Earth's Rocks and Soils: Types, Horizons, and Sampling

Earth's Crust and Rock Types

The crust is the layer of the Earth that contains soils and the various rock types that might exist.

Igneous Rocks

Igneous rocks are of deep origin and are formed by cooling of plutonic and volcanic magma. Their properties often include low porosity and variable density; they can act as thermal insulators.

Sedimentary Rocks

Sedimentary rocks are formed due to erosion and other agents that deposit material in layers. Over time, these layers consolidate and their particles join together.

Metamorphic Rocks

Metamorphic rocks originate from preexisting rocks and undergo physical and chemical transformations caused by temperature, pressure, or contact with other mineral elements.

Weathering Agents

Physical agents: sun, water, wind.

Chemical agents: oxidation, carbonation, hydration.

Soil Horizons and Profile

When considering a cross section of soil at different depths, one observes distinct layers with different appearances; each layer is called a horizon, and together they form the soil profile. The clarity of distinction depends on the age and particle composition. There are three main horizons: the main material layer (topsoil), the subsoil, and the bedrock.

Soil Classification: Texture and Particle Shape

Soils are classified according to particle shape, texture, and size.

• Round: Length, width, and thickness are of the same magnitude. There are various forms of rounded particles, often produced by abrasion and crushing.
• Subangular: Edges and corners are somewhat worn but still retain some angularity.
• Subrounded: Vertices and edges are worn but the particle still retains a recognisable original shape.
• Well-rounded: All traces of the original stone have disappeared, and the particle becomes nearly spherical or ovoid.
• Laminar: Particles are flat and plate-like—thin and elongated.
• Acicular: Needle-like particles with a high length-to-width ratio; they tend to be least resistant to stress.

Organic and Unstable Soils

Humus, peat, and agricultural soils are often unstable and should be removed or treated before construction.

Soil Sample Properties and Aggregates

Soil samples consist of three phases: solid, liquid, and gaseous (air). The relative proportions of these phases define many properties of the sample.

Particle aggregates are portions of soil that are removed for analysis.

Soil Survey and Sampling Methods

Soil survey begins with visual reconnaissance. Trenches and reconnaissance pits are types of sampling exposures: they are excavated to a certain depth to allow direct inspection of the soil to be studied.