B2 · Publication Volume 7

Igneous Processes, Metamorphism and Rock Evolution

Connects igneous and metamorphic processes, pressure–temperature conditions and mineralisation.

Purpose of this book

Igneous and metamorphic rocks are records of energy, material transfer and changing boundary conditions. A basalt may preserve the products of mantle melting, crystal separation, storage, ascent and eruption. A granite may contain crystals inherited from more than one magma pulse and may have exchanged heat and material with its wall rock. A schist may preserve a prograde mineral assemblage in one domain, a later deformation fabric in another and retrograde reaction along fractures. No single name contains that history.

This book develops the reasoning needed to reconstruct such histories without turning a classification diagram, mineral pair or chemical plot into an automatic answer. The central questions are: what changed, what remained, which evidence records the change, and which competing process could produce a similar record? Field relations, petrography, mineral chemistry, whole-rock chemistry, phase equilibrium, geochronology and uncertainty are therefore treated as connected evidence rather than separate specialties.

The sequence begins with melting and magma sources, follows crystallisation and open-system evolution, maps intrusive and volcanic products, and then develops metamorphic controls, facies, reactions, settings and fluid–rock interaction. The final lesson integrates these strands into an auditable magmatic–metamorphic–hydrothermal event sequence.

General and institution-neutral scope

This is a general, institution-neutral tutorial with no affiliation to, sponsorship by or endorsement from any company or individual. It is not written for a named mine, laboratory, software package, commercial classification or private dataset. Examples use synthetic outcrops, samples and analytical tables so that the method can be transferred to any region where the required evidence exists.

Standards bodies, public geological agencies and researchers named in source notes identify technical sources only. Their inclusion does not imply participation, approval or a relationship with this tutorial. Classification names are used only with their decision path and version stated. Software output is evidence only when inputs, thermodynamic data, activity models, analytical uncertainty and modelling choices are preserved.

The website that hosts this material is only a delivery surface. It is not presented as the textbook's author, publisher, sponsor, scientific authority or curriculum subject.

The diagrams are explanatory models, not measured sections or universal tectonic cartoons. Pressure–temperature fields are intentionally schematic. Site-specific petrological, mineral-exploration, volcanic-hazard or engineering decisions require current local observations, qualified review and the standards applicable in the relevant jurisdiction.

What you should already know

You should be able to describe minerals and rocks, distinguish a field observation from an interpretation, read maps and cross-sections, use relative-age principles and work with ratios, mass fractions, logarithms, graphs and uncertainty. Familiarity with plate tectonics, crystallography, mineral chemistry, phase diagrams and sedimentary contacts is helpful. Every quantitative method used here is introduced conceptually, but the book does not replace a full course in thermodynamics, geochemistry, structural geology or geochronology.

The most important prerequisite is evidence discipline. A sample must retain location, orientation, scale, geological context, preparation history, analytical method and identifiers. A numerical result without those links cannot reliably constrain a geological event, however precise its printed digits appear.

Learning outcomes

By the end of the book, you should be able to:

  • explain how decompression, volatile addition and heat transfer can initiate partial melting in different source rocks;
  • distinguish equilibrium crystallisation, fractional crystallisation, crystal accumulation, recharge, mixing and assimilation;
  • use contacts, chilled margins, xenoliths, fabrics and cross-cutting relations to order intrusive events;
  • separate lava, primary pyroclastic deposits and reworked volcaniclastic sediment using stated observations;
  • select an appropriate modal or chemical igneous classification and keep it separate from petrogenetic and tectonic interpretation;
  • explain how temperature, pressure, fluid, time, deformation and bulk composition control metamorphic products;
  • use facies, index minerals, reactions and mineral compositions to build a qualified pressure–temperature path;
  • compare contact, regional and dynamic metamorphism without treating them as mutually exclusive labels;
  • quantify possible gains and losses during metasomatism with explicit precursor and immobility assumptions; and
  • reconstruct a rock-evolution sequence that distinguishes observation, calculation, inference and unresolved alternatives.

Evidence discipline

Every lesson uses a seven-part chain:

  1. Context: where is the rock, and what are its contacts, geometry and structural relations?
  2. Material: which minerals, glass, clasts, veins and alteration products are present?
  3. Texture: what crystal sizes, shapes, fabrics, zoning and replacement relations were observed?
  4. Composition: which mineral and whole-rock measurements are representative, and how were they corrected and normalised?
  5. Process: which melting, crystallisation, deformation or reaction mechanisms are consistent with the observations?
  6. Sequence: what relative or numerical ages constrain the order and duration of events?
  7. Uncertainty: what was not preserved, sampled, analysed or uniquely explained?

The chain is directional. A magma process can produce a chemical pattern, but the pattern alone may not identify that process. A mineral may be stable in a pressure–temperature field, but its presence does not prove equilibrium with every neighbouring grain. A vein may cut a metamorphic fabric, but that relation alone does not identify the fluid source or economic significance.

Classification and naming discipline

Names should compress observations, not replace them. Coarse igneous rocks with measurable modal proportions may follow a QAPF route; many volcanic rocks for which a representative whole-rock analysis exists may follow a TAS route. Metamorphic rocks may be named from protolith, mineral content, structure or a recognised special term. In each case, record the classification authority, diagram or boundary version, required normalisation, analytical basis and qualifiers.

Classification, petrogenesis and tectonic setting are different claims. “Granite” is a compositional or modal class under a declared scheme. “Crustal melt” is a source hypothesis. “Continental collision” is a tectonic interpretation. Evidence may connect the three, but no arrow may be silently reversed.

How to use the figures

Each lesson contains one purpose-built schematic. Recreate it using a real or instructor-provided dataset. Replace every conceptual arrow with an observation, calculation or explicitly labelled hypothesis. Where a diagram shows a field boundary, use the authoritative source before plotting measured data. Where it shows a sequence, add evidence identifiers and confidence beside each event.

Colours do not encode universal rock types, grades or hazards. Distances, temperatures, pressures and times are qualitative unless values are printed and sourced. The figures are designed to expose reasoning structure, not to provide a lookup chart.

Completion task: two competing rock-evolution histories

Assemble a small, traceable dataset containing a geological map or sketch, at least two outcrops or drill intervals, oriented structural observations, representative hand specimens or thin-section images, mineral descriptions, a whole-rock analytical table and at least one age constraint. First classify each material without assigning a tectonic setting. Then construct two event histories that could explain the same evidence.

For each history, identify the source and melting mechanism, magma evolution, intrusive or eruptive geometry, metamorphic conditions, deformation, fluid–rock reactions and timing. Tag every statement as observed, calculated, inferred or unknown. Include one mass-balance calculation and one pressure–temperature argument. Finish with the new observation or analysis that would most efficiently discriminate between the histories.

The completion standard is not certainty. It is a versioned argument in which another reader can trace every conclusion to evidence, reproduce every calculation and see why the rejected alternative is currently less well supported.

Core sources