4.5 The Theory of

Plate Tectonics

Objectives

• Explain the theory of plate tectonics

• Describe the three types of plate boundaries.

Engage/Explore • What is a plate?• Think of other contexts in which the word is used.

• Metal plates that cover machinery• Home plate in baseball, • a reptile’s plates or scales• plate of photographs in a textbook

Introduction• J. Tuzo Wilson - Canadian scientist.

• He discovered that there are cracks in the continents similar to those on the ocean floor.

• Wilson proposed that the lithosphere is broken into separate sections called plates.

• Plates can carry both continents or parts of the ocean floor.

Great Rift Valley in East Africa

A Theory of Plate Motion

• Wilson combined the thoughts of sea-floor spreading, Earth’s plates, and continental drift into a theory.

• A scientific theory is a well-tested concept that explains a wide range of observations.

• Plate tectonics is the geological theory that state that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. It explains the formation, movement, and subduction of Earth’s plates.

How can Earth’s plates move?

• The plates of the lithosphere float on top of the asthenosphere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere.

Earth’s Lithospheric Plates

Affects of the plates• Collide• Pull apart• Grind past each other

• Volcanoes• Mountain ranges• Deep-sea trenches

Earth’s Lithospheric Plates

• P. 133 Figure 23• Which plates include only ocean floor?

• Which plates include both continents and ocean floor?

Plate Boundaries • Demonstration• Materials - 2 woooden blocks• Three types of blocks1. Transform boundary - slide past 2. Divergent boundary - pull away

3. Convergent boundary - push two blocks together.

• Draw the blocks with labels and arrows showing the direction of each block’s movement.

Flat rock demonstration

• Demonstrate the boundaries using rocks.

• Why do you think earthquakes occur frequently at transform boundaries?

• Answer: The plates cannot move smoothly past one another because of the irregular nature of faults.

• How fast do you think Earth’s plates are moving?

• Answer: Only a few centimenters per year.

Plate Boundaries

• Faults - breaks the Earth’s crust where rocks have slipped past each other which form along these boundaries.

• Three kinds of boundaries

Transform Boundary• A place where two plates slip past each other, moving in opposite directions.

• Earthquakes occur along these boundaries.

• San Andreas Fault is an example. The Pacific plate is sliding past the North American plate.

• Crust is neither created nor destroyed.

San Andreas Fault

San Andreas Fault

Divergent Boundary• The place where two plates move apart.

• Most occur at the mid-ocean ridge and on land.

• The boundary forms rift valleys.

• Example - Great Rift Valley has a 3000 km crack.

Rio Grande Rift• Extendsfrom central Colorado to El Paso, Texas.

Exploring Exercise - Plate Tectonics

Plate Tectonics - p. 134-135What is magma?What is the magma coming from that is shown erupting through the mid-ocean ridge and the rift valley?

Answer: asthenosphereWhat new process are shown?

Convergent Boundaries• Two plates come together. This is called a collision

• When two continental plates collide, why isn’t one subducted beneath the other?

Appalachian Mountains formed when two continental plates collided.

-The density of plates determines which plate comes out on top.

Appalachian Mountains

Appalachian Mountains

Convergent Boundaries

Oceanic/oceanic Subduction occurs

Oceanic/continental Oceanic plate sinks

Continental/continental Mountain ranges form

Candy Bar Inquiry Challenge

• Material - candy bar, paper towel• Procedure

– Wash hands– Give out Snickers– How can you use this candy bar to model what happens at divergent and convergent boundaries?

– Make a sketch of what you see.

The Continents’ Slow Dance

• The continents move at slow rates: one to ten centimeters per year.

• Pages 136-137 Read Figure 26• Pangea began to drift apart about 225 million years ago.