Physical/Mechanical Weathering

Discover mechanical weathering! Learn how ice, water, and wind break rocks. Perfect for science students.

Why is mechanical weathering so important here? Why is mechanical weathering so important here? The high mountains of the Sierra Nevada are made of granite. The climate is cold. Mechanical weathering dominates the weathering here. Can you find signs of mechanical weathering in the photo? Can you see evidence of chemical weathering in the photo? Why is mechanical weathering so much more important than chemical weathering here? How is mechanical weathering altering the landscape in the high Sierra? Mechanical Weathering Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they're just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression “a chip off the old block“ to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind. What are the three agents of weathering mentioned in the reading? Click all that apply. ice water dust wind Physical Weathering does notchange the composition (make up) of the rock. True False Ice Wedging Rocks can break apart into smaller pieces in many ways. Ice wedging is common where water goes above and below its freezing point (Figure below). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions like the Sierra Nevada pictured above. Ice Wedging (Continued) This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can. (For the record, water is very unusual. Most substances get smaller when they change from a liquid to a solid.) Ice Wedging (Continued) Ice wedging happens because water expands as it goes from liquid to solid. When the temperature is warm, water works its way into cracks in rock. When the temperature cools below freezing, the water turns to ice and expands. The ice takes up more space. Over time, this wedges the rock apart. Ice wedging is very effective at weathering. You can find large piles of broken rock at the base of a slope. These rocks were broken up by ice wedging. Once loose, they tumbled down the slope. Liquid water fills the space in cracks of rock and freezes. Once the water freezes it _ . gets smaller expands Ice wedging can wear away rocks and break it apart. False True Abrasion Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. Moving water causes abrasion; it moves rocks so that they bump against one another (Figure below). Strong winds cause abrasion by blasting sand against rock surfaces. Finally, the ice in glaciers cause abrasion. Pieces of rock embedded in ice at the bottom of a glacier scrape against the rock below. If you have ever collected beach glass or pebbles from a stream, you have witnessed the work of abrasion. What is abrasion? Rocks being affected by weather Rocks bumping other rocks What causes abrasion? Click all that apply. Moving water Gravity Sunny days Wind Plants and Animals in Mechanical Weathering Sometimes plants or animals cause mechanical weathering. This can happen slowly. A plant’s roots grow into a crack in rock. As the roots grow larger, they wedge open the crack (Figurebelow). Burrowing animals can also cause weathering. By digging for food or creating a hole to live, in the animal may break apart rock. Humans and Mechanical Weathering Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses. What are examples of how humans cause mechanical weathering? Click all that apply. building homes Hiking Building roads Playing at the park Weathering breaks down rocks AND minerals? True False What is physical weathering? Breaking down of rock by natural forces Rocks changing composition. How do trees break down solid rock? A tree will not grow straight if a rock is in the way. Roots grow in the cracks of rocks and further extend to break the rock. What causes the most common type of physical weathering? Heavy wind blowing between the cracks Constant freezing and thawing of rocks Changes in weather What percent does water expand? 8 15 10 11 How does water break apart a rock? Water in cracks freeze and expand which puts pressure on the rocks. Liquid water keeps collecting in the cracks and over time the cracks expand. Match the terms with the definitions abrasion Form of mechanical weathering that occurs whenever one rock hits another. ice wedging Water enters a crack, expands as it freezes, and wedges the rock apart. mechanical weathering Weathering that breaks rocks into smaller pieces without altering their chemical composition.