Built by a geologist

Plate Tectonics

Plate Tectonics Plate tectonics is the unifying theory of geology. It explains earthquakes, volcanoes, mountain ranges, and even why continents move. The Big Idea Earth's lithosphere is broken into about 15 major tectonic plates that float on the semi-fluid asthenosphere. These plates move — slowly, typically 2–10 cm per year — driven by heat from Earth's interior. Continental Drift In 1912, German meteorologist Alfred Wegener proposed that continents were once joined in a supercontinent called Pangaea meaning "all lands" . His evidence: - Coastline fit — South America and Africa fit together like puzzle pieces - Fossil matches — Identical fossils found on continents now separated by oceans - Rock matches — Mountain belts and rock sequences align across continents - Climate evidence — Glacial deposits found in tropical regions, and tropical plant fossils found near the poles Wegener was ridiculed because he couldn't explain how continents moved. The mechanism — seafloor spreading — wasn't discovered until the 1960s. Types of Plate Boundaries Divergent Boundaries Plates Moving Apart - New crust is created as magma rises to fill the gap - Example: Mid-Atlantic Ridge, East African Rift - Creates: Rift valleys, mid-ocean ridges, volcanic islands Convergent Boundaries Plates Colliding Three subtypes: - Ocean-ocean : One plate subducts beneath the other → volcanic island arcs e.g., Japan, Philippines - Ocean-continent : Oceanic plate subducts beneath continental plate → volcanic mountain ranges e.g., Andes - Continent-continent : Neither subducts → massive mountain belts e.g., Himalayas Transform Boundaries Plates Sliding Past - Crust is neither created nor destroyed - Example: San Andreas Fault, Alpine Fault New Zealand - Creates: Earthquakes often large and shallow What Drives Plate Motion? - Mantle convection — Hot rock rises, cool rock sinks, creating circulation cells - Ridge push — Elevated mid-ocean ridges push plates outward by gravity - Slab pull — Dense subducting plates pull the rest of the plate behind them considered the dominant force