The Southern Alps in the South Island were formed by tectonic uplifting and pressure as the Pacific and Indo-Australian Plates collided along the island's western coast. The uplifting continues, raising Aoraki / Mount Cook an average of each year. However, erosive forces are also powerful shapers of the mountains. The severe weather is due to the mountain's jutting into powerful westerly winds of the Roaring Forties which run around approximately 45°S latitude, south of both Africa and Australia. The Southern Alps are the first obstacle the winds encounter after South America, having moved east across the Southern Ocean.