the theory of - wallace's...

Plate

tectonics

The theory of ...

What are tectonic plates?

• Pieces of the lithosphere that move around on top of the asthenosphere.

• They fit together like puzzle pieces.

Facts about tectonic plates

• Plates can be made up of oceanic crust, continental crust, or both types of crust

• All tectonic plates have names. • Not all tectonic plates are the same size. • The LITHOSPHERE is like a JIGSAW PUZZLE,

and the tectonic plates are the puzzle pieces.

…like ice cubes in water!!!

Taken from pg. 195 in your textbooks:

Think about ice cubes floating in a bowl of water. If there are enough cubes, they will cover the surface of the water and bump into one another. Parts of the ice cubes are below the surface of the water and

displace it.

…like ice cubes in water!!!

Taken from pg. 195 in your textbooks:

Tectonic plates “float” on the asthenosphere in a similar way. The plates cover the surface of the

asthenosphere, and they touch one another and move around. The lithosphere displaces the asthenosphere.

Why do they move?!? • Because of CONVECTION CURRENTS

• mantle material near the core heats up and expands, becoming less dense

• less dense material rises toward the crust

• the material cools as it gets further from the core and becomes more dense

• more dense material sinks back toward the core, and the process repeats itself, creating convection currents

Plate boundaries

• A place where two or more tectonic plates touch. • Three main types:

1. CONVERGENT 2. DIVERGENT 3. TRANSFORM

Convergent plate boundaries •

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divergent plate boundaries

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transform plate boundaries

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The theory of ...

Continental

drift

What is continental drift?

• Theory that states that the continents once formed a single landmass, broke up, and drif ted to their present locations.

• Originally hypothesized by Alfred Wegener (pronounced VEH-GUH-

NER) in the ear ly 1900s.

What Did Alf. Weg. propose?!?

• That all of the present continents were once joined in a single, huge continent

called PANGAEA.

• Pangaea is Greek for, “all ear th.”

More about pangaea

• 245 million years ago Pangaea was one supercontinent. 180 million years ago it separated into two landmasses: Laurasia and Gondwana.

Clues that support cont. drift

1. PUZZLE-LIKE FIT: The continents seem to fit together like puzzle pieces.

2. FOSSIL CLUES: Fossils of identical plants and animals were found on continents separated by oceans.

3. ROCK CLUES: Similar types of rock were also found on continents separated by oceans.

4. CLIMATE CLUES: Evidence of similar climates have been found on different continents as well.

The Nay-sayers

•Even with these clues, nobody seemed convinced by Wegener!!! The crust just seemed too thick to be able

to move the way Wegener was proposing.

•In fact, it wasn’t until after Wegener’s death that more concrete evidence was found to support his

theory.

Seafloor

spreading

Introducing...

• Seaf loor spreading is the process by which new oceanic lithosphere is formed as magma rises toward the sur face and

solidifies.

• As tectonic plates move away from one another (DIVERGE), the seaf loor

spreads apar t and magma fills the gap.

What is seafloor spreading?

Why is seafloor spreading important?

• The discovery of seaf loor spreading helped suppor t Wegener’s theory of

continental drif t by explaining some of his initial obser vations (the 4 “clues”)

Where does s.s. occur?

• Seafloor spreading takes place at mid-ocean ridges

• Mid-ocean ridges are underwater mountain chains that run through Earth’s ocean basins

• THE OLDER ROCKS ARE FARTHEST AWAY FROM THE MID-OCEAN RIDGE DURING SEAFLOOR SPREADING.

Evidence of s.s.

•Throughout history, the north and south magnetic poles have changed places many times. When this happens, the polarity of Earth’s magnetic poles

changes.

•This is called a magnetic reversal.

•The rocks on the seafloor contain magnetic minerals (one of the special properties of some

minerals…remember?!?). We can see evidence of S.S. in the pattern of mineral alignment on the seafloor.

Every time there is a magnetic reversal, the alignment reverses as well.

How do we know magnetic reversals actually occur?