YEAR 9 GEOGRAPHY REVISION NOTES
STRUCTURE OF THE EARTH & PLATE TECTONICS
1. Structure of the Earth
The Earth has three main layers.
Layer
Description
Crust
The outermost and thinnest layer where we
live.
Mantle
The thick middle layer made of hot semimolten rock (magma).
Core
The innermost and hottest layer made mostly
of iron and nickel.
Types of Core
•
Outer Core – liquid iron and nickel
•
Inner Core – solid due to high pressure
2. The Lithosphere
The lithosphere is made up of:
•
The crust
•
The upper part of the mantle
It is broken into large pieces called tectonic plates.
3. Plate Tectonics
Plate tectonics is the theory that the Earth’s lithosphere is divided into large plates that move
slowly.
These plates float on the semi-molten mantle.
What causes plates to move?
Plate movement is caused by convection currents in the mantle.
Convection currents occur when:
•
Hot magma rises
•
It cools and sinks
•
This movement pushes the plates
4. Types of Plate Boundaries
There are three main types of plate boundaries.
1. Divergent Boundary (Constructive Boundary)
Plates move away from each other.
Features formed:
•
Mid-ocean ridges
•
Rift valleys
•
New crust is created.
Example:
Mid-Atlantic Ridge
2. Convergent Boundary (Destructive Boundary)
Plates move towards each other.
One plate may sink under another plate.
This process is called subduction.
Features formed:
•
Deep ocean trenches
•
Volcanoes
•
Fold mountains
Example:
The Himalayas
3. Transform Boundary (Conservative Boundary)
Plates slide past each other.
•
No crust is created or destroyed
•
Causes earthquakes
Example:
San Andreas Fault
5. Important Plate Tectonic Features
Mid-Ocean Ridge
A long underwater mountain chain formed at divergent boundaries.
Rift Valley
A long narrow valley formed when plates pull apart on land.
Example:
East African Rift Valley
Ocean Trench
A deep narrow depression in the ocean floor formed by subduction.
6. Subduction
Subduction occurs when:
A denser plate sinks below a less dense plate into the mantle.
This happens at convergent boundaries.
7. Earthquakes
An earthquake occurs when:
•
Plates get stuck
•
Pressure builds
•
The plates suddenly slip and release energy
This causes ground shaking.
8. Volcanoes and the Ring of Fire
Many volcanoes occur around the Pacific Ocean.
This area is called the Pacific Ring of Fire.
It has:
•
Many volcanoes
•
Many earthquakes
This is because many convergent plate boundaries are located there.
9. Continental Drift
Continental drift is the idea that:
The continents were once joined together and slowly drifted apart over millions of years.
This idea was proposed by Alfred Wegener.
The continents were once joined as a supercontinent called Pangaea.
10. Magnetic Polarity
Rocks on the ocean floor show magnetic stripes.
These stripes help scientists prove that new crust is formed at mid-ocean ridges and spreads
outward.
This is evidence for seafloor spreading.
11. Geological Phenomena Caused by Plate Movements
Plate movements can cause:
•
Earthquakes
•
Volcanoes
•
Fold mountains
•
Ocean trenches
•
Rift valleys
Plate Boundary Plate Movement
Features Formed
Divergent
Plates move apart
Mid-ocean ridges, rift
valleys
Convergent
Plates move together
Volcanoes, trenches,
mountains
Transform
Plates slide past
Earthquakes