Incredible rock formations called hoodoos can be seen at Bryce Canyon National Park in Utah.

Erosion What are the agents of erosion? Erosion acts through weathering, the force of gravity, and through the movement of streams, groundwater, glaciers, wind, and waves.

Erosion is the process that wears down and carries away rock and soil. Erosion is the destructive process that has shaped Earth’s surface over hundreds of millions of years. The end product of erosion is sediment.

Weathering What causes mechanical and chemical weathering? Weathering is the process by which rocks are chemically altered or physically broken down into fragments at or near Earth’s surface. There are two forms of weathering: mechanical and chemical. They cause rocks to disintegrate or decompose.

Weathering Mechanical Weathering Mechanical weathering is the process of physically breaking rock into smaller fragments. Mechanical weathering occurs through frost wedging, abrasion, growth of plant roots, and other processes.

Weathering Frost wedging occurs when water freezes to form ice and the water expands. The expansion pries open cracks. Frost wedging is an important cause of weathering in mountains, where water often freezes at night and melts during the day.

Weathering When water expands to form ice, it can pry open cracks in rock. This is called frost wedging.

Weathering Abrasion is a form of mechanical weathering that occurs when rocks scrape or grind against one another. For example, abrasion occurs when sand carried by water or wind causes bits of rock to break off as if they were being sandblasted.

Mechanical weathering also occurs in other ways. Plant roots can grow into cracks in a rock. Roots exert a powerful force that can slowly pry the rock apart. Erosion removes material from the surface of a mass of rock, reducing pressure on the rock that remains. The rock expands, causing the outside of the rock to crack and flake off.

Weathering Chemical Weathering Chemical weathering is a process in which rock is broken down by chemical reactions. Chemical reactions dissolve the minerals making up rock or change them into new minerals. Eventually, the rock crumbles and disintegrates.

Water is the main agent of chemical weathering. Water is an effective solvent. All minerals dissolve in water, though most do so very slowly. Chemical weathering also occurs because rain is slightly acidic.

Weathering One form of chemical weathering occurs through oxidation. Iron-rich minerals become rusted like this old bicycle.

Weathering Chemical weathering also occurs when rainwater dissolves or reacts with the minerals in rocks, as with this statue of a lion.

Weathering One type of chemical weathering, the rusting of minerals that are rich in iron, involves oxidation.

Rates of Weathering What factors affect the rate at which rocks weather? The rate at which mechanical and chemical weathering take place depends on three main factors: temperature, the availability of water, and the type of rock.

Rates of Weathering Chemical weathering occurs more rapidly in places with high temperatures and abundant rainfall. These conditions generally speed up chemical reactions. Mechanical weathering occurs faster in places where temperature conditions frequently alternate between freezing and thawing.

Rates of Weathering Some rocks, such as limestone and marble, undergo rapid chemical weathering. These rocks are composed primarily of calcite, a mineral that reacts readily with carbonic acid.

Rates of Weathering Different rocks weather at different rates. Even though the slate tombstone is older, it is much less weathered than the marble one (left).

Mass Movement What force causes mass movement? Through the process of mass movement, gravity moves loose material down a slope.

Mass Movement Once weathering loosens particles of rock, the particles, or sediment, do not stay in the same place. Mass movement is the downward movement of rock and soil due to gravity.

Mass Movement Landslides The rapid movement of large amounts of rock and soil is a landslide. Landslides often occur after heavy rains or after earthquakes loosen materials on a steep slope.

Mass Movement Mudflows Rapid mass movements of soil and other sediment mixed with water are called mudflows. Mudflows tend to occur in areas where fine sediment has collected in thick layers. When it rains, water loosens the sediment and increases its weight.

Mass Movement Creep Creep occurs when soil gradually moves down a slope. Creep often occurs because of the formation of ice. Each time the ground freezes, the soil expands outward. Creep happens so slowly that it’s hard to notice.

Mass Movement Creep can be caused by the alternate freezing and thawing of water in soil. Slumping often occurs when soil that is rich in clay is soaked by water.

Mass Movement Slumping Slumping occurs when weak layers of soil or rock suddenly move downslope as a single unit. Gravity acting on water-saturated soil and rock causes slumping. Slumping often leaves a curved scar.

Assessment Questions Which of these factors tends to increase the rate of chemical weathering? freezing temperatures strong winds abundant rainfall presence of minerals such as quartz

Assessment Questions Which of these factors tends to increase the rate of chemical weathering? freezing temperatures strong winds abundant rainfall presence of minerals such as quartz ANS: C

Assessment Questions Which type of mass movement involves the gradual movement of soil down a slope? landslides mudflows creeps lumping

Assessment Questions Which type of mass movement involves the gradual movement of soil down a slope? landslides mudflows creeps lumping ANS: C

Assessment Questions Erosion breaks rocks down by two processes: mechanical weathering and chemical weathering. True False

Assessment Questions Erosion breaks rocks down by two processes: mechanical weathering and chemical weathering. True False ANS: T