Clay soil has a limited geologic range.
Clay and related minerals are found in a limited geologic range. "Most clay minerals form where rocks are in contact with water, air, or steam," according to Nora Foley of the U.S. Geological Survey. They form near soil horizons (distinct soil layers) or in volcanic deposits and geothermal fields. They also form where old sediments and rock formations are weathered.
Soil Formation Basics
Weathering of iron soils makes clay.
Clays and other soils are formed from parent materials like rock and organic deposits. Weathering, pressure and oxidation cause the parent materials to be reformed into soil. Different geographic areas have different climates and different organisms present which affect the type of soil formed.
Depending on temperature extremes and rainfall, new minerals are made during soil formation. Clay is formed when silica combines with oxides, either iron or alumina. The sheets of silicates bond with metallic atoms (alumina and iron), oxygen and hydroxyl ions to form clay.
Where Different Clays Form
Tropic region clay dries out easily
Silicates, mica, iron and oxides of aluminum are all found in clay. Clays formed in the tropic zones tend to be iron and aluminum clays. Those formed between the tropics and polar regions tend to be silicate clays.
Clays formed in hot tropical regions are called two-layer clays. They are easily dried out and don’t hold water or nutrients well. Clays formed in mid-latitudes (between the tropics and poles) are three-layer clays. They are weakly bound and retain moisture easily.
Clay Formation by Erosion and Transport
Clay dunes are a form of transported clay.
As older rocks and soils erode they are transported from their original sites and deposited in layers called sediments. Clays laid down by erosion and transport are called transported clays. Clay formed in place by diagenesis or weathering is called residual clay.
Deposited sediments, called muds, contain clay-size particles (2 micrometers or less). The clays are a mix of clay-sized mineral crystals and fine-grained clay minerals. Sedimentation continues today in lakes, deltas and ocean basins. Clays are being formed by eroded particles washed down to these locations and compacted by settling.
Clay Formation by Diagenesis
Clay forming in rocky soil
Diagenesis is caused by compaction (squeezing out water), cementation (chemical cement precipitation) and recrystallization (new crystal grain growth). In other words water is removed, chemicals bond together the minerals left and new clay-sized mineral crystals are formed.
Clay Formation by Weathering
Weathered sediments
Weathering is the decomposing of materials near earth’s surface like rock. It is caused by changes in temperature and pressure (physical weathering) or by mineral elements being added to or removed from existing minerals (chemical weathering).
Weathering into clay takes place progressively. Minerals break down unevenly over time and clay can have different stages of breakdown in the same layer. The types of minerals present when weathering begins determine the type of clay produced.
Rocks like granite containing silica and alumina decompose to form clay. Rocks like limestone containing clay minerals are dissolved by water or the chemical action of plant roots. Their clay does not dissolve and is deposited in place. Clay transformed to shale rock by lithification is released when shale disintegrates and becomes a solution called neo-formed clay.
