19.2 Rock Cycle, Landforms & the Geologic Time Scale

Rocks record processes through texture, minerals, structures, fossils, chemistry and age. Mining engineers use that record to predict geometry, hardness, weathering, groundwater, geotechnical behavior and mineralization.

Three Rock Groups

Igneous

Igneous rocks crystallize from magma or lava. Slow underground cooling produces coarse phaneritic texture; rapid surface cooling produces fine aphanitic texture; quenching can form glass; two-stage cooling creates porphyritic texture. Composition ranges broadly from felsic to ultramafic. Intrusive contacts, dikes, volcanic centers and alteration can guide porphyry, epithermal, skarn and magmatic deposit models.

Sedimentary

Sedimentary rocks form by deposition and lithification of clasts, chemical precipitation, or biological accumulation. Grain size, sorting, rounding, bedding, sedimentary structures and fossils indicate transport and environment. Permeable sandstone, soluble limestone, weak shale and coal seams present different groundwater and stability behavior.

Metamorphic

Metamorphism changes rock through heat, pressure, stress and fluids without wholesale melting. Foliation develops from aligned minerals under differential stress; non-foliated rocks can form where mineralogy or stress does not produce layering. Metamorphic grade and index minerals record conditions. Shear zones can focus fluid and mineralization but may also create weak ground.

Rock Cycle

Any rock can be uplifted, weathered, eroded and deposited; buried sediment can lithify; heat and pressure can metamorphose rock; sufficient melting forms magma; magma crystallizes into igneous rock. The cycle has many paths, not one fixed circle. Weathering of ultramafic rock under tropical conditions can produce nickel laterite; erosion and hydraulic sorting can concentrate placer gold or heavy minerals.

Landform Processes

• Tectonic: fault scarps, uplifted ranges, subsiding basins.
• Volcanic: cones, calderas, lava plateaus and lahar channels.
• Fluvial: valleys, floodplains, terraces, alluvial fans and deltas.
• Karst: sinkholes, caves and underground drainage in soluble rock.
• Coastal: cliffs, beaches, bars and marine terraces.
• Mass wasting: falls, slides, flows and creep driven by gravity.

Landform interpretation helps locate alluvium, regolith, drainage anomalies, erosion surfaces and hazards. A straight stream reach may reflect a fault, but field evidence is required.

Relative Dating Principles

1. Superposition: in an undeformed sequence, younger beds overlie older beds.
2. Original horizontality: sediments are deposited approximately horizontal; tilting occurred later.
3. Lateral continuity: a layer originally extends laterally until it thins or meets a boundary.
4. Cross-cutting: a fault, dike or vein is younger than the rock it cuts.
5. Inclusions: fragments are older than the rock containing them.
6. Fossil succession: fossil assemblages occur in recognizable temporal order.

An unconformity represents missing time from erosion or non-deposition. Angular unconformity separates tilted older rocks from younger overlying strata; disconformity lies between parallel sedimentary beds; nonconformity places sedimentary rock over eroded igneous or metamorphic basement.

Radiometric Dating

Radioactive parents decay at a constant probability to daughter isotopes. After one half-life, half the original parent remains; after two, one quarter. The age calculation requires a closed system and appropriate mineral-isotope pair. Alteration can reset or disturb some systems, so age interpretation must identify what event the mineral records—crystallization, cooling or alteration.

Geologic Time

Largest to smaller units are eon, era, period, epoch and age. Precambrian time includes Hadean, Archean and Proterozoic eons. The Phanerozoic Eon includes Paleozoic, Mesozoic and Cenozoic eras. Major biological and geological transitions help define boundaries. Fossils provide correlation, while numerical ages calibrate the scale.

Exploration Time Sequence

Construct a relative sequence:

1. host volcanic unit deposited;
2. intrusive stock cuts host;
3. hydrothermal veins cut intrusion and host;
4. fault offsets veins;
5. weathering creates laterite;
6. alluvium covers the valley.

This sequence predicts where mineralization may continue under cover and whether later weathering upgraded or dispersed metals.

Exam Trap

A dike cutting a sedimentary unit is younger than the unit, but its numerical age is unknown without dating. A fossil can correlate strata but does not automatically give an exact age. Distinguish relative order from numerical time.

Half-Life Example

If a closed mineral originally contained a parent amount normalized to 100 and now contains 25, two half-lives have elapsed: 100 to 50 to 25. If the isotope half-life is 1.25 billion years, the idealized elapsed time is 2.50 billion years. Real geochronology measures isotope ratios and initial daughter components with uncertainty; the simple fraction illustrates exponential decay but does not replace method-specific interpretation.

Preserve the Event Sequence

Build a relative chronology before assigning ages: deposition, intrusion, deformation, metamorphism, alteration, mineralization, weathering and fault reactivation may overlap or repeat. A radiometric date records closure of a particular mineral-isotope system, not automatically the age of every visible event. Tie the dated mineral to mapped textures and cross-cutting relationships before using the number in an exploration model.