Rock Types & Geological Study Notes

Rock Classification Overview

• Earth’s solid materials are grouped into $3$ fundamental rock classes:
– Igneous
– Sedimentary
– Metamorphic
• Each class reflects a unique origin, texture, composition, and place in the geologic cycle.

Igneous Rocks (Fire-Formed)

• Definition: Rock that crystallised from molten material (magma or lava).
• Abundance: Compose the bulk of Earth’s crust even though they are commonly hidden beneath sedimentary cover.
• Mineralogy: Dominated by silicate minerals combined with metallic elements; grains are interlocking (no cement).
• Textural key: Crystal size records cooling rate.
– Slow cooling → large, easily visible crystals.
– Rapid cooling → microscopic to glassy textures.

Examples & Images Mentioned

• Granite, basalt, obsidian, pumice (Page 4).

Felsic–Mafic Continuum (Page 5)

• Intermediate rocks (andesite, diorite) sit between the two end-members.

Global Distribution (Page 6)

• Continents are predominantly felsic.
• Oceanic crust is overwhelmingly mafic.
• Map coordinates $\left(\pm80^{\circ}\text{ lat},\ \pm180^{\circ}\text{ long}\right)$ highlight continental vs. oceanic DEM contrasts.

Cooling, Texture & Setting (Pages 7–9)

• Slow cooling at depth → Intrusive (plutonic) bodies with coarse crystals.
– Granite (felsic), diorite (intermediate), gabbro (mafic).
• Fast cooling at/near surface → Extrusive (volcanic) lavas with fine crystals or glass.
– Rhyolite (felsic), andesite (intermediate), basalt (mafic).

Intrusive Geometry (Page 11)

• Pluton: any mass of intrusive igneous rock.
• Batholith: composite plutons >$100$ km²; feed large volcanic arcs.
• Dike: vertical discordant sheet cutting country rock.
• Sill: horizontal concordant sheet.
• Radiating dike swarms emanate from volcanic conduits.
• Magma conduit links batholith to surface volcano & lava flows.

Famous Localities (Pages 10, 12 & 13)

• Rhyolite – Yellowstone.
• Basalt – Hawaii and global ocean floor.
• Andesite – Andes volcanic belt.
• Granite – Sierra Nevada batholith, New Zealand plutons.
• Gabbro – Oceanic lower crust sections & ophiolites.
• Many “Famous extrusive/intrusive rocks” slides allude to textbook field sites (e.g.
Devil’s Tower columnar basalt, Half Dome granite, but specific names not listed in transcript).

Sedimentary Rocks

• Definition: Rocks produced by weathering, erosion, transportation, deposition, compaction, and cementation of pre-existing material.
• Sediment: unconsolidated mineral/organic material moved by wind, water, ice; settles mainly at low elevations forming strata (layered beds).
• Surface dominance: Although volumetrically small, sediments/ sedimentary rocks blanket the majority of Earth’s land surface.
• Fossils: Common; provide biological & chronological records.

Three Genetic Classes (Pages 16–20)

1. Clastic (Detrital)
   – Source: Physical fragmentation of parent rocks.
   – Grain size spectrum: boulder → clay.
   – Common rocks: sandstone, shale, mudstone.

2. Chemical
   – Source: Ions in solution precipitating inorganically or via micro-organisms.
   – Products: Limestone (calcite), evaporites such as halite & gypsum.

3. Organic (Biogenic)
   – Source: Accumulation of plant/animal remains.
   – Products: Coal, coquina (shell hash limestone).

Representative Imagery & GIS (Page 19)

• Transcript lists Google Earth coordinate $15^{\circ}18'04.57''\,\text{N},\ 98^{\circ}13'01.72''\,\text{W}$, elevation $-4363\text{ ft}$, eye altitude $2107.5\text{ mi}$—demonstrates remote sensing of sedimentary basins.

Notable Outcrops

• “Famous sedimentary rocks” slide (Page 21) references world-class strata such as the Grand Canyon, White Cliffs of Dover, but individual names absent in transcript.

Metamorphic Rocks

• Definition: Pre-existing rocks transformed by elevated temperature and/or pressure while remaining solid (no melting).
• Typical environment: Deep crustal roots of mountain belts, contact aureoles around intrusions, subduction zones.
• Key changes: Re-crystallisation, new mineral assemblages, foliation or banding, increased hardness.
• Common examples: Slate (low-grade, derived from shale), schist (medium-grade, micas visible), gneiss (high-grade banded rock from granite or sediment precursor).

Metamorphic Agents & Processes (Page 24)

• Heat: geothermal gradient, magmatic intrusions.
• Pressure: Lithostatic (confining) & directed (differential) stresses.
• Fluids: Hot water and volatiles accelerate chemical reactions (implied though not explicitly stated).

Field & Hand-Sample Identification (Page 27)

Decision flowchart provided:

• Are individual mineral grains visible?
– Yes → proceed; No → could be mudstone, basalt, or glassy igneous.
• Grain fabric
– Interlocking → igneous/metamorphic.
– Cemented → sedimentary.
• Layering or foliation?
– Distinct stripes/flat sheets → likely metamorphic slate/schist/gneiss or sedimentary shale/limestone bedding.
• Scaly or glittery sheen (micas) → schist.
• Very dark (mafic) → basalt, gabbro, or dark schist.
• Acid fizz test ($HCl + CaCO<em>3 \rightarrow CO</em>2\uparrow$) distinguishes limestone & marble.

Soil & Mineral Groups (Pages 28–29)

• Transition from rock to soil involves chemical/physical weathering and biological processes (no transcript detail beyond “Soil”).

• Major mineral classes enumerated:
– Native elements
– Halides
– Oxides & hydroxides
– Sulfates
– Sulfides
– Phosphates & “arsenates” (OCR garble)
– Silicates (most abundant in crust)
– Organic minerals (e.g.
whewellite).

Conceptual & Practical Connections

• Rock cycle: Igneous → weathering → sedimentary → burial → metamorphic → melting → igneous (implied holistic context).
• Plate tectonics:
– Divergent boundaries create mafic ocean floor basalts/gabbros.
– Convergent margins generate andesitic volcanism, batholiths, and regional metamorphism.
• Economic significance:
– Igneous intrusions focus ore deposits (Cu, Au).
– Sedimentary strata host hydrocarbons, coal, aquifers.
– Metamorphic rocks contain gemstones, slate roofing material.
• Environmental & ethical angles: Quarrying and mining of rock resources demand sustainable practices; understanding stratigraphy aids groundwater & fossil fuel stewardship.

Quick-Reference Numerical & Statistical Nuggets

• $3$ primary rock types.
• Batholith size criterion: >$100\text{ km}^2$ areal extent.
• Latitude/longitude grid on global composition map spans $\pm80^{\circ}$ lat, $\pm180^{\circ}$ long.
• Coordinate example: $15^{\circ}18'04.57''\,\text{N};\ 98^{\circ}13'01.72''\,\text{W}$ (Page 19).
• Elevation example: $-4363\text{ ft}$; eye altitude $2107.5\text{ mi}$.

Study Tips & Mnemonics

• "Felsic is FELlow-SIlica-Colored Light"; "Mafic MAkes IRon-Magnesium Dark."
• Intrusive = “in” the crust (large crystals); Extrusive = “exit” to surface (tiny crystals).
• Clastic sizes: Gravel > Sand > Silt > Clay ("Grandma Sang Sweetly, Calmly").
• Metamorphic grade sequence: Shale → Slate → Schist → Gneiss ("She Sells Shiny Garnets").