China's topography was formed around the emergence of the Qinghai-Tibet Plateau created millions of years ago in the collision of the Indian and Eurasian plates. The Qinghai-Tibet Plateau rose continuously to become the "roof of the world," averaging more than 4,000 m above sea level. The terrain in China then gradually descends from west to east like a staircase. The second step of the staircase includes the gently sloping Inner Mongolia Plateau, the Loess Plateau, the Yunnan-Guizhou Plateau, the Tarim Basin, the Junggar Basin and the Sichuan Basin, with an average elevation of between 1,000 m and 2,000 m. The third step, dropping to 500-1,000 m in elevation, begins at a line drawn around the Greater Hinggan, Taihang, Wushan and Xuefeng mountain ranges and extends eastward to the coast of the Pacific Ocean. Here, from north to south, are the Northeast Plain, the North China Plain and the Middle-Lower Yangtze Plain. Interspersed among the plains are hills and foothills. To the east is the fourth step of the staircase, land that consists of the vast continental shelf formed by the shallows together with the islands on the rim of the mainland.