5.1 Physical and chemical weathering processes

Weathering breaks down rocks through physical and chemical processes, shaping Earth's surface over time. Physical weathering cracks and splits rocks, while chemical weathering alters their mineral composition. These processes work together, influenced by climate, rock type, and biological activity.

Understanding weathering is crucial for grasping how landscapes form and change. It's the first step in producing the sediments that eventually form new sedimentary rocks and soils. Weathering also drives nutrient cycling, releasing minerals that support soil fertility and plant growth.

Physical Weathering Processes

Mechanical Weathering

Mechanical weathering physically breaks rocks into smaller pieces without changing their chemical composition. Physical forces like wind, water, ice, and gravity apply stress to rocks, exploiting pre-existing weaknesses such as cracks, joints, and bedding planes.

The result is sediment that ranges in size from large boulders down to fine grains of sand and silt. How effective mechanical weathering is depends on climate, rock type, and how exposed the rock is to weathering agents. A key point: the mineral makeup stays the same. You just get smaller fragments of the original rock.

Frost Wedging and Thermal Expansion

Frost wedging is one of the most powerful forms of mechanical weathering. Here's how it works:

1. Water seeps into cracks, joints, or pore spaces in rock.
2. When temperatures drop below freezing, that water expands by about 9% in volume as it turns to ice.
3. The expanding ice exerts tremendous pressure on the surrounding rock, widening the cracks.
4. When the ice melts, more water flows into the now-larger crack.
5. Repeated freeze-thaw cycles progressively split the rock apart.

This process is most effective in climates with frequent freeze-thaw cycles, such as mountainous regions and high-latitude areas where temperatures regularly cross 0°C.

Thermal expansion and contraction works differently. In environments with large daily temperature swings (deserts are a classic example), rocks heat up and expand during the day, then cool and contract at night. Over time, this repeated stress causes cracking and flaking of the rock surface. Because the outer layers of rock heat and cool faster than the interior, the surface tends to break away first.

Exfoliation

Exfoliation occurs when outer layers of rock peel off in curved sheets or slabs. The most common cause is pressure release (also called unloading):

1. A rock body forms deep underground under enormous pressure from the overlying material.
2. Erosion or uplift gradually removes the overlying rock.
3. With the weight removed, the buried rock expands outward.
4. This expansion causes curved fractures to form parallel to the surface.
5. Sheets of rock progressively detach and peel away.

This process creates distinctive rounded landforms called exfoliation domes. Half Dome in Yosemite is a famous example, formed in granite. Exfoliation is especially common in massive, homogeneous rocks like granite because they lack internal planes of weakness, so the stress from expansion gets released as these broad, curved fractures instead.

Chemical Weathering Processes

Chemical Reactions

Chemical weathering changes the actual mineral composition of rocks through reactions with water, air, and acids. Rather than just breaking rock into smaller pieces, it dissolves minerals or transforms them into entirely new minerals (most commonly clays).

The primary agents are water ($H_2O$), oxygen ($O_2$), and carbon dioxide ($CO_2$). Chemical weathering is most rapid in warm, humid climates because heat speeds up chemical reactions and water is the medium in which most of these reactions occur. Abundant vegetation in such climates also contributes acidic compounds to the soil, further accelerating the process.

Hydrolysis and Dissolution

Hydrolysis is a reaction between minerals and water that breaks apart chemical bonds within the mineral structure. Water ionizes into $H^+$ and $OH^-$ ions, and these ions replace the metal cations (like potassium or sodium) held within the mineral's crystal lattice. The result is a new, softer mineral, typically a clay.

Hydrolysis is particularly effective at breaking down silicate minerals. Feldspar, one of the most abundant minerals in Earth's crust, weathers through hydrolysis into kaolinite, a common clay mineral. This is why clay is so widespread in soils around the world.

Dissolution is more straightforward: the mineral simply dissolves completely in water. Highly soluble minerals like halite ($NaCl$) and gypsum ($CaSO_4 \cdot 2H_2O$) dissolve readily. Calcite ($CaCO_3$), the main mineral in limestone, also dissolves, especially in slightly acidic water.

Over time, dissolution of limestone creates karst topography, a landscape characterized by sinkholes, caves, disappearing streams, and underground drainage systems. The karst regions of Kentucky and Florida are well-known examples.

Oxidation and Carbonation

Oxidation occurs when minerals react with oxygen ($O_2$), often in the presence of water. It most commonly affects iron-bearing minerals like pyrite ($FeS_2$) and olivine. The iron in these minerals combines with oxygen to form iron oxides, which are essentially rust.

You can spot oxidation in the field by color: iron oxides give weathered rock surfaces a distinctive reddish, orange, or yellowish stain. This is what produces the red color in many desert landscapes and ancient "red bed" sedimentary formations.

Carbonation involves carbonic acid ($H_2CO_3$), which forms naturally when $CO_2$ dissolves in water:

$CO_2 + H_2O \rightarrow H_2CO_3$

This weak acid reacts with calcium-bearing minerals like calcite to produce soluble calcium bicarbonate:

$CaCO_3 + H_2CO_3 \rightarrow Ca(HCO_3)_2$

Because the product is soluble, it gets carried away in solution. Carbonation is the primary chemical process responsible for dissolving limestone and forming karst landscapes. Rainwater is naturally slightly acidic from dissolved atmospheric $CO_2$, so carbonation is happening wherever rain falls on carbonate rocks.

Biological Weathering

Weathering by Organisms

Biological weathering is caused by living organisms, and it bridges both physical and chemical categories. Plants, animals, and microorganisms all contribute.

Physical effects of organisms:

• Plant roots grow into cracks in rock, gradually widening them and prying the rock apart. Even small roots can exert surprising force as they grow.
• Burrowing animals (earthworms, rodents, ants) mix and turn soil, exposing fresh rock surfaces to air and water.

Chemical effects of organisms:

• Roots and decaying organic matter release organic acids into the soil, which chemically attack minerals.
• Fungi and bacteria produce acids and enzymes that break down rock at the microscopic level.
• Lichens, a symbiotic partnership between fungi and algae (or cyanobacteria), colonize bare rock surfaces and secrete acids that slowly etch and pit the rock beneath them. Lichens are often among the first organisms to begin breaking down newly exposed rock.

Biological weathering is most significant in warm, humid climates where vegetation is dense and microbial activity is high. Tropical rainforests, for example, have some of the most intensely weathered soils on Earth, in large part because biological and chemical weathering work together so aggressively in those conditions.