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.
Kenya, according to a 2008 report from the Joint United Nations Programme on HIV/AIDS, had the third largest number of individuals in Sub-Saharan Africa living with HIV. It also had the highest prevalence rate of any country outside of Southern Africa. Kenya's HIV infection rate dropped from around 14 percent in the mid-1990s to 5 percent in 2006, but rose again to 6.2 percent by 2011.