Short Topics (choose 4 of 7; all 7 answered): 04-BS-14, December 2016, Q4

Question 4: Short Topics (choose 4 of 7; all 7 answered) (40 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

25. Permafrost profile and temperature–depth graph

Permafrost is ground (soil or rock) that remains at or below 0°C continuously for two or more years. From the surface downward, a permafrost site has three characteristic zones: (1) the active layer — the top zone (typically from a few tenths of a metre to a few metres thick; thinnest in the coldest, continuous-permafrost regions and thickest toward the warmer, discontinuous margin) that thaws every summer and refreezes every winter; (2) the permafrost table/permafrost zone below it, remaining frozen year-round; and (3), at depth, the base of permafrost where geothermal heat flow eventually raises ground temperature back above 0°C. Within the permafrost, an intermediate zone of annual/seasonal temperature variation dampens with depth until the level of zero annual amplitude is reached, below which ground temperature is essentially constant year-round and controlled by the regional geothermal gradient.

Idealized permafrost profile: active layer, permafrost (with a damped seasonal-variation zone above the level of zero annual amplitude), and unfrozen ground below the permafrost base.

Summer and winter temperature–depth curves converge toward the same permafrost temperature at depth; only the active layer (above the dashed line) swings across 0°C seasonally.

26. Four drainage patterns and the geology that produces them

Pattern	Description	Underlying geology
Dendritic	Irregular, tree-like branching, tributaries joining the trunk stream at acute (variable) angles.	Develops on flat-lying rock of uniform resistance to erosion (e.g. flat sedimentary strata or homogeneous crystalline rock) with no strong structural control — the stream network simply follows the steepest available slope in every direction.
Trellis	Parallel main streams joined by short tributaries meeting them at right angles, giving a lattice/vine-trellis look.	Forms on alternating bands of resistant and weak, tilted or folded strata (classic in fold-and-thrust belts) — main streams follow the strike valleys of weak rock, tributaries cut directly down the dip slope through the resistant ridges.
Rectangular	Streams and tributaries bend at right angles in a grid-like pattern.	Controlled by an orthogonal joint or fault set in the bedrock; streams preferentially erode along the pre-existing fracture planes rather than cutting new courses.
Radial	Streams diverge outward in all directions from a single high point.	Typical of an isolated conical/domal landform — a volcano, dome, or laccolith — where the topographic high itself dictates outward-radiating drainage.

Four drainage patterns: dendritic (uniform rock), trellis (alternating dipping strata), rectangular (orthogonal jointing/faulting), radial (isolated conical high).

27. Four factors that affect mass wasting

Factor	Effect
Slope angle (gradient)	Steeper slopes have a larger down-slope component of gravity relative to the normal (resisting) component, so shear stress rises toward the material's shear strength — steeper slopes fail more readily and at lower additional trigger than gentle ones.
Water content / pore-water pressure	Water adds weight, reduces the effective normal stress (and hence frictional shear strength, per Terzaghi's effective-stress principle), and can lubricate slip surfaces or liquefy loose granular/silty material — a dominant trigger for slides, flows and debris flows after heavy rain or snowmelt.
Material type and internal structure	Unconsolidated or poorly cemented material, and rock with bedding, joints or foliation planes oriented parallel/sub-parallel to the slope face (daylighting), fail far more easily than massive, well-cemented, or favourably-oriented rock.
Vegetation cover	Root systems mechanically reinforce soil and increase apparent cohesion, while canopy interception and transpiration reduce infiltration; removal of vegetation (fire, logging, development) measurably raises mass-wasting susceptibility.
External triggers (seismic shaking, undercutting, loading)	Earthquake shaking imposes transient additional shear stress; stream/wave/human undercutting removes toe support; added load (construction fill, buildings) increases driving stress — any of these can push a marginally stable slope past failure.

(Five factors are tabulated so any four form a complete, well-justified answer; slope angle, pore-water pressure, material/structure and vegetation are the four most commonly cited in the introductory geology/geotechnical literature, with external triggers as the usual proximate cause of a specific failure event.)

28. The hydrologic cycle

The hydrologic (water) cycle is the continuous, solar-energy-driven circulation of water between the oceans, atmosphere and land. Evaporation from oceans and surface water (plus transpiration from vegetation, together evapotranspiration) puts water vapour into the atmosphere; the vapour condenses and falls as precipitation (rain/snow). On land, precipitation partitions into surface runoff (feeding streams and rivers back to the ocean), infiltration into the subsurface (recharging soil moisture and groundwater, which itself flows slowly toward discharge points such as springs, streams and the ocean), and evaporation back to the atmosphere; a fraction of precipitation falling as snow accumulates in ice sheets/glaciers as long-term storage before eventually melting and re-entering the cycle. The cycle is a closed system at the scale of the whole Earth, but individual reservoirs (atmosphere, ocean, groundwater, ice) exchange water at very different residence times.

Hydrologic cycle: evaporation/transpiration → atmospheric moisture → precipitation → runoff and infiltration → groundwater discharge back to surface water/ocean.

29. Three types of glaciers

Type	Key characteristics
Alpine (valley) glaciers	Confined within a mountain valley, flowing down-valley from an accumulation zone (cirque) at high elevation; erode classic U-shaped valleys, cirques, arêtes and horns; relatively small, respond quickly to climate change.
Continental ice sheets	Vast (sub-continental scale, >50,000 km²), dome-shaped masses that bury underlying topography and flow radially outward from a central thick region under their own weight, largely independent of local relief (e.g. Antarctic and Greenland ice sheets, and Pleistocene Laurentide ice sheet); primary agents of regional-scale continental glacial erosion/deposition (drumlins, till plains, outwash).
Piedmont glaciers	Form where one or more valley glaciers emerge from confining mountain terrain onto a broad lowland/plain and spread out laterally into a wide lobe (e.g. Malaspina Glacier, Alaska); intermediate in character between alpine and ice-sheet glaciation.

(Ice caps — smaller, dome-shaped ice masses covering <50,000 km² of highland, such as on Iceland or Baffin Island — are a fourth recognized type, intermediate between an alpine glacier complex and a full ice sheet.)

30. Stress–strain behaviour: brittle vs. ductile

A stress–strain curve plots applied stress against resulting strain as a material (rock) is progressively loaded. Both brittle and ductile materials show an initial linear (elastic, fully recoverable) segment obeying $\sigma = E\varepsilon$ up to the yield point; beyond yield the two diverge sharply. A brittle rock accumulates very little permanent (plastic) strain beyond yield before it fractures suddenly at, or only slightly past, the yield stress — the curve rises steeply, peaks, then drops abruptly to zero (loss of cohesion). A ductile rock instead continues to accumulate substantial permanent strain beyond yield at a much lower, often near-constant or slowly rising stress (strain hardening) before eventually failing at large total strain — the curve flattens into a long plastic plateau before an eventual, much-delayed failure.

Stress–strain curves: both share the initial elastic segment; brittle rock fractures with little post-yield strain, ductile rock flows through a long plastic plateau before eventual failure.

31. Glacial erosional and depositional features

Feature	Type	Description
Cirque	Erosional	A bowl-shaped, steep-walled amphitheatre carved at the head of an alpine glacier by rotational abrasion/plucking; the classic source area of a valley glacier.
U-shaped (glacial trough) valley	Erosional	A valley reshaped from an originally V-shaped fluvial valley by glacial erosion into a broad, steep-sided, flat-floored U-profile, often with truncated spurs and hanging tributary valleys.
Moraine (e.g. terminal/end moraine)	Depositional	A ridge of unsorted glacial till bulldozed and dumped at the margin (terminal), sides (lateral) or base (ground) of a glacier, marking its maximum or still-stand position.
Esker	Depositional	A long, sinuous ridge of stratified sand and gravel deposited by meltwater flowing in a subglacial or englacial tunnel; left standing as a ridge once the surrounding ice melts away.

Topic: Periglacial (permafrost) processes, fluvial drainage-pattern controls, mass-wasting mechanics, the hydrologic cycle, glaciology, and rock rheology (brittle vs. ductile behaviour).

Key relations: Effective-stress principle ($\sigma' = \sigma - u$) governs pore-pressure control of slope stability; drainage pattern reflects underlying structural/lithologic control (uniform rock → dendritic, structural grain → trellis/rectangular, isolated high → radial); elastic modulus $\sigma=E\varepsilon$ below yield, then brittle fracture vs. ductile flow above it; mass/energy conservation drives the hydrologic cycle's evaporation–precipitation–runoff–infiltration loop.

Why this works: Every sub-question here asks for a process or landform to be tied back to its controlling variable (temperature profile for permafrost zonation, bedrock structure for drainage pattern, temperature/pressure/strain-rate for ductile vs. brittle behaviour) — naming the feature is necessary but the mechanism is what the marking scheme rewards.

Common pitfall: Describing landforms without linking them to the controlling geology/process (e.g. naming “trellis drainage” without explaining the alternating-resistance structural control); confusing erosional (removes/reshapes existing material) with depositional (creates new landforms from transported sediment) glacial features.

Source: Marshak, Earth: Portrait of a Planet, Ch. 16 (mass wasting), Ch. 17 (streams and drainage), Ch. 19 (glaciers and permafrost), Ch. 10 (rock deformation).

Question 4 of 4: Short Topics (choose 4 of 7; all 7 answered)