1. The Dynamic Earth

2. Why is the Earth not flat?
Or other questions such as
Why are the continents where they are?
Why are they the shapes they are
Why are there mountains? How are they formed?
In 1912 Alfred Wegener proposed the Theory of Continental Drift
Pangea (all lands) and Panthalassa (single ocean)

3. Evidence Puzzle pieces Fossils
Africa and South America seem to “fit” together
Fossils
Eg. Mesosaurus (270 M years ago)

5. Evidence Geological Climate changes Age and type of rock formations
Appalachians and similar mountains in Scotland and northern Europe
Climate changes
Evidence of glaciation in S.A and Africa
Coal deposits in US, Europe and Siberia

6. Mechanisms
WW II introduced sonar – noticed that the ocean floor was not flat
1947 started mapping the Mid-Atlantic Ridge
part of a series of mid ocean ridges that extends around the world
rock they brought up was younger than continental rocks
none more than about 170 M years old

7. Mechanisms sea floor spreading
Harry Hess suggested the theory/hypothesis that the valley at the center of the ridge was actually a rift or break in the Earth’s crust and that new rock was welling up from deep within the Earth

8. Mechanisms Paleomnagnetism
As magma rising from the mantle cools, any magnetic mineral particles in it orient themselves to align with the Earth’s magnetic field
Scientists examined the cores from the ocean floor and found rocks with magnetic orientation pointing south, not north
When the rocks were dated it was found that those with the same magnetic variations were the same ages
The pattern was the same on each side of the ridge

9. Mechanisms Paleomagnetism con’t
The intervals were not of the same length each time making the pattern unique
1965 scientists finally accepted that the Earth’s magnetic pole does regularly reverse itself
How does Iceland provide good evidence for this theory?

11. Plate Tectonics
Is the name given to the modern theory that is based on Wegener’s original idea but that now incorporates so much more
This theory is not just about the continental plates moving but also includes the mechanisms that drive this movement
This explains the formation of many features of the Earth’s crust

12. Plate tectonics There are two types of crust
Oceanic crust and continental crust
The two crusts combine to form the lithosphere – the thin outer shell of the Earth
This rides over the asthenosphere which is the layer of liquid or plastic rock that flows under pressure

13. Plate tectonics
The plates of the lithosphere ride on the more fluid asthenosphere
Most plates are made up of oceanic and continental crust
About 30 plates have been identified and they vary is size
How the plates interact with each other has created the major surface features of the Earth

14. The major plates

15. Types of Plate Boundaries

17. Divergent Boundary Two plates moving away from each other
When this happens the gap is filled with material from the asthenosphere
The Mid-Atlantic ridge is one example (all ocean ridges are divergent)
Rift valleys form where the plates separate, for example Red Sea, Great Rift Valley (Kenya)

19. Composite heat-flow model obtained by merging age-dependent model heat-flow from Stein & Stein [1992]. Color scale is selected to show detail in continental areas of relatively low heat-flow.  All spreading ridges were assigned uniform 0.3 W/m2 conductive heat flow along their centers.

20. Divergent plates
Mid ocean rift valleys are typically broken into segments called fracture zones
These are perpendicular to the ridge
Movement along the fracture zones are a major cause of earthquakes along mid ocean ridges

21. Divergent plates
Mid ocean ridges are also the location for hydrothermal vents on the ocean floor that contain formerly unknown life forms who ecology is based on chemosynthesis and not photosynthesis

22. Convergent Boundaries
These are plates that are colliding with each other
Three different possibilities exist to deal with the excess of material at a single location
-Subduction boundaries
Ocean trenches and Island arcs
-Collision boundaries

23. Subduction boundaries
This creates a region known as a subduction zone
This occurs when oceanic crust converges with continental crust
Oceanic crust is denser so it is subducted or forced beneath the lighter continental crust

24. Subduction zones
Other consequences of subduction include the formation of volcanoes and island arcs

25. Volcano formation
When the ocean crust moves under the continental crust heat and pressure plus ocean water cause the solid material to melt
For this reason chain of volcanoes are often found associated with convergent plates.
One of the best known examples is the Pacific “Ring of Fire”
Costal mountains in North America such as Mount St. Helens

28. Ocean Trenches
These are created where plates converge and are some of the deepest places on the planet
E.g Marianna Trench off the coast of the Marianna Islands near Japan
11,000 m or 36,000 ft or over 6.5 miles deep

30. Island arcs
These are formed when the volcanoes at the subduction zone are not under a continent but instead form a ring of volcanic islands.
Examples – Aleutian Islands, Japan, Philippines, Indonesia

31. Collision Boundaries
When two continental crusts converge the material cannot be subducted instead the continental masses pile on top of each other forming mountain ranges
Himalayas, Rocky Mountains etc.

32. The Himalayas

34. Rocky Mountains

35. Transform Boundaries
This occurs when two plates are moving past each other
Since this not a smooth passing they tend to grind against each other
Pressure builds up as the plates scrape past each other
Movement tends to be in a series of jerks, often released with great force, known as earthquakes

36. Transform Boundaries
Mid ocean fractures are also examples of transform boundaries.
The San Andreas fault is one of the best known due to its location in such populated areas in California
It moves at about 5 cm year in some areas
Some have not moved in over 100 years
San Francisco 1906

38. San Andreas Fault

40. Mechanism of plate motion
Convection is considered the most likely mechanism that causes plates to move
There are three possible hypotheses that may account for these movements
Mantle convection
Ridge push
Slab pull

41. Mantle convection
Cooler plate material sinks down at a subduction boundary
This balances out the new material being produced at the mid-ocean ridge
This hypothesis may not explain the enormous force needed to move the plates
Magma that is hotter and less dense than its surroundings rises upwards at a mid-ocean ridge
This creates one side of a convection cell
As it moves it drags the lithospheric plate with it.

43. Ridge push
As new magma rises at the mid-ocean ridge it is less dense and floats
As it cools it becomes more dense and gravity causes it to slide away from the ridge and new molten magma rises
Computer models suggest that cooling subsiding rock could help drive plate movement – called ridge push

44. Slab pull
At subduction boundaries one plate is denser and heavier than the other
Since this is heaver and cooler than the mantle it continues to sink pulling the rest of the plate with it.
The force of sinking edge exerts on the rest of the plate is called slab pull.

46. Plate tectonics and Supercontinents
About 250 million years ago all the continents were formed into one landmass
Previous combined landmasses had existed
Data for many is reduced to evidence from cratons
The ancestors to the modern continents were smaller and land mass has increased over time
Whether surface rock is increasing, decreasing or constant is not certain at this time
Once complete super continent cycle takes 300 – 500 million years

47. Plate tectonics and Supercontinents
The oldest ocean floor is about 170 m years but the oldest rock is dated to 4 b years ago.
This means that knowing the history of landmasses comes from piecing together geological bits – either cratons (core continental rock) or terranes (from collisions)
Deep sea and river sediments and igneous rock also help build continents

48. Laurentia, the North American Craton
Some of the oldest rock in the world is found here (3.96 b yrs)
The craton shows the approximate shape 1 b yrs ago
The exposed parts are known as Canadian Shield, the rest is covered by sediment

49. Previous continents Nena – about 1.8 b yrs ago
Columbia (also called Nuna) – about b yrs ago
Rodinia –about 1.1b m years ago)
Pannotia – about m years ago
Euramerica – about 300 m years ago
Pangea – about m years ago
Laurasia – about m years ago
Gondawana- about m years ago

50. Supercontinent breakup
Pangea broke into two smaller masses called Gondwana and Laurasia about 180 m yrs ago
Over time each of these broke up into smaller landmasses who shapes began to resemble present continents.

52. Microplate terranes
A terrane is a piece of lithospheric plate with a unique geological history
This will include rock types, fossils, faults, folds, magnetic properties etc.
For example, there are over 10 different terranes in northern California!

53. Ancient terrane distributions
Terranes on Cuba

54. Deep Sea sediments
These can be added to the edges of a continent when an oceanic plate plunges under a continental plate and sediment from the ocean floor is scraped off and left behind on the edge of the continent

55. Igneous rock
Magma that rises to the surface and cools adds new rock to the continent
Volcanoes add rock mass at the edges of continents along subduction zones
Volcanic islands (island arcs) may become part of the continent due to plate movements

56. River sediment
Material deposited by river sediments that flow across continents can extend the edges of continents
The Mississippi River is doing this right now at the edge of the North American continent

57. Landsat TM false-color image of the modern Mississippi Delta
Landsat TM false-color image of the modern Mississippi Delta. Pink and red depict active, emergent vegetation; cyan shows water rich in suspended sediment (mud); dark blue to black is relatively clear water. Image obtained from NASA