Mechanical weathering occurs when a rock is physically broken into smaller pieces. It may also be called _________________ and does not involve any change in a rock’s _________________. Changes are only in the size and sometimes the shape of the rock. Rocks and minerals break down into smaller pieces. There are 5 types of mechanical weathering: temperature & frost wedging, pressure, organic activity, gravity, and abrasion. Physical Weathering Composition
Temperature plays a significant role in mechanical weathering through heating and cooling. During the day rocks are heated up and the outer layers begin to expand. The inside of the rock however remains cool and does not expand. At night the outer layer contracts when cooled. This cycle can cause pieces of the rock to crack or flake off.
Expands Ice wedging Water may get into cracks in the rock. When water freezes, it expands by 9%. When the water freezes and expands, the surrounding rock may be broken apart.
The next time water freezes in the cracks the rock will be broken apart even further. _________________ is the repeated thawing and freezing of water in the cracks of rocks. This is how potholes are formed. Frost Wedging
Bedrock at great depths is under _________________ from the overlying rock layers. When the overlying layers are removed the pressure of bedrock is reduced. The bedrock surface is then able to expand. Pressure
Joints are are long, curved cracks that may occur parallel to the surface of the rock. Over time when the outer rock layers are stripped away it is called exfoliation.
Wind-blown sand wears away rocks in a process called abrasion. Mars; polished rock due to wind abrasion
Animals burrowing may also loosen rock material.
Organic activity: Plants and animals can cause mechanical weathering. The roots of trees and other plants can wedge themselves into the cracks of rocks. As the roots grow, pressure is exerted on the surrounding rock and the rock will begin to split. This is called root pry.
Sometimes gravity pulls loosened rocks down mountain cliffs in a landslide, causing them to break apart
The process by which rocks and minerals undergo changes in their composition as the result of chemical reactions is called chemical weathering There are 5 types: Water, oxidation, carbonation, acid precipitation, and plant acids.
Water is the most important agent. The reaction of water with other substances is known as hydrolysis. An example of this would be the decomposition of a silicate mineral such as potassium feldspar into kaolinite.
Oxygen in the air can combine with iron and through oxidation may form rust. An example would be when the iron in magnetite combines with atmospheric oxygen to form the iron in hematite. Example: 2Fe3O4 (magnetite) + ½ O2 -- 3Fe2O3 (hematite)
Carbonic acid is formed when carbon dioxide (CO2) combines with water in the atmosphere. That then falls to the Earth’s surface as precipitation. (acid rain) This acid reacts with minerals such as limestone or marble and breaks them down.
The oxidation of sulfur dioxide and nitrogen oxides that are released into the atmosphere may form acid rain. Grandfather Mt. NC
The sulfur dioxide forms from the burning of fossil fuels and the nitrogen oxides are emitted from motor-vehicle exhaust.
The pH of water is neutral and has a value of seven(7).
Plants produce weak acids that can dissolve certain minerals in rocks. The acids seep into rocks and dissolve some of the minerals and the rock will begin to break apart. Lichens and mosses are examples
The warmer and wetter the climate, the faster chemical weathering can occur. Dry, cool climates promote mechanical weathering.
In general, sedimentary rocks are more easily weathered than the harder igneous and metamorphic rocks.
20. The more total surface area available, the easier it is for a rock to be chemically weathered.
Section Assessment 1.Match the following terms with their definitions. ___ weathering ___ frost wedging ___ exfoliation ___ hydrolysis A.the process by which rocks on Earth’s surface break down and change B.the process by which outer rock layers are stripped away C.the repeated freezing and thawing of water in the cracks of rocks D.the reaction of water with other substances A C B D
Section Assessment 2.What is the difference between mechanical and chemical weathering in relation to rock composition? Mechanical weathering does not involve any change in a rock’s composition. Chemical weathering is the process in which rocks undergo changes in their composition.
________ Acid precipitation has a pH lower than 5.6. ________ Hydrolysis can result in the formation of clay minerals. ________The highest rate of chemical weathering occurs in higher latitudes. ________ Trees can play a part in physical weathering. ________ Iron is not affected by oxidation. Section Assessment 3.Identify whether the following statements are true or false. true false True false