightly different representation from
This Dynamic Earth
USGS poster
Shading (red) shows diffuse zones of crustal deformation, most of these are associated with (adjacent to ) plate boundaries and are areas of high earthquake activity
Red dots are locations of “hot spot” volcanism, many of these are within plates

6. IV. Modern Plate Tectonics
Major plates and plate margins
Divergent (constructive) margins
mid-ocean ridges (=spreading centers)
Continental rift zones (incipient divergent margins)
Divergent plate margins are also referred to a constructive plate margins; this is because igneous activity at these plate margins generates new lithospheric plates including new oceanic crust

7. IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
mid-ocean ridges (=spreading centers)

8. IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
mid-ocean ridges (=spreading centers)
elevated ridge but with an axial rift (faulted) valley along the ridge axis
active magmatism including sea floor volcanism
highest rate of heat loss (surface heat flow) on Earth
magmatism generates new oceanic crust and lithospheric plates
Mid Atlantic Ridge is a slow spreading ridge
Water depth or depth below sea-level

9. IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
mid-ocean ridges (=spreading centers)
East Pacific Rise is a fast spreading ridge
highest rate of heat loss (surface heat flow) on Earth
magmatism generates new oceanic crust and lithospheric plates

10. IV. Modern Plate Tectonic Theory
B. Divergent (constructive) margins
mid-ocean ridges (=spreading centers)
East Pacific Rise is a fast spreading ridge
Another look at an axial rift valley

11. Major Tectonic Plates & Plate Boundaries
Divergent (constructive) margins
Continental rift zones (incipient divergent margins)
Magmatism and volcanic activity accompanies continental rifting (ex. East Africa Rift)
Such rifting can lead to development of a new plate margin and ultimately a new ocean basin (ex. Red Sea)

12. Major Tectonic Plates & Plate Boundaries
Divergent (constructive) margins
Continental rift zones (incipient divergent margins)
East African Rift Zone
Red Sea is a new ocean basin
Active
volcanoes
Image from USGS plate tectonics web site

13. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins (two main types)
Subduction zones (two sub-types)
Ocean-ocean
Ocean-continent
Continent-Continent collision zones
Transform or Strike Slip margins
What drives it all?

14. IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones
ocean-ocean
ocean-continent
Deep-sea trench is site where
oceanic lithosphere starts to descend
into mantle
Ocean-continent
Ocean-ocean

15. IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones are sites where an oceanic tectonic plate descends into the mantle
Overriding plate may also be an oceanic tectonic plate (ocean-ocean) or may be a continental tectonic plate (ocean-continent)
Chain of volcanoes are found on the overriding plate
Island arcs (ocean-ocean)
Continental volcanic arcs (ocean-continent)
Ocean-continent
Ocean-ocean

16. IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones are sites of deepest earthquakes on earth (to 700 km)
Array of earthquakes marks the subducting lithosphere = Benioff Zone
Earthquake array starts at trench (shallow) and gets progressively deeper beneath the over riding plate
Japan

17. IV. Modern Plate Tectonic Theory
Convergent margins
Subduction zones are sites of deepest earthquakes on earth (to 700 km)
Array of earthquakes marks the subducting lithosphere = Benioff Zone
Earthquake array starts at trench (shallow) and gets progressively deeper beneath the over riding plate
Benioff Zone = earthquakes within the cold subducting lithospheric plate

18. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins (two main types)
Subduction zones (two sub-types)
Ocean-ocean
Ocean-continent
Continent-Continent collision zones
Transform or Strike Slip margins
What drives it all?

19. IV. Modern Plate Tectonic Theory
Convergent margins
Continent-Continent collision zones
Himalayas (India & Tibet)
Zargos (Middle East)
Alps (Southern Europe)
Image from USGS plate tectonics web site

20. IV. Modern Plate Tectonic Theory
Convergent margins
Continent-Continent collision zones
Escape tectonics
Clay block model
Rigid indenter

21. IV. Modern Plate Tectonic Theory
Convergent margins
Continent-Continent collision zones
Escape tectonics

22. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip plate margins
At mid-ocean ridges
San Andreas fault zone

23. IV. Modern Plate Tectonic Theory
Transform or Strike Slip Plate Margins
San Andreas fault zone
At mid-ocean ridges
Image from USGS plate tectonics web site

24. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hotspots

25. IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A volcanic hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Examples include:
Hawaii
(oceanic hotspot)
Yellowstone
(continental hotspot)

26. IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.

27. IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Emperor-Hawaii Seamount Chain
Hotspot (mantle plume stationary with respect to overriding tectonic plate)

28. IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Pacific plate has moved over the Hawaii hotspot for over 60 million years (Ma) leaving a hotspot track of now extinct volcanoes, all similar to Hawaii
The Hawaii-Emperor Volcanic Chain

29. IV. Modern Plate Tectonic Theory
E. Volcanic Hotspots
A hotspot is mantle plume that is stationary with respect to overriding tectonic plate.
Yellowstone-Snake River Plain
Hot Spot (mantle plume stationary with respect to overriding tectonic plate)

30. IV. Modern Plate Tectonic Theory
Major plates and plate margins
Divergent (constructive) margins
Convergent margins
Transform or Strike Slip margins
Volcanic Hot Spots
What drives it all?
1. Earth’s internal heat engine
2. Density “currents” in plastic mantle

31. IV. Modern Plate Tectonic Theory
Volcanoes
IV. Modern Plate Tectonic Theory
F. What drives it all?
1. Earth’s internal (endogenic) heat engine
Geothermal gradients (evidence of hot interior)
Temperature increases with depth in solid earth, most obvious surface expressions of geothermal gradients
Volcanic eruptions, geysers and hot springs, fumaroles
Radioactive heat production and residual heat of planet formation
Hot springs
Radioactive heat production declining with time
Only during earth’s formation
Rocks are poor heat conductors
Earth is still cooling
Heat production with time
Heat produced by impact during planet formation

32. IV. Modern Plate Tectonic Theory
F. What drives it all?
2. Density “currents” in plastic mantle
Ridge push (rising mantle)
Slab pull (sinking plate)
Mid-ocean spreading ridges have the highest surface heat flow on earth
Subduction zone trenches have the lowest surface heat flow on earth
Slab pull
New (young) oceanic lithospheric plates are hot, but they cool over 10’s of millions of years as they move away from the oceanic spreading ridge.
As the oceanic plates cool they become more dense, eventually becoming dense enough to sink into the mantle.

33. Age of the Crust
Shield areas on the continents are Precambrian in age (0.54 – 4.2 billion years)
Remainder of continental crust is Phanerozoic in age (Recent to 542 million years)
Oceanic crust is Mesozoic and Cenozoic in age (170 million years [Jurassic] to Recent)

34. Age of the Crust
Oceanic crust is generated by igneous activity at mid-ocean ridges and eventually is recycled back into the mantle at subduction zone (due to its higher density)

35. Age of the Crust
Igneous activity above subduction zones (island arcs and continental volcanic arcs) generates lower density continental crust
This lower density crust is too buoyant to be subducted
The inability to subduct explains why much of the continental crust is so old (billions of years)