Plate Tectonics and Continental Drift Theory

Historical Development of Plate Tectonics

• Early Earth scientists believed in catastrophism, which stated that present landscapes were created by sudden cataclysms
• Uniformitarianism, a contrasting concept, proposes that internal and external Earth processes occur gradually, not suddenly
• This paradigm shift occurred after 1830, rejecting catastrophism

Continental Drift Theory

• Alfred Wegener, a German climatologist, proposed Continental Drift in 1912
• The theory suggests that continents and landmasses have shifted positions over millions of years
• Pangaea, a single supercontinent, existed around 255 million years ago
• Pangaea Ultima (Neopangaea) formed around 250 million years ago

Evidence Supporting Continental Drift

• Continental Fit: Opposing coastlines of continents fit together
• Similarity of Mountain Ranges: Mountain ranges of the same age in adjoining locations on opposite continents match
• Similarity of Rock Sequences
• Paleoclimate Evidence: Glacial deposits formed 300 million years ago in India, Australia, South America, and Southern Africa match those found in Antarctica today
• Fossil Evidence: Fossils of species like Cynognathus, Mesosaurus, Lystrosaurus, and Glossopteris were found in different continents, supporting continental drift

Seafloor Spreading Theory

• Proposed by Harry Hess in 1960
• Sea floor moves outward from Mid-Oceanic Ridges

Theory of Plate Tectonics

• Involves the interaction of two layers: Lithosphere (outer layer of the mantle) and Asthenosphere (inner layer of the mantle)
• Lithosphere is a 100km thick shell of hard, strong rock that floats on the hot, plastic asthenosphere
• Plate Boundaries separate one plate from another, characterized as:
  • Divergent Boundaries: Moving away from each other
  • Convergent Boundaries: Moving towards each other
  • Transform Boundaries: Plates slide past one another

Laws of Geology

• Law of Original Horizontality: Rock layers are originally laid down horizontally and can later be deformed
• Law of Superposition: Oldest layer is at the base
• Principle of Cross-Cutting Relationships: Cuts or disrupts
• Idea of Unconformities: Continuous until they encounter other solid bodies

Dating Methods

• Relative Dating: Tells how old something is, but does not provide a specific year or date
• Absolute Dating: Provides a specific calendar year
• Radioactive Decay: Unstable atomic nucleus loses energy by emitting radiation
• Half-Life: Time needed for half of the sample of radioactive substance to undergo radioactive decay
• Radiometric Dating: Estimates how long ago rocks are formed

Geologic Time Scale

• Eon: Phanerozoic, Proterozoic, Archean, Hadean (derived from the Greek word "hades")
• Era
• Period
• Epoch