12.1 Weathering12.1 Weathering

Key Idea: Over time, the rocks are Key Idea: Over time, the rocks are broken down by mechanical and broken down by mechanical and

chemical weathering.chemical weathering.

Lesson objectives:Lesson objectives:

1. Describe how mechanical weathering breaks down rocks.

2. Describe how chemical weathering breaks down rocks.

3. Name three factors that affect weathering rates.

Weathering? What’s That?Weathering? What’s That?

• Weathering is the breakup of rock due to exposure to processes that occur at Earth’s surface. The weathering processes belong to two major groups:

A) Mechanical weathering, also called Physical weathering

B) Chemical weathering

Mechanical WeatheringMechanical Weathering

• Mechanical weathering or Disintegration takes place when rock is split into smaller pieces of the same material without changing its composition. This type of change (in size and shape) is also called physical weathering.

Mechanical WeatheringMechanical Weathering

Mechanical WeatheringMechanical Weathering

What Causes Mechanical What Causes Mechanical Weathering?Weathering?

Mechanical of physical weathering processes include:

1. Frost wedging

2. Wetting and drying abrasion

3. Plant and animal activity

4. Exfoliation as a result of upward expansion

1. Frost Wedging1. Frost Wedging

• When water freezes its volume increases by about 10%. Most natural materials shrink as the temperature drops, but water does the opposite.

• When rain water enters existing cracks in rocks and freezes, the ice expands, wedging the rock apart.

Frost WedgingFrost Wedging

Frost WedgingFrost Wedging

• Frost wedging occurs in places where there are frequent freezes and thaws. Bare mountaintops are especially susceptible to frost wedging.

• The vast fields of sharp cornered boulders on mountaintops are pieces of the mountain that has been broken off by frost wedging.

• Frost wedging also creates potholes in the pavement; when the water in the ground freezes, it lifts up the pavement; when the ice thaws, the pavement collapses, leaving a pothole.

2. Abrasion2. Abrasion

• Water, wind, and ice are capable of moving rocks. • Water can tumble boulders, pebbles, and sand along

streambeds. • Wind can blow sand and small pebbles across a rocky

plain. Glaciers, which are moving masses of ice carry rocks over great distances.

• As moving sand, pebbles, and larger rocks grind and scrape against one another, thee rock materials are worn away. This type of mechanical weathering is called abrasion.

• The sand on beaches is a product of abrasion.

3. Plants and Animals 3. Plants and Animals (Biological Weathering)(Biological Weathering)

• The growth of plants and the activity of animals contribute to mechanical weathering of rocks.

• When mosses and other small plants grow on rocks they wedge their tiny roots into pores and crevices. As the roots grow, the rock splits; larger plants may grow in the cracks, continuing to open the cracks.

• Insects, worms, and larger animals dig holes in the soil. These holes allow air and water to reach the bedrock and weather it.

3.Biological Weathering3.Biological Weathering

3.Biological Weathering3.Biological Weathering

4. Upward Expansion4. Upward Expansion

• The upward expansion of rocks that are formed deep underground may result in a mechanical weathering process called exfoliation. For example magmatic rocks formed deep in the ground eventually become exposed when the magmatic rocks are lifted up and the overlaying rocks are eroded away.

• The removal of the overlaying rocks reduces the pressure along the surface of the granite. , causing the granite to break along curved joints that are parallel to the surface. Such joints can be seen in exposed peaks.

Exfoliation Exfoliation

B). Chemical weatheringB). Chemical weathering

• Chemical weathering involve a change in the chemical makeup of the rock. Such changes occur under the influence of water, either in liquid or gaseous state (water vapors), acids, and oxygen.

• Water interacts with the minerals in the rocks in two ways:

a) it dissolves some minerals and removes them from the rock and

b) Water is added to the molecules of some minerals through a process hydrolysis. Through hydrolysis the newly formed minerals become softer, and are removed easier.

Chemical WeatheringChemical Weathering

• The water often combines chemically with gasses from the atmosphere and forms acids, such as:

-carbonic acid, (water + CO2) which dissolves limestone.

-sulfuric acid (water + SO2 ), which is pretty aggressive, and dissolves a larger array of minerals, as well as concrete, metal structures, etc.

Caves form due to chemical Caves form due to chemical weatheringweathering

Oxygen and Chemical weatheringOxygen and Chemical weathering

• Iron is present in the chemical make up of many minerals. When iron combines with oxygen, it forms a iron oxide, or rust.

• Many rock formations display beautiful yellowish, or reddish colors when iron oxides are present. There are two common iron oxides: limonite (yellow) and hematite (red).

The red coloration of the rocks is The red coloration of the rocks is given by iron oxidesgiven by iron oxides

Rates of WeatheringRates of Weathering

• The weathering is a very slow process. The following conditions conditions can accelerate the decay of the rocks though:

1. surface exposure

2. composition of the rock

3. Climate

1. Surface Exposure1. Surface Exposure

• The rate at which a rock is affected by weathering increases as the surface exposed to weathering becomes larger.

Surface ExposureSurface Exposure

Rates of WeatheringRates of Weathering

• The composition of the rock

Some rocks are more resistant to weathering than others, depending on the minerals that are found in these rocks. Quartz is a very resistant mineral, therefore rocks that contain a lot of quartz are very resistant to weathering.

Calcite reacts with weak acids and the rocks containing calcite (limestone) are easily weathered.

The rock formation was exposed to the same conditions, The rock formation was exposed to the same conditions, yet different layers of rocks responded differently to yet different layers of rocks responded differently to

weathering (Palo Duro Canyon, Texas)weathering (Palo Duro Canyon, Texas)

3. Climate Influences Weathering3. Climate Influences Weathering

-Humid and climates, with lots of rain favor chemical weathering; water and heat accelerate chemical reactions, and some minerals are rapidly removed from rocks.

-Dry climates, on the other hand favor mechanical weathering. In deserts, for example, the temperature drop below freezing, and during day time is very, very hot. The rocks dilate and contract repeatedly, and crumble into small pieces. Ice wedging is also an important factor.