Rocks and Weathering Practice Flashcards

The Structure of the Earth and Elementary Plate Tectonics

The theory of plate tectonics describes the Earth as being composed of several distinct layers. On the exterior is a very thin crust. Beneath the crust lies the mantle, which accounts for $82\%$ of the Earth’s total volume. At the greatest depth is the core, which is characterized as being extremely hot and very dense. Near the Earth's surface, the rocks are primarily solid and brittle; this layer, which includes both the crust and the upper portion of the mantle, is known as the lithosphere and extends to a depth of approximately $70\,km$.

The Earth’s crust is categorized into two primary types: continental crust and oceanic crust. The continental crust has an average thickness between $35\,km$ and $70\,km$, consists of very old rocks (mainly exceeding $1500 \times 10^6$ years), and is light in color. It is lighter in weight with an average density of $2.6$ and is rich in minerals such as silica, aluminium, and oxygen. Granitic rock is the most common type found in the continental crust. In contrast, the oceanic crust is much thinner, averaging between $6\,km$ and $10\,km$. Its rocks are geologically young, mostly under $200 \times 10^6$ years old, and dark in color. The oceanic crust is heavier, with an average density of $3.0$, and contains minerals like silica, iron, and magnesium. It consists of fewer rock types, primarily basaltic.

Plate Boundaries and Movement Theories

Global earthquake zones have defined six major plates and several minor plates. The major plates include the Pacific, African, Australian-Indian, North American, South American, Eurasian, and Antarctic plates. Minor plates and features noted include the Juan de Fuca Plate, Nazca Plate, Cocos Plate, Caribbean Plate, Scotia Plate, Hellenic Plate, Anatolian Plate, Arabian Plate, Iranian Plate, and Philippine Plate. Specific oceanic features include the Aleutian Trench, Tonga Trench, East Pacific Rise, Peru-Chile Trench, Mid-Atlantic Ridge, Japan Trench, Mariana Trench, and Java Trench.

There are three primary theories regarding the movement of these plates:

1. Convection Current Theory: This suggests that massive convection currents occur within the Earth’s interior, driven by radioactive decay in the core. Magma rises through the mantle to the surface and spreads out at mid-ocean ridges.
2. Dragging Theory: This posits that plates are subducted or dragged by their oldest edges, which have become cold and heavy. While plates are hot at mid-ocean ridges, they cool as they move away; complete cooling takes approximately $1 \times 10^6$ years. As these cold plates descend into trenches, pressure causes the rock to become heavier.
3. Hotspots: A hotspot is a plume of lava rising vertically through the mantle. While most are near plate margins, the Hawaiian hotspot (the world's most abundant lava source) is not. Movement may be caused by the outward flow of viscous rock from the hotspot center, creating a drag force on the plates.

Sea-Floor Spreading and Ridge Dynamics

In the early 1960s, Dietz and Hess proposed that continents move due to the growth of oceanic crust between them. This was confirmed by Vine and Matthews, who discovered symmetrical magnetic anomalies on either side of the Mid-Atlantic Ridge axis. This phenomenon, known as paleomagnetism, occurs because magnetic grains in cooling lava align with the Earth’s magnetic field at that time.

Mid-ocean ridges differ based on their spreading rates. Slow-spreading ridges, like the Mid-Atlantic Ridge, feature a pronounced central rift and are fed by small, discontinuous magma chambers, leading to a wide variety of basalt types. Fast-spreading ridges, such as the East Pacific Rise, lack a central rift, possess a smooth topography, and are fed by large, continuous magma chambers. These produce more uniform magmas and higher rates of magma discharge, making sheet lavas more common. Spreading rates vary significantly: from a few millimeters per year in the Gulf of Aden to $1\,cm/yr$ in the North Atlantic near Iceland, and $6\,cm/yr$ for the East Pacific Rise. Ridges remain elevated because they consist of rock that is hotter and less dense than the older, colder surrounding plates.

Processes at Plate Boundaries and Landmark Features

Plate boundaries are divided into three main types: spreading (constructive), colliding (destructive), and conservative. Spreading ridges are typically mid-oceanic and produce shallow earthquakes (< 50\,km deep). Colliding boundaries involve subduction, where one plate is forced into the mantle, creating deep-sea trenches, fold mountains, and island arcs. Deep earthquakes (up to $700\,km$) occur here. Examples include the trenches off the Andes and the Aleutian Islands. If a thick continental plate meets an oceanic plate, the descending oceanic plate partially melts, forming volcanic island arcs like those in the Caribbean. Conservative boundaries, or transform faults, involve plates sliding past each other without creating or destroying crust, associated with shallow earthquakes like the San Andreas Fault.

Mountain building is a core process at convergent boundaries. Where oceanic and continental plates meet, the less dense continental plate buckles into mountains (e.g., the Andes). When two continental plates collide, subduction does not occur; instead, the crust is crushed and folded into young fold mountains like the Himalayas (formed by the Eurasian and Indian plates). Mountain building often involves crustal thickening and deformation, though volcanic activity is minimal in the Himalayas.

Ocean ridges are the longest linear uplifted features on Earth, forming submarine chains over $60,000\,km$ long and $1000\text{--}4000\,km$ wide. Their crests rise $2\text{--}3\,km$ above ocean basins that are $5\,km$ deep. Ocean trenches are long, narrow depressions ($6000\text{--}11,000\,m$ deep), often asymmetric with the steep side toward land. Subduction zones typically dip at angles between $30^{\circ}$ and $70^{\circ}$, with older crust dipping more steeply. The volume of subduction generally balances the production of new crust at constructive margins.

Weathering Processes and Controls

Weathering is the in situ decomposition and disintegration of rocks. It is categorized into three types:

1. Physical Weathering: Produces smaller, angular fragments (e.g., scree). Processes include:    - Freeze-thaw (frost shattering): Water in cracks freezes at $0^{\circ}C$, expanding by $10\%$ and exerting pressure up to $2100\,kg/cm^2$ at $-22^{\circ}C$.    - Insolation (Heating/Cooling): Diurnal temperature changes cause expansion and contraction. Granite may undergo granular disintegration, while single-mineral rocks experience block disintegration. In deserts (> 40^{\circ}C day), poor heat conduction causes stresses in outer layers, leading to exfoliation (peeling).    - Salt Crystallization: Sodium sulfate ($Na_2SO_4$) and sodium carbonate ($Na_2CO_3$) expand by $300\%$ at temperatures between $26\text{--}28^{\circ}C$. Evaporation also leaves crystals that exert pressure on rock joints.    - Wetting and Drying: Effective on shales.    - Pressure Release: Overlying rock/ice removal causes underlying rock to fracture parallel to the surface.
2. Chemical Weathering: Most effective sub-surface due to water and organic acids. Processes include:    - Hydrolysis: Affects rocks with orthoclase feldspar (granite). Feldspar reacts with acidic water to form kaolin (china clay), silicic acid, and potassium hydroxyl.    - Hydration: Minerals absorb water. Anhydrite becomes gypsum, expanding by $0.5\%$. Shales/mudstones can increase in volume by $100\%$.    - Carbonation-Solution: Rainfall and $CO_2$ form weak carbonic acid. Calcite reacts with this to form soluble calcium bicarbonate.    - Oxidation: Iron compounds react with oxygen ($FeO \rightarrow Fe_2O_3$), creating a reddish-brown coating, common in well-drained tropical soils.
3. Biological/Organic Weathering: Plant roots and bacterial respiration increase $CO_2$ levels. Chelation involves roots absorbing insoluble minerals through hydrogen ion exchange. Humic acids come from decomposing vegetation.

Controls on weathering include climate (moisture and temperature), geology (chemical composition, cements, and joints), vegetation (organic acids and root growth), and relief. According to Van’t Hoff’s law, the chemical weathering rate increases $2\text{--}3$ times for every $10^{\circ}C$ temperature rise (up to $60^{\circ}C$). Peltier’s diagram illustrates these climatic relationships. Coarse-grained rocks weather quickly due to permeability, while fine-grained rocks provide more surface area for chemical attack.

Specialized Landforms: Tors and Karst

Granite is a crystalline igneous rock containing resistant minerals like quartz, mica, and feldspar. Tors are isolated bare rock masses up to $20\,m$ high. Linton (1955) suggested they form via chemical weathering of joint planes in warm, humid Tertiary conditions; unweathered "corestones" remain as tors after denudation. Another theory attributes tor formation to periglacial frost shattering and solifluction. This is an example of equifinality, where different processes yield the same result.

Limestone (Karst) landscapes form due to the rock's permeability and solubility. Carboniferous limestone is massively jointed. Carbonation-solution creates surface features like clints (blocks) and grikes (grooves) forming limestone pavements. Depressions include swallow holes (sinks) and large dolines (> 30\,m diameter). Resurgent streams appear where limestone meets impermeable rock. Reversibility of the process creates speleothems (stalactites from the ceiling, stalagmites from the floor) and tufa.

Slope Processes and Development

Slopes are shaped by several factors: geology (faulting and folding), climate (arid slopes are jagged; humid slopes are rounded), and regolith. Regolith is unconsolidated surface material (soil, scree, weathered rock). Clay-rich regoliths are unstable due to water retention. Aspect (the direction a slope faces) is also influential; in the Northern Hemisphere, south-facing slopes are warmer. Slopes with mixed rock types are vulnerable to landslides due to differential erosion.

Mass movements are large-scale movements without a moving agent (like a river). Movement occurs when the safety factor (ratio of resistance to downslope force) is compromised by gravity, slope angle, or pore pressure. Types include:

• Heave (Creep): Slow, winter-dominant process. Wetting, heating, or freezing pushes particles perpendicular to the surface; they fall vertically upon drying/cooling, resulting in net downslope movement and terracettes.
• Falls: Occur on steep slopes (> 40^{\circ}) due to weathering/erosion of joints. Debris forms straight or concave scree/talus slopes.
• Slides: The entire mass moves along a slip plane (faults, joints, or bedding planes). Landslides are sensitive to water, which adds weight and increases pore pressure to push particles apart.
• Slumps: Rotational movement along a curved slip plane, common in clay. They have higher water content than slides.
• Flows: Continuous, fluid movements of fine-grained material (like deeply weathered clay). Mudflows are faster than earthflows.
• Avalanches: Rapid movements of snow, ice, rock, or earth. They occur on slopes (> 22^{\circ}), particularly north-facing ones where snow is less stable.

Human Impact and Environmental Management

Human activity significantly alters landforms and processes. Urbanization increases chemical weathering due to $SO_2$ emissions (from fossil fuels) which produce sulfuric acid and salts like calcium sulfate. In limestone caves, artificial lighting promotes plant growth, leading to biological weathering. Deforestation and fossil fuel use increase $CO_2$ levels, accelerating carbonation-solution.

Mass movements are often triggered by human excavation, undercutting, or overloading (piling waste). However, humans also manage slopes. Techniques for falls include grading/benching, drainage, grouting with cement, anchor bolts, and steel mesh. For slides and flows, we use surface ditches, sealing cracks, sub-surface drainage, buttresses, retaining walls, and pilings. In mining, opencast methods involve removing "overburden" (overlying material), leading to habitat destruction and waste disposal issues (tailings). For example, producing $1\,tonne$ of copper generates over $100\,tonnes$ of waste rock.

Acidification and Pollution

Pollution is the contamination of environmental systems to toxic or harmful levels. Acidification is caused by $SO_2$ and $NO_x$ emissions. Dry deposition occurs near the source as particles or gases. Wet deposition (acid precipitation with pH < 5.0) occurs when oxides are oxidized into sulfuric ($H_2SO_4$) and nitric acid ($HNO_3$) and fall as rain, snow, or mist. Impacts were first noted in Scandinavia in the 1960s; $18,000$ lakes in Sweden are acidified, and some wells show aluminium levels of $1.7\,mg/l$ (WHO safe limit is $0.2\,mg/l$). Acid rain causes needle drop in coniferous trees, interferes with photosynthesis, and corrodes historic buildings. Solutions include using low-sulfur fuels, flue gas desulfurization (FGD), and neutralising water with lime.

Questions & Discussion

• Q1: Outline the main differences between continental crust and oceanic crust. (Answers involve thickness, age, density, and mineral composition).
• Q2: Name the six major plates shown in Figure 3.1. (Pacific, North American, South American, African, Eurasian, Australian-Indian, Antarctic).
• Q3: Briefly explain what is meant by (a) paleomagnetism and (b) sea-floor spreading.
• Q4: What processes happen at (a) a mid-ocean ridge and (b) a subduction zone?
• Q5: With which types of plate boundary is volcanic activity associated? (Spreading and colliding boundaries).
• Q6: Describe the main features of an island arc system. (Arc-shaped volcanic islands formed in subduction zones).
• Q7: Briefly explain how island arcs are formed.
• Q8: Define physical weathering.
• Q9: What are the factors that make freeze-thaw weathering effective? (Plentiful moisture and frequent fluctuations above/below $0^{\circ}C$).
• Q10: Describe the process of exfoliation. Why is it characteristic of hot desert environments? (Due to large diurnal temperature ranges and poor heat conduction in outer rock layers).
• Q11: Compare the character of rocks affected by mechanical weathering with those affected by chemical weathering.
• Q12: In which climatic zone is the weathering depth greatest? (Humid tropical).
• Q13: In which other climatic zones is there also some intense weathering?
• Q14: Describe and explain how the intensity of chemical weathering varies with climate. (Increases with moisture and heat).
• Q15: How useful are mean annual temperature and mean annual rainfall as a means of explaining freeze-thaw weathering? (Cycles are more important than averages).
• Q16: What are the two main theories about tor formation? (Linton’s chemical theory vs. periglacial theory).
• Q17: What is equifinality?
• Q18: What are the main processes affecting limestone? (Carbonation-solution, freeze-thaw, fluvial/glacial erosion).
• Q19: Explain the formation of swallow holes.
• Q20: Briefly describe two ways in which geology affects slope development. (Faulting, folding, and rock resistance).
• Q21: State one difference between a rockslide and a mudflow. (Particle size and water content).
• Q22: Define the term mass movement.
• Q23: Suggest how mass movements can be classified. (Speed, water content, and movement type such as heave, fall, slide, or flow).
• Q24: Define the terms shear strength and shear stress. (Strength/resistance vs. forces pulling mass downslope).
• Q25: Compare and contrast the characteristics of falls and slides.
• Q26: How does a slump differ from a slide? (Slumps are rotational and occur on curved slip planes).
• Q27: Explain the terms rotational slide and avalanche.
• Q28: Outline how avalanches are formed. (Snow/ice/rock/earth moving rapidly, often on slopes over $22^{\circ}$).
• Q29: How can human activity reduce the risk of rockfalls? (Grading, grouting, mesh, etc.).
• Q30: What are the environmental impacts of opencast mining? (Habitat destruction, waste/tailings disposal).
• Q31: What impacts can underground mining have? (Subsidence and water pollution).
• Q32: Outline the natural and man-made causes of acidification. (Volcanoes vs. burning fossil fuels).
• Q33: In what ways is it possible to manage the effects of acidification? (Liming, FGD, low-sulfur fuel).