How Does Soil Become Clay?

What is Clay Soil?

Clay soil is a type of soil that consists of tiny particles less than 0.002 mm in diameter.

The small particle size and plate-like shape of the particles result in properties such as plasticity, shrinking and swelling when wet, and hardness when dry or fired. Clay particles are made of clay minerals such as kaolinite, illite, and montmorillonite.

Due to the small particle size, clay soils have a very high surface area compared to sand. This allows the soil to adsorb significant amounts of water. The plate-like shape of the particles also promotes extensive surface contact between particles.

As a result, clay soils are sticky, plastic, and easily molded when wet, but are hard and brick-like when dry. They also shrink and crack on drying, and swell when wet. The shrinking and swelling can damage building foundations and roads.

How Clay Forms

Clay forms through the weathering of rock over long periods of time. As rock is exposed to water, air, acids, and physical abrasion, it begins to break down into smaller particles. Feldspar is one of the most common minerals found in igneous and metamorphic rocks, and its composition includes silica, alumina, and potash. As feldspar weathers, it breaks down into clay minerals like kaolinite, which contain silica, alumina, and water. The potash is usually leached away in the weathering process.

Over hundreds and thousands of years, rocks like granite are weathered into particles of clay. As rainwater and streams transport these tiny clay particles, they eventually settle and accumulate in lakes, oceans, rivers, and soil. The very smallest clay particles, less than 0.002 mm in size, are deposited farther away, while larger silt particles drop out first. This transportation and deposition process concentrates and sorts the clay minerals, creating clay deposits. With time, accumulation, and compaction, these clay sediments eventually lithify into shale and mudstone.

Parent Materials

The starting ingredients that lead to clay formation are called parent materials. There are three main types of rocks that serve as parent materials for clay:

Sedimentary rock like shale – Shale is a very fine-grained sedimentary rock that weathers into clay minerals. As shale breaks down, it releases its clay mineral components like illite and montmorillonite.

Granite, basalt and other igneous rocks – While igneous rocks like granite and basalt do not contain clay minerals themselves, they contain minerals like feldspar that break down into clay. The feldspar in granite weathers into kaolinite clay.

Metamorphic rock – Parent metamorphic rocks like mica schist can also be a source of clay minerals. As mica schist decomposes, it liberates clay minerals like illite.

Climate

The climate in an area plays an important role in clay formation. Environments that have high rainfall promote the leaching of minerals from the upper soil layers into lower layers. As these minerals accumulate in the lower layers over time, they help create clay soils.

Warm, humid climates also facilitate clay development. The abundant moisture supports chemical weathering processes that break down minerals like feldspar into clay particles. The warm temperatures accelerate these chemical reactions. Areas with mild, wet climates are therefore more likely to have clay soils than arid or cold regions.

How Topography Contributes to Clay Formation

Topography, or the shape and elevation of land, plays an important role in clay formation. Depressions and low-lying areas collect eroded clay particles and promote clay accumulation because they receive water runoff from surrounding areas. Water erosion selectively sorts soil particles by size, washing the larger sand and silt particles away while depositing the microscopic clay particles in low spots. Over time, these low areas accumulate higher and higher percentages of clay, creating clay soils. Therefore, clayey soils tend to develop in flat plains, valley bottoms, depressions, and other low-lying landforms that collect water runoff. The longer water stands in an area and deposits clay particles, the heavier the soil texture becomes. Topography creates the ideal conditions for clay accumulation in certain landscape positions.

Time

The formation of clay soils takes a very long time. It can take hundreds to thousands of years for rocks like granite to break down into smaller particles under the effects of weathering. This produces particles like sand, silt, and clay.

Once these particles are eroded and transported by water, wind, or gravity, they are deposited in layers. Over repeated cycles of erosion and deposit, thicker layers of clay sediments accumulate in an area. The sheer time and repetition of these geologic processes is essential for generating the high clay content of clay soils.

Biological Factors

Plants, animals, and microbes all help weather rock and develop clay soils through their biological processes and activities. As plants grow, their roots penetrate into rock crevices and help break apart rock through root wedging. Plant roots also release organic acids that react with minerals in the parent material to produce clays.

Burrowing animals like earthworms, moles, and prairie dogs mix up soil layers, bringing deeper soils to the surface and burying surface soils deeper. Their burrowing helps aerate the soil and speeds up chemical weathering. Worm castings and animal waste enrich the soil with nutrients for clay formation.

Microbes like bacteria and fungi secrete acids and enzymes that chemically break down rocks and minerals. Bacterial metabolism helps dissolve rock and release nutrient elements. The byproducts of microbial decomposition become integrated into the developing soil profile.

Human Activities

Human activities like agriculture, deforestation, and mining can influence how clay soils form. For example, clearing trees and vegetation exposes the underlying rock and soil to weathering and erosion. Tilling the soil for farming also mixes and reworks the different soil layers.

Large-scale irrigation is another significant human activity. As water moves across irrigated lands it picks up clay particles and redeposits them downstream. Over time, this can create thick clay deposits in areas with intensive irrigation.

Mining, like strip mining, scrapes away surface layers and brings deeper clay deposits closer to the surface. The exposed clay is then more prone to weathering into soil. So while clay forms naturally, human activities like farming, irrigation, and mining can accelerate clay formation and redistribution.

Identifying Clay Soils

There are a few simple tests that can be done to identify clay soils. The most common are:

Ribbon Test

This involves taking a moist soil sample and rolling it between your fingers to form a ribbon shape. Clay soils with a high clay content can form a long ribbon before breaking, generally 2.5cm or more. The longer the ribbon, the higher the clay content.

Soil Texture Triangle

Using the soil texture triangle, you can identify clay soils based on the percentage of sand, silt and clay particles. Clay soils fall in the upper left part of the triangle, with at least 30% or more clay content.

Stickiness and Plasticity

Clay soils are sticky when wet and can be molded into different shapes. When dry, they become very hard and dense. The high plasticity and stickiness helps distinguish them from other soils.

Summary

Clay soils form over long timescales through the gradual chemical and physical breakdown of rocks and minerals. Key factors in clay formation include parent material, climate, topography, and biological activity. In most cases, clay accumulates in low-lying areas where drainage is poor. This allows the small clay particles to settle out of the soil solution instead of being washed away by water. A humid, warm climate accelerates chemical weathering processes that produce clay minerals from the parent material. The presence of organic acids from plant roots and microorganisms also promotes chemical transformations that yield clay. In summary, clay minerals originate from the weathering of rocks under specific conditions of landscape, drainage, climate and biological influences over geological timescales. The accumulation and composition of clay reflects this complex interplay between the soil, the topography, the climate, and living organisms.