Landscape Evolution

“For purple mountain majesties above the fruited plains” are the famous lyrics in the America The Beautiful song heard and sung by millions of people. Are these words describing the beautiful landscapes that make up the San Francisco Peak and the Colorado Mountains? You have learned that the movements of tectonic plates help produce mountains and rift valleys. Water and wind also have an effect on the outward appearance of the Earth. In this lesson, you will learn about how water, mass wasting, weathering, and erosion have help shape the landforms on Earth.

Essential Questions:

  • What are the two main types of weathering that can occur?
  • What are the physical processes that cause weathering?
  • How does physical weathering occur?
  • What are the chemical processes that cause weathering?
  • How does chemical weathering occur?
  • What role does water play in both physical and chemical weathering?
  • How does soil form?
  • How does mass wasting affect people?
  • How do ice, water, and wind contribute to a landform features?

Module Minute

The process in which rocks and minerals are broken down or change their chemical composition is called weathering. Rocks that are broken down into smaller pieces have undergone the process of physical weathering. The physical and chemical composition of rocks are changed due to natural occurrences in their environment. This process is known as chemical weathering. Soil is a product of these weathered rocks and minerals. Over time many layers of soil have formed on the Earth. Rocks and soil can move from one location to another by processes called mass wasting and erosion. The downward force of gravity can reshape the appearance of mountains, hills, and slopes on land. The effects of wind, water, and glacial erosion have also helped shape the surface of the Earth by producing various landforms.

Key Terms

Weathering – The transformation or breaking down of rocks.

Chemical Weathering – A process that results in rocks being changed into one or more new compounds.

Mechanical weathering – The physical forces that cause rocks to break into smaller pieces without changing the mineral composition of the rocks.

Erosion – The process in which weathered rocks and materials are transported by wind, water, or ice.

Soil – A mixture of weathered rocks, organic matter, water, and gases.

Soil Horizon – A horizontal layer of soil.

Water Cycle – The continuous movement or circulation of the Earth’s water supply.

Glacier – A large moving mass of ice.



photo of rocks erodingThe picture to the left demonstrates how wind and water can reshape a landform due to weathering. Weathering is defined as the transformation or breaking down of rocks. This is a major part of the rock cycle. There are two types of weathering that occur simultaneously: mechanical (or physical) weathering and chemical weathering. Mechanical weathering involves physical forces that cause rocks to break into smaller pieces without changing the mineral composition of the rocks. Physical processes that cause weathering are frost wedging, movement of surface water, unloading, and biological activity.

Frost wedging occurs when water runs into cracks on the earth’s surface and freezes. Since water expands when it freezes the cracks are forced to expand. Over many freeze/thaw cycles the rocks may break into small pieces and form piles called talus.

The movement of surface water depends on stream flow. Stream flow is the act of a stream carrying water to the ocean. The ability of a stream to erode and transport materials relies heavily on its velocity, which is the distance water travels in a period of time. The steeper the gradient the more energy the stream possesses as it flows. A stream channel is the pathway the stream follows as it flows. As the water flows down the stream channel it experiences friction, which slows the movement of the water while simultaneously widening the channel through erosion and weathering.

Unloading occurs as a result of reduction in pressure on igneous rock as overlying rock is removed via erosion and uplifting. When pressure decreases, sheets of the outer rock surface, separate, and break into smaller rock pieces. This process is called exfoliation. Stone Mountain in Georgia is an example of a geologic feature formed through uploading and exfoliation.

Biological activity occurs any time a living organism causes a change in the earth’s surface which has the potential to accelerate the process of weathering. Some examples include a plant’s roots causing cracks to form in the earth’s surface and deforestation by humans. Take notes on erosion and weathering as you watch the interactive.

Chemical Weathering is a process that results in rocks being changed into one or more new compounds.  Water is the most important means of chemical weathering.  It absorbs gases from the ground and atmosphere, which then react with other minerals.  Carbon dioxide is absorbed by rain when it passes through the atmosphere. It also can be absorbed from decaying organic minerals. The reaction of water with carbon dioxide can form carbonic acid. When granite, which is composed primarily of feldspar and quartz, is exposed to water that contains carbonic acid the feldspar is converted to clay minerals. Water present in the atmosphere also absorbs sulfur oxides and nitrogen oxides, which are produced by the burning of coal and petroleum. The reaction of water vapor with these compounds results in sulfuric acid and nitric acid respectively. Both of these acids contribute to acid precipitation. Silicate Minerals undergo chemical weathering when groundwater removes the sodium, calcium, potassium, and magnesium contained in silicate minerals and carries these elements away. Spheroidal weathering occurs when the edges of rocks weather at a faster rate causing the physical shape of a rock to take on a spherical appearance because of chemical weathering.