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 The number of AIDS-related deaths in sub-Saharan Africa fell by 39 percent between 2005 and 2013.  In the 24 countries where the U.S. Government focuses its maternal and child health efforts, the maternal mortality ratio decreased almost 25 percent from 420 per 100,000 in 2005 to 321 per 100,000 in 2013.