Plate Tectonics

What you’ll learn

• Earth's rigid outer shell (the lithosphere) is broken into tectonic plates that slowly ride on the softer, flowing asthenosphere below.
• Alfred Wegener's continental drift was rejected for decades because he could not explain the mechanism moving the continents.
• Fossils (Mesosaurus, Glossopteris), matching rock formations, and puzzle-piece coastlines are the classic evidence that continents were once joined.
• Seafloor spreading, confirmed by magnetic striping and seafloor age, supplied the missing engine and proved the theory.
• The three plate boundaries, divergent, convergent, and transform, produce earthquakes, volcanoes, mountains, and trenches.
• The Pacific Ring of Fire hosts about 75% of Earth's active volcanoes and roughly 90% of its earthquakes.