Download Earth Science Chapter 7: Weathering, Erosion, and Soil Chapter

Earth Science
Chapter 7: Weathering, Erosion, and Soil
Chapter Overview
Section 1: Weathering
The process by which rocks on or near Earth’s surface break down and change is called
weathering. The removal and transport of weathered material from one location to another is
known as erosion
1. Mechanical Weathering
The process by which rocks and minerals break down into smaller pieces is mechanical
weathering, also called physical weathering. Mechanical weathering does not involve any
change in a rock’s composition, only changes in the size and sometimes the shape of the
rock. A variety of factors are involved in mechanical weathering
• Temperature
Temperature plays a significant role in mechanical weathering. When water freezes its
volume increases by about 9%. So when liquid water seeps into cracks in rocks or
between rock layers and then freezes, its volume will increase and put pressure or the
rock. When the temperature increases the ice will melt. The repeated thawing and
freezing of water in the cracks of rocks is called frost wedging
• Pressure
Pressure is another factor in mechanical weathering. Rock layers that are buried at great
depth are under a great deal of pressure. When the overlying layers are removed the
pressure on the buried rock layer is reduced allowing the rock to expand, and long,
curved cracks can form. Over time, the outer layers of rock are stripped away. The
process by which outer rock layers are stripped away is called exfoliation. Exfoliation
often results in dome shaped formations
2. Chemical Weathering
The process by which rocks and minerals undergo changes in their composition as the result
of chemical changes is called chemical weathering. Significant agents of chemical
weathering include water, oxygen, carbon dioxide, and acids.
• Water
Water is an important agent in chemical weathering because it can dissolve many kings
of minerals and rocks. The reaction of water with other substances is known as
hydrolysis
• Oxygen
The chemical reaction of oxygen with other substances is called oxidation. The oxygen
in Earth’s atmosphere will react with certain metals in minerals causing those minerals to
undergo a chemical change
• Carbon Dioxide
When carbon dioxide combines with water in the atmosphere a weak acid, carbonic acid,
is formed. Carbonic acid can fall to Earth’s surface as precipitation and react with
minerals such as calcite in limestone and marble to dissolve rocks. Carbonic acid can
also react with silicate minerals to form clay minerals. In high enough concentrations,
carbonic acid can aid in the formation of caves
• Acid Precipitation
Human activity can also contribute to chemical weathering. Through the combustion of
fossil fuels, tremendous quantities of sulfur and nitrogen oxides are released into Earth’s
atmosphere. Once in the atmosphere, these oxides will react with water to form sulfuric
and nitric acids. These acids can fall as rain and have a significant negative impact on
the environment and on man-made structures. Acid rain can damage large areas of
forests, increase the acidity of lakes and streams making it impossible for certain species
of amphibians and fish to survive, react with the limestone and marble in structures and
statues, and increase the rate of corrosion of metals.
3. What Affects the Rate of Weathering
The natural weathering of Earth materials occurs very slowly. It may take 2000 years to
weather 1 cm (less than half an inch) of limestone, and most rocks weather at even slower
rates.
• Climate
The climate of an area is a major influence on the rate of physical and chemical
weathering of Earth’s materials. Variables of climate include precipitation, temperature,
and evaporation, and the interaction between temperature and precipitation has the
greatest effect on a region’s rate of weathering. Chemical weathering occurs readily in
climates with warm temperatures, abundant rainfall, and lush vegetation. Physical
weathering occurs readily in cool, dry climates. Physical weathering rates are highest
where water undergoes repeated freezing and thawing
• Rock type and composition
The characteristics of rocks, including how hard or resistant they are to being broken
down, depend on their type and composition. In general, sedimentary rocks are more
easily weathered than harder igneous and metamorphic rocks.
• Topography and other variables
Materials on level areas are likely to remain in place as they undergo changes, whereas
materials on slopes have a greater tendency to move as a result of gravity. As material
moves down a slope, it exposes underlying rock surfaces and provides more
opportunities for weathering to occur. Another factor that influences the rate of
weathering is the presence of organic matter. Decaying organic matter releases carbon
dioxide, which reacts with water to form carbonic acid.
Section 2: Erosion and Deposition
Erosion is the process that transports Earth materials form one place to another. A number of
different agents transport weathered materials on Earth. At some point the movement of
transported materials will slow down and the materials are dropped in another location in a
process known as deposition
1. Gravity’s Role in Erosion
Gravity is associated with many erosional agents. Gravity tends to pull all materials down
slope, so is responsible for mass movements such as landslides, mudflows, and avalanches
2. Erosion by Running Water
The erosion of running water in small channels on the side of a slope is called rill erosion.
When a channel becomes deep and wide it can evolve into gully erosion
• Coastal Deposition and Erosion
Each year streams and rivers carry billions of tons of sediments and weathered materials
to coastal areas. In the ocean weathering and erosional processes continue. The work of
ocean currents, waves, and tides carves out cliffs, archers, and other features along t he
continents’ edges
3. Glacial Erosion
The erosional effects of glaciers are large scale and dramatic. Glaciers scrape and gouge out
large sections of Earth’s landscape, and have the capacity to carry huge rocks and piles of
debris over great distances. The landscape features left in the wake of a glacier include
valleys and variously shaped deposits of sediments
4. Wind Erosion
Wind is a major erosional agent in areas on Earth that experience both limited precipitation
and high temperatures. Such areas typically have little vegetative cover to hold the soil in
place.
Section 3: Soil
1. Soil Formation
Soil – the loose covering of weathered rock particles and decaying organic matter overlying
the bedrock of Earth’s surface, and serves as a medium for the growth of plants
•
Soil is the product of thousands of years of chemical and mechanical weathering and
biological activity
2. Soil Layers
During the process of soil formation, layers develop in the soil. Most of the volume of soil is
formed from the weathered products of a source rock.
• Residual soil – a soil whose parent material is the local bedrock.
• Transported soil – soil that develops from parent material that has been moved for from
its original location
• Soil profile – a vertical sequence of soil layers. It can take tens of thousands of years for
distinct layers to form in a soil
• Soil horizon – a distinct layer within a soil profile. There are typically four major soil
horizons in mature soils, designated as O,A,B, and C
O-horizon – the top layer of organic material which is made of humus and leaf litter
A-horizon – a layer of weathered rock combined with a rich concentration of dark brown
organic material.
B-horizon – called the zone of accumulation, is a red or brown layer that has been
enriched over time by clay and minerals deposited by water flowing the layers above or
percolating upward from layers below
C-horizon – contains little or no organic matter, and is often made of broken-down
bedrock
3. Factors of Soil Formation
Five factors influence soil formation: climate, topography, parent material, biological activity,
and time. These factors combine to produce different types of soil.