02/05/2026

 

Samuel Clifford

 

Chapter 6: Weathering and Soils

 

6.1

 

Weathering is the breaking down (disintegration) and chemical alteration (decomposition) of rock near the earth's surface. Weathering goes on all around us and actually helps with the cycling of carbon as carbon released from the rocks goes into the atmosphere or oceans where it is the used by the shells of marine organisms and then once again becomes a part of rock cycle as sediment.

 

There are two components of weathering:

 

1. Mechanical Weathering: this doesn’t change the rocks chemical composition but breaks rocks into smaller and smaller pieces.

 

2. Chemical Weathering: this transforms the rock chemically into one or more new compounds.

 

6.2

 

Mechanical weathering results in many small pieces from a single large rock. Mechanical weathering then creates more surface area for chemical weathering to occur. 

 

In nature, there are four physical processes that cause the fragmentation of a rock:

 

1. Frost Wedging: Liquid water expands about 9% when frozen. This is the basis for frost wedging. If water works its way into the cracks of rocks and then freezes, it can cause the cracks of rocks to expand and then eventually break off. 

 

2. Salt Crystal Growth: Sometimes dissolved salts seep into tiny cracks and pore spaces within rock, then expand as they crystallize. In coastal environments, sea spray delivers salty water into these openings, while in arid regions the source is often salty groundwater. When the water evaporates, the dissolved salts begin to crystallize, and as those crystals grow, they exert outward pressure on the surrounding mineral grains. Over time this repeated expansion widens existing fractures and can eventually split the rock apart.

 

3. Sheeting: Sheeting is a type of mechanical weathering in which large, coherent slabs of rock peel away from the surface in layers, almost like the pages of a book separating. It happens most commonly in massive, homogeneous rocks such as granite. Deep underground, these rocks form under enormous confining pressure. When erosion removes the overlying material and the rock is brought closer to the surface, that pressure is released. The sudden drop in stress allows the rock to expand slightly, and because the expansion is greatest near the surface, it creates curved fractures parallel to the ground. Over time, these fractures widen and entire sheets detach.

 

4. Biological Activity: Finally, weathering can be caused by biological activity such as animals, plant roots, and humans. 

 

6.3

 

Mechanical weathering can affect chemical weathering as it creates more surface area. However, chemical weathering can affect mechanical weathering by weakening the outer portions of various rocks.

 

Chemical weathering breaks down rock by altering the minerals themselves, changing their composition rather than just breaking them into smaller pieces. Water is the main agent, often carrying dissolved substances like acids or oxygen that react with minerals. Three of the most important processes, dissolution, oxidation, and hydrolysis, each attack minerals in different ways and leave distinct signatures in the landscape.

 

1. Dissolution

 

Some minerals dissolve directly into water, especially when the water is slightly acidic. Rainwater naturally contains carbonic acid formed when CO₂ dissolves into it, and this weak acid reacts with minerals like calcite in limestone. The mineral breaks apart into ions that are carried away in solution. Over long periods, this process creates caves, sinkholes, and karst landscapes because entire volumes of rock can be removed simply by being dissolved.

 

2. Oxidation

 

Oxidation is essentially the rusting of rock. Oxygen—often dissolved in water—reacts with iron‑bearing minerals such as olivine, pyroxene, and biotite. The iron changes from Fe²⁺ to Fe³⁺, forming iron oxides or hydroxides. These new minerals are weaker, softer, and often reddish or yellowish in color. As oxidation progresses, the rock becomes crumbly and breaks apart more easily. This is why weathered basalts and other mafic rocks often develop a rusty surface.

 

3. Hydrolysis

 

Hydrolysis is a reaction between minerals and acidic water that transforms the original mineral into a new, more stable one. Feldspars are the classic example: when hydrogen ions in acidic water replace some of the mineral’s ions, the structure breaks down and forms clay minerals such as kaolinite. This process weakens the rock, increases its porosity, and produces the fine-grained clays that make up many soils. Hydrolysis is one of the dominant ways silicate rocks weather on Earth’s surface.

 

Spherical Weathering- “Any weathering process that tends to produce a spherical shape from an initially blocky shape.” (Tarbuck, Lutgens, and Linneman 179)

 

6.4

 

What Affects The Rate of Weathering?

 

1. Rock characteristics shape how quickly and how intensely a rock weathers because they control both what the rock is made of and how easily water can get inside it.

 

2. Climate also can affect weathering rates through temperature and precipitation. High temperatures can increase the rate of chemical weathering while low temperatures can increase frost wedging in mechanical weathering.

 

Chapter 7: Sedimentary Rocks

 

7.1 

 

Sediments and sedimentary rock make up around 75% of Earth’s solid material at the surface. The ocean floor represents 70% of the earth's surface and is composed almost entirely of sediment. Sedimentary rock is also economically significant as coal is classified as a sedimentary rock and other energy sources like oil and natural gas is trapped in sedimentary rock. 

 

Sedimentary rock forms after igneous, metamorphic, or other sedimentary rock goes through weathering and is transported. After the sediment is transported it is deposited and then eventually goes through lithification and becomes sedimentary rock.

 

There are three categories of sedimentary rock: 

 

1. Clastic Sedimentary Rock- accumulations of rocks that were weathered by both mechanical and chemical processes

 

2. Chemical Sedimentary Rock- produced solely by chemical processes. 

 

3. Organic Sedimentary Rock- these rocks are formed by carbon rich remains of organisms.

 

7.2

 

Clastic sedimentary rocks form from the accumulation and lithification of broken fragments of pre‑existing rocks. These fragments, called clasts, are transported in various ways such as water, wind, ice, or gravity, then deposited and compacted over time. Their texture, grain size, and sorting reveal the energy and environment of the system that carried and deposited them.

 

Shale is a fine‑grained clastic sedimentary rock composed mostly of clay minerals and very small silt particles. It forms in quiet, low‑energy environments such as deep lakes, lagoons, and offshore marine basins where tiny particles can settle out of suspension. Shale typically breaks into thin, flat layers due to its internal alignment of clay minerals. Fissility refers to a rock’s tendency to split into thin, planar sheets. In shale, this property arises because clay minerals align parallel to bedding during compaction, creating planes of weakness. Fissility distinguishes shale from mudstone, which has similar composition but lacks this layered splitting behavior.

 

Oil shale is a sedimentary rock rich in kerogen, an organic precursor that must be heated (pyrolyzed) to release usable hydrocarbons. In contrast, shale oil refers to actual liquid petroleum trapped within shale formations and extracted directly through drilling and hydraulic fracturing. The key difference is that oil shale contains potential hydrocarbons requiring processing, while shale oil is already formed and simply needs to be liberated from the rock.

 

Sandstone is a clastic sedimentary rock composed mainly of sand‑sized grains, typically quartz, feldspar, or rock fragments. It forms in a wide range of environments, from beaches and deserts to river channels, where sand accumulates and becomes cemented by minerals like silica or calcite. The composition, sorting, and cement type help geologists interpret the rock’s depositional history. Sorting describes the uniformity of grain sizes within a sediment or sedimentary rock. Well‑sorted sediments contain grains of similar size, indicating steady, consistent transport energy such as wind or long‑distance water flow. Poorly sorted sediments contain a mix of grain sizes and typically reflect rapid deposition, high‑energy environments, or short transport distances.

 

Particle shape refers to the angularity or roundness of sediment grains, which reflects how far and how long the particles have been transported. Angular grains indicate minimal transport and proximity to the source rock, while rounded grains suggest prolonged abrasion during movement. Shape helps reconstruct the energy and distance of sediment transport.

 

Conglomerate and breccia are coarse‑grained clastic rocks composed of gravel‑sized clasts larger than 2 mm. Conglomerate contains rounded clasts, showing that particles were transported long enough to become smoothed by abrasion. Breccia contains angular clasts, indicating deposition close to the source, often from landslides, debris flows, or fault zones.

 

7.3

 

Chemical sediments come from ions that are carried via solution to lakes and seas where it is then deposited to form chemical sedimentary rocks. The precipitation of the material can occur in two major ways:

 

1. Inorganic, coming from evaporation or chemical activity.

 

2. Organic, coming from water-dwelling organisms.

 

Limestone is the most abundant chemical sedimentary rock, making up roughly a tenth of all sedimentary material on Earth. It is composed primarily of calcite and can form either through inorganic precipitation or through the accumulated hard parts of marine organisms. Even though all limestone shares the same basic mineral composition, it appears in many varieties because it forms in a wide range of environments.

 

Carbonate reefs develop when corals, small invertebrate animals, secrete external skeletons made of calcium carbonate. Although each coral polyp is tiny, they grow in dense colonies that build enormous reef structures over long periods of time. These reefs are further strengthened by algae that also secrete calcium carbonate, cementing the framework into a solid mass.

 

Coquina and chalk are two easily recognized forms of biochemical limestone. Coquina forms from loosely cemented shells and shell fragments, creating a coarse, porous rock that clearly displays its biological origins. Chalk, by contrast, is soft, fine-grained, and composed almost entirely of the microscopic hard parts of marine organisms. Although it looks uniform and powdery, chalk represents the accumulated remains of countless tiny creatures that once lived in ancient seas.

 

Evaporites form in restricted marine basins where seawater continually flows in but evaporates faster than it can be replenished. As the water becomes increasingly concentrated, dissolved minerals begin to precipitate and settle out as layers of chemical sediment.

 

Dolostone is closely related to limestone but is composed of the mineral dolomite, which contains both calcium and magnesium. Although dolostone and limestone can look similar, they behave differently when tested with dilute hydrochloric acid: limestone fizzes vigorously, while dolostone reacts weakly unless powdered. The origin of dolostone has long puzzled geologists because no marine organisms produce dolomite directly, and dolomite rarely precipitates from seawater under normal conditions.

 

Chert is a hard, compact rock made of microcrystalline quartz and appears in a wide range of colors depending on the trace elements it contains. Varieties such as flint and jasper are well known, with flint’s dark color coming from organic material and jasper’s red tones produced by iron oxides. Chert can also form through the replacement of organic material, as seen in petrified wood, where silica-rich groundwater gradually substitutes quartz for the original wood structure

 

7.5

 

Diagenesis refers to all the physical, chemical, and biological changes that sediments undergo after they are deposited but before they become solid rock. These changes occur at relatively low temperatures and pressures, typically within the upper few kilometers of Earth’s crust. During diagenesis, sediments may compact, minerals may dissolve or recrystallize, pore fluids may circulate and alter the chemistry, and organic material may break down. These processes can modify grain size, mineral composition, porosity, and even the overall appearance of the sediment. Diagenesis is a broad term that includes everything from early-stage compaction to more advanced chemical transformations that gradually push loose sediment toward becoming a coherent rock.

 

Lithification is the final stage of diagenesis in which loose sediment is converted into solid sedimentary rock. It involves two main processes: compaction and cementation. Compaction occurs as layers of sediment accumulate, squeezing grains closer together and reducing pore space. Cementation follows when minerals, commonly calcite, silica, or iron oxides, precipitate from groundwater moving through the sediment and bind the grains together. These cementing minerals act like a natural glue, locking the particles into a rigid framework. Once lithified, the sediment becomes a true sedimentary rock such as sandstone, shale, or limestone, preserving the conditions and materials of its original depositional environment.