Plate Tectonics

Chapter 4

1. Explain Wegener’s hypothesis of continental drift

Section 4.1 Objectives

3. Describe seafloor spreading

2. List evidence for Wegener’s hypothesis of continental drift

4.1 Continental DriftIn 1912, Alfred Wegener proposed his continental drift hypothesis.

It stated that the continents once formed part of a single landmass, which he named Pangaea, which means all lands.

Wegener thought that Pangaea began breaking up into smaller continents about 200 million years ago, and drifted to their present locations.

EVIDENCE:

1. Fit of the shapes of the continents

2. Fossils of plants and animals found on different continents that are separated by oceans

Evidence 1: Geologic Fit of the continents

1. Age and type of rocks match in West Africa and Brazil

2. The Appalachian mountains match up with the mountain range that runs through Scotland and North Europe.

Mesosaurus fossils date back to 270 million years ago in South America and West Africa. Did it swim that

far? There is not evidence of a land bridge.

EVIDENCE:

3. Glacial grooves show that glaciers looked like they moved from sea to land

EVIDENCE:

4. Climatic patterns shown by rock layers:

Some rock types only form in certain climates, for example coal, which forms in warm, very wet (rainy) environments. If coal is found in a place that is not warm and rainy,

then either the climate has changed or the rock has moved.

The Dance of the Continents…

GO>

The evidence that Alfred was looking for was found …On the Ocean floor!

• In 1947, a group of scientists set out to map the Mid-Atlantic Ridge. This is part of an 80,000 km.-long system of mid-ocean ridges.

• The oldest rocks found on the sea floor were less than 175 million years old. The oldest continental rocks are about 4 billion years old. WHY IS THIS INFORMATION SURPRISING?

Seafloor Spreading Hypothesis

Seafloor Spreading Hypothesis

1. The valley at the center of the MOR is a rift (a long fracture in the crust)

2. Magma is coming up from inside the earth into the rift.

3. This magma can find space in the crust because the ocean floor is moving away from both sides of the MOR

PALEOMAGNETIc REVERSALS• As magma solidifies

to form rock, the magnetic fields of iron-rich minerals align with the earth’s magnetic field, just like a compass.

• Scientists have discovered that this is not always the case. they have found minerals that point south.

• Throughout the earth’s history, the magnetic field has reversed itself many times.

• Such reversals have come at irregular intervals, averaging about every 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows

PALEOMAGNETIc REVERSALS

PALEOMAGNETISM of THE OCEAN FLOOR

1. Scientists discovered magnetic patterns locked into the rocks of the ocean floor

2. These patterns showed alternating bands of normal and reversed magnetism

3. As molten rock rises from the rift in an MOR, it quickly cools and hardens and its magnetic orientation becomes fixed.

Paleomagnetism of the ocean floor

Sea Floor Spreading

1. Summarize the theory of plate tectonics.

Section 4.2 Objectives

4. Summarize the theory of microplate terranes.

3. Explain the possible role of convection currents in plate movement.

2. Compare the characteristic geologic activities that occur along the three types of plate boundaries.

ooo

Plate Tectonic Theory: Summary

.

1.Earth’s outer layer is broken into about 30 sections called plates.

2.The plates are composed of uppermost mantle and either continental crust or oceanic crust (lithosphere).

3.The plates ride on the asthenosphere.

4.The plates move because of convection currents flowing in the mantle below the plates.

Earth’s Tectonic Plates

Types of Crust1. oceanic crust 2. continental

crust

Lithosphere/Asthenosphere• The oceanic and continental crust and the rigid upper mantle make up the lithosphere.

• The lithosphere forms a thin outer shell that lies above the plastic rock of the asthenosphere.

Divergent Boundary

African PlateArabian Plate

1. The Red Sea occupies an area that contains a divergent boundary.

2. Seafloor is pushing the African and Arabian plates away from each other.

3. A rift valley runs down the center of the sea.

African Plate Arabian Plate

Red Sea

Divergent Boundary

Boundary

Birth Ocean

ooo

Convergent Boundary• convergent boundary: the direct

collision of one plate with another.

• Three types of convergent boundaries:

1. ocean to ocean2. continent to ocean3. continent to continent

Convergent Boundary3. Ocean To Ocean: oceanic plate

subducting under another oceanic plate

Watch Video

OCEAN TO OCEAN

Convergent Boundary

Convergent Boundary2. Continent To Ocean: • when the oceanic crust slides beneath continental crust. • this is a process called subduction. • subduction zones create deep trenches… as the plate is

subducted (tucked under) heat is released, the mantle partially melts, and magma rises to the surface: VOLCANOES

Watch Video

Convergent Boundary1. Continent to Continent Neither plate is subducted because

they have the same density. Instead the land is crumpled and uplifted creating MOUNTAIN RANGES!

Watch Video

Transform Boundary• Transform boundary: two plates

are grinding side-by-side past each other

Transform BoundaryThese boundaries have sudden spurts of activity and then

periods of no motion

Plate Boundaries

1 23

4

How many plates?

Convergent boundaries?

Divergent boundaries?

Transform boundary?

Causes Of Plate Motion• Scientists think that the movement of lithospheric

plates is due to convection.

Watch Video

Microplate Terranes• Theory of Microplate Terranes: continents are

actually a patchwork of crustal blocks, called terranes. Each block has its own distinct geological history.

• Terranes are regions that are bounded by faults and have rocks of different character (age, type, fossils) than in surrounding regions.

• Terranes form in another part of the world and are moved by plate motion to their present locations

Microplate Terranes of Virginia