MEV 012: Unit 02 - Plate Tectonics

 UNIT 2: PLATE TECTONICS

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2.0 Introduction

Plate tectonics is the unifying theory of geology explaining the dynamic nature of Earth's outer shell. It describes how Earth's lithosphere is divided into rigid plates that move over the asthenosphere, leading to the formation of continents, oceans, mountains, earthquakes, and volcanoes.

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2.1 Objectives

After completing this unit, you will be able to:

• Explain the formation of continents and ocean basins.
• Understand the process of sea-floor spreading.
• Describe the theory of plate tectonics.
• Identify different types of plate boundaries.
• Analyze the mechanism of plate movements and mantle convection.

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2.2 Formation of Continents and Ocean Basins

2.2.1 Introduction

The Earth’s crust has not always looked the way it does now. The continents and ocean basins have changed positions due to tectonic forces over millions of years.

2.2.2 Continents

The idea of moving continents was first proposed by Alfred Wegener in his theory of continental drift (1912). He proposed that all continents were once part of a supercontinent called Pangaea, which broke apart ~200 million years ago.

Evidence for continental drift:

• Fit of coastlines (e.g., South America and Africa)
• Fossil correlations
• Similar rock formations and mountain chains
• Paleoclimatic evidence

2.2.3 Oceans

Ocean basins formed as continents drifted apart. As tectonic plates diverge, magma rises to form new oceanic crust at mid-ocean ridges, expanding ocean basins over time.

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2.3 Sea Floor Spreading

2.3.1 Introduction

Proposed by Harry Hess in the 1960s, sea-floor spreading explains how new oceanic crust forms at mid-ocean ridges and moves outward.

2.3.2 Evidences and Mechanism

Key evidence:

• Magnetic striping: Symmetrical patterns of magnetic reversals on either side of mid-ocean ridges.
• Age of oceanic crust: Younger near ridges, older further away.
• Heat flow: Highest near mid-ocean ridges.

Mechanism:

• Magma rises from the mantle.
• It cools and solidifies, forming new oceanic crust.
• Older crust moves away, pushing continents with it.

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2.4 Plate Tectonics

2.4.1 Introduction

The theory of plate tectonics (developed in the late 1960s) integrates sea-floor spreading and continental drift, describing Earth’s lithosphere as a mosaic of plates floating over the asthenosphere.

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2.4.2 Major Plates

There are seven major plates and several minor plates. Major plates include:

1. Pacific Plate
2. North American Plate
3. South American Plate
4. African Plate
5. Eurasian Plate
6. Indo-Australian Plate
7. Antarctic Plate

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2.4.3 Movement of Lithospheric Plates

Plate movements are categorized into:

• Divergent boundaries: Plates move apart (e.g., Mid-Atlantic Ridge).
• Convergent boundaries: Plates collide (e.g., Himalayas).
• Transform boundaries: Plates slide past each other (e.g., San Andreas Fault).

Rates of movement: Typically 2–15 cm/year.

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2.4.4 Mantle Convection and Plate Tectonics

Mantle convection is the driving force behind plate movements. Hot, less dense material rises while cooler, denser material sinks, creating convection currents in the mantle.

Other contributing mechanisms:

• Ridge push: Gravity pulls plates away from mid-ocean ridges.
• Slab pull: Sinking of cold, dense oceanic plate at subduction zones.

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2.4.5 Plate Boundaries and Hot Spots

Plate Boundaries:

• Divergent: New crust formation (e.g., ocean ridges).
• Convergent: Crust is destroyed/subducted (e.g., trenches).
• Transform: Crust is neither created nor destroyed.

Hot Spots:

• Localized zones of upwelling magma.
• Form volcanic islands (e.g., Hawaiian Islands).
• Stationary relative to moving plates.

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2.5 Let Us Sum Up

This unit explored how continents and ocean basins formed through tectonic processes, including sea-floor spreading and the theory of plate tectonics. The Earth's lithosphere consists of several moving plates whose interactions create geological features like mountains, earthquakes, and ocean trenches. Mantle convection, slab pull, and ridge push are key mechanisms behind these movements. Understanding plate tectonics is essential to grasp Earth’s dynamic structure and evolution.

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2.6 Keywords

·         Continental Drift-Movement of continents over geological time.

·         Pangaea-Supercontinent that existed ~200 million years ago.

·         Sea-Floor Spreading-Formation of new oceanic crust at mid-ocean ridges.

·         Plate Tectonics-Theory describing Earth's outer shell as broken into plates.

·         Lithosphere-Rigid outer layer of Earth (crust + upper mantle).

·         Asthenosphere-Semi-fluid layer beneath the lithosphere.

·         Convergent Boundary-Plate boundary where plates move toward each other.

·         Divergent Boundary-Plate boundary where plates move apart.

·         Transform Boundary-Plates slide horizontally past each other.

·         Mantle Convection-Heat-driven circulation in Earth’s mantle.

·         Slab Pull-Sinking of a cold, dense plate pulling the rest of the plate.

·         Ridge Push-Gravity-driven force pushing plates from ridges.

·         Hot Spot-Stationary volcanic region fed by mantle plume.