WHAT IS A ROCK SAVE ME GEOLOGY MIDTERM

Stratigraphic Profile

A vertical column or cross-section that describes the sequence of rock strata, formations, and contacts over time in a given area (such as the Grand Canyon).

Unconformity

A surface representing a time gap in the rock record caused by a period of erosion or nondeposition.

Angular unconformity

An unconformity where horizontal sedimentary strata are deposited on top of tilted, folded, and eroded rock layers, representing a massive gap in geologic time.

Nonconformity

An unconformity in which younger sedimentary rock strata cap an eroded surface of older crystalline (igneous or metamorphic) basement rocks.

Disconformity

An unconformity between parallel sedimentary rock layers caused by a pause in deposition or a period of erosion (often due to sea-level changes).

Uniformitarianism

Articulated by James Hutton ("The present is the key to the past"), the principle stating that geologic processes occurring today are the same as those that operated in the past.

Original Horizontality

The geologic principle stating that sediments settle out of a fluid by gravity in flat, horizontal layers

Superposition

The geologic principle stating that in an undeformed sequence of layered rocks, each bed is older than the one above it and younger than the one below it (oldest on bottom, youngest on top).

Lateral continuity

The geologic principle stating that strata originally form laterally extensive horizontal sheets that continue until they thin out or meet a barrier, even if later dissected by erosion.

Cross-cutting relationships

The geologic principle stating that younger features (such as faults, igneous dikes, or erosion surfaces) truncate or cut across older preexisting rock features.

Baked contacts

Thermal metamorphism where an igneous intrusion "cooks" the surrounding invaded country rock

Inclusions

Rock fragments (such as weathering rubble or xenoliths) enclosed within another rock layer or intrusion

Lithification

The process that transforms loose sediment into solid sedimentary rock through burial compaction (squeezing out water/air) and cementation (minerals like quartz or calcite precipitating in pore spaces).

Isotopic (Absolute) dating

A quantitative method of dating geological materials (primarily igneous rocks) that determines the actual numerical age in years by measuring the radioactive decay of parent isotopes to daughter isotopes over half-lives.

Relative dating

A qualitative method of dating geological materials based upon their order of formation using physical principles and fossils to determine older vs. younger relationships without assigning a specific numerical age.

Definition of a Mineral

A naturally occurring, inorganic, solid substance formed geologically with a definite chemical composition and an ordered internal crystalline structure (crystal lattice).

Muscovite mica

A common sheet silicate mineral with a vitreous/pearly luster that exhibits perfect cleavage in one direction, peeling into thin, flexible, transparent sheets

Fracture

The way a mineral breaks when atomic bond strength is equal in all directions (lacking cleavage planes), such as the smooth, curved conchoidal fracture seen in quartz and volcanic glass.

Cleavage

The tendency of a mineral to break along flat, shiny, parallel planes of weaker atomic bonding, described by the number of planes and the angles between them.

Mohs Hardness Scale

A relative scale from 1 to 10 measuring the scratching resistance of a mineral based on atomic bond strength: 1-Talc, 2-Gypsum, 3-Calcite, 4-Fluorite, 5-Apatite, 6-Orthoclase/Microcline, 7-Quartz, 8-Topaz, 9-Corundum/Ruby, 10-Diamond.

Igneous rock classification

Igneous rocks are classified based on their mineral composition/silica content (Felsic, Intermediate, Mafic, Ultramafic) and their texture/cooling history (Intrusive/coarse-grained vs. Extrusive/fine-grained, glassy, or pyroclastic).

Felsic rock examples

Light-colored igneous rocks with high silica content (~70% SiO2) rich in quartz, potassium feldspar, and sodium-rich plagioclase

Granite & Rhyolite

Felsic igneous rock pair: Granite is a coarse-grained, intrusive (plutonic) rock formed from slow cooling underground

Mafic rock examples

Dark-colored igneous rocks with lower silica content (~50% SiO2) rich in iron, magnesium, pyroxene, and calcium-rich plagioclase

Gabbro & Basalt

Mafic igneous rock pair: Gabbro is a coarse-grained, intrusive (plutonic) rock formed from slow cooling

Common metamorphic indicator minerals

Index minerals that form within specific temperature and pressure ranges, indicating the grade of metamorphism (from low to high temp): Chlorite -> Muscovite -> Biotite -> Garnet -> Andalusite -> Sillimanite.

Foliated metamorphic rocks

Metamorphic rocks that exhibit parallel alignment of platy/inequant mineral grains or compositional banding due to differential stress (compression/shear)

Nonfoliated metamorphic rocks (and why?)

Metamorphic rocks that lack a planar fabric or layering (e.g., Marble, Quartzite, Hornfels, Anthracite) because they were not subjected to differential stress, are dominated by equant minerals (like quartz or calcite), and lack platy minerals (like clays or micas).

Transgression

A geologic event where sea level rises relative to the land and the shoreline migrates inland, resulting in deeper-water marine facies (shale, then limestone) being deposited on top of shallower nearshore facies (sandstone).

Regression

A geologic event where sea level falls relative to the land and the shoreline migrates seaward, resulting in shallower nearshore facies (sandstone) being deposited on top of deeper-water facies (limestone and shale).

Pumice vs. Scoria

Both are vesicular (gas-bubble rich) extrusive glassy igneous rocks: Pumice is felsic, light-colored, frothy, and light enough to float on water

Clay minerals (1:1 & 2:1)

Fine-grained hydrous sheet silicates (<2 microns): 1:1 clays (e.g., Kaolinite) have 1 tetrahedral and 1 octahedral layer, low substitution, low CEC, and are non-swelling

Conglomerate

A coarse-grained clastic sedimentary rock composed of rounded gravel-sized clasts (pebbles, cobbles, boulders), indicating transport by flowing water over a significant distance from the source.

Sandstone

A clastic sedimentary rock composed of sand-sized particles (0.063 to 2 mm), predominantly quartz, commonly formed in beach, dune, and river settings.

Marble

A coarse-grained, nonfoliated metamorphic rock composed of recrystallized calcite or dolomite, formed from the regional or contact metamorphism of a limestone or dolostone protolith.

Granite

A coarse-grained (phaneritic), intrusive felsic igneous rock composed primarily of quartz, potassium feldspar, sodium plagioclase, and biotite, forming the bulk of continental crust.

Basalt

A fine-grained (aphanitic), extrusive mafic igneous rock composed primarily of pyroxene and calcium-rich plagioclase, forming the bulk of oceanic crust.

Gabbro

A coarse-grained (phaneritic), intrusive mafic igneous rock composed primarily of pyroxene and calcium-rich plagioclase

Tuff

A pyroclastic igneous rock with a fragmental texture formed from consolidated volcanic ash and shattered rock fragments ejected during a violent volcanic eruption.

Evidence of continental drift

Proposed by Alfred Wegener, the 5 main lines of evidence are: 1) Jigsaw puzzle fit of continents, 2) Matching fossil evidence across oceans (Mesosaurus, Glossopteris, Lystrosaurus, Cynognathus), 3) Glacial striations/deposits in modern tropical regions, 4) Matching rock types (lithologies) and mountain belts across oceans, and 5) Apparent magnetic polar wandering paths.

Forces of plate tectonics

The driving mechanisms of plate motion: mantle convection currents (rising warm mantle at ridges and sinking cooled mantle) combined with ridge push at elevated mid-ocean ridges and slab pull from cold, dense subducting lithosphere sinking into trenches.

Seafloor spreading

The process occurring at divergent mid-ocean ridges where rising magma injects, cools, and creates new oceanic lithosphere that spreads outward laterally, confirmed by symmetrical marine magnetic reversal stripes and increasing rock age away from the ridge.

Mid-ocean Ridges

Underwater mountain ranges (divergent plate boundaries, e.g., Mid-Atlantic Ridge) where tectonic plates pull apart, magma rises to form new oceanic crust, and the youngest seafloor is located.

Types of plate boundaries

1) Divergent (plates move apart, creating new crust at mid-ocean ridges or continental rifts), 2) Convergent (plates collide, resulting in subduction trenches, volcanic arcs, or continental collision mountain belts like the Himalayas), and 3) Transform (plates slide horizontally past one another along strike-slip faults like the San Andreas).

Hot spots

Stationary plumes of hot rising mantle rock that burn through the overlying moving lithospheric plate, creating a linear chain of progressively older extinct volcanoes, islands, and seamounts/guyots (e.g., the Hawaiian Emperor Chain or Yellowstone).

Plate tectonic stresses

The three fundamental types of directed force at plate boundaries: Compression (squeezing together at convergent boundaries), Tension (pulling apart at divergent boundaries), and Shear stress (sliding past each other at transform boundaries).

Geology stress & strain

Stress is the force applied per unit area on a rock (compression, tension, or shear)

Anticline

An arch-like (A-shaped) fold in rock layers produced by compressional stress, in which the oldest rock strata are exposed at the center (core) of the fold after erosion.

Syncline

A trough-like (U-shaped) fold in rock layers produced by compressional stress, in which the youngest rock strata are exposed at the center (core) of the fold after erosion.

Dome

A circular or oval upward-bulging geologic structure in which rock layers dip away from a central point in all directions, exposing the oldest rocks in the center.

Basin

A circular or oval bowl-shaped depression in which rock layers dip inward toward a central point from all directions, exposing the youngest rocks in the center.

Normal fault

A dip-slip fault caused by tensional (extensional) stress in which the hanging-wall block moves downward relative to the footwall block.

Reverse fault

A steep dip-slip fault (typically ~60 degree dip) caused by compressional stress in which the hanging-wall block moves upward relative to the footwall block.

Thrust fault

A low-angle reverse fault (typically ~30 degree dip or less) caused by severe compressional stress in which the hanging-wall block is pushed up and over the footwall block.

Oblique-slip fault

A fault that exhibits a combination of both vertical (dip-slip) and horizontal (strike-slip) displacement along the fault plane.

Strike-slip fault

A nearly vertical fault caused by horizontal shear stress in which rock blocks slide horizontally past one another parallel to the strike of the fault.

Horsts & Grabens

Fault-block structures formed by tensional stress and normal faulting: a Graben is a downdropped valley block bounded by normal faults, while a Horst is an uplifted ridge block standing high between grabens.

Direction of plunge in folds

In a plunging fold (where the fold axis is tilted into the ground), the V-shaped or U-shaped contact lines on an eroded map surface point toward the direction of plunge in a plunging anticline, and open (point away from) the direction of plunge in a plunging syncline.

19th Century Milestones in climate science

1) Joseph Fourier (1820s) describes the atmosphere's heat-trapping contribution to planetary temperature

Cation Exchange Capacity (CEC)

The measure of how well a clay mineral or soil particle can attract, retain, and exchange positively charged ions (cations like Ca2+, Mg2+, K+, NH4+) on its negatively charged surface, measured in meq/100g

Paraconformity

A type of unconformity in which strata are parallel and there is no obvious erosional surface, making the time gap detectable only through fossil or age records.

Isostasy

The gravitational equilibrium between Earth's lithosphere (continental and oceanic crust) and the denser underlying asthenosphere/mantle, where less dense continental crust (granite, rho=2.8) floats higher and has a deeper root than denser oceanic crust (basalt, rho=2.9) above a ~50 km compensation depth.

Magnetic Declination & Inclination

Declination is the horizontal angle between magnetic north and true geographic north (varies with longitude)

Wilson Cycle

The cyclical opening and closing of ocean basins driven by plate tectonics: A) Stable craton -> B) Early continental rifting -> C) Full ocean basin with seafloor spreading -> D) Subduction zone initiation -> E) Closing remnant ocean basin -> F) Continental collision orogeny -> G) Peneplained mountain belt.

Polymorph

Minerals that share the exact same chemical composition (such as pure carbon, C) but have different internal crystalline structures and physical properties due to different pressure/temperature formation conditions (e.g., Diamond vs. Graphite).

Streak

The color of the mineral powder produced by scraping a mineral across an unglazed porcelain plate

Specific Gravity

A measure of the density ("heft") of a mineral, defined as the weight of the mineral divided by the weight of an equal volume of water (e.g., Galena feels very heavy at SG 7.60 compared to Quartz at SG 2.65).

Effervescence

The fizzing reaction (release of CO2 gas bubbles) when carbonate minerals like Calcite (CaCO3) or rocks like Limestone and Marble react with dilute hydrochloric acid.

Silica Tetrahedron

The fundamental building block of all silicate minerals (SiO4^4-), consisting of one central silicon cation surrounded by four oxygen anions in a pyramid-like tetrahedral arrangement.

Porphyritic Texture

An igneous rock texture featuring large crystals (phenocrysts) embedded in a finer-grained matrix, indicating a two-stage cooling history (initial slow cooling underground followed by rapid cooling upon eruption).

Bowen's Reaction Series

The sequence in which silicate minerals crystallize from cooling magma as temperature drops: Olivine -> Pyroxene -> Amphibole -> Biotite (discontinuous branch) and Ca-rich to Na-rich Plagioclase (continuous branch), followed at lowest temperatures by Potassium Feldspar, Muscovite, and Quartz.

Diorite & Andesite

Intermediate igneous rock pair (~60% SiO2, composed of plagioclase and amphibole): Diorite is coarse-grained intrusive ("salt and pepper" appearance)

Peridotite & Komatiite

Ultramafic igneous rock pair (<45% SiO2, composed mostly of olivine and pyroxene): Peridotite is coarse-grained intrusive rock that makes up the upper mantle

Breccia

A coarse-grained clastic sedimentary rock composed of angular gravel-sized rock fragments, indicating minimal transport and deposition very close to the source (e.g., talus at the base of a cliff).

Arkose

A clastic sedimentary rock rich in sand and gravel-sized quartz and abundant feldspar, commonly deposited in alluvial fans after short transport.

Shale & Siltstone

Fine-grained clastic sedimentary rocks deposited in quiet water settings (lagoons, floodplains, deep basins): Siltstone is lithified silt (0.0039 to 0.063 mm), and Shale is compact, brittle, fissile lithified clay (<0.0039 mm).

Chert

A hard, cryptocrystalline quartz (SiO2) sedimentary rock formed either biochemically from the silica skeletons of marine plankton or chemically as replacement nodules/beds in limestone (flint, agate, petrified wood).

Evaporites

Chemical sedimentary rocks (such as Rock Salt/Halite and Rock Gypsum) precipitated directly from evaporating seawater or lake water.

Limestone & Dolostone

Carbonate sedimentary rocks: Limestone (CaCO3) forms biochemically from cemented shell/coral fragments or micrite mud

Slate, Phyllite, Schist, Gneiss, Migmatite

The sequence of foliated metamorphic rocks formed from a shale protolith under increasing regional heat and pressure: Slate (slaty cleavage) -> Phyllite (silky sheen) -> Schist (visible micas, schistosity) -> Gneiss (light and dark compositional banding) -> Migmatite (partially melted gneiss).

Quartzite

A very hard, nonfoliated metamorphic rock formed by the regional or contact metamorphism of quartz sandstone, in which quartz grains fuse and break with conchoidal fracture across grains.

Hornfels

A fine-grained, nonfoliated metamorphic rock formed by contact metamorphism (baking) of country rock next to a hot magma/plutonic intrusion.

Hanging Wall & Footwall

In an inclined fault, the Footwall is the block of rock below the fault plane (where a miner would stand), and the Hanging Wall is the block of rock above the fault plane (where a miner would hang a lantern).