Chapter 5: Weathering, Soil, and Mass Movements Section 1: Weathering

Key Questions - Objectives What is the difference between Mechanical and Chemical weathering? What rock characteristics change the rate of weathering? What environmental factors change the rate of weathering?

Notes Outline Mechanical Weathering Frost Wedging Unloading Biological Activity Chemical Weathering Water Weathering of Different Materials Rate of Weathering Rock Characteristics Climate Differential Weathering

Mechanical Weathering Mechanical weathering is the physical destruction or breakdown of rock into smaller pieces called sediments. Mechanical weathering does not change the chemical composition of a rock. Think-Pair-Share There are human causes and natural causes for mechanical weathering. Can you think of some human causes of physical/mechanical weathering?

Mechanical Weathering Frost Wedging Frost wedging occurs when water freezes in a crack in a rock, and by expanding, breaks the rock. How quickly the rock breaks depends on how many freeze-thaw cycles occur in a given amount of time (year, month, day).

Mechanical Weathering Unloading Recall the vocabulary term Regional Metamorphism. The great weight of the land can metamorphose rocks. What happens when the weight is lifted?

Mechanical Weathering Unloading Unloading occurs when intrusive igneous rock reaches the surface and breaks loose from it’s host bedrock. Load (or weight) of surrounding rock on top. Intrusive Igneous Rock

Loading

Loaded

Unloading

Unloaded

Exfoliation is the term used to describe the specific type of unloading where rock separates in thin layers.

Biological Activity There are many ways that animals and plants can mechanically weather rocks. One of the most impressive ways is called Root Wedging.

Chemical Weathering When chemical reactions change the molecular form of rock into something else, it is called chemical weathering. What are other real world examples that show the difference between chemical and physical changes?

Chemical Weathering Water The most prominent and powerful agent of chemical weathering is water. Water absorbs dissolved gasses like CO2 and O2 Water provides constant contact to the rock surface for dissolved gasses to act on rocks Water reacts with some compounds (like CO2 ) to make weak acids

The Weathering of Different Materials Weathering of Granite When granite is chemically weathered, the quartz is resistant to carbonic acid (made by CO2). The feldspar in granite will be mostly converted into clays. The quartz will fall away (sand). Dissolving Rocks Magnesium, Calcium, Potassium, and Sodium present in rocks are dissolved by water and carried away, leaving behind Iron, Aluminum, other non-dissolvable elements which react with oxygen.

Have you ever wondered why the edges of rocks are often smooth? This is because corners are weathered more rapidly than the rest of the rock.

Rates of Weathering Three things effect how quickly or slowly rocks will be weathered. Climate Rock Characteristics Mechanical Weathering Mechanical weathering increases the amount of surface area available for chemical reactions. This speeds up the weathering process Chemical reactions are the source of Chemical Weathering.

Rock Characteristics Mineral composition and shape will change how quickly rocks are weathered. Rocks with cracks will weather more quickly due to mechanical weathering. The mineral (chemical) composition of rocks changes the rate by which those rocks weather. Granite weathers more slowly than marble.

Climate More rainfall, and more temperature cycling will cause rocks to weather more quickly. Dry areas weather very slowly.

END 5.1