1. Earth Part 3
G. Yannakoulias

2. Understanding Earths Surface
By examining its surface can better understand what is underneath crust
Observing surface of land is easy
Observing bottom of oceans is difficult

3. Early Maps of Earth’s surface
Early map of Earth (1595)
Does this look much like the Earth as we know it to be today?

4. Mapping Sea Floor cont…
Initially used long ropes with weights (pre 1920’s)
Sonar was used later (post 1920’s)
Found that the bottom of the oceans contained many features that were also found on land—i.e. mountain ranges (ridges) and valleys (trenches)

5. Mid-Ocean Ridges

6. Mid-Ocean Ridge Features
Underwater mountain range
Can be very high (up to 3000 m high)
Can be quite long (e.g. mid-atlantic ridge is 16,000 km long)
Earthquakes and volcanic eruptions are common along these ridges
Have Rift Valleys in the middle of these ridges

7. Rift Valleys These are valleys in the middle of mid-oceanic ridges
They are cracks or rifts in the oceanic crust forming the sea floor
Molten rock or magma rises from the mantle to fill these cracks

8. Rift Valley

9. Trenches Deepest parts of the ocean
Are long and narrow depressions on sea floor
Can be thousands of km long and many kilometers deep
Marianas trench is deeper than mount Everest is tall!

10. Oceanic Trenches

11. Sea-Floor Spreading
Ocean rock is younger than land (i.e. continental) rock
Ocean floor near mid-ocean ridges is younger than ocean floor further away from these mid- ocean ridges
Reason: new oceanic rock made at mid-ocean ridges
This idea is known as sea-floor spreading

12. How Are These Ridges and Trenches Formed?

13. Sea-Floor Spreading

14. Mechanism of Sea-Floor Spreading
Convection Currents in Upper Mantle pull apart oceanic plates at mid-oceanic ridges
Gaps in ocean floor replaced by magma (molten rock from mantle fills in gaps in rift valleys along mid-ocean ridges)
New rock helps to push plates away from ridge
Results in undersea volcanoes and thus mid-ocean ridges

15. Mechanism of Sea-Floor Spreading cont….
Molten rock from a rift in a mid-ocean ridge cools, hardens forming new sea floor
Convection curents in mantle continuously pushes older rock in the opposite direction on both sides of the ridge
Continents can be carried by the widening ocean floor in much the same way that objects are moved by a conveyor belt

16. Mechanism of Sea Floor Spreading

17. Where Do These Oceanic Plates End Up?

18. Subduction Zones
Ocean plates are thinner and more dense than continental plates
When collide with continental plates that are less dense and lighter they slide under them
These regions are known as subduction zones

19. Subduction Zones

20. How Trenches Are Formed
Trenches occur where oceanic plates slide under the continental plates and there is pulling of the continental plate with the subducting oceanic plate
This is due to friction
This results in the formation of deep sea trenches
Subducting oceanic plates are recycled in Earth’s mantle

21. Oceanic Plate Subducting Under Contintental Plate
Notice how the oceanic plate descends down to the upper mantle and melts to become magma again
Also notice how the oceanic plate pulls the edge of the continental plate forming a trench at the plate boundary

22. Evidence For Sea-Floor Spreading
Mid-oceanic ridges and volcanic eruptions along these mid-ocean ridges

23. Evidence For Sea-Floor Spreading cont…
Age of rock as you move from mid-ocean ridges outward

24. Evidence For Sea-Floor Spreading cont…
Magnetic striping:
Earth’s magnetic poles reverse every 200, ,000 years
These reversals are evident in new rock formations on the sea floor
as tectonic plates move apart, new rock is formed and locks in the direction of the magnetic field at the time

25. Magnetic Striping