Paleomagnetism

Paleomagnetism

Paleomagnetism

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

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.

Seafloor Spreading Theory

HESS – DIETZ HYPOTHESIS

Sea-Floor Spreading

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

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

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.

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).

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.

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.

•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.

PROOF THAT THE SEA FLOOR IS SPREADING1. 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.

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:

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.

•How does magnetic stripes support the theory of Sea floor spreading?

2. Evidence from Magnetic Stripes

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

In Order to understand Magnetic Stripes lets talk about the Earth’s

magnetic North!

What exactly is the North Pole?

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

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.

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.

How the seafloor got it’s stripes!

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.

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.

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

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.

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

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.

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

Sea-Floor SpreadingSubduction – 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

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

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. 

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

•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.

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

Subduction Zone

YOU NEED TO LOOK AT THE ARROWS AND LEARN HOW

TO READ THEM FOR PLATE MOVEMENT

→← CONVERGING

←→ DIVERGING

↑↓SLIDING PAST

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

increasesyoungest

3. ____________________________________drive plate movement.

Convection currents