Weathering, Erosion, and Soils

What is weathering? Weathering is the physical or chemical breakdown of rock into smaller pieces Two types of weathering: – Mechanical – Chemical

Mechanical Weathering Physical breakdown of rock into smaller pieces Only SIZE changes, not composition Examples… – Frost wedging: water freezes, expands, breaks rock apart

Mechanical Weathering Physical breakdown of rock into smaller pieces Only SIZE changes, not composition Examples… – Exfoliation: removal of pressure causes it to expand and crack; layer wears away

Mechanical Weathering Physical breakdown of rock into smaller pieces Only SIZE changes, not composition Examples… – Root wedging: plant roots act like ice to split rock

Chemical Weathering Chemical reactions break the minerals/rocks into different materials Composition changes Examples… – Oxidation: mineral reacts with oxygen and break off (e.g., iron turns to rust)

Chemical Weathering Chemical reactions break the minerals/rocks into different materials Composition changes Examples… – Hydrolysis: chemicals in rock react with water, causing some minerals to break away

Chemical Weathering Chemical reactions break the minerals/rocks into different materials Composition changes Examples… – Carbonation: CO2 and water make carbonic acid that washes away some minerals (especially limestone)

Chemical Weathering Chemical reactions break the minerals/rocks into different materials Composition changes Examples… – Acids (in living things or due to pollution) can crack rock

What is Erosion? Erosion is the movement of weathered materials from one location to another (often downhill) Caused by: – Wind – Gravity – Running water – Glaciers – Plants, animals, people

Weathering vs. erosion Weathering breaks down rocks, which ultimately makes soil Erosion carries it away!

Wind erosion

Gravity erosion

Running water erosion

Glacial erosion

Plant, animal, human erosion

What is soil? Soil is a mixture of weathered rock, organic matter (living/decaying), air, and water Made from… – Materials weathered from bedrock (underlying “parent” rock), – Materials deposited from erosion – Organic matter made by the actions of living things

Soil types Type of soil mainly depends on the parent material (bedrock) – Example: quartz-rick rocks produce sandy soil – Example: iron-rich rocks produce red soil Also depends on climate (temperature and rainfall) – Determines amount of organic matter

Soil textures Particle size – Sand – Silt – Clay If particles are too small, soil can’t hold much air and water If particles are too large, water runs right through!

Soil fertility How well a soil can support plant growth Factors include minerals/nutrients, organisms living in soil, precipitation, topography, acidity

Soil profiles Soil profiles are the vertical sequence of layers – The layers are called horizons Soils in different areas produce different profiles Climate affects the horizons

Soil erosion Soil erosion is the movement of soil from one place to another – Caused by wind, water, glaciers, animals/plants, human activity Weathering creates soil, erosion destroys it!

Human causes of soil erosion Development for housing/buildings

Human causes of soil erosion Development for housing/buildings Deforestation Farming Widespread and devastating problem around the world – “Desertification” – Dust Bowl of the 1930s

Dust Bowl, 1930s

Destitute Pea Pickers in California. Mother of Seven Children. by Dorothea Lange

How can we prevent it? Plant trees and groundcover Improve farming practices – Contour plowing – Strip cropping – Terracing – Crop rotation – Cover cropping

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