Theory of Continental Drift

Why doesn’t the Earth keep getting bigger?…Subduction Zones

• The ocean floor is also being destroyed
• Parts of the ocean floor are going back down into the mantle at subduction zones. These trenches are the deepest part of the ocean.(Ex: Marianas Trench)

• Subduction Zone

The Theory of Plate Tectonics explains the formation, movements, collisions, and destruction of the Earth’s crust.

• Areas of Sea Floor Spreading

• Key Subduction Zones

How do the plates move?

• One hypothesis is that large convection currents within the mantle move the plates.
• Movement is caused by differences in temperature causing a rising and sinking cycle.

• Convection Currents

The lithosphere is divided into 7 major and minor plates. Plates move at different speeds in different directions. Plate boundaries are where two plates meet.

• Major Plates

Three Types of Plate Boundaries:

• Divergent—Boundary where plates move apart (ex: Great Rift Valley—Africa; Mid ocean Ridges)
• Convergent—Boundary where plates come together (ex: Himalayan mountains, Japan, and Philippines)
• Transform—Boundary where two plates slide along side each other (ex: San Andreas Fault—San Francisco)

• Divergent Boundaries occur where
• plates move apart. There are two types
• of divergent or constructive boundaries:
• **Mid-Ocean Ridge
• **Rift Valley

• Divergent Boundary

• Mid- Ocean Ridge- New ocean floor, mountains, earthquakes and volcanic action occur when an ocean plate spreads apart (Mid-Atlantic Ridge: largest mountain range in world.)      

• Mid-Atlantic Ridge

• Mid-Ocean Ridge

• Rift Valley- Continent starts to split apart forming a rift valley and eventually a seaway and then an ocean. Volcanic activity and earthquakes occur in these areas. (Ex: African Rift Valley)

• Rift Valley

• The Great African Rift Valley