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.
There are two main ways that rocks can break apart into smaller pieces. The way that is most common in cold climates is called ice wedging. Ice wedging is the main form of mechanical weathering in any climate that regularly cycles above and below the freezing point. Some places where this happens include Earth’s polar regions and mid latitudes. It also happens in the colder climates of higher elevations, like mountainous regions.
This is how it works. When water changes from a liquid into a solid (ice), it increases in volume. This is a very unusual property. Most substances contract (get smaller) as they change from a liquid to a solid, but water does just the opposite. You may have already experienced this if you ever filled an ice cube tray all the way to the top with water and then put it into the freezer. The ice cubes will be much larger than the amount of water you first put in. You may have also made the mistake of putting your favorite soda into the freezer to cool it down quickly. If you leave your drink in the freezer too long, it will expand so much that it bends or pops the can. Ice wedging happens for the same reason.
Water works its way into cracks and fractures in rock, and then expands as that water freezes. The ice
takes up more space than the water did, which wedges the rock apart, physically breaking the rock into pieces. Ice wedging breaks apart so much rock that you will find large piles of broken rock at the base of a cliff or mountain, as broken pieces separate and tumble down its sides. Ice wedging will work quickly, breaking apart lots of rock in areas that go above and below the freezing point every night and day, and also in areas that cycle with the seasons.
Abrasion is another form of mechanical weathering. Abrasion can happen anywhere. All that is needed is one rock bumping against another rock. Gravity can cause abrasion as a rock tumbles down a mountainside or cliff. Moving water causes abrasion as particles carried in the water collide and bump against one another. Strong winds can pick up pieces of sand and blast surfaces with those sand grains. Finally, the ice in glaciers carries many bits and pieces of rock. As the glacier moves, pieces of rock embedded in the ice scrape against the rocks below. Broken pieces of rock tumbling down a mountain stream or tossed about by waves crashing onto the shore, will become smooth and rounded as abrasions smooth and round the sharp or jagged edges. If you have ever collected beach glass or cobbles from a stream, you have benefited from the work of abrasion.
Scientists talk about a few other types of mechanical weathering but ice wedging and abrasion are the two most important types. Without these two types of mechanical weathering, very little rock would break apart and that would slow down the rate of chemical weathering as well. Sometimes biological elements can do the work of mechanical weathering. This could happen slowly as a plant’s roots grow into a crack or fracture in rock and gradually grow larger, wedging open the crack. Burrowing animals can also break apart rock as they dig for food or to make living spaces for themselves. Today, of course, human beings do quite a bit of mechanical weathering, whenever we dig or blast into rock to build homes, roads, subways, or to quarry stone for construction or other uses.
Actually whenever there is mechanical weathering, it increases the rate of chemical weathering. This happens because as rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur. Let’s say you wanted to make some hot chocolate on a cold day. You can imagine how hard it would be to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have and the easier it is to dissolve in the milk.
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