The continents move apart 1 yard per century. That means when geologist Alfred Wegner discovered Pangaea the continents were together. Over time the continents were moving. But they were moving at the speed of a snail. People didn't know that the land was separating. According to the plate-tectonics theory, scientists believe that Earth's surface is broken into a number of shifting plates, which average about 50 miles in thickness. The boundaries of lithosphere plates are active.