Around 225 million years ago, Earth was home to the supercontinent Pangaea and the massive sea Panthalassa. In fact, Earth's land and water existed in several configurations before today's familiar continents and oceans formed. Readers of this book will get an accessible introduction to plate tectonics. This key scientific theory explains why Earth's landmasses have changed over time. The theory posits that the planet's crust is broken up into plates that are constantly, if slowly, on the move. The book also examines the impact of plate tectonics on volcanoes, earthquakes, and the formation of mountains and rift valleys.