Weathering, Erosion, and Soil Formation

Learning Objectives

Distinguish between mechanical (physical) and chemical weathering.

Describe several types of weathering, such as frost wedging and oxidation.

Define erosion and identify its major agents.

Outline the basic process of soil formation.

Shaping the Earth's Surface

The Earth's surface is constantly being broken down and reshaped by the processes of weathering and erosion.

Weathering: Breaking Down Rocks

Weathering is the in-situ breakdown of rocks, soil, and minerals at or near the Earth's surface. It does not involve movement.

1.Mechanical (Physical) Weathering: The breakdown of rocks into smaller pieces (sediment) without changing their chemical composition.

Frost Wedging: Water seeps into cracks in rocks, freezes, expands, and pries the rock apart.

Abrasion: Rocks are worn down by the scraping action of other rock particles carried by wind, water, or ice.

Exfoliation: Sheets of rock peel off an exposed pluton due to the release of pressure.

2.Chemical Weathering: The breakdown of rocks through chemical reactions, which change the mineral composition of the rock.

Oxidation: A reaction between minerals and oxygen. The most common example is the rusting of iron-bearing minerals.

Dissolution: Some minerals, like calcite (in limestone), dissolve in water, especially if the water is slightly acidic. This is responsible for forming caves.

Hydrolysis: A chemical reaction between water and minerals that breaks them down. For example, feldspar weathers into clay minerals.

Erosion: Moving the Pieces

Erosion is the process that transports the products of weathering (sediment) from one place to another.

The Major Agents of Erosion:

Water: The most significant agent. River and stream flow, coastal wave action.

Wind: Can transport sand and dust, especially in arid regions.

Ice: Glaciers are immense 'rivers of ice' that can carve valleys and move huge amounts of rock and sediment.

Gravity: The downslope movement of rock and soil, known as mass wasting (e.g., landslides, rockfalls).

Soil Formation

Soil is the mixture of minerals, organic matter, gases, liquids, and organisms that together support plant life.

Formation Process: Soil forms from the top down.

1.It begins with the weathering of a parent material (bedrock).

2.Organic matter (from decaying plants and animals) accumulates and is mixed in by organisms like earthworms.

3.Over hundreds or thousands of years, distinct layers called soil horizons develop. A typical soil profile consists of the O (organic), A (topsoil), B (subsoil), and C (weathered parent material) horizons.

Key Terms

Weathering: The breaking down or dissolving of rocks and minerals on Earth's surface.
Erosion: The geological process in which earthen materials are worn away and transported by natural forces such as water or wind.
Mechanical Weathering: The process of breaking big rocks into little ones without any change in the chemical composition.
Chemical Weathering: The erosion or disintegration of rocks caused by chemical reactions (chiefly with water and substances dissolved in it).
Soil: The upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.