Rock Cycle and Soil Formation: Weathering, Erosion and Soil Structure

The Rock Cycle

Lava and magma cool and solidify, forming igneous rocks. Igneous rocks subjected to weathering and erosion break down, and their particles are washed away by erosive agents and deposited as sediments.

Through lithification, these sediments transform into sedimentary rocks. When pressure and temperature increase, they become metamorphic rocks. Metamorphic rocks may in turn melt back into magma, and the cycle begins again with igneous rocks.

Weathering

Weathering is the process of breaking down rocks into smaller pieces by the action of erosive agents.

• Mechanical weathering: physical destruction of rocks by agents such as:
  • Thermal: changes in temperature.
  • Freeze-thaw cycles: water freezes in cracks, expands, and fractures the rock.
  • Salt crystallization: dissolved salts crystallize within pores and crack the rock.
• Chemical weathering: _{action of dissolving agents, such as water dissolving minerals, solutions passing through salts, oxidation by atmospheric oxygen and water, and decomposition by biological chemical agents that transform rocks into compounds usable by organisms.}

Erosion

Erosion is the set of exogenous or external processes acting on the Earth's surface that help modify its forms.

Soil

Soil is the superficial layer of the continental Earth's crust, resulting from the alteration of the underlying lithologic substrate.

Pedogenesis

The following factors intervene in pedogenesis:

• Climate: temperature and water availability.
• Living agents: vegetation, bacteria, fungi, and microorganisms with their biological activity.
• Time: over the time elapsed, the soil reaches equilibrium with vegetation and climate, becoming a mature soil called climax soil.
• Bedrock: provides the initial characteristics of the soil.
• Relief: in deep, flat terrains soils tend to form, while on slopes soils originate with greater difficulty.

Soil Structure

Soil is structured in horizontal layers called horizons, which together form the soil profile.

• O horizon: organic accumulation of matter, the topsoil, composed of accumulated organic matter (humus).
• A horizon: mineral accumulation of organic matter, enriched with elements leached from the upper layer, with a darker color and less clay than the layer below.
• B horizon: zone of accumulation of minerals and clays leached from upper horizons.
• C horizon: partially weathered and altered bedrock.