Page 469 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 433
(a) Exfoliated granite, White Mountains, New Hampshire.
(b) Exfoliation forms characteristic granite domes such as Half Dome in Yosemite, California.
▲Figure 14.10 Exfoliation in granite. exfoliation loosens slabs of rock, freeing them for further weathering and downslope movement. [(a) and (c) Bobbé Christopherson. (b) Robert Christopherson.]
When some minerals undergo hydration, they ex- pand, creating a strong mechanical wedging effect that stresses the rock, forcing grains apart in a physi- cal weathering process. A cycle of hydration and dehy- dration can lead to granular disintegration and further susceptibility of the rock to chemical weathering. Hy- dration also works with other processes to convert feldspar, a common mineral in many rocks, into clay minerals. The hydration process is also at work on the sandstone niches shown in Figure 14.9.
▼Figure 14.11 Chemical weathering of sandstone. The Saint Magnus Cathedral, in Kirkwall, Scotland, shows the signs of
almost nine centuries of chemical weathering. [Bobbé Christopherson.]
(c) Rock sheeting exposed along Beverly Sund, Nordaustlandet Island, in the Arctic Ocean.
Hydration and Hydrolysis Chemi- cal decomposition of rock by water can occur by the simple combina- tion of water with a mineral, in the process of hydration, and by the chemical reaction of water with a mineral, in the process of hydrolysis. Hydration, meaning “combination with water,” involves little chemi- cal change (it does not form
new chemical compounds) but does involve a change in structure. Water becomes part of the chemical compo- sition of the mineral, form- ing a hydrate. One such hydrate is gypsum, which is hydrous calcium sulfate (CaSO4 • 2H2O).