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Plate tectonics is the scientific theory that describes the large-scale motion of Earth's lithosphere. The 's outer shell is into several plates glide over semi- mantle. plates are in constant, slow , moving about the same speed fingernails grow. The interactions at the boundaries of these plates are responsible for major geological events like earthquakes, volcanic eruptions, and mountain formation. This theory also explains how the continents have drifted to their current positions over millions of years.