Soil Formation, Weathering, and Igneous Rock Textures & Types
Weathering Factors
- Mineralogical Composition: Basic minerals with ionic bonds weather more easily than those with covalent bonds.
- Climate: Temperate, humid climates are most favorable for weathering; arid climates lack sufficient water.
- Rock State: Fragmented rocks weather faster due to increased surface area.
Soil Formation and Evolution
Soil, the mantle of change, forms from physical and chemical weathering of bedrock, sometimes transported from higher elevations.
Components
- 50% Mineral matter
- 50% Organic matter (decomposed plants and animals, forming a black paste; darker soils have more organic matter)
Soil Formation Factors
1. Climate
Temperate-humid climates are conducive to soil development, potentially forming complete soils in 200 years. Increased temperature and humidity accelerate organic matter decomposition and chemical reactions.
2. Relief
Flat ground is ideal for soil accumulation, while slopes cause water runoff and prevent material retention, leading to less differentiated soil layers.
3. Orientation
Umbria (shaded) areas retain moisture better than sunny areas, impacting soil development.
4. Rock Composition
Basic rocks enhance soil formation.
5. Time
Soil formation time varies; arid or cold climates may take thousands of years, while humid climates may take only 200 years.
6. Living Activity
Plant roots convert organic matter into mineral matter. Decomposition rates vary: fast in tropical climates, slow in glacial climates.
Soil Profile
More developed soils have more differentiated layers (horizons):
- A Horizon (Superficial): Contains living matter.
- A1: Undecomposed and partially decomposed organic matter.
- A2: Developed humus.
- B Horizon (Precipitation): Accumulates minerals leached from A horizon, including oxides and hydroxides; tree roots reach this layer.
- C Horizon: Fragmented bedrock undergoing decompression.
- D/R Horizon: Undecomposed bedrock.
Soil Types
Polar Areas
Permafrost: Characteristic soil of periglacial areas, where the surface layer thaws in summer. Melting forms Mollisol (very white). Less cold areas form Podzol, characterized by intense washing in the A horizon due to abundant rainfall, concentrating minerals in the B horizon. Typical vegetation is taiga.
Temperate Zones
Brown soils are typical of temperate deciduous forests, with a well-developed A horizon rich in organic matter. Varieties include red soils, Chernozem, and Mediterranean soils.
Arid Areas
Arid areas have limited precipitation, with evaporation dominating. Minerals are deposited in the A horizon, and if the B horizon is rich in carbonates, caliche crusts form in the A horizon.
Equatorial Areas
Heat and rain lead to a thick A horizon but a thin B horizon. Humus decomposes rapidly and accumulates in the B horizon.
Magmatic Differentiation and Assimilation
1. Gravitational Differentiation
Basic magmas solidify first due to higher density, settling to the bottom of the magma chamber. Residual magma, rich in silica, rises. Basic rocks remain below acidic rocks. Crystals smaller than 5mm cannot settle due to magma viscosity; large crystals cannot stay at the top due to weight. Small crystals at the top crystallized earlier and were brought up by magma currents.
2. Filtered Water
High pressure in the magma chamber forces lighter elements to areas of lower pressure.
3. Gas Transport
Gas bubbles carry elements to the top.
Magmatic Assimilation
Magma sometimes incorporates fragments of surrounding rock, which may disappear completely or partially, altering the magma’s composition. Remaining solid fragments are called xenoliths.
Igneous Rock Textures and Types
Porphyritic
Contains phenocrysts formed before magma eruption. If cooled in a crack, it forms a matrix of microliths.
Pegmatite
Filonian rock texture with large crystals due to growth in eutectic proportions.
Aplite
Fine-grained texture based on microcrystals.
Volcanic
Formed from lava solidification during volcanic eruptions. Lava can be acidic or basic. In some regions, basic lava forms Pahoehoe (smooth, ropy surface) and acidic lava forms AA (rough, blocky surface). Rhyolite is a glassy acidic rock. Pumice is a vacuolar rock from gas-rich magma. Basalt can have a glassy or porphyritic texture, sometimes with olivine crystals.
Pyroclastic
Formed from volcanic eruptions: Volcanic bombs are large fragments ejected from the crater; Lapilli are small fragments; Cinder is pulverized lava dust.
Igneous Rock Forms (Site)
Concordant
Follows existing strata.
- Sill: Igneous rock mass intruded parallel to strata.
- Laccolith: Mushroom-shaped intrusion causing uplift.
- Lopolith: Concave, dish-shaped intrusion.
Discordant
Cuts across existing strata.
- Batholith: Large igneous rock mass often exposed by erosion.
- Dike: Magmatic material intruded in any direction.