Continental Drift Theory — Wegener's Pangaea Theory

Key concepts: • Pangaea (Pangea): The single supercontinent that existed ~200–300 million years ago • Panthalassa: The single ocean surrounding Pangaea • Drift: The gradual movement of continents over millions of years

Timeline of continental breakup:

1. ~200 million years ago (Mesozoic Era): Pangaea begins to break
2. Pangaea splits into: • Laurasia (northern supercontinent): North America, Europe, Asia • Gondwanaland (southern supercontinent): South America, Africa, Antarctica, Australia, Indian subcontinent
3. Laurasia and Gondwanaland further split and drifted apart
4. India separated from Gondwanaland and drifted north, colliding with Asia → formed the Himalayas
5. Present day: 7 continents in their current positions

Wegener's publication: • Book: 'The Origin of Continents and Oceans' (Die Entstehung der Kontinente und Ozeane), 1915 • Initial reception: rejected by most geologists of the time • No mechanism proposed for how continents drifted

Wegener provided multiple lines of evidence:

1. Jigsaw Fit (Geometric fit): • The eastern coastline of South America fits remarkably well against the western coastline of Africa — like puzzle pieces • This 'fit' is most prominent between South America and Africa

2. Fossil Evidence: • Identical plant and animal fossils found on continents now separated by oceans • Mesosaurus (a freshwater reptile): found in both South America and Africa • Glossopteris (a fern fossil): found in Africa, South America, Antarctica, India, and Australia → shows they were once connected • These organisms could not have crossed the present-day oceans

3. Rock Type Similarity: • Rock formations of the same age and composition found on different continents • Ancient mountain ranges in North America align with ranges in Scotland and Norway (Appalachians = Caledonians)

4. Climatic Evidence: • Coal deposits (tropical forests) found in Antarctica → Antarctica was once in a tropical position • Glacial deposits found in tropical Africa and India → these regions were once near the poles

5. Orientation of Glacial Striations: • Glacier movement marks suggest ice moved from the South Pole and covered parts of South America, Africa, India, and Australia simultaneously — only possible if they were joined

Continental Drift evolved into Plate Tectonics:

In the 1960s, scientists developed the Theory of Plate Tectonics: • Earth's outer shell (lithosphere) is divided into about 15 major tectonic plates • These plates float on the semi-molten asthenosphere • Plates move due to convection currents in the mantle • This provided the mechanism Wegener lacked

Types of plate boundaries:

1. Convergent (collision): Plates move toward each other → Mountains form: Indo-Australian plate + Eurasian plate → Himalayas → Oceanic trench forms when oceanic plate dives under continental plate (subduction)
2. Divergent (separation): Plates move apart → Mid-ocean ridges form: Mid-Atlantic Ridge
3. Transform (sliding): Plates slide past each other → Earthquakes: San Andreas Fault (California)

India's movement: • The Indian plate was part of Gondwanaland • It moved northward over millions of years • Collided with the Eurasian plate ~40–50 million years ago • The collision formed the Himalayas — still rising • India continues to move north at ~5 cm/year

Importance of Continental Drift Theory: • Explains distribution of earthquakes and volcanoes • Explains formation of mountains, ocean trenches, and mid-ocean ridges • Explains why similar fossils are found on different continents • Explains past climate changes