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.
Women aged between 30 and 34 and males aged 35 to 39 had the highest infection rates: 36% of females and 28.8% of males in these respective age groups contracted HIV. Provincially, KwaZulu-Natal has the highest HIV prevalence (16.9%) and the Western Cape the lowest (5%). The survey is the fourth in a series of national HIV household surveys conducted by a consortium of scientists led by the HSRC â the previous surveys were conducted in 2002, 2005 and 2008.