What is Weathering?
Weathering is the breakdown of rocks into smaller particles by natural processes. It occurs whenever rocks are exposed to the air, water, and living things at or near Earth's surface.
Weathering is distinct from erosion, which involves the movement of weathered materials by agents like water, wind, or ice.
Physical Weathering: Breaking Rocks Without Chemical Change
Physical weathering involves processes that break rocks into smaller pieces without altering their chemical composition.
Frost Action
- Water seeps into cracks in rocks.
- When temperatures drop below 0°C, the water freezes and expands by about 9%.
- This expansion exerts immense pressure on the rock, widening cracks.
- Repeated freeze-thaw cycles eventually break the rock apart.
Frost action is common in temperate climates with frequent freeze-thaw cycles, such as in New York State or high mountain regions.
Look for talus slopes—piles of rock fragments at the base of cliffs—as evidence of frost action.
Abrasion
- Rocks are worn down by rubbing against each other.
- This occurs in streams, where water carries rock fragments that collide and erode surfaces.
- Wind-driven sand can also abrade rocks, smoothing and rounding them over time.
Think of how pebbles in a river become smooth and rounded due to constant abrasion.
Exfoliation
- Involves the peeling of thin layers from rock surfaces.
- Often occurs in coarse-grained rocks containing feldspar.
- Moisture penetrates the rock, causing feldspar to chemically transform into kaolin (a type of clay).
- The expansion of kaolin pries apart mineral grains, causing layers to flake off.
Exfoliation is a physical process triggered by a chemical change, highlighting the interplay between physical and chemical weathering.
Plant and Animal Action
- Plants: Plant roots grow into cracks, widening them as they expand.
- Animals: Burrowing animals like earthworms, ants, and moles expose rock to air and water, accelerating weathering.
Humans also contribute to physical weathering through activities like quarrying, road construction, and mining.
Changes in Temperature
- Rocks expand when heated and contract when cooled.
- Extreme temperature changes, such as those from wildfires, can cause rocks to crack or flake.
Pressure Unloading
- Rocks formed deep underground are under immense pressure.
- When exposed at the surface, the release of pressure causes them to expand and crack.
- This process is common in areas where glaciers have melted, reducing the weight on underlying rocks.
Chemical Weathering: Transforming Rocks Through Reactions
Chemical weathering involves the breakdown of rocks by altering their chemical composition. Key agents include oxygen, water, and carbon dioxide.
Oxidation
- Oxygen reacts with minerals, especially those containing iron, to form oxides.
- Oxides weaken the rock's structure, similar to how rust weakens metal.
The reddish or yellow-brown color of weathered rocks often comes from iron oxides like hematite or goethite.
Hydration, Hydrolysis, and Solution
- Hydration: Water combines with minerals, causing them to expand and weaken.
- Hydrolysis: Water breaks down minerals by replacing their ions.
- Solution: Water dissolves minerals like halite (rock salt) and gypsum, weakening the rock.
Carbonation
- Carbon dioxide dissolves in water to form carbonic acid
- This weak acid reacts with minerals, especially calcite in limestone, dissolving the rock.
Carbonation is responsible for forming limestone caves and karst landscapes.
Other Chemical Factors
- Organic Acids: Produced by decaying plants or secreted by lichens, these acids dissolve minerals in rocks.
- Acid Rain: Human activities release pollutants like sulfur and nitrogen oxides, which form strong acids in rainwater, accelerating chemical weathering.
Factors Affecting Weathering
Climate
- Warm, wet climates: Favor chemical weathering due to higher temperatures and moisture.
- Cold, moist climates: Promote physical weathering, especially frost action.
Cleopatra's Needle, an ancient obelisk, weathered rapidly in New York's moist climate compared to its original dry environment in Egypt.
Particle Size
- Smaller particles weather faster because they have more surface area exposed to weathering agents.
A cube of rock broken into smaller pieces has a greater total surface area, increasing the rate of chemical reactions.
Exposure
- Rocks fully exposed to air, water, and weathering agents break down faster than those shielded by soil or vegetation.
- Steep slopes expose fresh rock as weathered material is removed by erosion.
Mineral Composition
- Rocks with minerals that react readily with water, acids, or oxygen weather quicker.
- Softer rocks abrade easier than harder, crystalline rocks.
Quartz is highly resistant to weathering, while minerals like feldspar and calcite are more susceptible.
Time
- Weathering is a slow process, but over long periods, even the hardest rocks break down completely.
The Products of Weathering
Sediments
Weathering produces rock fragments called sediments, classified by size:
- Clay: 0.0004 cm
- Silt: 0.0004–0.006 cm
- Sand: 0.006–0.2 cm
- Pebbles: 0.2–6.4 cm
- Cobbles: 6.4–25.6 cm
- Boulders: 25.6 cm
Soils
- Soil is a mixture of weathered rock, water, air, and organic material (humus).
- It forms through the interaction of weathering and biological activity.
Soil is classified as residual if it forms directly from underlying bedrock, or transported if it forms from material moved by erosion.
Soil Horizons
Soils develop distinct layers, or horizons, over time:
- Horizon O: Organic matter (topsoil).
- Horizon A: Leached minerals and organic material.
- Horizon B: Accumulated clay and minerals from Horizon A.
- Horizon C: Partly weathered bedrock.
- Parent Material (R): Unaltered bedrock.
The type of soil that forms depends on factors like parent material, climate, and biological activity.
Why Does Weathering Matter?
- Soil Formation: Weathering creates soil, a critical resource for agriculture and ecosystems.
- Landscape Evolution: Weathering shapes landforms, from mountains to valleys.
- Human Impact: Understanding weathering helps us manage erosion, preserve historical structures, and mitigate the effects of acid rain.
