The Earth's Crust and Resources
Crust, minerals and rocks
Understand the distinction between a mineral (naturally occurring, inorganic, definite composition, ordered atomic structure) and a rock (aggregate of minerals). Know the three rock classes and their formation pathways, and how the rock cycle links them through melting, cooling, weathering, burial and metamorphism.
Resources
- Renewable versus non-renewable is about rate of replenishment relative to use, not about being 'natural'.
- Ore deposits form by magmatic segregation, hydrothermal precipitation, sedimentary concentration or residual weathering. Grade and tonnage determine economic viability, and viability shifts with price and technology.
- Fossil fuels are stored chemical energy from ancient biomass; coal seams, oil and gas traps require source, reservoir, seal and migration pathway.
- Mining impacts: acid mine drainage from sulfide oxidation, tailings, subsidence, dust, and rehabilitation requirements.
Common traps
- Calling coal a mineral - it is a rock-like organic sedimentary deposit.
- Confusing reserves (economically extractable, demonstrated) with resources (total, including undiscovered or uneconomic).
- Assuming metamorphism always involves melting - it does not; melting produces magma.
- Ignoring that exploration and extraction have environmental costs even for 'clean' technologies.
- Misreading a cross-section: younger units overlie older ones unless the sequence is overturned.
Worked example: reserve versus resource
A deposit holds 500 Mt of copper ore at 0.4% Cu. At current prices only material above 0.6% Cu is profitable. Rule of thumb - resource is the whole 500 Mt; reserve is only the portion that is currently economic and demonstrated. If prices rise, reserve can grow without any new discovery.