[E1] Elective 1: Dynamic Earth (Rock)
[E1] Elective 1: Dynamic Earth (Rock)
Types of Rocks
Igneous Rocks | ||
Intrusive / Plutonic | ||
Granite | Properties | - Slow cooling and solidification of magma → compact + coarse-grained + less resistant - Absence of layers → non-stratified - Not formed of remains of dead marine organisms → without fossils - Long crystallization process → large crystals - Formed of quartz, feldspar, mica → light color - Slow and less contraction |
Formation | Magma intrudes into cracks and bedding planes in earth crust → magma cools and solidifies slowly at great depth → slow and less contraction → found in batholiths → exposed on earth surface after erosion / removal of overlying rocks | |
Extrusive / Volcanic | ||
Tuff | Properties | - Fast cooling and solidification → compact + fine-grained + more resistant - Not formed of remains of dead marine organisms → without fossils - Short crystallization process → small crystals - Formed of quartz, feldspar, mica → light color - High silica content → slightly acidic (pH < 7) - Vertical joints - Hexagonal columnar structures - Rapid and more contraction |
Formation | Lava pours out onto earth surface → lava cools and solidifies rapidly with contacted air → a series of volcanic eruptions → large-scale collapse of crater → caldera filled with volcanic ash and lava | |
Sedimentary Rocks | ||
Clastic (mechanically formed) | ||
Formation | 1. Deposition / sedimentation: sourcing and sorting of sediments 2. Compaction: sediment layers squeezed 3. Cementation: minerals dissolved in water → glue grains tightly 4. Lithification: sediments hardened into rocks | |
Common properties | - Non-crystalline - Formed of remains of dead marine organisms → contain fossils | |
Shale (siltstone) | Properties | - Dark grey to reddish brown color - Presence of layers → stratified - Thin layers - Less compact // resistant than igneous rocks |
Conglomerate | Properties | - Red / white color - Top: extrusive rocks // others: sedimentary rocks - Sediments poorly stored - Absence of layers → non-stratified - Thick layers - More resistant |
Non-clastic (chemically/organically formed) | ||
Rock salt | Formation | Precipitation of minerals from mineral solution due to evaporation of water → formed under arid conditions |
Limestone | Formation | Remains of living organisms in shallow seas Sea animals die → shell and skeletons (mainly CaCO3) sink into mud of bottom ocean → chalk → plate movements → earth layers sink further → high pressure → recrystallize → turn into harder limestone / marble (ref. form 3 Chemistry) |
Metamorphic Rocks | |
Thermal metamorphism | |
Properties | - Changes completely from other rocks - Banded structure - Harder and more compact than before - Non-foliated - Aureole is found - Affect small area |
Formation | Great heat → rock contact with lava or magma directly |
Dynamic metamorphism | |
Properties | - Changes completely from other rocks - Banded structure - Harder and more compact than before - Foliated - Affect large area |
Formation | Great pressure → earth movement (folding / faulting) |
Internal Processes
Folding | |
Compressional force | |
Formation | Rocks bend → form folds |
Example | - Ma Shi Chau - Lai Chi Chong |
Faulting | |
Tensional force | |
Formation | Line of weakness → rock displacement → linear topographic depression → penetration of water weathering → enhanced erosion concentrate along faults |
Example | - Tolo Channel Fault - Sha Tau Kok Fault - Tai Lam Chung Valley |
Volcanism | ||
Intrusive | ||
Results | - Batholiths - Dykes - Sills - Laccoliths - Intrusive igneous rocks | |
Examples | - Batholiths: low hills → Castle Peak - Resistant dykes: spurs // waterfalls - Less resistant dykes: narrow inlets along coast | |
Extrusive | ||
Results | - Rocks produced through solidification of volcanic ash and lava → resistant to weathering | |
Examples | - Volcanoes // volcanic islands: Tai Mo Shan - Lava plateaux: Lantau Peak | |
External Processes
Weathering | ||
Mechanical / physical | ||
Block disintegration | Pressure release of well-jointed rock → outer layer of rock removed by erosion → rainwater seeps through joints → inner layer of rock mass expands → sheeting occurs → vertical joints appear on rock | |
Granular disintegration | Pressure release of coarse-grained rock → outer layer of rock removed by erosion → inner layer of rock mass expands → different minerals expand and contract at different rates | |
Exfoliation | Pressure release of fine-grained rock with a uniform structure → outer layer of rock removed by erosion → inner layer of rock mass expands → sheeting occurs → growth of salt crystals forces the cracks to widen | |
Honeycomb | Salt crystallization → seawater with salt content seeps into cracks of volcanic rocks → crystals formed after moisture evaporates → crystals grow inside cracks exert pressure on rock → cracks formed on rock → different expansion and contraction rates of rock | |
Biological (cracks with roots) | Roots inside cracks exert pressure on rock → large physical pressure helps break the rock along joints → dense presence of roots | |
Chemical | ||
Spheroidal | Hot and wet climate // heavy rain → produce many loosened weathered materials → top soil exposed to surface → easily carried away by erosion agents (rainfall) → mass wasting → soil erosion → → undergo oxidation // hydrolysis → unweathered corestones pile up → smaller upper corestones and larger lower corestones | |
Mass wasting | |||
Rockfall | |||
Result | Evidence and Explanation | ||
Large amount of rock debris / talus found at the base of slope | - Steep gradient → vertical steep slope surface → great stress on slope - Weathered rock → rusty color → rock structure weakened - Presence of joints → vertical columnar joints → loosened weathered materials → lower internal strength - Presence of overhanging rocks → rock pieces fall to base of slope under gravity → lower strength at slope - Shear stress → shear strength | ||
Landslide | |||
Result | Type | Evidence and Explanation | |
Scar can be seen | Climate | - Hot and wet climate // heavy rainfall → add weight → increase shear stress // pore water pressure increase → saturated soil → decrease shear strength | |
Gradient | - Steep and hilly relief → cut slope // fill slope → increase gravitational force → → high resistance of rock → increase shear stress | ||
Vegetation cover | - Dense vegetation cover → induce biological / chemical weathering | ||
Geology | - Granite + hot and wet climate // coarse-grained volcanic rock + hot and wet climate → well-jointed → easily chemically weathered → weathering agents seep along joints into rock mass → many loosened weathered materials → unstable slope → decrease shear strength | ||
Human activities | - Removal of vegetation → no roots to hold soil → soil easily washed away by rain → decrease cohesion → decrease shear strength - Cut slope → unstable slope → steeper slopes → improper maintenance → increase shear stress | ||
Poor maintenance | - Cracks on surface // damage on drainage systems → water seeps into slope materials → reduce friction → decrease cohesion → decrease shear strength → water adds weight to slope materials → increase shear stress | ||
Measures on Mass Wasting
Landslide | |||
治標 | Boulder fence / flexible barriers | Stop boulders from rolling downslope | |
Debris barriers / check dams | Resist large-scale debris flow → reduce rate of surface runoff → regulate flow of water → trap sediments | ||
治本 | Impermeable layers | Prevent rainwater from infiltrating into slope → prevent saturation of soil + reduce pore water pressure → reduce weathering on slopes → decrease shear stress + increase shear strength | |
Soil nails | Anchor soil to slope → increase cohesion → increase shear strength | ||
Retaining walls | Support steep slope surfaces → increase shear strength | ||
Weepholes | - Shotcrete: increase slope cohesion → reduce water infiltration - Drainage: reduce pore water pressure → reduce slope shear stress | ||
Reclamation
Rock fill (weathered granitic rocks) | |||
Advantages | Abundant supply of rock by levelling of islands —> weathered materials near land of surface —> easy to be excavated —> reducing transport cost of reclamation materials | ||
Disadvantages | Noise pollution | Machines produce noise —> scare away birds // disturb people | |
Air pollution | Extraction creates dust —> harm respiratory system | ||
Water pollution | - Block drainage channels - Increase sediment content | ||
Ecosystem destruction | - Destroy habitats and cut food - Barely support vegetation | ||
Visual pollution | - Leave scars on green landscape | ||
Marine sand fill (marine sand) | |||
Advantages | Bring about less air pollution | ||
Disadvantages | - Increase suspended sediment content - Reduce ability of sea animals to find food - Remove contaminated mud deposits - Increase seawater pollution (heavy metals) - Bring less hazards to marine ecosystem // habitat // foodweb | ||
Pubic fill (demolition // construction waste) | |||
Advantages | - Less damage on marine ecosystem - Help alleviate pressure on landfills - Reused for reclamation - Conserve natural resources | ||
Disadvantages | - Limited and unreliable supply - Requires on-site sorting | ||
Rock Cavern
Formation | Large man-made spaces created by excavating rocks | ||
Favorable factors | Abundance of igneous rocks (granite // tuff) | ||
Explanation | Well-jointed —> easy to excavate —> less resistant to denudation // weathering | ||
Unfavorable factors | Near major fault lines and zones of deep weathering | ||
Explanation | Risk of rock displacement // movement —> require supporting structure // strengthening structure |
