The outside of the Earth is not even. There are high places called mountains, and high flat places called plateaus. There are low places called valleys and canyons. For the most part, moving air and water from the sky and seas damages rocks in high places and breaks them into small pieces. The air and water then move these pieces to lower places. Because of this, the Earth would have been very flat a long time before now. The fundamental cause of the differences in the Earth's surface is plate tectonics. The shape of the entire planet itself is not even a ball. Because of its velocity, Earth has a slight bulge at the Equator. Other than that, Earth is shaped more like a pear than an actual sphere.