5.1 Physical and chemical weathering processes

Weathering Processes

Rocks at Earth's surface are constantly being broken down by weathering. Physical weathering splits rocks into smaller pieces without changing what they're made of, while chemical weathering actually transforms their minerals into new substances. Understanding the difference matters because each type produces different products and dominates in different climates.

Physical vs. chemical weathering

Physical (mechanical) weathering breaks rock into smaller fragments while keeping the same mineral composition. The main types:

• Frost wedging occurs when water seeps into cracks in rock, freezes, and expands. Since water expands by about 9% when it becomes ice, this generates enormous pressure that widens the crack over time. Repeated freeze-thaw cycles can split boulders apart. You'll see this most in climates where temperatures regularly cross the freezing point.
• Exfoliation (sometimes called onion-skin weathering) happens when overlying rock or sediment is removed, releasing pressure on the rock below. The rock expands upward and outer layers peel off in curved sheets. Granite domes like those in Yosemite are classic examples.
• Abrasion is the physical wearing down of rock surfaces through friction and impact. Wind-blown sand can sandblast exposed rock, and tumbling rocks in a river grind each other smooth over time.

Chemical weathering changes a rock's mineral composition through chemical reactions, producing new minerals and dissolved substances. The three main reactions are hydrolysis, oxidation, and carbonation (each covered in detail below).

Water's role in weathering

Water is involved in nearly every weathering process, both physical and chemical.

In physical weathering, water contributes by:

• Frost wedging: Water in cracks freezes and expands (~9% volume increase), prying rock apart. A related process called ice segregation draws additional water toward the freezing front, amplifying the pressure even further.
• Salt crystallization: When saltwater seeps into rock pores and evaporates, salt crystals grow and exert pressure from within. This is especially common in coastal and arid environments.

In chemical weathering, water is even more central:

• It acts as a reactant in hydrolysis and carbonation, directly participating in the chemical reactions that break down minerals.
• It serves as a solvent, dissolving ions and carrying them away, which keeps reactions moving forward.
• Rainwater is naturally slightly acidic (around pH 5.6) because atmospheric $CO_2$ dissolves into it, forming carbonic acid. This makes rainwater more chemically reactive than pure water.

Temperature effects on weathering

Temperature changes stress rocks in several ways, all falling under physical weathering.

Thermal expansion and contraction happen because rock expands slightly when heated and contracts when cooled. Repeated cycling builds up stress along grain boundaries and existing fractures, eventually causing cracks. This is sometimes called thermal shock.

Thermal fatigue takes this a step further. Different minerals within the same rock expand at different rates when heated. Over many heating-cooling cycles, these mismatched expansions weaken the bonds between mineral grains, causing the rock to crumble grain by grain. This process is called granular disintegration.

Insolation weathering is particularly effective in deserts, where daytime surface temperatures can be scorching while nights are cold. The outer surface of the rock heats and cools faster than the interior, and this uneven stress causes the surface to crack and flake off.

Chemical reactions in weathering

Three reactions do most of the work in chemical weathering:

Hydrolysis is the most common chemical weathering reaction. Water reacts with minerals, and hydrogen ions ($H^+$) replace other cations in the mineral structure.

• Example: Feldspar (the most abundant mineral group in Earth's crust) reacts with water to produce clay minerals plus dissolved ions like $K^+$, $Na^+$, or $Ca^{2+}$. This is why granite, which is rich in feldspar, weathers into clay-rich soil.

Oxidation occurs when oxygen reacts with minerals, especially iron-bearing ones. Think of it as rusting.

• Example: Pyrite ($FeS_2$) reacts with oxygen and water to produce iron oxide (rust) and sulfuric acid. That reddish-brown staining you see on rock surfaces is usually iron oxide from oxidation.

Carbonation is the reaction between carbonic acid and calcium-bearing minerals. It's the main process dissolving limestone and forming caves. It works in two steps:

1. Atmospheric $CO_2$ dissolves in water to form carbonic acid: $CO_2 + H_2O \rightarrow H_2CO_3$
2. Carbonic acid reacts with limestone: $CaCO_3 + H_2CO_3 \rightarrow Ca(HCO_3)_2$

The product, calcium bicarbonate ($Ca(HCO_3)_2$), is soluble in water, so the limestone literally dissolves and gets carried away. This is how karst landscapes with sinkholes and caverns form.