Abstract: There are three kinds of basic erosion periods in the Loess Plateau. The first is caused by climate, the second by tectonic movement and the third by human activities. During the warm and moist climatic period, there was rich precipitation and the vegetation grown luxuriantly, which held back the activity of sandstorm. So in this period, the accumulation of aeolian dust became slow or came to stop. But because of much precipitation, the acting force of the running water became stronger and the formed loess would be somewhat eroded. This period is the climate-caused erosion period of the Loess Plateau in theory. In fact, the loess became soil easily and the warm and moist climate benefited the vegetation growing, the erosive action was held back. So in this period, the loess erosion was not very strong, which is proved by a lot of red-brown paleosol formed during the warm and moist period. During the cold and dry climatic period, wind-force became strong; it was helpful to the Loess Plateau's development. If no cold and dry climate, there would be no Loess Plateau. Although the erosion was stronger in cold and dry period than in warm and moist period, the eroded volume was less than the accumulated volume. So the cold and dry climatic period isn't absolutely the erosion period of the Loess Plateau. Tectonic uplifting caused the base level of erosion to descend and strengthened water erosion made erosion fast along the edge of the loess tableland. This period was the tectonic erosion period. The observation on the terraces indicates that the Loess Plateau has experienced six tectonic erosion periods in its developing processes. Although the erosion scale in the tectonic erosion period was different, it is the main reason that causes the Loess Plateau to break up and naturally erode. The unreasonable human activities deteriorated the natural vegetation and the structure of the soil, which resulted in the extensive erosion of the Loess Plateau in modern times. Besides, there were erosion periods comprehensively caused by climate and tectonic movement and by tectonic movement and human activities. It is the human activity that speeds up the erosion in present days. It is necessary to take biological and engineering measures to control the soil erosion in the Loess Plateau.

Key words: Loess Plateau, climatic erosion period, tectonic erosion period, human-caused erosion period