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.
1 HIV is the worldâs leading infectious killer. 2  According to WHO, an estimated 39 million people have died since the first cases were reported in 1981 and 1.5 million people died of AIDS-related causes in 2013. According to WHO, sub-Saharan Africa is the most affected region, with 24.7 million people living with HIV in 2013. 2  Seventy-one percent of all people who are living with HIV in the world live in this region.