LESSON 2: PLATE MOVEMENTS AND PLATE BOUNDARIES
Video Lesson
Lesson Objective
At the end of this lesson, you will be able to:
- Identify the major tectonic plates and their boundaries;
- Draw a diagram to demonstrate the movement of plates;.
- Identify the major continental and oceanic plates; and
- Explain how geological processes and plate boundaries affect human lives.
- distinguish between endogenic and exogenic forces of the earth;
- explain processes like folding and faulting and their effect on the earth surface;
- draw a map to locate the major young fold mountains of the world;
- draw a diagram to show the formation of horst mountains and rift valleys;
Brainstorming Question
Which of the tectonic plates do you think are oceanic? List down all the plates and group them as either continental or oceanic?
As the plates move, they may:
1. Converge, or come together; forming convergent (Destructive) boundary;
2. Diverge, or spread apart; forming divergent (Constructive) boundary or
3. Slide past each other along fractures called transform (Conservative) boundary.
Key terms and concepts
- Convergent Plate Boundary
- Divergent Plate Boundary
- Transform Fault Boundary
- Subduction Zone
- Rift Valley
A type of plate boundary where two tectonic plates move toward each other. One plate may be forced under the other in a process known as subduction, leading to the formation of mountains, volcanic activity, and earthquakes.
A type of plate boundary where two tectonic plates move away from each other. This often occurs at mid-ocean ridges, where new oceanic crust is formed through seafloor spreading, leading to the creation of rift valleys and volcanic activity.
A type of plate boundary where two tectonic plates slide past each other horizontally. This movement can cause earthquakes. A well-known example is the San Andreas Fault in California.
An area where one tectonic plate is being forced under another. This occurs at convergent boundaries where an oceanic plate meets a continental plate or another oceanic plate, leading to volcanic activity and the formation of deep ocean trenches.
A lowland region formed by the divergence of two tectonic plates. As the plates pull apart, the land between them sinks, creating a valley. An example is the East African Rift Valley.
Major Tectonic Plates and Their Boundaries
The Earth’s lithosphere is divided into several large tectonic plates. The major plates include:
- African Plate
- Antarctic Plate
- Eurasian Plate
- North American Plate
- South American Plate
- Pacific Plate
- Indo-Australian Plate
Types of Plate Boundaries
There are three main types of plate boundaries:
A Convergent (Destructive) Boundaries
A convergent plate boundary is a place on Earth where two or more tectonic plates collide, with one plate eventually sliding beneath the other. This process is called subduction.
- Where plates move towards each other
- Oceanic-Continental: Oceanic plate subducts under continental plate
- Oceanic-Oceanic: One oceanic plate subducts under another
- Continental-Continental: Collision, forming mountain ranges
- Features: Trenches, volcanoes, mountain ranges
Examples include:
- The collision of the Indo-Australian and Eurasian plates, forming the Himalayas
- The subduction of the Nazca Plate under the South American Plate, creating the Andes Mountains
Figure1.6: Convergence of plate boundary
When two oceanic plates converge, one may be forced under the other, creating a volcanic island arc, such as the Mariana Islands. Similarly, when two blocks of continental crust converge, the denser block might slide beneath the other, lifting it rather than sinking deeply. An example of this is the collision between the Indo-Australian Plate and the Eurasian Plate, which led to the formation of the Himalayas, the highest mountain range on Earth.
Divergent (Constructive) Boundaries
Where plates move apart. Examples include:
- The East African Rift Valley, which is gradually splitting the African continent.
- The Mid-Atlantic Ridge, where new oceanic crust is being formed.
Figure1.7: Diverging Plates
Figure 1.8 Seafloor spreading
Divergence can also happen on continents, producing fractures called rift valleys. A modern example is the East African Rift Valley. Over millions of years, the continental crust may separate completely, with the area between flooding with water to become a new ocean.
Transform Fault boundaries
- Where plates slide past each other horizontally
- Example: San Andreas Fault
- Features: Earthquakes, offset streams
Figure 1.9: Transform Fault
Driving Forces of Plate Tectonics
- Convection currents in the mantle
- Ridge push: Gravity-driven sliding of plates away from oceanic ridges
- Slab pull: Downward force of subducting plates
Figure 1.9: Convection currents in the Asthenosphere
Evidence Supporting Plate Tectonics
- Distribution of earthquakes and volcanoes along plate boundaries
- Seafloor spreading and magnetic striping of the ocean floor
- Paleomagnetism: Frozen magnetic orientation in rocks
- GPS measurements of plate movements
Geological Phenomena Explained by Plate Tectonics
- Mountain building (orogeny)
- Formation of oceanic trenches and island arcs
- Distribution of earthquakes and volcanoes
- Continental drift and the supercontinent cycle
Impact on Earth Systems
- Climate change: Altering ocean currents and atmospheric circulation
- Biodiversity: Creating and destroying habitats, influencing evolution
- Natural resources: Formation of mineral deposits and fossil fuel reservoirs
MAJOR GEOLOGICAL PROCESSES
Geological processes are driven by two types of forces:
5.1 Endogenic Forces
Internal forces originating from within the Earth, including:
- Plate tectonics
- Volcanism
- Earthquakes
5.2 Exogenic Forces
External forces acting on the Earth’s surface, including:
- Weathering
- Erosion
- Deposition
Geological Processes and Their Effects
3.1 Folding
The bending of rock layers due to compressional forces, resulting in:
- Anticlines: Upward-arching folds
- Synclines: Downward-arching folds
Examples: Formation of fold mountains like the Alps and the Appalachians
Figure1.10: Anticline, syncline
If compression continues then simple folds are changed into asymmetrical folds, over folds and over thrust folds. (Figure 1.10)
Asymmetrical fold: one limb is steeper than the other.
Over fold: One limb is pushed over the other limb.
Over thrust fold: When pressure is very great, a fracture occurs in the fold and one limb is pushed forward over the other limb.
Types of Fold Mountains
The Fold Mountains of the world are grouped into two: They are:
1. Young fold mountains
2. Old fold mountains
How can we classify Fold Mountains into categories?
Young Fold Mountains
Major young fold mountain ranges include:
- The Alps (Europe)
- The Andes (South America)
- The Himalayas (Asia)
- The Atlas Mountains (North Africa)
- The Zagros Mountains (Middle East)
Figure1.11: Major fold mountains of the world
FAULTING
Fracturing of rock layers due to stress. Types of faults include:
- Normal faults: Where the hanging wall moves down relative to the footwall.
- Reverse faults: Where the hanging wall moves up relative to the footwall.
- Strike-slip faults: Where blocks move horizontally past each other.
1. Rift valleys
2. Block mountains
Rift Valley Formation
Rift valleys form when continental crust is pulled apart. The East African Rift System is a prime example, stretching from Ethiopia to Mozambique.
Block Mountains: They are formed when the land between two parallel faults is pushed upward. A block mountain is called horst. For example: the Afar Horst.
Volcanic Activity
Volcanoes often form at plate boundaries. Examples in Ethiopia include:
- Erta Ale: An active shield volcano in the Afar Region
- Dallol: A cinder cone volcano in the Danakil Depression