Chapter 5.1 Weathering

Weathering Weathering is the breaking down and changing of rocks at or near Earth’s surface. There are the types of weathering: mechanical and chemical.

Mechanical Weathering Mechanical weathering takes place when physical forces break rock into smaller and smaller pieces without changing what the rock is made of. When mechanical weathering takes place it creates more exposed surface on the rock. There are 3 physical processes that produce mechanical weathering: frost wedging, unloading, and biological activity.

Frost Wedging When water freezes it expands and creates an outward force. Water can work it’s way into cracks in the rock and that water will freeze, expand, and then enlarge the crack in the rock. After many freeze-thaw cycles it will break a rock. This is frost wedging. Frost wedging is common in mountain areas. Talus – a large pile of rocks that forms at the base of steep, rocky cliffs.

Unloading Igneous rock is exposed to the outside by uplift (being pushed up) and erosion of rock that is over it. This movement of rock toward the Earth’s surface decreases pressure. This is called unloading. This will cause the rock on the outer layers to expand more than rocks below so the outer layers will separate and break loose. This is called exfoliation. This makes dome shaped rock formations.

Biological Activity This is the activity of organisms – like plants, burrowing animals, and humans. Plant roots can grow into rocks and cause cracks in the rock.

Chemical Weathering This is the transformation of rock into one or more new compounds. (we are making something new)

Water Water produces chemical weathering by absorbing gases from the atmosphere and ground. These gases can react with minerals in rock and produce new compounds. Water can absorb carbon dioxide from the air and ground. That can make carbonic acid that can react with many minerals. Acid precipitation can also cause weathering or rocks.

Chemical Weathering of Granite Granite is made mainly of feldspar and quartz. When granite is exposed to carbonic acid (in water) the feldspar is converted to clay minerals. The quartz stays the same so over time the quartz will be lost to the environment because the clay cannot hold it.

Chemical Weathering of Silicate Minerals When silicate minerals are exposed to chemical weathering it dissolves sodium, calcium, potassium, and magnesium. The remaining minerals produce clay minerals.

Spheroidal Weathering This is change to the physical shape of the rock. When water goes through cracks in rocks it weathers corners and edges so the rock becomes rounded. It can also cause the outer layers of rock to separate from the rock’s main body.

Rate of Weathering Mechanical weathering can affect the rate of chemical weathering. By breaking rock into smaller pieces it speeds up the rate of chemical weathering because more surface is exposed to the elements. 2 other factors that affect the rate of weathering are rock characteristics and climate.

Rock Characteristics One main characteristic are the amount of cracks in a rock, this increases weathering. The type of rock also affects the rate. Different types of rock weather at different rates. Granite vs. Marble – granite does not weather as fast as marble.

Climate Temperature and moisture have a strong effect on weathering. These can affect the freeze-thaw cycle. Climate that favors chemical weathering has high temperature and a lot of moisture.

Differential Weathering Different parts of the same rock can weather at different rates. This is differential weathering. This is because there is a difference in mineral composition and variations in the number and spacing of cracks.