Geology GEOG110: Rocks and Weathering (Week 2)

The Three Rock Types: What, Where, and How

  • Geology is the study of rocks and structures.

  • Three rock types based on formation process:

    • Igneous: formed when magma cools and solidifies.

    • Sedimentary: formed from fragments of other rocks and minerals that are cemented together.

    • Metamorphic: formed when an existing rock undergoes changes due to heat and/or pressure.

  • Building blocks of rocks are minerals.

    • Over 2500 minerals exist on Earth, but only a few are common in most rocks.

    • Common minerals you should know:

    • Feldspars (incl. Plagioclase and Orthoclase)

    • Quartz

    • Mica (Muscovite, Biotite)

  • Importance of mineral composition for rock properties and identification.

The Rock Cycle

  • The rock cycle shows how rocks are interrelated and transform through processes:

    • Erosion, transportation, and deposition

    • Burial and heating

    • Melting and input of new melt into the crust from the mantle

    • Return of material to the mantle by subduction

  • Sedimentary rocks form from fragments and sediments deposited and lithified (compaction and cementation).

  • Igneous rocks form from cooling of magma (below the crust = intrusive; on the surface as lava = extrusive).

  • Metamorphic rocks form from pre-existing rocks that are altered by heat, pressure, and/or chemically active fluids without melting.

  • The cycle operates over geological timescales and depth ranges, linking surface processes to deep crustal/mantle processes.

Igneous Rocks

  • The most abundant rock type on Earth.

  • The lithosphere is largely igneous; surface rocks include sedimentary and metamorphic but igneous material dominates beneath.

  • Magma is the parent material; it is less dense than surrounding solid rock, so it tends to rise toward the surface.

  • Igneous rocks formed from magma have little to no void space and are crystalline (solid crystals).

  • Can form beneath the crust (intrusive/plutonic) or on the surface as lava (extrusive/volcanic).

Extrusive Igneous Rocks (Volcanic)

  • Also called volcanic igneous rocks.

  • Form when lava erupts onto the surface and cools rapidly in air or sea water.

  • Rapid cooling produces very small crystals; crystals grow quickly but remain small.

  • Very rapid cooling can yield a glassy texture (no crystalline structure) in rocks such as obsidian.

  • Most common extrusive rock: Basalt.

  • Visual example (video reference in slides): Obsidian and Basalt.

Intrusive Igneous Rocks (Plutonic)

  • Also called plutonic igneous rocks.

  • Form when magma cools underground, crystallizing slowly beneath overlying rocks.

  • Slow cooling allows crystals to grow larger.

  • Most common intrusive rock: Granite (think of a granite kitchen bench).

  • Intrusive rock formations include:

    • Dykes: cross-cut host rocks.

    • Sills: run parallel to existing rock layers.

    • Laccolith: intrusion that uplifts overlying rocks creating a dome with a flat base.

Notable Localities and Examples

  • Salisbury Crags, Edinburgh, UK

    • Exposed sandstone; site highlights exposure of Pleistocene materials (~100{,}000 to 11{,}700 years ago).

  • Glass House Mountains, SE Queensland

    • Formed from East Australia hotspot activity; intrusions formed ~26–27 million years ago (Tertiary).

    • Features: radial dykes and volcanic plugs; rock types include igneous intrusive and Trachyte.

  • Coolum, Queensland (Laccolith example)

    • Mountain formed from a laccolith intrusion.

  • The Newer Volcanics Province of south-eastern Australia (map context)

    • Includes sub-provinces such as Western Plains, Mt Gambier, Portland/Colac area, and others.

    • Most recent mainland eruptions: Mt Gambier, Mt Schank, SA (~5000 years ago).

Identification of Igneous Rocks

  • Key criteria to classify igneous rocks:

    • Size of crystals indicates cooling history:

    • Extrusive rocks cool quickly; crystals are microscopic.

    • Intrusive rocks cool slowly; crystals grow large.

    • Colour as an indicator of mineral content:

    • Light-coloured rocks are high in silica (e.g., quartz).

    • Pink minerals indicate high potassium feldspar (orthoclase).

    • Dark green colours indicate olivine.

  • Diagrams often contrast intrusive (coarse-grained) vs extrusive (fine-grained) textures with mineral symbols (e.g., Feldspar, Quartz, Mica, Pyroxene).

  • Practical note: Rock textures reflect history of cooling and mineralogy, not just appearance.

Sedimentary Rocks

  • Sediments are produced by weathering of rocks and minerals, plus accumulation of organic remains (plants, coral, shells, etc.).

  • Weathered material and other solids are transported via wind, water, gravity (slopes), and glaciers.

  • Sediments accumulate in topographic lows: oceans, lakes, base of slopes, floodplains, valley floors.

  • Sedimentary rocks form when sediments are compacted and cemented over time, usually in distinct layers (beds/strata).

  • They are generally not as hard as igneous or metamorphic rocks.

  • If layers remain undisturbed, the oldest beds are at the bottom and the youngest are on top (superposition principle).

Sedimentation, Diagenesis, and Cementation (Diagenesis)

  • Sedimentation: accumulation of particles such as sand, clay, and other materials in various environments (oceans, lakes, rivers, deserts).

  • Diagenesis: the process turning sediments into rock, primarily through two steps:

    • Compaction: weight of overlying material compresses sediment, reducing pore space.

    • Cementation: minerals precipitate from pore waters and bind sediment grains together (common precipitated minerals include calcite, silica, iron oxides).

  • Sedimentary rocks typically have pore spaces, unlike most igneous rocks.

  • Fossils may be present within sedimentary layers.

  • For more on stratigraphy and fossilisation, see educational resources on stratigraphy and fossilisation processes.

Sedimentary Rock Types

  • Classified into three broad groups:

    • Clastic (detrital): rocks formed from weathered fragments transported and deposited as solid particles.

    • Organic: rocks formed from plant or animal material.

    • Chemical: rocks formed by precipitation of minerals from aqueous solutions.

Clastic Sedimentary Rocks

  • Most common type: 70–90% of all sedimentary rocks.

  • Formed in layered beds (strata).

  • Classified by particle size:

    • Shale: clay-sized particles (invisible to naked eye).

    • Siltstone/Mudstone: silt and mud-sized particles.

    • Sandstone: sand-sized particles (> 63 μm63 \ \mu m, visible to the naked eye).

    • Conglomerates and breccias: larger fragments (gravel > 2 mm2 \ \text{mm}) cemented into rock.

  • Particle roundness matters: conglomerates have rounded particles; breccia has angular particles.

  • Progressive increase in grain size corresponds to texture and energy of the depositional environment.

Organic Sedimentary Rocks

  • Formed by diagenesis of organic materials.

  • Examples:

    • Limestone: composed of carbonates from coral, shells, snails, etc.

    • Chalk: remains of micro-marine organisms.

    • Coal/Peat: formed from plant remains; peat is incompletely decayed plant material; coal forms when peat experiences more burial, temperature, and pressure.

Organic Sedimentary Environments

  • Coal seam examples (e.g., Hunter Valley, NSW).

  • Peat bogs (examples in Ireland).

Chemical Sedimentary Rocks

  • Form by precipitation of minerals out of solution.

  • Mineral sources: weathering releases salts such as Na, Ca, K, Mg; halite (NaCl) and gypsum (CaCO3) are common chemical precipitates.

  • Cave deposits include stalagmites and stalactites, primarily calcite (CaCO3).

  • Example cave deposit data: stalagmites in Southern Cambodia ~0.5 m long; growth occurred over ~12–13{,}000 years, stopped during droughts (e.g., Middle Ages).

Identification of Sedimentary Rocks

  • If stratified (layered), classify as clastic by particle size (e.g., sandstone).

  • If composed entirely of plant or animal material, classify as organic.

  • If composed of a single mineral in pure form, classify as chemical.

Metamorphic Rocks

  • Etymology: morph means “to change” – metamorphic rocks are changed rocks.

  • Formed when any existing rock is altered by heat and/or pressure.

  • Major agents of metamorphism:

    • Temperature: approximately Trom250o750<br>oundextCT rom 250^ o750^{<br>ound} ext{C} (approximate range)

    • Pressure: typically exceeding P > 10\ ext{kbar}

    • Chemical environment changes (fluid activity) and deformation can contribute.

  • Metamorphism occurs in zones beneath weathering and surface processes where rocks are not exposed to surface conditions.

  • Example: Limestone protolith becomes Marble; fossils may be visible in some samples; interlocking calcite crystals form via recrystallisation.

Nature of Metamorphic Rocks

  • Texture and mineralogy change due to the action of heat, pressure, migrating fluids, and deformation.

  • Grains/Crystals can grow larger (recrystallisation).

  • Mineral composition can change (metamorphic mineral assemblages form).

  • Metamorphic rocks often have exotic minerals and a shiny appearance.

  • Metamorphism occurs while rocks remain solid (no melting).

  • Rocks typically form at significant depths beneath the surface.

Types of Metamorphic Rocks

  • Two main metamorphic environments:

    • Contact metamorphism: intrusion of hot magma into surrounding cooler rocks.

    • Regional metamorphism: metamorphism caused by large-scale temperature and/or pressure increase during deep burial.

Recrystallisation and Deformation

  • Recrystallisation occurs when rocks are subjected to high temperature and pressure but do not melt.

  • Processes include:

    • Mineral growth

    • Grain-size changes

    • Changes in mineral composition

    • Rearrangement of crystals

  • This can lead to texture changes such as foliation (preferred mineral orientation).

Foliation and Deformation

  • Deformation leads to foliated textures in metamorphic rocks:

    • Slatey cleavage: in fine-grained rocks rich in mica/olivine (e.g., slate) – parallel cleavage.

    • Schistosity: in medium-to-coarse mica-rich rocks; visible shimmering mica; bedding is less perfect.

    • Gneissosity: in coarse-grained rocks with conspicuous light/dark banding (biotite or amphibole often present).

  • Foliation reflects directed pressure and metamorphic strain.

Metamorphic Rocks and Their Protoliths (Table Concepts)

  • Non-foliated vs foliated textures;

  • Protoliths (original rocks before metamorphism) and resulting metamorphic rocks:

    • Sandstone → Quartzite (non-foliated to foliated context depending on deformation)

    • Limestone → Marble (non-foliated in classic calcite-dominated cases)

    • Shale → Slate (foliated); Shale → Phyllite → Schist → Gneiss with increasing metamorphic grade and foliation complexity.

  • Example relationships illustrate how heat and pressure reorganize grains and minerals.

Practical Takeaways and Connections

  • Igneous rocks reveal cooling history and mantle/crustal processes; intrusive rocks indicate deeper, slower cooling; extrusive rocks indicate surface eruptions and rapid cooling.

  • Sedimentary rocks record surface processes, climate, hydrology, and life; they preserve fossils and sedimentary structures (beds, grading, cross-bedding).

  • Metamorphic rocks reveal the pressure-temperature history of rocks, deep crustal conditions, and deformation histories; foliated textures point to directional stress, while non-foliated textures point to more uniform pressure or localized metamorphism.

Quick Reference: Key Numbers and Terms (LaTeX)

  • Temperature range for some metamorphism: T≈250−750∘CT \approx 250-750^{\circ}\mathrm{C}

  • Pressure for metamorphism: P > 10\ \text{kbar}

  • Crystal size indicators: extrusive cooling -> microscopic crystals; intrusive cooling -> larger crystals

  • Age references in localities:

    • Pleistocene exposure events: 100,000 to 11,700 years ago100{,}000 \text{ to } 11{,}700\,\text{years ago}

    • Intrusions at Glass House Mountains: 26−27 Ma26-27\,\text{Ma} (million years ago)

  • Particle sizes (clastic rocks):

    • Sand: > 63\ \mu\text{m}

    • Gravel: > 2\ \text{mm}

  • Dominance of clastic sedimentary rocks: roughly 70%−90%70\%-90\% of sedimentary rocks

  • Chemical sediment terminology: calcite (CaCO3), silica (SiO2), iron oxides

  • Common mineral indicators in igneous rocks:

    • Light colours = high in silica (e.g., quartz)

    • Pink = high in K-feldspar

    • Dark green = olivine

END OF NOTES