DSST Introduction to Geology cheat sheet

Earth's internal layers by composition: crust (oceanic ~5-10 km, basaltic; continental ~30-70 km, granitic), mantle (ultramafic peridotite), core (iron-nickel). By physical state: lithosphere (rigid), asthenosphere (plastic), mesosphere, outer core (liquid), inner core (solid).

Moho is the seismic boundary between crust and mantle; the Gutenberg discontinuity separates mantle from core, and the Lehmann discontinuity separates inner from outer core. S-waves cannot pass the outer core, which is how its liquid state was inferred.

Bowen's reaction series: discontinuous branch olivine -> pyroxene -> amphibole -> biotite; continuous branch Ca-plagioclase -> Na-plagioclase; both converge on K-feldspar, muscovite, quartz (last to crystallize).

Igneous classification: felsic (high silica, low density, light color), intermediate, mafic (low silica, high density, dark), ultramafic. Texture reflects cooling rate: phaneritic = slow/intrusive, aphanitic = fast/extrusive, porphyritic = two-stage, glassy = very rapid, vesicular = gas-rich.

Sedimentary rock types: clastic (shale, sandstone, conglomerate) classified by grain size; chemical (limestone, rock salt, chert); organic (coal, some limestone). Grain size order: gravel > sand > silt > clay.

Metamorphic grade indicators: slate -> phyllite -> schist -> gneiss (increasing grade); index minerals chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Foliation is perpendicular to maximum compressive stress.

Relative dating principles: superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, faunal succession. Unconformities: angular (tilted then eroded), disconformity (parallel erosion surface), nonconformity (sediment over igneous/metamorphic).

Absolute dating: half-life is time for half the parent atoms to decay. After n half-lives, fraction parent = (1/2)^n. Common systems: U-238 -> Pb-206 (4.5 Ga), K-40 -> Ar-40 (~1.25 Ga), C-14 (~5,730 yr, useful only for organic material < ~50,000 yr).

Mineral properties used for identification: hardness (Mohs 1 talc to 10 diamond), cleavage vs fracture, luster (metallic/nonmetallic), streak, color, specific gravity, crystal habit. Calcite fizzes in dilute HCl; halite tastes salty; graphite marks paper.

Chemical weathering agents: hydrolysis (feldspar -> clay), oxidation (iron -> rust), dissolution (limestone in carbonic acid), carbonation. Physical weathering: frost wedging, salt wedging, exfoliation, root wedging, abrasion. Warm/wet climates favor chemical; cold/dry favor physical.

Soil horizons: O (organic), A (topsoil, humus), E (eluviated/leached), B (zone of accumulation), C (weathered parent material), R (bedrock). Laterite forms in tropical climates by intense leaching.

Stream discharge Q = cross-sectional area A x velocity v. Gradient = elevation change / horizontal distance. A stream's competence is the largest clast it can carry; capacity is total load. Deposition occurs when velocity drops.

Base level is the lowest elevation a stream can erode to (sea level for most streams). A graded stream is in equilibrium between erosion and deposition. Meanders migrate laterally; oxbow lakes form when a meander is cut off.

Drainage patterns: dendritic (uniform material), trellis (folded strata), rectangular (jointed rock), radial (dome/volcano), deranged (glaciated). A drainage divide separates watersheds.

Groundwater: water table is the top of the saturated zone; aquifer is permeable rock storing/transmitting water; aquitard is impermeable. Porosity is storage capacity; permeability is ability to transmit. Darcy's law: Q = K i A.

Glaciers: continental ice sheets vs alpine (valley) glaciers. Erosional features: U-shaped valleys, cirques, arêtes, horns, striations, roches moutonnées. Depositional: till, moraines (terminal, lateral, medial), eskers, drumlins, kames.

Wind processes: deflation, abrasion, saltation. Features: ventifacts (faceted stones), yardangs, dunes (barchan, transverse, longitudinal, star), loess deposits. Wind sorts fine material and rounds grains.

Coastal processes: longshore drift moves sediment parallel to shore; wave refraction concentrates energy on headlands. Erosional features: sea cliffs, sea stacks, wave-cut platforms. Depositional: beaches, spits, tombolos, barrier islands.

Plate boundaries: divergent (mid-ocean ridges, continental rifts - normal faults, shallow quakes), convergent (subduction zones - reverse/thrust faults, deep quakes, volcanic arcs; continental collision - fold mountains), transform (strike-slip faults, shallow quakes, no volcanism).

Seafloor spreading evidence: magnetic striping (Vine-Matthews), age of ocean floor increasing away from ridges, pillow basalts, heat flow, sediment thickness increasing with distance from ridge.

Subduction zone features: trench, accretionary wedge, forearc, volcanic arc, back-arc basin. Oceanic-continental subduction produces andesitic volcanism (e.g., Andes); oceanic-oceanic produces island arcs (e.g., Japan, Aleutians).

Hot spots: fixed mantle plumes produce chains of volcanoes as the plate moves over them (Hawaii, Yellowstone). The bend in the Hawaiian-Emperor chain records a change in Pacific plate motion.

Stress types: compressional (folds, reverse faults), tensional (normal faults, rifts), shear (strike-slip faults). Folds: anticline (up, oldest in core), syncline (down, youngest in core), monocline (one-limb bend).

Fault types: normal (hanging wall drops, extension), reverse/thrust (hanging wall rises, compression), strike-slip (lateral motion, shear). Fault slip direction follows the stress regime.

Earthquake waves: P-waves (compressional, fastest, travel through solids and liquids), S-waves (shear, slower, solids only), surface waves (Love, Rayleigh - slowest, most damaging). Epicenter is the surface point above the focus (hypocenter).

Locating an earthquake: difference in P- and S-wave arrival times at three or more stations, plotted as circles, intersect at the epicenter. Richter scale is logarithmic (each integer = ~10x amplitude, ~32x energy); moment magnitude (Mw) is more accurate for large quakes.

Volcano types: shield (low-viscosity basalt, gentle slopes, e.g., Hawaii), stratovolcano/composite (viscous andesite, steep, explosive, e.g., Fuji, Vesuvius), cinder cone (small, pyroclastic), caldera (collapsed magma chamber, e.g., Yellowstone).

Fossil fuels: coal forms from peat in swamps (rank: peat -> lignite -> bituminous -> anthracite with increasing heat/pressure); oil and gas form from marine organic matter in source rock, migrate into reservoir rock, and are trapped by cap rock (anticline, fault, salt dome traps).

Mineral resources: ore is rock with economically recoverable minerals. Examples: bauxite (aluminum), hematite/magnetite (iron), chalcopyrite (copper), galena (lead), sphalerite (zinc), cinnabar (mercury), uraninite (uranium). Placer deposits form by density sorting in streams.

Groundwater contamination and management: recharge areas must be protected; cone of depression forms around a pumping well; saltwater intrusion occurs in coastal aquifers when pumping exceeds recharge. Porosity vs permeability control flow.

Earthquake hazard assessment uses seismic gap theory, recurrence intervals along faults, paleoseismology (trenching), and shaking amplification in unconsolidated sediment (e.g., Mexico City, San Francisco Bay mud). Liquefaction occurs in saturated loose sand during shaking.

Volcanic hazards: pyroclastic flows (deadliest, fast, hot), lahars (volcanic mudflows), ash fall (aviation and respiratory hazard), tsunamis from flank collapse, and gas emissions (CO2, SO2). Monitoring uses seismicity, tilt, gas, and thermal data.

Mass wasting (landslide) triggers: steep slopes, water saturation, earthquakes, undercutting, removal of vegetation. Types: creep, slump, debris flow, rockfall, lahar. Angle of repose is the maximum stable slope angle.

Karst and sinkholes: dissolution of limestone creates caves, sinkholes, disappearing streams, and karst topography. Groundwater overpumping and leaking utilities accelerate sinkhole collapse.

Plate tectonics applied to hazards: subduction zones (e.g., Cascadia, Japan, Chile) produce megathrust earthquakes and tsunamis; transform boundaries (San Andreas) produce shallow strike-slip quakes; intraplate quakes (New Madrid) occur far from boundaries.

Geologic time and resources: fossil fuels and many ores are nonrenewable on human timescales; the rock cycle links igneous, sedimentary, and metamorphic processes, so resource distribution follows tectonic and depositional history (e.g., oil in passive-margin basins, porphyry copper at convergent margins).