Plate Tectonics: Continental Drift, Seafloor Spreading, and Plate Boundaries
Chapter 7 – Plate Tectonics
Alfred Wegener’s Theory of Continental Drift
Alfred Wegener proposed the theory of Continental Drift, suggesting that a supercontinent called Pangaea existed around 200 million years ago and gradually broke apart, with the continents “drifting” to their current positions. He presented several pieces of evidence to support his theory:
- Fit of the continents: The shapes of South America and Africa appear to fit together like puzzle pieces.
- Fossil evidence: Matching fossils of extinct organisms are found on different continents, suggesting they were once connected.
- Rock types and structures: Similar rock types and geological structures, such as mountain ranges, are found on different continents.
- Ancient climates: Evidence of past climates, such as glacial deposits, suggests that continents were once located in different climatic zones.
Seafloor Spreading and Plate Tectonics
Seafloor spreading provided crucial evidence for plate tectonics. Studies, including the Deep Sea Drilling Project (DSDP) and the International Ocean Drilling Project (ODP), confirmed key observations:
- Age of the seafloor: The oldest seafloor rocks are approximately 280 million years old, much younger than the Earth’s age of 4 billion years. This is because new seafloor is constantly being created at mid-ocean ridges.
- Paleomagnetism: The study of ancient magnetism preserved in rocks revealed Earth’s magnetic field reversals. These reversals are recorded in seafloor basalts as they form at oceanic ridges, creating a symmetrical pattern that supports seafloor spreading.
Paleomagnetism and Magnetic Reversals
Paleomagnetism is the study of Earth’s past magnetic field, preserved in iron-rich rocks like basalts. Magnetic reversals, where the Earth’s magnetic field flips polarity, are recorded in these rocks. The current magnetic field orientation is “normal,” with the north magnetic pole near the geographic North Pole. Seafloor basalts record the magnetic field at the time of their formation, providing evidence for seafloor spreading and plate movements.
Types of Plate Boundaries
There are three main types of plate boundaries, each associated with different stresses and geological processes:
1. Divergent Boundaries
- Stress: Tensional stress, pulling plates apart.
- Processes: Seafloor spreading, creation of new oceanic crust.
- Features: Mid-ocean ridges, rift valleys.
- Example: Mid-Atlantic Ridge.
2. Transform Boundaries
- Stress: Shear stress, plates sliding past each other horizontally.
- Processes: Earthquakes.
- Features: Transform faults.
- Example: San Andreas Fault.
3. Convergent Boundaries
Convergent boundaries involve the collision of plates, with three main types:
a) Oceanic-Continental Convergence
- Processes: Subduction of the denser oceanic plate beneath the continental plate, forming a deep-sea trench and volcanic arc on the overriding continental plate.
- Features: Trenches, volcanic mountains.
- Example: Andes Mountains.
b) Oceanic-Oceanic Convergence
- Processes: Subduction of the older, denser oceanic plate, forming a volcanic island arc on the overriding plate.
- Features: Trenches, volcanic island arcs.
- Example: Japanese Islands.
c) Continental-Continental Convergence
- Processes: Collision of two continental plates, neither subducting due to their low density, forming mountain ranges.
- Features: Mountain ranges.
- Example: Himalayas.
Hot Spot Volcanism
Hot spots are volcanic regions fed by rising plumes of hot mantle material. They are not directly associated with plate boundaries. As the tectonic plate moves over the stationary hot spot, a chain of volcanoes forms, with the age of the volcanoes indicating the direction of past plate movement. Examples include the Hawaiian Islands and Yellowstone.