Course:EOSC311/2025/The Geological Roots of Our Cities
Overview
The foundations of our cities are rooted underground in the shifting bedrock beneath us. From every skyscraper to every sidewalk, the geological foundation beneath is bedrock, one of the less obvious factors that make urban life sustainable. However, this foundation is not as static as it may seem. Over time, bedrock undergoes a transformation known as weathering, a process that fractures and alters the rock as a result of contact with the atmospheric conditions at the Earth’s surface[1]. This phenomenon, while seemingly invisible to us, plays a large role in shaping the environments we live in today.
Bedrock weathering is more than just a geological occurrence; it’s the intersection between geology, ecology, chemistry, and urban planning. In cities, weathered rock serves as the parent material for soil, influences groundwater behavior, and determines the stability or vulnerability of a slope during heavy rainfall. Its chemical products affect plant growth and nutrient cycles, while its physical properties shape how water moves and where infrastructure is built. In other words, understanding weathering is essential for understanding cities themselves.
Statement of Connection
From the perspective of Integrated Sciences, particularly in the unification of ecology, inorganic chemistry, and geographical sciences in the greater context of urbanization, bedrock weathering is an occurrence that allows for the investigation of how natural processes interact with manmade development. Ecologically, the process creates soil conditions that impact biodiversity and green infrastructure. Chemically, bedrock weathering governs the release and cycling of various elements, shaping soil pH, and nutrient availability. In the realm of geographical sciences, weathering controls landscape evolution and influences the stability of slopes, especially in regions with high rainfall or complex topography. Throughout history, these disciplines, combined with geology, have gone hand in hand with each other to shape our world; specifically, in the urbanization of our modern world.
I’ve tailored my Integrated Sciences degree towards the central themes of urbanization and green infrastructure. To portray the interconnected nature of these disciplines, I have chosen to highlight the significance of bedrock weathering in the context of these core themes. Despite its importance, the role of bedrock weathering is often overlooked in urban development conversations. Failing to account for it can lead to catastrophic slope failures, foundation instability, and poor soil health. However, when integrated into urban planning, it can also be a tool for enhancing stormwater management, supporting vegetation in green infrastructure, and even sequestering carbon through techniques like Enhanced Rock Weathering (ERW) [2].
Understanding Bedrock Weathering
What is Bedrock Weathering?
Bedrock weathering refers to the chemical and physical processes that alter the surface of the lithosphere through interactions with the atmosphere, hydrosphere, and biosphere at relatively low temperatures [3]). These processes transform solid rock into saprolites (decomposed rock) and soil. There are two main types of weathering: chemical weathering and physical weathering.
Chemical weathering involves mineral decomposition through reactions with water, oxygen, and organic acids. Some key mechanisms of chemical weathering include hydrolysis, oxidation, and ion exchange [1]. Physical weathering focuses on mechanical breakdown without chemical change, as seen in freeze-thaw cycles, which allow water migration into fractures in rocks [4] .
The two main types of weathering are quite distinct. Chemical weathering induces mineral transformations through chemical reactions, while physical weathering is characterized by structural degradation. Despite these significant distinctions, the two processes are able to interact. Chemical reactions weaken mineral bonds, making rocks more susceptible to mechanical breakdown, which is physical weathering [1].
Soil Formation and Products of Weathering
As seen in Figure 2, through the process of weathering, soil is produced. There are typically 3 distinct layers in the process of weathering: corestone, saprolite, and soil. Corestone is the bottom layer of solid, unaltered bedrock, while saprolite is the middle weathered zone where the rock has decomposed but still retains its original structure, and soil is the top layer of fully weathered material mixed with organic matter. The saprolite layer is particularly interesting because it depicts isovolumetric weathering, meaning the rock decomposes in place while maintaining its original texture and structure [5].
Bedrock weathering marks the first step in soil formation, initiating the slow transformation of solid rock into the aggregates of minerals and organic material that sustain terrestrial ecosystems and urban landscapes [6]. This process begins with the breakdown of bedrock, typically through either physical weathering or chemical weathering, as previously mentioned. This breakdown is generally a result of factors such as water infiltration and temperature fluctuations. Over time, these forces fragment the rock and release essential ions, forming the loose material that serves as the soil’s mineral base [1].
Key Environmental and Urban Impacts
Bedrock weathering plays a foundational role in shaping both the natural environment and the built environment, particularly because of its influence on hydrological behavior, slope stability, and subsurface structure [7]. As rock weathers, its permeability and porosity evolve, creating conduits for water infiltration and zones of variable saturation [8]. These properties are vital in urban hydrology, especially for managing stormwater and assessing infrastructure risk. Slope stability and infrastructure risk will be covered in more detail in the “Case Studies in Urban Contexts” section.
Weathering also governs the morphology of landscapes. Over long geological timescales, chemical and physical weathering alter topography, influence river path development, and dictate sediment movement [9]. In urban planning, this matters because underlying bedrock characteristics can determine where construction is feasible, which areas are prone to erosion, and how subsurface conditions affect tunneling, drainage, and foundation design.
Moreover, the products of weathering (clays, oxides, leached minerals, etc.) can accumulate in ways that either promote or hinder plant growth, stormwater infiltration, or pollutant transport. As cities aim to integrate more nature-based solutions, such as permeable pavements, understanding how weathered bedrock regulates water movement and mechanical support becomes essential [10]. Geology doesn't just form the physical base of a city; it influences its sustainability, resilience, and long-term safety. Whether supporting a green roof or managing stormwater, the legacy of bedrock weathering continues to influence the functionality of soil long after the rock itself has disappeared from view.
Weathering Through an Ecological Lens
Biological Weathering
Biological weathering occurs when organisms such as fungi, bacteria, lichens, and plant roots chemically or physically break down minerals, which is known as mineral dissolution [11]. Microbes and fungi have been labelled as major contributors to biological weathering across various scales (nano to global) through mechanisms such as proton release, enzymatic dissolution, and organic acid secretion [11]. For example, siderophores (small molecular organisms that bind to iron) produced by fungi (e.g. Aspergillus fumigatus; Figure 3) have been shown to accelerate the dissolution of potassium feldspar, mobilizing K, Ca, and Fe [11]. These species-specific interactions create localized weathering zones that fuel nutrient cycles which, in turn, allow our ecosystems to thrive.
Soil and Plant Relationships
The altered mineral conditions of weathered bedrock directly influence nutrient availability for plants and microbial communities, since parent rock type determines nutrient profiles. For instance, soil derived from basalt often contains higher levels of Mg, Ca, and micronutrients compared to granitic soils [12]. Early ecological succession systems demonstrated that plants growing on different types of rocks (specifically basalt, rhyolite, and schist) exhibited variable mineral release and biomass accumulation [12]. Therefore, plant community composition and ecosystem productivity directly depend on subsurface geology.
The presence of healthy and stable soil for urban infrastructure is crucial for sustainability. In cities, the interactions between plants, soil, and rocks manifest in amenities like green roofs and gardens, along with the obvious biodiversity like trees. Effective green infrastructure must highlight a clear understanding of weathering geology to ensure factors such as successful soil fertility and water management. For instance, knowledge of local soil chemistry, which is derived from weathering, in the selection of plants for landscaping benefits local ecosystems.
Positive Feedback Loop
Vegetation plays two main roles in the process of bedrock weathering. It (1) accelerates weathering and (2) is shaped by it. Plant roots exert physical pressure that fractures rock and release carbon dioxide into the soil, altering the chemical environment. At the same time, microbial communities associated with plant roots significantly enhance mineral dissolution rates [11]. These biologically driven processes not only accelerate weathering, but also enrich the soil, enabling further root penetration and growth. Together, these interactions form what’s known as a positive feedback loop, where increased weathering improves plant access to nutrients and water, which in turn promotes deeper root development and more biological activity. Within the Earth's “critical zone” (the outer layer of the Earth from the atmosphere and vegetation to the soil), this feedback loop helps shape soil depth, structure, and biogeochemical cycles, with lasting implications for ecosystem productivity and consequently land use planning [11].
Inorganic Chemistry of Weathering
Chemical Reactions in Weathering
Weathering is governed by inorganic reactions that transform primary minerals into nutrients and secondary minerals, as mentioned above. Hydrolysis, which is the driving force in the conversion of feldspar to kaolinite, releases essential ions [13]. On the other hand, oxidation-reduction reactions control the release of elements like Fe and Mn. Under acidic conditions, minerals like pyrite can dissolve due to chemical weathering through redox reactions [13]. These reactions modify mineral structures and liberate ions into soil and water.
Influence on Soil Chemistry
A key determinant of the resulting soil’s properties is the composition of the parent rock. For instance, soil produced from granite tends to be acidic and low in nutrients due to high quartz content, while soil produced from basalt is typically richer in iron, calcium, and magnesium [12]. The mineral makeup of the original bedrock is the foundation for nutrient availability, pH buffering capacity, and soil fertility [1].
Let’s consider minerals such as feldspar, biotite, and olivine. As they weather, they undergo chemical transformations, particularly hydrolysis, oxidation-reduction, and ion exchange, that release elements like potassium and calcium into the developing soil [14]. These chemical reactions not only generate nutrients for plant life but also form secondary minerals such as clays, oxides, and oxyhydroxides [15]. Organic matter, introduced later through plant colonization and microbial activity, binds with these weathered products to form humus-rich topsoil layers [1].
Geographical Sciences and Weathering
Landscape Evolution
Bedrock weathering sculpts landscape topography over geological timescales. During weathering, softer minerals typically erode faster than more resistant ones, which is one of the driving factors in the creation of the ridges and valleys we see today. A recent study showed how weathering depth correlates with ridge and valley topography and influences near-surface water storage [9]. In urbanizing regions, these landscape forms determine the suitability and feasibility of development.
Hydrology and Slope Stability
Weathered bedrock layers significantly alter subsurface water flow. Models have shown water tables forming at the interface between soil and bedrock, concentrating pore water in depressions, and triggering shallow landslides [16]. This hydrological behavior is vital to account for in urban planning, particularly in steep terrain where root-supported soils and green infrastructure must manage infiltration load. Failure to account for these conditions leads to slope instability and infrastructure risk [16].
Geohazards
Weathered bedrock increases the complexity and difficulty of accounting for geohazards. It’s important not to overlook subsurface variations in structure and composition, so as not to leave risk for slope failure or drainage inefficiency. Engineers now use Geographic Information Systems (GIS) and geotechnical surveys to map saprolite thickness, bedrock depth, and weathering variance [16]. Landslide hazard models also incorporate subsurface topography to predict failure zones [16]. These tools are essential for ensuring urban resilience, allowing for safer placement of buildings, roads, and green infrastructure.
Case Studies in Urban Contexts
Case Study A: Weathering as a Hazard in Hong Kong
Hong Kong’s dense urbanization atop steep, weathered igneous bedrock offers a striking example of how geological processes can quietly shape urban infrastructure risks. While much of the city is engineered to withstand natural hazards, the deeply weathered nature of its subsurface presents hidden dangers. In weathered bedrock, pore spaces can range from microscopic fractures to interconnected voids, resulting in varying subsurface conditions that influence how water moves underground. In cities like Hong Kong, intense weathering of granitic bedrock creates saprolite layers with high porosity and variable permeability. In an investigation by Jiao et al. (2005) [7], it was determined that confined groundwater zones can develop within weathered areas, posing hidden dangers to slope stability. During periods of intense rainfall, these zones rapidly pressurize, leading to extremely elevated pore pressures and resulting in landslides, which are intensified by the increased surface loading and altered drainage patterns associated with urbanization. This concealed threat can be overlooked by urban planners and engineers, resulting in unexpected slope failures, or landslides such as that seen in Cusco, Peru in Figure 5.
This instability stems in part from the chemistry of weathering. The igneous bedrock in Hong Kong undergoes hydrolysis reactions that transform feldspars into kaolinite, a clay mineral with very low permeability [7]. As these reactions proceed under the region’s humid subtropical climate, thick layers of kaolin-rich saprolite form. While these layers may appear structurally stable from the surface, they act as chemical and physical confining barriers. During storms, water infiltrates through more permeable soil above, encounters the kaolinite layer, and becomes trapped, resulting in elevated pore pressure in the underlying fractured rock [7]. The result is the aforementioned rapidly pressurizing subsurface zone that becomes subject to landslides.
_02.jpg)
From a geographical science perspective, this case illustrates how long-term weathering alters the subsurface in ways that interact with contemporary land use. Many of Hong Kong’s urban developments are situated on or adjacent to steep terrain (as seen in Figure 6) where weathering profiles are highly variable in depth, composition, and hydraulic behavior [7]. When urbanization alters natural drainage pathways, such as through the construction of impermeable surfaces, roads, and retaining walls, the water bodies of Hong Kong’s hilly terrain are disturbed; therefore, increasing the likelihood of slope instability.
Case Study B: Enhanced Rock Weathering as a Resource for Urban Farming
On a different note, bedrock weathering can also have a positive impact on our ecosystems, specifically, in the realm of urban agriculture. As cities search for sustainable ways to mitigate climate change while simultaneously improving infrastructure, the concept of enhanced rock weathering (ERW) has emerged as a geological solution with ecological and chemical benefits. In a 2021 study by Haque et al. [17], the application of ERW in urban environments by integrating alkaline silicate minerals, specifically wollastonite (Figure 7), into urban farming systems was proposed. This approach is a method of atmospheric carbon sequestration which also enhances soil health, increases crop productivity, and aligns with sustainable land use and urban planning strategies.
This application of ERW relies on fundamental inorganic chemical reactions between silicate minerals and CO₂. When calcium- and magnesium-rich silicate rocks like wollastonite are added to soil, they react with CO₂ and water to form stable carbonate minerals, effectively storing atmospheric carbon in the ground. In their trials, the authors found that amending soil with wollastonite led to the sequestration of up to 1.28 kg C/m² as inorganic carbon in soil, in addition to 0.70 kg C/m² stored as organic carbon in plant biomass over a two-month period of growth [17]. These pathways for carbon storage, geochemical and biological, demonstrates a powerful intersection of geology with inorganic chemistry and urban ecology.
Ecologically, the application of weathered rock in urban soils fosters more favorable conditions for plant and microbial communities. The weathering of wollastonite improves soil structure, raises pH in acidic conditions, and releases essential elements such as calcium and silica [17]. These changes benefit root development, nutrient uptake, and microbial activity, all of which are critical to the health and productivity of urban green spaces. Therefore, ERW supports ecosystem functions by chemically altering the soil environment in a way that benefits both microbial and plant life, boosting biomass growth and effectively mimicking long-term natural weathering in an accelerated yet controlled urban context.
Rooftops alone can represent 20–25% of urban surface area in cities like Sacramento, CA, and globally, around 17 million hectares of green roof space could be developed for ERW agriculture [17]. If just 1% of urban land used for farming contained wollastonite-amended soil, 13.6 million tons of carbon could be sequestered annually [17]. This has implications for urban zoning, infrastructure policy, and climate adaptation planning. Underutilized urban land, such as rooftops and balconies, can be reinvented as assets in green infrastructure worldwide and used to shape healthier, more sustainable cities.
Conclusion
Overall, the intersection between geology and my integrated disciplines is central to our environment and the very ground on which we stand. Through the scope of bedrock weathering, it's clear how geology actively impacts ecological and chemical processes, along with the geographical landscapes surrounding us. In the context of urbanization and sustainable development, geological processes like bedrock weathering become increasingly important as cities expand and climate pressures consequently intensify. Understanding these concepts and processes is crucial to building stable and sustainable infrastructure with efficient geohazard mitigation. This project has shown me that geology isn't just relevant to my degree; rather, it's foundational to it.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Carroll, D. (2012). Rock Weathering. Monographs in Geoscience. Springer.
- ↑ Jagoutz, O; Krol, A. (2023, November 9). "Enhanced rock weathering". MIT Climate Portal. Check date values in:
|date=(help) - ↑ Weaver, C (1990). "Clays, Muds, and Shales". Developments in Sedimentology. Elsevier Health Sciences. 44.
- ↑ Lv, Y., & Xie, J. (2021). Effects of Freeze–Thaw cycles on water migration, microstructure and protein oxidation in cuttlefish. Foods, 10(11), 2576. https://doi.org/10.3390/foods10112576
- ↑ Riebe, C. S., Callahan, R. P., Granke, S. B., Carr, B. J., Hayes, J. L., Schell, M. S., & Sklar, L. S. (2021). Anisovolumetric weathering in granitic saprolite controlled by climate and erosion rate. Geology, 49(5), 551–555. https://doi.org/10.1130/g48191.1
- ↑ Šamonil, P., Phillips, J., Daněk, P., Beneš, V., & Pawlik, L. (2020). Soil, regolith, and weathered rock: Theoretical concepts and evolution in old-growth temperate forests, Central Europe. Geoderma, 368, 114261. https://doi.org/10.1016/j.geoderma.2020.114261
- ↑ 7.0 7.1 7.2 7.3 7.4 Jiao, J. J., Wang, X.-S., & Nandy, S. (2005). Confined groundwater zone and slope instability in weathered igneous rocks in Hong Kong. Engineering Geology, 80(1), 71–92. doi:10.1016/j.enggeo.2005.04.002
- ↑ Ho, K. K. S., Cheung, R. W. M., & Wong, C. Y. S. (2016). Managing landslide risk systematically using engineering works. Proceedings of the Institution of Civil Engineers - Civil Engineering, 169(6), 25–34. https://doi.org/10.1680/jcien.15.00079
- ↑ 9.0 9.1 Pedrazas, M. A., Hahm, W. J., Huang, M., Dralle, D., Nelson, M. D., Breunig, R. E., Fauria, K. E., Bryk, A. B., Dietrich, W. E., & Rempe, D. M. (2021). The relationship between topography, bedrock weathering, and water storage across a sequence of ridges and valleys. Journal of Geophysical Research Earth Surface, 126(4). https://doi.org/10.1029/2020jf005848
- ↑ Upper Midwest Water Science Center. (2019, March 17). Evaluating the potential benefits of permeable pavement on the quantity and quality of stormwater runoff. USGS. https://www.usgs.gov/centers/upper-midwest-water-science-center/science/evaluating-potential-benefits-permeable-pavement
- ↑ 11.0 11.1 11.2 11.3 11.4 Finlay, R. D., Mahmood, S., Rosenstock, N., Bolou-Bi, E. B., Köhler, S. J., Fahad, Z., … Lian, B. (2020). Reviews and syntheses: Biological weathering and its consequences at different spatial levels -- from nanoscale to global scale. Biogeosciences, 17(6), 1507–1533. doi:10.5194/bg-17-1507-2020
- ↑ 12.0 12.1 12.2 Zaharescu, D. G., Burghelea, C., I., Dontsova, K., Presler, J. K., Maier, R. M., Domanik, K. J., Hunt, E. A., Amistadi, M. K., Sandhaus, S., Munoz, E., Gaddis, E. E., Vaquera-Ibarra, M. O., Palacios-Menendez, M. A., Galey, M., Castrejón-Martinez, R., Roldan-Nicolau, E., Li, K., Reinhard, C. T., & Chorover, J. (2019). Ecosystem-bedrock interaction changes nutrient compartmentalization during early oxidative weathering. arXiv (Cornell University). https://doi.org/10.48550/arxiv.1810.05128
- ↑ 13.0 13.1 Panchuk, K. (2019). Physical Geology, First University of Saskatchewan Edition. Physical Geology, First University of Saskatchewan Edition. https://www.saskoer.ca/physicalgeology/
- ↑ Wilson, M. J. (2004). Weathering of the primary rock-forming minerals: processes, products and rates. Clay Minerals, 39–39, 233–266. https://doi.org/10.1180/0009855043930133
- ↑ Warren, J., & Spiers, G. (2021, November 25). 3.1: Soil mineralogy. Geosciences LibreTexts. https://geo.libretexts.org/Bookshelves/Soil_Science/Digging_into_Canadian_Soils%3A_An_Introduction_to_Soil_Science/03%3A_Digging_Deeper/3.01%3A_Soil_Mineralogy#:~:text=While%20primary%20minerals%20form%20by,specific%20surface%20area%20(SSA).
- ↑ 16.0 16.1 16.2 16.3 Lanni, C., McDonnell, J., Hopp, L., & Rigon, R. (2012). Simulated effect of soil depth and bedrock topography on near‐surface hydrologic response and slope stability. Earth Surface Processes and Landforms, 38(2), 146–159. https://doi.org/10.1002/esp.3267
- ↑ 17.0 17.1 17.2 17.3 17.4 Haque, F., Santos, R. M., & Chiang, Y. W. (2021). Urban Farming with Enhanced Rock Weathering As a Prospective Climate Stabilization Wedge. Environmental Science & Technology, 55(20), 13575–13578. https://doi.org/10.1021/acs.est.1c04111
 |
This Earth Science resource was created by Course:EOSC311. |