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 1.2 million people died of AIDS in 2014, a 42% decrease since 2004. 2  Deaths have declined due in part to antiretroviral treatment (ART) scale-up. 3  HIV is a leading cause of death worldwide and the number one cause of death in Africa. Deaths have declined due in part to antiretroviral treatment (ART) scale-up. 2  HIV is a leading cause of death worldwide and the number one cause of death in Africa. 3  New HIV infections globally have declined by 35% since 2000. 4  In 61 countries, new HIV infections have decreased by more than 20%.