Paleomagnetism

1. Paleomagnetism

2. Paleomagnetism

3. Paleomagnetism

4. Paleomagnetism
• 1950’s: discovery of
ancient rocks had
magnetic directions
and inclinations that
did not correspond
with the present
magnetic pole position.

5. Rejection of Continental Drift
• Wegener concentrated on developing evidence that continents
had drifted not on what caused them to move.
• Two alternative possibilities
1.Continents plow their way through oceanic crust, shoving
it aside as a ship plows through water.
2.Continental crust slides over oceanic crust.
• Physicists proved that both mechanisms were impossible.
1.Oceanic crust is too strong for continents to plow through
it.
2.Frictional resistance is too great for continents to slide over
oceanic crust.

6. Seafloor Spreading Theory
HESS – DIETZ HYPOTHESIS

7. Sea-Floor Spreading

9. Mid-Ocean Ridge – the longest chain of mountains in
the world---these are divergent plate boundaries.

10. During World War
II, geologist Harry
Hess became
interested in
mapping the ocean
floor using sonar.

11. Sonar - a device that
bounces sound waves
off under-water objects
and then records the
echoes of these sound
waves.
The time it takes for the
echo to arrive indicates
the distance to the
object.

12. Unexpected Finding:
Long mountain ranges existed
throughout the floors of the
oceans called mid-ocean
ridges.
The longest one runs through
the middle of the Atlantic
Ocean and is called the Mid
Atlantic Ridge (MAR).

13. Hess devised a theory
called sea-floor
spreading which states
that at mid-ocean
ridges, molten material
rises from the mantle
and erupts. It then
spreads out, pushing
older rock to both sides
of the ridge.

14. THE PROOF-----SEAFLOOR SPREADING
•An American scientist named Harry Hess
proposed the seafloor spreading theory in
1962.
•The Sea floor spreading theory states that
new ocean crust is being created at mid-
ocean ridges (which are large mountain
chains underwater) and destroyed at deep-
sea trenches.
•This is proof that the plates are moving
along on a “conveyor belt” so Wegner’s
idea on continent drift was correct.

15. • Ocean floor moves like a conveyor belt carrying
continents with it.
• New ocean floor forms along cracks in the ocean crust
as molten material erupts from the mantle spreading
out and pushing older rocks to the sides of the crack.
New ocean floor is continually added by the process of
sea-floor spreading.

16. PROOF THATTHE SEA FLOOR IS SPREADING
1. The oldest ocean floor rocks ever found are 180
million years old. Remember the Earth is 4.6 billion
years old. This proves that ocean floor is being
destroyed therefore all ocean floor rocks are young
compared to the age of Earth.
2. The rocks closest to the ocean ridge were younger
than the rocks found further from the ridge. This
means that new rocks are formed at the ridges and
push the older rocks away from the ridge.

17. 1. Evidence from Molten
Material
– Rocks shaped like
pillows(rock pillows)
show that molten
material has erupted
again and again from
cracks along the mid-
ocean ridge and cooled
quickly
Three Types of Evidences:

18. Molten Material
• Scientist aboard a
submersible called
Alvin dove to the
sea floor and
discovered rocks
called pillow lavas
along the mid
ocean ridges. Such
rocks can only
form from cooled
molten material.

20. • How does magnetic stripes support
the theory of Sea floor spreading?
2. Evidence from
Magnetic Stripes

21. 2. Evidence from
Magnetic Stripes
– Rocks that make
up the ocean floor
lie in a pattern of
magnetized stripes
which hold a
record of the
reversals in
Earth’s magnetic
field

22. In Order to understand Magnetic
Stripes lets talk about the Earth’s
magnetic North!

23. What exactly is the North Pole?

24. The Earth acts like a giant magnet because of it’s
metallic core!

25. Magnetic Reversal
– a change in Earth’s magnetic field in which the north
magnetic pole becomes the south magnetic pole and vice
versa; has occurred on average every 500,000 years over
the past 65 million years.
• Normal Magnetic Polarity – magnetic orientation the
same as that of Earth’s current field.
• Reversed Magnetic Polarity – magnetic orientations in
rock that are opposite to the current orientation of
Earth’s magnetic field.

26. Magnetic Stripes
• Rocks that make up the ocean floor lie in a pattern of
magnetized stripes, which show a record of reversals of
Earth’s magnetic field.

28. How the seafloor got it’s stripes!

31. Magnetic Reversals occur
• When magma cools, the iron cools into the mineral magnetite. It lines up parallel to the Earth’s
present magnetic field. This iron is like compass needles, pointing north. So when the rock hardens,
a record of the Earth’s magnetic field at that time is locked in stone.

32. 2. Studies of these magma flows have showed a
magnetic reversal over geologic time.
3. A magnetic reversal (called a FLIP) is a change in
Earth’s magnetic field. North pole becomes south pole
and south becomes north.

33. 4. A magnetic field that has the same orientation as
Earth’s present field is said to have a normal polarity
5. A magnetic field that is opposite to the present field
has reversed polarity

34. 3. Evidence from
Drilling Samples
– Core samples from
the ocean floor show
that older rocks are
found farther from
the ridge; youngest
rocks are in the
center of the ridge.

35. Drilling Samples
• In the year 1968, a drilling ship called the Glomar
Challenger was used to gather samples of rocks
from the ocean floor.

36. Scientists discovered that the further the rocks were from
the mid ocean ridges, the older they were. The youngest
rocks were always at the center of the ridges.

37. Resulting Question: Are all of the oceans getting
bigger?
• Suggested Answer: Ocean floors do not keep
spreading. Parts of the oceanic crust get plunged
into deep-ocean trenches due to the process of
subduction

38. Sea-Floor Spreading
Subduction – Process
by which the ocean
floor sinks beneath a
deep-ocean trench
and back into the
mantle; allows part of
the ocean floor to
sink back into the
mantle

39. Subduction-process by which the ocean floor sinks
beneath a deep-ocean trench and back into the
mantle.

40. Subduction in the Pacific Ocean:
There are many trenches, so the Pacific Ocean
is getting smaller.
Subduction in the
Atlantic Ocean:
There are fewer
trenches, so the
Atlantic Ocean is
getting wider.

42. The force responsible for driving or moving the plates is
_____________________.
Convection Currents occur within the mantle of the earth when
hot magma rises and cool magma sinks
convection currents

43. •Magma, because it is hotter and less
dense, is forced up towards the ocean floor
cooling off when it touches water.
•When the magma hardens, a small amount
of new ocean floor is added to the Earth’s
crust. As more magma rises and cools it
pushes the new sea floor away from the
ridge. This process is called sea floor
spreading.

44. Sea-Floor Spreading: Subduction
zone
Deep-Ocean
Trench – Occurs
at subduction
zones. Deep
underwater
canyons form
where oceanic
crust bends
downward

45. Subduction Zone

46. YOU NEED TO LOOK AT THE ARROWS AND
LEARN HOW
TO READ THEM FOR PLATE MOVEMENT
→← CONVERGING
←→ DIVERGING
↑↓SLIDING PAST

47. AMAZING EARTH SCIENCE FACTS
1. ____________________is created at mid oceanic ridges.
2. Age of rock ____________ as distance from ridge increases (____________ rock is at the ridge).
New sea floor
increases
youngest
3. __________________________
__________drive plate movement.
Convection
currents