Physical Science

Question: How can magnetism of rocks provide evidence of

continents moving?

Wednesday

Connecting the rube to the physics assignmentAnd Answers to 10 class questions

Rube Check

continental drift evidence

Plate Tectonics

Homework assignment• Use the internet and book to find descriptions of the

following concepts:

Pangaea Divergent boundaryPanthalassa Convergent BoundaryContinental drift Rift ValleyMesosaurus Subduction ZoneMid-Atlantic Ridge Ocean TrenchMagma Transform BoundarySea-floor spreadingPaleomagnetism

What evidence is there that the continents have moved in the

past?In other words, what evidence is used to support the continental

drift theory

Evidence1)

2)

3)

4)

5)

Evidence1) Physical shape of continents

2) Rock evidence of change in climate

3) Fossil evidence, including Mesosaurus

4) Difference in ages of oceanic crust vs continental crust

5) Paleomagnetism of rock on either side of the Mid Atlantic ridge

Alfred Wegener

Determining the magnetic orientation of ocean rock

In the 1950’s and 1960’s….

• Used Sonar (invented during WWII) to map the Ocean Floor

To Produce underwater maps for Submarines

Found that there was a huge ridge of mountains in the middle of the Atlantic ocean

Have found many examples since

• Of these mid Oceanic ridges around the world

• Volcanically active (but not wholly destructive)

• Ocean floor is not smooth as once thought

Paleomagnetism

• Study of the Earth’s magnetic record over time

• Minerals in hot rocks do not have a set magnetic direction

• As it cools, the magnetic orientation of those minerals becomes fixed pointing in the direction of the “North Pole”

This magnetic orientation can and has be measured

What is magnetic orientation?

Means the direction that the rock makes a compass point as it passes over

What has been discovered…

Discovered bands of rock with different magnetic orientation as the ship moves away from ridge

• Evidence that the orientation reverses every 200,000 to several million years

• Matched bands with the same magnetic orientation are found on both sides of a ridge

Orientation of rocks around a ridge

Different colors represent rocks of different geologic age

Iceland on a diverging plate boundary

Who wants to act?

What is the best explanation for the matched bands of rock?

• New ocean floor is being produced from the ridge

• Older material material is pushed away from ridge by newer material

• This in turn moves the continents apart

Mid ocean Ridges

• Are the place of creation of new plate material

• Boundary between plates

• As plates are created items on the plates are moved farther away from the ridge

• Divergent boundary

Sea-floor Spreading

• Driven by the introduction of new material at the ridges, the older sea floor is pushed away from the ridges.

• The oldest ocean rock is farthest away from the ridges

• The coldest and most dense rock is farthest away from the ridges

If new crust material is being created…

Old material must be destroyed because the surface area of the

earth is not increasing

Where do plates come together?

• Convergent boundary

• Subduction zones

• Most have a lot of volcanic and earthquake activity associated with their interaction

What happens at a subduction zone?

• The density of material coming together determines what happens

• The most dense material is pushed under the least dense material

Variations in Convergent boundaries

Ocean to continental crust

Continental to continental

Ocean to Ocean crust

Oceanic meets Continental crust

The west Coast of the US around Seattle and Portland

West Coast

Ocean-vs-Continent

• More dense ocean crust is push beneath Continental crust

When it is pushed deep enough it begins to melt

The melted material is pushed upward, supplies material for volcanos

Magma and Lava

• Magma

• Lava

Continental material meets other continental material

The mountains between India and China

Subduction zone –The Himalayas

Continent –vs-Continent

• Nothing is pushed down much, similar density

• Build Mountains

• Keep building as long as pushing occurs

• No volcanos

Ocean-vs-Ocean

• All based on age of rock, the oldest material is pushed under

• Creates volcanic islands, or island ars

Subduction zone - Japan

One final type of boundary interaction

TRANSFORM BOUNDARY

Southern California

• San Andreas Fault

Transform

• Moves past each other

• Not creating or destroying material

• Earthquakes

Plate Tectonics

• Describes continental movement

• Describes why and how continents move

Plates and Continents

Pangaea and Panthalassa