Soil and Rock Formation

Soil Formation

• Soil develops from parent material through soil formation
• Soil formation from hard rock (e.g., granite) occurs in two stages: rock weathering and soil formation
• Mineral matter passed down from rocks constitutes soil parent material
• Organic soil principal parent materials derive from plant material decomposition
• Parent materials texture and mineral composition dictates its soil properties

Rock Formation

• Rocks come from magma cooling
• Basalt and granite are common igneous rocks
• Basalt is the main hard rock of ocean basins
• Weathering of basalt produces clay, but less stone

Rock Weathering

• Granite breaks down into coarse-grained, sandy soil
• Granite yields deeper soil than basalt
• Igneous rocks make up 10% of the Earth's soil area
• Sedimentary or metamorphic rocks make up the other 90% of land area

Sedimentary and Metamorphic Rocks

• Sedimentary rocks result from the cementation of materials deposited by wind, water, ice, or gravity
• The cementing agents include iron, aluminum, silicon, and CaCO3
• Shale, limestone, and quartz stones are examples of sedimentary rocks
• Metamorphic rocks form under heat and pressure
• Sandstone becomes quartzite, shale becomes slate, and limestone becomes marble through these processes

Soil Formation Processes

• Weathering forms the regolith, which leads to soil formation
• Weathering combines both destruction and synthesis processes
• Weathering forces act upon rock fragments and minerals, with a continuous decrease in particle size and release of soluble constituents

Weathering Processes

• Weathering has two processes: mechanical disintegration and chemical decomposition

Mechanical Weathering

• Differential expansions of minerals, frost action, and exfoliation are all temperature dependent
• Erosion and deposition via wind, water and ice
• Plant and animal influences

Mechanical Weathering Forces

• Temperature fluctuations lead to rock disintegration
• Freezing water widens cracks and dislodges rock fragments
• Water cuts through valleys
• Ice breaks down rocks
• Wind transports dust

Biological Weathering

• Mosses and lichens help disintegrate rocks through organic material production and root growth in crevices
• Repeated warming and cooling disintegrates parent materials effectively

Chemical Weathering

• Hydrolysis is a key decomposition reaction for feldspars and mica
• Hydration involves the attachment of H+ and OH- ions to compounds

Key Chemical Reactions

• 2Fe2O3+3H2O yields 2Fe2O3
• Hematite (red) converts to Limonite (Yellow)
• Hydrogen ion, carbonic acid, HNO3, and H2SO4 drive carbonation
• CaCO3 reacts with H2CO3 to form Ca(HCO3)2: Calcite + Carbonic Acid results in Soluble Bicarbonate
• Oxidation converts Fe++ to Fe+++
• 4FeO+O2 becomes 2Fe2O3

Factors Influencing Soil Formation

• Climate, especially temperature and precipitation
• Native vegetation (living organisms)
• Parent material (texture and structure of soil)
• Chemical and mineralogical composition
• Topography
• Time

Soil Profile

• The horizontal layers of soil are called soil horizons
• The vertical sequence of horizons forms the soil profile
• Soil forming processes is grouped under four categories: O, A, B, and C

Soil Layers

• O Group: Organic horizons above mineral soil
• A Group: Mineral horizons at or near the surface, leached
• B Group: Accumulation of materials from above or formed in place

Horizon Details

• Accumulation of iron/aluminum oxides & silicate clays occurs in this region
• In arid areas, calcium carbonate and calcium sulphate accumulate here
• The A & B horizons are collectively referred to as the solum
• C-Horizon consists of unconsolidated material, where least weathered accumulations of Ca/Mg carbonate and cementation occur
• R-Horizon: the bed rock (consolidated), it might be the parent rock to the soil above