Regents Prep: Earth Science: W. E. D., & Landforms:
Weathering
Weathering is the chemical and physical processes that change the characteristics of rocks on the Earth’s surface. It is also known as the preparation for erosion. In order for weathering to occur, the environment of a rock sample must change and the rock needs to be exposed to some form of water and the air. Human processes such as pollution, which can be a large factor in acid rain, along with the acts of other living organisms, can cause chemical weathering to occur at faster rates. 

Weathering Process
The weathering process occurs when rocks are exposed to the hydrosphere (water) and atmosphere (air). These weathering agents can change the physical and chemical characteristics of rocks. As rocks are broken down (weathered) they can be classified as different types of sediments, which are: boulders, cobbles, pebbles, sand, silt, clay, and colloids. The chart below from page 6 of the Earth Science Reference Tables explains the sizes of each of these sediments.

Physical Weathering
Physical weathering occurs when rocks are broken in to smaller pieces without changing the chemical composition of the rock. Think of a physical change (e.g., ripping a piece of paper) where the sample will change in size but all its other characteristics will remain the same. There are a few types of physical weathering such as:

Frost action/ice wedging is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water. Water can seep into the cracks of a rock and as the climate cools the water freezes and expands breaking the rock apart. A very similar process occurs on roads, which causes potholes.

Abrasion is the physical wearing down of rocks as they rub or bounce against each other. This process is most common in windy areas, under glaciers, or in stream channels.

Exfoliation is the peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica.

Chemical Weathering
Chemical weathering occurs when a rock is broken down by chemical action resulting in a change in the composition of a rock. Such as the change a piece of paper would go through after being burned. The main agents of chemical weathering are oxygen, rainwater, carbon dioxide, and acids produced by decaying plants and animals that leads to the formation of soil. There are a few types of chemical weathering such as:

Oxidation occurs when oxygen interacts chemically with minerals. For example, when a nail rusts oxygen combines with the iron in the nail to form iron oxide.

Hydration occurs when water interacts chemically with minerals. For example, when hornblende and feldspar unite with water they eventually form into clay.

Carbonation occurs when carbon dioxide interacts chemically with minerals. When carbon dioxide is dissolved in water, it forms weak carbonic acid. Carbonic acid when it comes in contact with the surface of the earth dissolves large masses of limestone, creating caves and caverns. Other common terms associated with carbonation are sink holes, karst topography, stalactites and stalagmites.

Weathering rates overall depend on 3 different factors:

  • Particle size/surface area exposed to the surface
  • Mineral composition
  • Climate

Soil Formation
One of the major products of weathering is soil. Soil is a combination of particles of rocks, minerals, and organic matter produced through weathering processes. Soil contains the necessary nutrients to support various forms of plant and animal life.

As a result of the weathering processes and biologic activity, soil horizons (layers) form. Soil horizons vary in depth depending on an areas climate and weathering rates.

The below diagram is a mature soil profile common to New York State:

A soil profile refers to the layers of soil; horizon O, A, B, and C, and D. Horizon O refers to the organic material on the upper most part of the profile (this layer is usually very thin). Horizon A refers to the upper layer of soil, right below the O layer. It is commonly known as topsoil. In the woods or other areas that have not been plowed or tilled, this layer would probably include organic material (humus), such as fallen leaves, twigs, decaying plant and animal remains. The material helps prevent erosion, holds moisture, and decays to form a very rich soil known as humus. Horizon A provides plants with nutrients they need for a great life.

The layer below horizon A is horizon B known as the subsoil. Organic material from layer A is not present in horizon B and therefore there is much less humus. Horizon B usually will contain a fair amount of clay and iron oxides, but also may contain some elements from horizon A because of the process of leaching. Leaching resembles what happens in a coffee pot as the water drips through the coffee grounds. Leaching may also bring some minerals from horizon B down to horizon C.

If horizon B is below horizon A, then horizon C must be below horizon B. Horizon C consists mostly of weatherized big rocks known as Parent material (the rock that he soil formed from).

Horizon D, which is not shown in this illustration, usually contains solid bedrock.

 

Created by Luke Carnicelli
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