Alfred Wegener studied astronomy and meteorology-and was even a record-holding balloonist-before he became famous for his theories on how the land and seas on Earth were formed and change. These ideas are continental drift and plate tectonics. Seeing that the continents fit together like a puzzle, Wegener proved the theory that all of Earth&&&^s continents were once connected. Although his theories weren&&&^t accepted until after his death, scientists use plate tectonics to explain volcanoes and many other changes on Earth.