Weathering breaks down and loosens the surface minerals of rock so they can be transported away by agents of erosion such as water, wind and ice. There are two types of weathering: mechanical and chemical. Mechanical weathering is the disintegration of rock into smaller and smaller fragments. Frost action is an effective form of mechanical weathering. When water trickles down into fractures and pores of rock, then freezes, its volume increases by almost 10 percent.
(July 2002) Despite the fact that sub-Saharan Africa contains only about 11 percent of the Earth's population, the region is the world's epicenter of HIV/AIDS. The numbers are daunting. Adult HIV prevalence is 1.2 percent worldwide (0.6 percent in North America), but it is 9.0 percent in sub-Saharan Africa.
HIV/AIDS in South Africa is a prominent health concern; South Africa is believed to have more people with HIV/AIDS than any other country in the world. The 2007 UNAIDS report estimated that 5,700,000 South Africans had HIV/AIDS, or just under 12% of South Africa's population of 48 million. In the adult population the rate is 18.5%. The number of infected is larger than in any other single country in the world.