PLATE TECTONICS IN ACTION Plate tectonics explains how Earth's surface is shaped by the movement of large plates in the outer shell of the Earth. These plates slowly move over a softer layer called the asthenosphere. At divergent boundaries, like the Mid-Atlantic Ridge, plates move apart and form new crust. At convergent boundaries, they collide to form mountains or cause earthquakes and volcanoes. At transform boundaries, they slide past each other, leading to earthquakes. Weathered sediments i