Notes on Plate Tectonics and Continental Drift

Alfred Wegener proposed the hypothesis of continental drift, suggesting all continents were once joined as a single landmass called Pangaea.
Continents have slowly moved to their current positions over millions of years.

Fossil evidence: Similar fossils (e.g., Mesosaurus, Glossopteris) found in continents now separated by oceans, suggesting they were once connected.
Geological similarities: Rock structures, such as those in the Appalachian Mountains, are similar to those in Greenland and Western Europe.
Climate clues: Fossils of warm-weather plants found in Arctic regions; glacial deposits found in now tropical regions indicate continental shifts.

Wegener could not explain what forces caused the continents to move.
He suggested Earth's rotation, but this was deemed insufficient by physicists.
He also proposed that continents plowed through oceanic crust, but this was argued against as the crust is too brittle.

Harry Hess proposed the theory of seafloor spreading in the early 1960s.
Hot material from beneath the Earth's crust rises at mid-ocean ridges; this pushes the seafloor apart.
Evidence gathered in 1968 showed younger rocks are near mid-ocean ridges, older rocks further away, supporting this theory.
The magnetic alignment in ocean floor rocks provides clues about past magnetic fields and seafloor formation.

Developed in the 1960s, this combines continental drift and seafloor spreading concepts.
Earth's lithosphere (crust + upper mantle) is divided into plates that float and move on the asthenosphere.
Plate movements are driven by convection currents within the mantle, where hot material rises and cooler material sinks.
Plate boundaries:
- Divergent: Plates move apart.
- Convergent: Plates move together.
- Transform: Plates slide past one another.