Geology — Rocks, Strata, Geologic Time, and Minerals

Minerals

Definition of a mineral:

• A naturally occurring, inorganic, crystalline solid
• Consistent chemical composition and an ordered atomic arrangement
• About 4,500 known species

Physical properties of minerals:

• Hardness: the Mohs hardness scale, 1–10 1 talc · 2 gypsum · 3 calcite · 4 fluorite 5 apatite · 6 orthoclase feldspar · 7 quartz · 8 topaz 9 corundum · 10 diamond
• Cleavage: splitting along planes of weaker bonding calcite: cleaves in 3 directions / mica: perfect cleavage in 1 direction
• Fracture: breaking irregularly (quartz)
• Streak: the color of the powder left on an unglazed porcelain plate pyrite: golden appearance, black streak (“fool’s gold”)
• Luster: glassy, metallic, resinous, pearly
• Specific gravity: density relative to water

Silicate minerals:

• Make up more than 95% of the crust’s minerals
• Basic structure: the SiO4 tetrahedron
• Major silicate minerals: quartz: SiO2, used to make glass and semiconductors feldspar: the most abundant mineral in the crust orthoclase (KAlSi3O8), plagioclase mica: sheet-like crystals, muscovite and biotite amphibole and pyroxene: contain iron and magnesium olivine: the dominant mineral of the upper mantle

Non-silicate minerals:

• Carbonates: calcite (CaCO3), dolomite form limestone and marble
• Sulfides: pyrite (FeS2), galena, sphalerite metal ores
• Oxides: magnetite, hematite (iron ore), corundum
• Halides: halite (NaCl), fluorite
• Sulfates: gypsum (CaSO4·2H2O), barite

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The Rock Cycle

Igneous rock:

• Forms from the cooling and solidification of magma
• Plutonic (intrusive) rock: cools slowly underground, large crystals granite: high SiO2, felsic, light-colored gabbro: low SiO2, mafic, dark-colored
• Volcanic (extrusive) rock: cools rapidly at the surface, small crystals basalt: dark-colored, dense or vesicular rhyolite: light-colored, fine-grained andesite: intermediate composition
• Hypabyssal rock (dikes and sills): an intermediate cooling rate

Sedimentary rock:

• Clastic: rock fragments deposited, compacted, and cemented conglomerate (gravel) · sandstone (sand) · shale (clay)
• Chemical: precipitated from solution limestone, rock salt, gypsum, chert (precipitated SiO2)
• Organic: accumulated remains of organisms limestone (shells, coral) · coal (plants) · chert (radiolarians)
• Bedding: horizontal sedimentary layers the law of superposition: lower layers are older fossils: record the era in which the organism lived

Metamorphic rock:

• The original rock is transformed by heat and pressure
• Types of metamorphism: contact metamorphism: caused by heat from a magma intrusion hornfels, marble, quartzite regional metamorphism: high temperature and pressure from mountain-building schist, gneiss, slate
• Metamorphic grade: low grade (lower temperature and pressure) to high grade slate < phyllite < schist < gneiss

The rock cycle:

• Igneous ↔ sedimentary ↔ metamorphic rock convert into one another
• Energy source: Earth’s internal heat + solar energy
• Matter is conserved: atoms cycle
• Cycle pathway: igneous rock → weathering and erosion → sediment → sedimentary rock → burial → metamorphic rock metamorphic rock → partial melting → magma → igneous rock

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Strata and Geologic Age

Determining relative age:

• The law of superposition: lower layers are older
• The law of cross-cutting relationships: an intrusion is younger than the rock it intrudes
• Fossil correlation: index fossils establish age index fossils: short-lived, widely distributed organisms trilobites (Paleozoic) · ammonites (Mesozoic) nummulites (Cenozoic, Eocene)
• Unconformity: represents a time gap, an erosion surface, and missing strata

Determining absolute age (radiometric dating):

• Radioactive decay: an unstable isotope decays into a stable one
• Half-life: the time for half of a radioactive element to decay
• Common dating pairs: carbon-14 (half-life: 5,730 years): organic material, archaeology potassium-40 → argon-40 (half-life: 1.3 billion years): igneous rock, older rock uranium-238 → lead-206 (half-life: 4.5 billion years): Earth’s oldest rocks
• Limits of dating: contamination, disturbance, unknown original composition

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Geologic Time

Divisions of geologic time:

• Precambrian (about 4.5 billion–540 million years ago): 88% of Earth’s history · unicellular to multicellular organisms late Proterozoic: the Ediacaran fauna (soft-bodied organisms)
• Paleozoic era (540–250 million years ago): the Cambrian explosion: the sudden appearance of major animal phyla Ordovician: the heyday of trilobites and armored fish Silurian: land plants and fish appear Devonian: “the age of fish”; amphibians move onto land Carboniferous: forests flourish, giant insects, coal forms Permian: reptiles diversify → the end-Permian mass extinction (96% of species extinct)
• Mesozoic era (250–66 million years ago): Triassic: dinosaurs and mammals appear Jurassic: the age of dinosaurs, Archaeopteryx, the supercontinent breaks apart Cretaceous: flowering plants flourish, the K-Pg mass extinction (dinosaurs go extinct)
• Cenozoic era (66 million years ago–present): Paleogene: mammals diversify Neogene: grasslands develop, apes emerge Quaternary: ice ages, human evolution

Mass extinctions:

• The “Big Five” mass extinctions: end-Ordovician (about 444 million years ago): 85% of species extinct late Devonian (about 375 million years ago): 75% of marine life end-Permian (250 million years ago): 96% of species (the largest ever) end-Triassic (200 million years ago): 76% end-Cretaceous (66 million years ago): 76% · dinosaurs extinct caused by the Yucatán Peninsula asteroid impact plus volcanic activity
• A sixth mass extinction: currently underway, driven by human activity

Korean peninsula geology:

• Precambrian metamorphic rock: the Gyeonggi and Yeongnam massifs (about 2.5 billion years old)
• Paleozoic: the Joseon and Pyeongan supergroups (limestone and coal)
• Mesozoic: sedimentary rock of the Gyeongsang Basin (dinosaur fossils) and granite intrusions
• Cenozoic: the volcanic formation of Jeju Island, Dokdo, and Ulleungdo
• Notable geological sites: Dutayeon and the Hantan River (basalt gorges) Chaeseokgang in North Jeolla Province (sedimentary cliffs) the Paleozoic limestone belt of Gangwon Province

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Frequently Asked Questions

Q. How do granite and basalt differ, and where does each form? A. Granite and basalt are distinguished on two grounds. First is chemical composition. Granite has a high SiO2 content (above 65%), classifying it as “felsic” or “acidic.” It’s rich in potassium and sodium and typically light-colored (pink, white, or gray). Basalt has a low SiO2 content (below 52%), classifying it as “mafic” or “basic,” rich in iron and magnesium, and dark-colored. Second is crystal size. Granite forms deep underground (as a plutonic rock), cooling slowly over millions of years, so its large crystals are visible to the naked eye. Basalt forms at the surface or on the shallow seafloor (as a volcanic rock), cooling quickly, so its crystals are very small or absent. In terms of where each forms, granite forms deep within continental collision zones and subduction zones — the granite of Korea’s Mesozoic era is a representative example. Basalt forms at mid-ocean ridges (most of the Pacific seafloor), hotspots (Hawaii, Jeju Island), and continental rift zones.

Q. How reliable is radiometric dating? A. The reliability of radiometric dating rests on physical law. Radioactive decay occurs at a constant rate regardless of temperature, pressure, or chemical environment — a natural law whose half-lives have been precisely measured in the laboratory. Confidence is strengthened three ways. First, cross-validation among independent dating methods: when uranium-lead, potassium-argon, and rubidium-strontium dating (among others) agree, confidence rises. Second, agreement with independent astronomical evidence: lunar rock and meteorite ages consistently align with Earth’s formation date (about 4.56 billion years ago). Third, verification against rocks of known age or historical artifacts of known age: radiocarbon (C-14) dates have been cross-checked against tree-ring chronologies. Contamination or isotope migration can of course introduce error, but the methodology for detecting such problems has also advanced considerably.