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Formation & Weathering Process

How is soil formed?

Soil is formed following the weathering of exposed rocks. Weathering is the decomposition of rocks under the influence of physical, chemical, and biological agents.

Physical weathering:

The mechanical breakup of the parent rock


Chemical weathering:

Chemical alteration or decomposition of rock due to water, oxygen, and carbon dioxide


Biological weathering:

Caused by an animal or plant. Photo credit

biological weathering.PNG

Soil forming factors:

parent material, topography, climate, organisms, and time

Photo taken July 14, 2013 on Brittanny's US Roadtip - Great Sand Dunes National Park & Preserve, Colorado

Grain Size

Soils are categorized by grain size, one side of the scale being 'coarse-grain soils' and the other being 'fine-grained soils'. While it may seem odd for something so small to be considered on the 'coarse' side of the scale, any soil grains larger than 0.075mm are indeed on this side. All soil grains finer then the 0.075mm size are considered fine-grained.

Coarse-grained soils: sand, gravel, cobbles, and boulders

Fine-grained soils: silt and clay

How the soil particles or mineral grains are arranged is considered when analyzing the structure. Within these arrangements, the interparticle forces that act between them are also considered to best understand the structure. Photo credit


Types of soil can be determined by the percentage of material, as shown in the diagram. It is important to determine these percentages in order to apply the date to the Unified Soil Classification System (USCS)


Particles less than 5 μm diameters are soils classified as clay. The two basic components of clay are Silica Tetrehedron and Alumina Octahedron. The crystalline materials AKA the minerals making up clay are Kaolinite, Illite, and Montmorillonite. Each crystal is surrounded by layers of water, known as 'adsorbed water' which is what determines the structure of clay soils. 

Montmorillonites volume can increase when absorbing water due to its very high shrink-swell capacity. This comes into play in the 'Sinking Cities' blog post.

Clay can become very problematic in engineering design because of its variations throughout climate changes (droughts, floods, etc.) and can result in an unstable foundation due to so much movement. 

Photo taken April 2021 on Brittanny's Canada Roadtip - Grasslands National Park, Saskatchewan
Grasslands National Park - Eagle Butte Trail.JPG
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