Question

2. How does rock turn into soil?

Answer 1

Rock becomes soil through weathering and the addition of organic material. Key steps:

1. Mechanical (physical) weathering — breaks rock into smaller pieces:
- Freeze–thaw: water in cracks freezes, expands and splits rock.
- Thermal expansion: heating and cooling cause rock to fracture.
- Abrasion: wind, water, ice and other rocks wear surfaces down.
- Root wedging: plant roots grow into cracks and pry rock apart.

2. Chemical weathering — changes minerals into new substances:
- Water, acids (including carbonic acid from CO2 + water) and oxygen react with minerals.
- Feldspars turn into clays (e.g., kaolinite); iron oxidizes (rusts).
- Dissolving of soluble minerals (limestone dissolves in weak acids).

3. Biological activity — mixes and enriches the material:
- Plants add dead leaves and roots that decompose into humus (organic matter).
- Microbes and animals (worms, insects) break down organic matter and mix it with mineral particles.
- Roots and burrowing organisms increase porosity and help chemical reactions.

4. Mixing and formation of horizons — a soil profile develops:
- Mineral particles (sand, silt, clay) from weathered rock combine with humus.
- Water moves materials up and down, creating layers (O, A, B, C horizons).
- Texture, structure and nutrient content stabilize into soil capable of supporting plants.

Timescale and influences:
- Climate (temperature and rainfall), parent rock type, organisms, topography and time all control how fast soil forms — from decades to thousands of years. Wet, warm climates speed chemical weathering; cold or dry climates slow it.

Summary: Weathering (physical and chemical) breaks rock into mineral particles; biological processes add and mix organic matter; together they form the layered, living material we call soil.