Mining methods and the geosphere: WACE Year 12 Earth and Environmental Science
“Compare mining methods and analyse their physical effects on the geosphere”
The mining method chosen depends on how deep the resource lies and its value. Open-cut mining strips overburden to reach shallow ore and reshapes the land most dramatically; underground mining follows deeper ore through shafts and can cause subsidence; dredging works wet deposits; and in-situ methods dissolve and pump out the resource. Each method changes the geosphere by removing rock, creating voids, and producing overburden, waste rock and tailings.
What this dot point is asking
SCSA wants you to compare the main mining methods and analyse how each physically alters the geosphere. This is the structural foundation for the broader impacts and rehabilitation content, so be precise about what each method does to the land and the waste it generates.
Choosing a method
The method follows the geology and economics of the deposit.
- Open-cut (open-pit) mining is used for large, shallow, lower-grade ore bodies. Overburden is removed to expose the ore, creating a widening, deepening pit. Most Pilbara iron ore and Darling Range bauxite are mined open-cut, and the Kalgoorlie Super Pit is one of the largest open pits in the country.
- Underground mining is used where ore is deep or rich enough to justify shafts and tunnels. It disturbs less surface area but leaves voids that can collapse.
- Dredging extracts loose sediment from rivers, beaches or shallow seas, used for some mineral sands and alluvial deposits.
- In-situ recovery dissolves the target mineral underground and pumps the solution to the surface, avoiding bulk excavation but risking groundwater contamination.
The single biggest control on method is depth. Shallow, bulky deposits favour open-cut because moving overburden is cheaper than tunnelling. Deep or high-value deposits favour underground mining despite higher cost. Recognising this trade-off is the key to comparison questions.
Physical effects on the geosphere
Whatever the method, mining changes the solid Earth in characteristic ways.
- Landform change. Open pits, waste dumps and tailings dams replace the original topography permanently.
- Overburden and waste rock. Material removed to reach ore must be stored, forming large dumps that alter drainage and can be unstable.
- Tailings. The fine waste left after the valuable mineral is separated is stored as a slurry behind dams, which must be engineered to prevent failure and leakage.
- Subsidence. Underground voids can collapse, causing the surface above to sink.
- Loss of soil and exposure to erosion. Stripping vegetation and topsoil exposes ground that erodes more easily.
Overburden: rock and soil removed to reach ore. Waste rock: uneconomic rock excavated with the ore. Tailings: fine processing waste, usually stored as slurry. Subsidence: sinking of the ground over underground voids. In-situ recovery: dissolving and pumping ore without bulk excavation.
Comparing footprint and waste
Open-cut mining has the largest surface footprint and produces the most overburden and waste rock, but is cheaper for bulk ore. Underground mining has a smaller surface footprint but adds subsidence risk and is costlier. Dredging churns aquatic sediment and increases turbidity, while in-situ recovery has the smallest surface disturbance but the highest groundwater risk. A strong comparison links each method to its dominant geosphere impact and the volume and type of waste it creates.
Given the depth, size and grade of a deposit, justify a method. Shallow and bulky with low grade: open-cut, accepting a large footprint and overburden. Deep and high grade: underground, accepting subsidence risk and higher cost. Soluble ore with groundwater nearby: weigh in-situ recovery against contamination risk. Always tie the choice to the dominant geosphere effect.
Stripping ratio and the economic limit of open-cut
A useful quantitative idea for comparison questions is the stripping ratio, the volume of overburden and waste rock that must be moved per unit of ore extracted. A low stripping ratio means little waste per tonne of ore, making open-cut cheap; as a pit deepens, the ratio rises because ever-wider walls must be cut back to keep the pit stable, so more waste is moved for each tonne of ore. Eventually the cost of moving overburden exceeds the value of the ore at depth, and this economic limit is often what triggers a switch from open-cut to underground mining on the same deposit, as happened as the Kalgoorlie Super Pit deepened. The stripping ratio therefore links the geology of a deposit directly to both the method chosen and the volume of geosphere disturbance.
- Treating all mining as open-cut
- Method depends on depth, grade and deposit type.
- Confusing overburden, waste rock and tailings
- Overburden is removed to reach ore, waste rock is uneconomic excavated rock, tailings are fine processing waste.
- Ignoring subsidence
- It is the signature geosphere impact of underground mining.
- Forgetting in-situ recovery's water risk
- Its small surface footprint comes with a groundwater contamination hazard.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style7 marksCompare open-cut and underground mining in terms of their physical effects on the geosphere, and recommend with justification a method for extracting a deep, high-grade gold ore body beneath a town.Show worked answer →
A 7 mark answer needs a structured comparison plus a justified recommendation tied to the scenario.
- Open-cut
- Strips overburden to expose shallow ore, creating a large pit and the biggest surface footprint, with major landform change and large overburden and waste-rock dumps. It is cheaper for bulk, low-grade ore.
- Underground
- Follows deep or rich ore through shafts and tunnels, disturbing far less surface area but creating voids that can cause subsidence, at higher cost.
- Recommendation
- For a deep, high-grade gold body beneath a town, underground mining is appropriate. The depth makes open-cut impractical (vast overburden removal) and the high grade justifies the cost of tunnelling. Crucially, the small surface footprint avoids destroying the town, though subsidence must be managed by engineering and backfilling voids.
Markers reward the footprint-versus-subsidence-and-cost comparison and a recommendation explicitly justified by depth, grade and the surface constraint (the town).
WACE 2023-style6 marksDistinguish between overburden, waste rock and tailings, and explain one geosphere management challenge associated with each.Show worked answer →
A 6 mark answer needs clear definitions plus a distinct challenge for each.
- Overburden
- The soil and rock removed to reach the ore. Challenge: it must be stockpiled in large dumps that alter surface drainage and can erode or become unstable; storing topsoil separately matters for later rehabilitation.
- Waste rock
- Uneconomic rock excavated alongside the ore. Challenge: it may contain sulfide minerals, so dumps can generate acid mine drainage and must be characterised and encapsulated.
- Tailings
- Fine processing waste stored as a slurry behind dams. Challenge: tailings dams must be engineered against failure and seepage, because collapse releases a destructive slurry and leakage contaminates water.
Markers reward correct distinctions (removed-to-reach versus excavated-uneconomic versus fine-processing-waste) and a relevant, distinct management challenge for each.