Earth Science 5.1 Weathering

Key Concepts we will cover today. . . Students will learn. . . . What is mechanical weathering What is chemical weathering What factors affect the rate of weathering

Weathering Earth’ surfaces are always changing. Internal forces gradually raise some parts of the surface through mountain building and volcanic activity. At the same time, external processes continually break down rock and move the sediments to lower locations. Weathering is the breaking down of rocks at or near Earth’s surface.

Mechanical Weathering Mechanical weathering occurs when physical forces break rocks into smaller and smaller pieces without changing the rock’s mineral composition. Each piece has the same characteristics of the original rock, it is just broken into a smaller pieces so that the surface area is increased.

Mechanical Weathering In nature, three physical processes are especially important causes of mechanical weathering: Frost wedging Unloading Biological activity

Frost Wedging When water freezes, it expands by about 9%, exerting great outward force. In nature, water works it’s way into every crack in a rock. When the water freezes and expands, it enlarges the crack in the rock. After many repeated cycles of this freezing-thawing, the rock will eventually split in two halves. This process is called frost wedging. Frost wedging is most common in mountainous regions in the middle latitudes.

Uploading Large masses of igneous rock may be exposed by the uplift of the rock and erosion over time. Reduced pressure on the rock, called unloading, causes the outer rock layers to expand more than the rock layers beneath. This causes the top rock layers to peel off like the layers of an onion in a process called exfoliation

Uploading Exfoliation is especially common in rock masses made of granite. It often produces large, dome-shaped rock formations.

Biological Activity The activities of organisms, including plants, burrowing animals, and humans, can cause mechanical weathering. Plant roots can grow into cracks in rocks and apply pressure over time, splitting rocks apart. Burrowing animals cam move rocks to the surface where weathering occurs more rapidly. Decaying organisms can produce compounds called acids that cause chemical weathering.

Chemical Weathering Chemical weathering is the transformation of rock into one or more new compounds. In mechanical weathering, the rock breaks into smaller pieces but is compositionally unchanged In chemical weathering, the rock undergoes a transformation in it’s composition as a result of the process.

Water Water is the most important agent of chemical weathering. Water promotes chemical weathering by absorbing gases from the atmosphere and the ground. These dissolved substances than chemically react with various minerals.

Water Oxygen dissolved in water reacts easily with certain minerals, forming oxides. Iron rich minerals get a yellow-reddish coating of iron oxide when they react, forming rust. Iron oxide is the rust that forms when objects with iron are exposed to water.

Water Water also absorbs carbon dioxide when rain falls through the atmosphere. Water that seeps through the ground also picks up carbon dioxide from decaying plant material. The carbon dioxide dissolved in water forms carbonic acid. This is the weak acid in carbonated soft drinks. Carbonic acid reacts with many common minerals.

Water Water in the atmosphere also absorbs sulfur oxides and nitrogen oxides. These oxides are produced by the burning of coal and petroleum. Through a series of chemical reactions in the atmosphere, these pollutants are converted into acid rain. Acid rain precipitation accelerates the chemical weathering of stone monuments and structures considerably.

Chemical Weathering of Granite Chemical weathering affects rocks in different ways depending on their compositional makeup. Lets look at granite as one example. When granite is exposed to water containing carbonic acid, the feldspar is converted mostly to clay materials. As the feldspar is converted to clay, the quartz grains are released from the granite. The quartz grains are deposited near the shore where they become the main component of beaches and sand dunes.

Chemical Weathering of Silicate Materials When silicate materials undergo chemical weathering, the sodium, calcium, potassium, and magnesium they contain dissolve and are carried away by groundwater.

Spheroidal Weathering Chemical weathering can change the shape of rock as well as it’s chemical composition. When water enters along the joints in a rock, it weathers the corners and edges most rapidly. The corners and edges break off and the rock becomes more rounded. The rock takes on a spherical shape and this process is called spheroidal weathering.

Rate of Weathering Mechanical weathering affects the rate of chemical weathering. By breaking rock into smaller pieces, mechanical weathering accelerates chemical weathering by increasing the surface area of rock exposed to the chemical components.

Rate of Weathering Rock Characteristics Climate Two other factors affect the rate of weathering: Rock Characteristics Climate

Rate of Weathering Rock Characteristics: Physical characteristics of rock, such as cracks, are important because they influence the ability of water to penetrate the rock. However, the rock’s mineral composition makes the most difference.

Rate of Weathering : Characteristics Gravestones made of granite are relatively resistant to weathering. In contrast, marble gravestones undergo much more rapid chemical weathering

Rate of Weathering: Climate Climatic factors, especially temperature and moisture, have a strong effect on the rate of weathering. These factors control the freeze-thaw cycle which affect the amount of frost wedging. Temperature and moisture affect the growth of plants which also contribute to breaking rock down so chemical weathering can infiltrate cracks and decay rocks further.

Differential Weathering Different parts of rock masses often weather at different rates. This process, called differential weathering, has several causes. Differences in mineral composition are one cause. More resistant rocks protrude as pinnacles or high peaks.

Concept Review. . . . Mechanical weathering occurs when physical force breaks rocks into smaller and smaller pieces without changing the rock’s mineral appearance. In nature, three physical processes are especially important causes of mechanical weathering: frost wedging, unloading, and biological activity. Chemical weathering is the transformation of rock into one or more new compounds. Two factors that affect the rate of weathering are rock characteristics and climate.