Weathering & soil

Technical Terms

Students need to understand the technical terms, be able to explain these terms, and should find the meaning in mother language (be able to explain in your own way)

A horizon (zone of leaching)

The top layer of soil, characterized by organic matters mixed with mineral material.

E horizon (zone of leaching)

leaching by downward percolating water.

B horizon (zone of accumulation)

A soil layer characterized by the accumulation of material leached downward from the A horizon above; also called zone of accumulation.

chemical weathering

The decomposition of rock resulting from exposure to water and atmospheric gases.

C horizon

A soil layer composed of incompletely weathered parent material.

clay mineral

A hydrous aluminum-silicate that occurs as a platy grain of microscopic size with a sheet silicate structure.

differential weathering

Varying rates of weathering resulting from some rocks in an area being more resistant to weathering than others.

erosion

The physical removal of rock by an agent such as running water, glacial ice, or wind.

exfoliation

The stripping of concentric rock slabs from the outer surface of a rock mass.

exfoliation dome

A large, rounded landform developed in a massive rock, such as granite, by the process of exfoliation.

frost action

Mechanical weathering of rock by freezing water.

frost heaving

The lifting of rock or soil by the expansion of freezing water.

frost wedging

A type of frost action in which the expansion of freezing water pries a rock apart.

hematite

A type of iron oxide that has a brick-red color when powdered; Fe₂O₃.

laterite

Highly leached soil that forms in regions of tropical climate with high temperatures and very abundant rainfall.

limonite

A type of iron oxide that is yellowish-brown when powdered; Fe₂O₃·nH₂O.

loam

Soil containing approximately equal amounts of sand, silt, and clay.

mechanical weathering

The physical disintegration of rock into smaller pieces.

O horizon

Dark-colored soil layer that is rich in organic material and forms just below surface vegetation.

pressure release

A significant type of mechanical weathering that causes rocks to crack when overburden is removed.

residual soil

Soil that develops directly from weathering of the rock below.

sheet joints

Cracks that develop parallel to the outer surface of a large mass of expanding rock, as pressure is released during unloading.

soil

A layer of weathered, unconsolidated material on top of bedrock; often also defined as containing organic matter and being capable of supporting plant growth.

soil horizon

Any of the layers of soil that are distinguishable by characteristic physical or chemical properties.

spheroidal weathering

Rounded by weathering from an initial blocky shape.

transportation

The movement of eroded particles by agents such as rivers, waves, glaciers, or wind.

transported soil

Soil not formed from the local rock but from parent material brought in from some other region and deposited, usually by running water, wind, or glacial ice.

weathering

The group of processes that change rock at or near Earth’s surface.

Volume expansion

Volume contraction

Oxidation

Hydration

Extra reading

Watch out! this reading part is from the main objectives of the lesson

1. Weathering creates sediment and soil by either mechanical or chemical processes. Erosion is the pick-up and removal of weathering products (transportation).

2. Mechanical weathering causes physical disintegration without compositional change. Frost wedging, frost heaving, and pressure release cause most mechanical weathering. Sheeting and exfoliation domes develop as a result of pressure release. Plant growth, burrowing organisms and salt crystal development are additional mechanical weathering processes. Differential weathering is also a common result of mechanical weathering

3.Chemical weathering causes rock decomposition and new mineral formation that reflects mineral instability because conditions of formation of the original minerals are significantly different from those of the earth's surface. Oxygen and slightly acidic rainwater (carbonic acid) are the agents of chemical weathering. The feldspars and ferromagnesian minerals weather to clay minerals, while quartz does not weather chemically. Calcite dissolves adding calcium and bicarbonate ions to ground water, while some silica is produced by chemical weathering of feldspars. This dissolved load may eventually be carried to the ocean.

4. Soil is the layer of unconsolidated weathered material on top of bedrock. Clay minerals make important contributions in holding water and nutrients on their surfaces because they are negatively charged.

5. Soil horizons develop as it matures. The O horizon is the top layer and consists of plant litter and other organic material. The A horizon is the next layer and is characterized by leaching downward. The B horizon is the zone of accumulation for material leached from the A horizon. The C horizon is transitional from soil to un-weathered bedrock. Most soils are residual, but transported soils can be deposited by ice, wind, and running water. Soils thicken with time.

6. The character of the soil depends on the parent material. Soils forming on granite are sandy, while those forming on basalt are never sandy. Soil types reflect climate. Soils containing large amounts of aluminum and iron oxides are found in wet climates. Soils formed in arid climates are thinner and contain higher concentrations of calcite. Hardpans form in either wet or dry climates and have thick B horizons. Laterites form in tropical regions, have thick A horizons and may be mined for aluminum.