Page 467 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 14 Weathering, Karst Landscapes, and Mass Movement 431
(a) Physical weathering along joints in sandstone produces discrete blocks in Canyonlands National Park, Utah. Note the differential weathering, as the softer supporting rock underneath the slabs has already weathered and eroded.
(b) Joint-block separation in slate at Alkenhornet, Isfjord, on Spitsbergen Island in the Arctic Ocean, where freezing is intense.
▲Figure 14.7 Physical weathering along joints. [(a) Robert Christopherson. (b) Bobbé Christopherson.]
by permafrost (discussed in further detail in Chapters 17 and 18).
As winter ends and temperatures warm in moun- tainous terrain, the falling of rocks from cliff faces occurs more frequently. The rising temperatures that melt the winter’s ice cause newly fractured pieces to fall without warning and sometimes start rockslides. Many such incidents are reported in the European Alps, and they appear to be on the increase. The falling
▲Figure 14.8 Rockfall. Shattered rock debris from a large rockfall in yosemite National Park. Freshly exposed, light-coloured rock shows where the rockfall originated. [Robert Christopherson.]
rock pieces shatter on impact—another form of physi- cal weathering (Figure 14.8).
salt-Crystal Growth (salt Weathering) Especially in arid climates where heating is intense, evaporation draws moisture to the surface of rocks, leaving behind previously dissolved minerals in the form of crystals (the process of crystallization). Over time, as the crystals accumulate and grow, they exert a force great enough to separate the grains making up the rock and begin breaking the rock to pieces, a process known as salt-crystal growth, or salt weathering.
In many areas of the U.S. Southwest, groundwater that meets an impermeable rock layer, such as shale, within sandstone rock strata will flow laterally until it emerges at a surface. The water then evaporates and leaves salt crystals that loosen the sand grains within the rock. Sub- sequent erosion of the grains by water and wind complete the sculpting process, forming alcoves at the base of sand- stone cliffs. More than 1000 years ago, Native Americans
Georeport 14.1 Rockfalls in Kluane
In alpine glacial regions, melting valley glaciers have an effect on weathering and mass movement processes. Similar to the physical weathering process of exfoliation described in this chapter, there is a release of pressure as thinner glaciers expose
valley slopes. No longer being buttressed by glacier ice, pieces of rock break off and move downslope as rockfalls or rock avalanches as the slopes attempt to reach an equilibrium with the new conditions. Denny Capps and Dan Shugar were graduate students at Simon Fraser University researching impacts of climate change on glaciers and mountains in Kluane National Park and Reserve, yukon, when they observed these processes firsthand. In one indelible incident observed by Capps, Shugar was collecting samples on a slope when a rock avalanche occurred and the debris hurtled towards him.* Luckily he was able to move to the side and escaped unscathed. (Note that our cover photo is from this same national park.)
*C. Montgomery, “When mountains crumble,” Canadian Geographic, 127, 3 (May/June 2007): 68–78.