as those resulting from plate tectonics, work to build huge mountains like the Himalayas or
majestic volcanoes like Mt Fuji, the forces of weathering gradually wear away rocks, changing
once tall mountains into hills and even plains. The Appalachian Mountains along the east
coast of North America were once as tall as the Himalayas! So what happened?
No human being can watch for millions of years as mountains are slowly built, nor can we
watch as those same mountains gradually wear away. However you probably have been able
to ride your bike or walk along a brand new sidewalk or road. What do you experience? The
new road or sidewalk is smooth and even. If it was made well, there won’t be any cracks or
bumps. Does that smooth surface stay that way? Certainly over millions of years, it will
completely disappear, but we don’t have to wait that long. If you live in a part of the world
that has cold winters, you may only have to wait one year to start seeing changes. We will
talk next about what types of weathering change that brand new, smooth and even sidewalk
into areas that are rough or cracked, chipped or buckled (Figure9.1).
Figure 9.1: You can see the once smooth road surface has cracks and fractures, plus a large
pothole. ( 11 )
Mechanical Weathering
Mechanical weathering (also called physical weathering) is the breaking of rock into
smaller pieces. These smaller pieces will be just like the bigger rock, the pieces will just
be smaller. That means the rock has been changed mechanically (or physically) without
changing its composition. The smaller pieces will have the same minerals, in just the same
proportions as the original rock. You could actually use the expression, ‘A chip off the old
block’ to describe mechanical weathering! The main agents of mechanical weathering are
water, wind, ice, and gravity. You will see how each of these works to break rock into smaller
pieces.