15: Glossary

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A

Absaroka Sequence - A group of North American sedimentary formations formed during marine transgressions in the Mississippian, Permian, Triassic, and Jurassic periods
abyssal plain – the vast, flat, and deep region of the ocean floor, typically found at depths of 3,000–6,000 meters, covered by fine-grained sediments
Acadian Orogeny - a mountain-building event affecting a more southern part of the east coast of Laurentia occurring during the late Silurian through the Devonian resulting in the second wave of Appalachian mountain-building
Acasta Gneiss Complex - located in Canada, it preserves the oldest rocks on Earth some of which where derived from the partial melting of 4.3 billion year old Hadean crust
accretion - increasing the mass of an object by adding material to it or the addition of crustal material through collisions
accretion(ary) - the process of small particles and gases within the solar nebula combining together to form larger particles and, eventually, planets
accretionary wedge - structure near an oceanic trench formed by the accumulation of sediment scraped off the subducting oceanic lithosphere and shoved onto the overriding oceanic or continental plate
active margin - a continental coastline that is marked by a plate boundary
adaptive radiation - the diversification of a group of organisms into forms filling different ecological niches
aeolian – refers to processes, sediments, or landforms produced or transported by wind, such as sand dunes deposits
aerobic - involving requiring oxygen
albedo - the ability of a surface to reflect sunlight
Alleghenian Orogeny - the third and final episode of Appalachian mountain-building causing folding in the southern part of the range and the Ouachita Mountains in Texas and Oklahoma to form occurring during the Carboniferous and Permian periods
alluvial fan – a fan- or cone-shaped deposit of sediment that forms where a river or stream flows out of a mountainous area onto a flatter plain
alpine glacier – a glacier that forms in mountainous regions and flows down valleys under the influence of gravity
amniotes - a group of tetrapod vertebrates comprising reptiles, birds, and mammals defined by their ability to lay eggs on land or retain fertilized eggs within the mother
amphibolite - non-foliated metamorphic rock made of amphibole (hornblende) formed from a mafic igneous protolith
anapsids - the group of sauropsids with reptile skulls with no openings in the rear of the skullavemetatarsalia - bird-line archosaurs
andesite - an aphanitic (intrusive), intermediate igneous rock
angular unconformity - a geological feature where younger, horizontal sedimentary rock layers are deposited on top of older, tilted or folded sedimentary rock layers, creating an angular discordance between the two sets of strata
anion - a negatively charged ion formed as the number of electrons exceeds the number of protons
anoxic - having no oxygen
Anthropocene - a proposed geological epoch defined by humanity's significant, transformative impact on Earth's climate, ecosystems, and geology
anticline – a fold in rock layers that is arch-shaped, with the oldest rocks located at the core of the fold and the limbs dipping away from the center
Antler Orogeny - first mountain-building event of the Cordilleran occurring at the end of the Devonian
archosaurs - a branch of diapsids that includes the last common ancestor of birds and crocodilians
arkose sandstone - clastic sedimentary rock composed of sand-sized feldspar particles
arthropods - a phylum of invertebrate animals characterized by a segmented body, a hard exoskeleton, and jointed appendages
artificial selection - the process by which humans deliberately breed organisms for specific and preferred traits
asthenosphere - the layer beneath the lithosphere, which is in a plastic, semi-solid state
atomic mass - the mass of an atom determined by adding the number of protons and neutrons in the atom
atomic number - the number of protons in an atom which defines what element the atom is
augite - a common single-chain silicate mineral rich in iron and magnesium though other elements often substitute in making its formula complex; the most common mineral of the pyroxene family
axial plane – an imaginary surface that divides a fold into two roughly symmetrical halves, passing through the center of each layer

B

background extinction - the normal, ongoing rate of species loss that occurs as a result of natural ecological and evolutionary processes
banded iron formation - unusual chemical sedimentary rocks with distinctive alternating layers of dark gray to black iron oxides and red to yellow chert that could only form in an ocean that is largely oxygen-free
basalt - an aphanitic (intrusive), mafic igneous rock
Basin and Range Geologic Province - a province in the western US containing multiple north-south trending mountain ranges separated by north-south trending valleys
beach – a gently sloping shoreline environment composed of unconsolidated sediment (sand, gravel, or pebbles) shaped by waves, tides, and currents
bed - a layer of sediment or sedimentary rock that is distinguishable from adjacent layers due to differences in its composition, texture, or other physical properties
bedding - the layered structure of sedimentary rocks, where layers, are separated by planes
bedform - a geological feature developed when a moveable bed interfaces with a fluid resulting in the material being shaped by the flow
binomial nomenclature - a system of naming plants and animals by assigning a two-part, Latin-based name to each, consisting of the genus and the species
biochemical sedimentary rock - sedimentary rock formed as shells and the body parts of underwater organisms lithify
biostratigraphy – a branch of stratigraphy that uses fossil content and fossil assemblages to divide and correlate rock layers, helping to establish their relative ages
biotite - a sheet silicate rich in iron and magnesium making it light in color
bioturbation - the physical churning and mixing of sediments by the activities of living organisms (such as burrowing)
bituminous coal - sedimentary rock formed by the breakdown of peat bog material
body fossil - a preserved remnant or part of an actual organism, such as bones, teeth, shells, leaves, or wood
Bouma Sequence - a geological model describing the idealized vertical succession of sedimentary structures and lithologies in a turbidite
Bowen's Reaction Series - a diagram that depicts the order in which minerals crystallize as molten material cools
braided river – a river system with multiple intertwining channels separated by bars or islands of sediment, usually formed where sediment supply is high and water amount and velocity varies
breccia - poorly sorted clastic sedimentary rock composed of angular gravel-sized particles
brittle deformation – the breaking or fracturing of rocks a a result of exposure to stress
Burgess Shale - a Cambrian lagerstätte that allowed scientists to study Cambrian animals in immense detail

C

calcite - the only "Big Ten" mineral that is not a silicate; composed of calcium, carbon, and oxygen; typically associated with sedimentary processes
Caledonian Orogeny - a mountain-building event affecting the northern part of the east coast of Laurentia occurring during the late Silurian through the Devonian
Cambrian Explosion - the sudden appearance of biological diversity at the start of the Paleozoic Era
carbonate rocks – sedimentary rocks made mostly of carbonate minerals (like calcite and dolomite), commonly formed in marine environments (e.g., limestone, dolostone)
carbonates - sedimentary rocks primarily composed of carbonate minerals like limestone (calcite or aragonite) and dolomite -
carbonization - a specific form of fossilization where organic matter (like plants) is converted into a thin, dark, flattened film of carbon due to heat and pressure, driving off volatile elements and preserving detailed impressions of the original organism
casts - replicas of organisms, essentially recreating their original shape
cation - a positively charged ion formed as the number of protons exceeds the number of electrons
chalk - sedimentary rock made of very small shells made of calcite
chemical sedimentary rock - sedimentary rock formed as minerals precipitate from ions dissolved in solution
chert - sedimentary rock made of very small crystals of quartz
Chicxulub Crater - a ~180‑kilometer‑wide impact structure buried beneath the Yucatán Peninsula in Mexico formed about 66 million years ago by the collision of a large asteroid, meteoroid or comet
chondrules - small grains found in certain types of meteorites formed in the early solar nebula as molten droplets quickly cooled and crystallized
chordata - a major animal phylum containing all vertebrates and some specialized invertebrates all sharing specific characteristics such as having a hollow dorsal nerve cord
chronostratigraphy – the study of rock strata in relation to geologic time, focusing on assigning absolute or relative ages to rock units and organizing them into a standardized time framework
clastic sedimentary rock - sedimentary rock composed of sediment compacted and cemented to form a new rock
clay - a group of silicate minerals formed through the weathering of pre-existing silicate minerals; also a very small size of material
claystone - clastic sedimentary rock composed of clay-sized particles
composition - the materials something is made of
compressional stress – a type of stress that squeezes rocks together, shortening and thickening the crust
confining pressure - pressure coming from all directions equally
conglomerate - poorly sorted clastic sedimentary rock composed of rounded gravel-sized particles
contact metamorphism - metamorphism that occurs in rock exposed to high temperatures resulting from the proximity to a magma body or lava flow
continental crust - thicker, less dense crust that when paired with the solid upper mantle creates continental landmasses
continental volcanic arc - a chain of volcanoes formed on a continental plate as a result of the subduction of oceanic lithosphere
convection - the movement of the mantle as it transfers heat from the hot core to the brittle lithosphere
convergent boundary - plate boundary type at which the two plates move toward one another
correlation – the process of demonstrating equivalence or continuity between rock units or strata at different geographic locations, based on lithology, fossil content, age, or other stratigraphic markers
covalent bond - a strong type of bond in which atoms share electrons to complete their valence shells rending them stable
craton - tectonically stable interiors of continents composed of the shield, which is crystalline basement rock like gneiss and greenstone, and the platform of sedimentary rocks covering the shield
cross section – a vertical slice or profile through the Earth, showing the arrangement and relationships of rock layers, structures, and features beneath the surface
cross-bed - a sedimentary structure where layers of sediment are deposited at an angle to the main, horizontal bedding plane
crystalline limestone - a sedimentary rock formed as calcite precipitates
cyanobacteria - single or multicellular bacteria that use chlorophyll to absorb energy from light and were most responsible for constructing stromatolites

D

daughter product – the stable atom that forms as a result of the decay of a radioactive parent isotope
debris flow - mixtures of mud, water, and coarse debris, moving down a slope due to gravity that lack fluid turbulence
Deccan Traps - one of Earth’s largest continental flood basalt provinces, located primarily in western India and erupted in multiple phases around 66 million years ago
decompression melting - melting of rock triggered as rock rises and pressure is lowered
delta – a wedge-shaped deposit of sediment formed at the mouth of a river where it enters a standing body of water, such as a lake or ocean
desert – a region that receives very little precipitation (typically less than 25 cm per year), characterized by sparse vegetation and dominance of wind and occasional water erosion
detrital pyrite - rounded, transported pyrite found in sedimentary rocks, serving as an indicator of an oxygen-free Earth atmosphere
diamictite - generic term for poorly sorted sediment; it could be from glacial activity but could also be from landslide or other sedimentary activities
diapir - blobs of magma that rise towards Earth's surface
diapsids - the group of sauropsids with two openings in the rear of the skull
differential stress - an unequal balance of forces in one or more directions
differentiation - material settling out according to density or the process of a planet developing a layered structure according to density and chemical composition
diorite - a phaneritic (intrusive), intermediate igneous rock
dip – the angle at which a rock layer, fault, or other planar feature is inclined from the horizontal, measured downward in the direction of maximum slope
disconformity - a gap in the geological record that occurs between parallel layers of sedimentary rock
discontinuous bed - a sediment ary bed that pinches out in one direction
divergent boundary - plate boundary type at which the two plates move away from one another
dolostone - a sedimentary rocks formed as magnesium replaces the calcium in limestone before lithification
dropstones - random large clasts found within fine-grained, thinly-bedded and laminated deepwater deposits of clay and silt that were transported out into the ocean by an iceberg that then melted and dropped it
ductile deformation – the bending, flowing, or folding of rocks without breaking when subjected to stress, usually at high temperature and pressure

E

Ediacaran biota - plants and animals that evolved in the late Proterozoic and represent the earliest known complex multicellular organisms
elastic deformation – a temporary change in the shape or size of a rock that is reversible; the rock returns to its original form once the stress is removed
electron - a negatively charged subatomic particle with essentially no mass that resides outside of an atom's nucleus
endosymbiosis - the process of two organisms merging into one to form a mutually beneficial relationship
epibole – the sudden, widespread appearance (or disappearance) of a fossil species or assemblage within the stratigraphic record, often used as a marker horizon for correlation
erosion - the removal of weathered rock material from its original location
eukaryotes - organisms with membrane-bound nuclei and include all animals, plants, fungi, seaweeds, and many unicellular organisms
eutrophication - the over-enrichment of water by nutrients, primarily phosphorus and nitrogen, causing excessive algae growth and oxygen depletion
evolution - the process of gradual change in the characteristics of a population leading to the development of new species over many generations
evolutionary fauna - a group of marine animal taxa with hard skeletons that appear, evolve, and go extinct together as a distinct, dominant community over a particular geological time
external mold - an impression of an organism’s outside shape
extrusive - igneous rock formed as molten material cools on the surface

F

fabric - the arrangement and organization of a rock's constituent parts and features
facies - Characteristics of a rock unit that reflect the origin of the rock and distinguish it from adjacent rock units. These characteristics include mineralogy, sedimentary structures, fossil content, and texture. These provide clues about the depositional environment in which a rock was formed
facies – the set of characteristics (such as rock type, fossils, sedimentary structures, and color) that reflect the conditions in which a sedimentary rock was deposited
feldspar - a framework silicate mineral with a wide variety of compositions as potassium, sodium, and calcium fit into the structure
felsic - a composition in which there are high (~70%) concentrations of silica; resultant rocks are typically light in color
foliated/foliation - the characteristic of a metamorphic rock by which there is a distinct alignment of minerals
footwall – the block of rock that lies beneath an inclined fault plane
foreset - an inclined layer of sediment in a bedform that dips in the direction of current flow, forming the steep part of the structure
formation – a distinct, mappable body of rock with unique physical characteristics, such as color, composition, or particle size, that differentiates it from the surrounding rock layers
formation – a fundamental lithostratigraphic unit of rock that is mappable, has a distinct set of characteristics (lithology, composition, etc.), and is thick and extensive enough to be recognized at the Earth’s surface or in the subsurface
fossil – any preserved remains, impression, or trace of an organism that lived in the past; must be older than 11,700 years old
fossiliferous limestone - sedimentary rock composed of calcite that contains an abundance of fossils

G

gabbro - a phaneritic (intrusive), mafic igneous rock
gene migration - the movement of genetic material from one population to another through the migration of individuals into a new area
genetic drift - a change in the frequency of an allele over time caused by some individuals reproducing more successfully than others
genetic mutations - changes in the sequence of nucleic bases in the coding portion of a DNA molecule
genetic symbiosis - the transfer of genetic material from one species to another due to close physical interactions
genotype - a unique sequence of DNA
Geologic Time Scale - a graphical representation of Earth's history based on the rock record that shows the timing and relationships of geologic and biologic events
glacial - time interval cold enough to support widespread glaciation
gneiss - a foliated metamorphic rock having minerals separated into light and dark bands
graded bedding - a sedimentary layer characterized by progressively finer-grained particles from the bottom upward
granite - a phaneritic (intrusive), felsic igneous rock
Great Ordovician Biodiversification Event (GOBE) - an event during the late Ordovician in which vertebrates and invertebrates became more diverse and complex at family, genus, and species level
greenstone - non-foliated metamorphic rock made of chlorite formed from a basalt protolith
greenstone belt - a linear or branching zone of metamorphosed sedimentary and igneous rock within Precambrian cratons; the name comes from the presence of green minerals like chlorite, actinolite, and eclogite
greywacke - clastic sedimentary rock composed of sand-sized particles that vary in their composition
group – a lithostratigraphic unit consisting of two or more related formations that share similar geologic features and are grouped together for convenience in mapping and description

H

Hadean - the earliest part of the Precambrian Eon starting at the conception of Earth up to 4 Ga
half-life – the amount of time it takes for half of the atoms in a sample of a radioactive isotope to decay into daughter products
halides - minerals consisting of chlorine bonded with sodium or other cations
hanging wall – the block of rock that lies above an inclined fault plane
hominids - a group of large-bodied primates that includes modern humans and the great apes
homology - a similarity in structure, genetics, or developmental pathways among different organisms that results from shared ancestry
horizontal gene migration - the movement of genetic material between organisms that are not parent and offspring
hornblende - a common double-chain silicate mineral; it has a complex formula due to its many possible substitututions

I

igneous rock - a rock that forms as minerals crystallize when molten material cools
imbrication - a primary depositional fabric where elongated or flat clasts are arranged in a overlapping pattern indicating flow direction
index fossil - organisms that lived for relatively short time periods and distributed over a wide geographic area
interglacial - time interval between glacials that does not support widespread glaciation
intermediate - a composition in which there are moderate (~60%) concentrations of silica; resultant rocks are typically medium in color
internal mold – a fossil formed when sediment fills the inside of a shell or other hollow structure; after the shell dissolves, the hardened sediment shows the internal shape
intrusive - igneous rock formed as molten material cools below the surface
invertebrate - an animal that does not have a backbone or spinal column
ionic bond - a type of bond in which atoms lose or gain electrons to become negatively or positively charged and thus attracted to one another
isotopes – atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different atomic masses

K

Kaskaskia Sequence - A group of North American sedimentary formations formed during marine transgressions from the Devonian through the Pennsylvanian periods
komatiite - an aphanitic (intrusive), ultramafic igneous roc
K–Pg mass extinction - also referred to as the K-T Extinction, this extinction occurred at 66 million years and eliminated roughly 75% of Earth’s species
Kuiper Belt - a doughnut-shaped region of icy objects beyond the orbit of Neptune; it is home to Pluto and most of the known dwarf planets and some comets

L

lacustrine – pertaining to lakes; describes sediments, deposits, or environments associated with lake basins
Lagerstätte - a site of exceptional fossil preservation that includes impressions of soft body parts
Lamarck's inheritance of acquired traits - the theory that an organism's offspring inherit characteristics that the parent acquired during its lifetime through the use or disuse of organs
Laramide Orogeny - a major mountain‑building event that affected western North America from roughly 80 to 40 million years ago
late Devonian extinction event - an extinction event at the end of the Devonian occurring as the oceans became too deficient in oxygen to support life
Late Heavy Bombardment - a period of extraordinary meteorite impacts at the start of the Archean, thought to have been triggered by the gravitational interaction of Jupiter and Saturn
Laurentia - a Proterozoic craton that forms the core of the North American continent
lava - molten material on Earth's surface
lenticular bed - a sedimentary bed that pinches out in both directions
lepidosaurs - a branch of diapsids known for overlapping scales
limb – the side or arm of a fold, consisting of rock layers that dip away from (or toward) the center of the fold
lithosphere - the outermost layer of Earth that includes the crust and solid upper mantle; this solid, rock-layer, is broken up into individual pieces called plates
lithostratigraphy – the branch of stratigraphy that classifies rock units based on their physical and lithologic characteristics, without direct reference to age

M

mafic - a composition in which there are low (~50%) concentrations of silica; resultant rocks are typically dark in color
magma - molten material below Earth's surface
magnetostratigraphy – a stratigraphic method that uses the record of Earth’s magnetic field reversals preserved in rocks to establish ages and correlate strata
mantle plume - long-lived narrow conduits of magma rising from Earth's core to make hot spots
map – a two-dimensional representation of the Earth’s surface used to show the distribution of rock units, structures, and landforms
marble - non-foliated metamorphic rock made of calcite or dolomite formed as limestone was metamorphosed
mass extinction - a widespread, rapid decrease in the Earth's biodiversity, where at least 75% of all species perish within a geologically short interval
mass number – the total number of protons and neutrons in the nucleus of an atom
meandering river – a river with a single, sinuous channel characterized by large bends (meanders) that migrate laterally across the floodplain
member – a lithostratigraphic subdivision of a formation, smaller than a formation but still mappable, that has distinctive properties
metamorphic rock - rock formed when elevated temperatures and pressures cause minerals to change into new minerals without going through the melting phase
micrite - sedimentary rock made of very fine grains of calcite
migmatite - a metamorphic rock that is partially melted showing the transition from metamorphic and igneous activity
mineral - naturally occurring, inorganic solid with an ordered atomic structure and definite chemical composition
monocline – a simple fold in which otherwise horizontal rock layers bend in one direction, creating a single step-like feature
mountain belt - a mountain chain formed as a result of the collision of two continental plates
muscovite - a sheet silicate rich in potassium making it light in color
N
native elements - minerals that are composed of one element, existing in a pure or nearly pure state
natural selection - the method by which organisms with traits better suited to their environment are more likely to survive and reproduce, passing those advantageous traits to their offspring
nebula - a massive cloud of gas and dust in space that is the byproduct of a previous star that suffered a supernova explosion
neo-Darwinian evolutionary theory - the theory that combines Charles Darwin's concept of natural selection with modern genetics, including Mendelian inheritance and the understanding of mutations
neutron - a subatomic particle with no charge having a mass of 1 atomic mass unit that resides inside an atom's nucleus
Nevada Orogeny - a Middle to Late Jurassic (roughly 170–140 million years ago) compressional event along the western margin of North Americ
nonconformity - a gap in the geologic record where sedimentary rock overlies igneous or metamorphic rock
non-foliated - the characteristic of a metamorphic rock by which there is no alignment of minerals
normal fault – a type of dip-slip fault where the hanging wall moves downward relative to the footwall, caused by tensional stress
nucleus - the central portion of an atom in which protons and neutrons reside
numerical dating - the type of geologic dating in which you assign a number age to a geologic event
Nuvvuagittuq Greenstone Belt (NGB) - located in Canada, rock from this has been dated back to 4.31 Ga making it the oldest preserved rock on earth (though the dating is controversial)

O

oceanic crust - thinner, more dense crust that when paired with solid upper mantle creates the ocean floors
oceanic ridge - mountain chain at the bottom of the ocean, often in the middle of the ocean, created at divergent boundaries where new crust is created
olivine - a common silicate mineral rich in iron and magnesium making up much of the ocean floor and the mantle
oolitic limestone - sedimentary rock formed from small grains of calcite
organic sedimentary rock - sedimentary rock formed as the remains of organic materials undergo lithification
ornithischians - herbivorous dinosaurs characterized by a “bird‑hipped” pelvic structure in which the pubis is rotated backward
orogeny - mountain building event occurring due to compression at a convergent plate boundary
outage – in stratigraphy, more commonly referred to as a hiatus, it is a gap in the geologic record representing a period of non-deposition or erosion between rock layers
overturned fold – a fold in which one or both limbs are tilted beyond vertical, so that the rock layers on one limb are upside down.
oxides - minerals consisting of metal ions covalently bonded with oxygen

P

Paleocene-Eocene Thermal Maximum - rapid global warming occurring part way through the Cenozoic Era
paleomagnetic - ancient magnetic orientation
paleomagnetism - the study of the Earth's ancient magnetic field, preserved in rocks and sediments
paleontology - the study of the history of life on Earth as based on fossils
parent – the original radioactive isotope that undergoes decay to form a daughter product
passive margin - a continental coastline that is not marked by a plate boundary
peridotite - a phaneritic (intrusive), ultramafic igneous rock
Permian-Triassic (P-T) extinction - mass extinction event at the end of the Paleozoic Era causes by global warming and drying that resulted from the assembly of Pangea
permineralization – a fossilization process where minerals carried by water fill the pores and empty spaces in bones, wood, or shells, preserving fine details of the original structure
phenotype - the set of observable characteristics of an organism, resulting from the interaction between its genetic makeup (genotype) and environmental factors
phosphates - minerals consisting of a PO4 2- cation bonded with various ions
photosynthesis - a system of biological processes where plants take in CO2 to convert sunlight into chemical energy which fuels their metabolism and releases oxygen to their surroundings
phyllite - a fine-grained foliated metamorphic rock in which the platy chlorite and mica minerals have grown larger and the surface of the foliation shows a sheen from light reflecting from the grains
planetesimals - the nuclei of planets formed in the early solar system from collisions with other objects in the solar system
plate tectonics - the theory that Earth’s rigid outer shell, the lithosphere, is broken up into individual pieces called plates, that move and interact with one another along plate boundaries, driving Earth’s constructive geologic processes
plates - pieces of lithosphere composed of oceanic and/or continental crust and the solid upper mantle
platform – a portion of a craton that is covered by relatively undeformed, horizontally lying or gently tilted sedimentary rocks, overlying older crystalline basement
plutonic - igneous rock formed as molten material cools below the surface
porphyroblast - a large crystal of a particular mineral in a metamorphic rock surrounded by small grains
principle of cross-cutting relationships – a geologic principle stating that a rock or geologic feature (such as a fault or intrusion) that cuts across another rock body must be younger than the rock it cuts through
principle of faunal succession – the concept that fossils succeed one another in a definite, recognizable order, allowing rock layers to be dated and correlated by the fossils they contain
principle of inclusions – states that any rock fragments (inclusions) contained within another rock must be older than the rock in which they are enclosed.
principle of lateral continuity – sedimentary layers originally extend in all directions until they thin out or are interrupted by a barrier, meaning that separated layers on opposite sides of a valley were once continuous
principle of original horizontality – states that layers of sediment are originally deposited in horizontal or nearly horizontal layers so tilting must have occurred after deposition
principle of superposition – in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest layers are at the top
prokaryotes - single-celled organisms that lack nuclei and predate eukaryotes
prokaryotes - organisms whose cells lack a nucleus and other organelles
protolith - the parent rock or the rock from which the metamorphic rock forms
proton - a positively charged subatomic particle having a mass of 1 atomic mass unit that resides inside an atom's nucleus
protoplanetary disk - ring of debris around a star
pseudosuchian - crocodile-line archosaurs

Q

quartz - a framework silicate mineral composed entirely of silicon and oxygen though small amounts of other elements can cause color changes
quartz sandstone - clastic sedimentary rock composed of sand-sized quartz particles
quartzite - non-foliated metamorphic rock made of quartz formed as quartz sandstone was metamorphosed

R

radioactive decay – the natural process by which an unstable atomic nucleus loses energy by emitting particles or radiation, transforming into a more stable form (often producing a daughter isotope)
recrystallization – a preservation process where the original minerals of a fossil change into more stable forms without changing the overall shape of the fossil
recumbent fold – a fold that has been rotated so much that its axial plane is nearly horizontal, causing the limbs to be almost parallel
regional metamorphism - metamorphism that occurs over a wide area resulting from the collision and compression of tectonic plates
regression – a geologic process in which the sea level lowers, exposing previously submerged areas and leading to the deposition of progressively shallower-water sediments over deeper-water sediments
regression - a time period when sea levels drop relative to land masses; caused by sea level drop or land level rise
relative dating – a method of determining the chronological order of past events by comparing rock layers and fossils, without determining their exact numerical age
replacement – a fossilization process where the original material of an organism is gradually dissolved and replaced molecule by molecule with minerals, creating a detailed copy
reverse fault – a type of dip-slip fault where the hanging wall moves upward relative to the footwall, caused by compressional stress
rhyolite - an aphanitic (intrusive), felsic igneous rock
ridge push - the elevation of ocean ridges, caused by rising magma, pushes the plates away from the high spot down toward the deeper ocean floor causing the plate to move
rift valley - elongated low spot created when a divergent boundary develops in continental lithosphere
rift zones - weak spots in Earth's crust where stretching is occurring creating low spots
rock - a solid composed of grains of one or more minerals or mineraloids
rock cycle - a diagram that describes the processes through which the three main types of rocks – igneous, metamorphic, and sedimentary – are created, transformed, and recycled over geological time
rock gypsum - sedimentary rock made of gypsum
rock salt - sedimentary rock made of halite
Rodinia - a supercontinent that formed and broke up in the late Proterozoic

S

San Andreas Fault System - a series of faults that mark the transform plate boundary that accommodates horizontal motion between the Pacific Plate and the North American Plate
Sauk Sequence - A group of North American sedimentary formations formed during marine transgressions in the late Proterozoic and Ordovician periods
sauropods - large, long‑necked, quadrupedal, "lizard-hipped" herbivorous dinosaurs known for their columnar limbs, elongated cervical vertebrae, and massive body size
sauropsids - the branch of amniotes that would eventually evolve into reptiles, dinosaurs, and birds
schist - a foliated metamorphic rock where minerals are aligned and visible as individual crystals
seafloor spreading - process where new oceanic crust is formed at oceanic ridges and moves away from the ridge
sedimentary basin – a low area of the Earth’s crust where sediment accumulates over time to form thick, laterally extensive sequences
sedimentary structures – features that form in sediments at or shortly after deposition, providing clues to the depositional environment (e.g. a current, alternating wet and dry environment, etc.)
sequence – a relatively conformable package of sedimentary strata bounded above and below by unconformities (erosional or non-depositional surfaces) that record a cycle of sea-level change and sediment deposition
Sevier Orogeny - a major compressional mountain‑building episode that occurred from the Late Jurassic through the Late Cretaceous (approximately 140–50 million years ago)
shale - clastic sedimentary rock composed of silt- and clay-sized particles that breaks into sheets
shear stress – a type of stress that causes parts of a rock body to slide past each other in opposite directions
shield – a large, exposed area of a craton composed of old crystalline igneous and metamorphic rocks, typically tectonically stable and deeply eroded
silica tetrahedron - a molecule composed of one silicon atom and four oxygen atoms arranged in a tetrahedron with the silicon in the middle
siliciclastic rocks – sedimentary rocks made mainly from silicate mineral fragments formed from the weathering and erosion of preexisting rocks
siltstone - clastic sedimentary rock composed of silt-sized particles
slab pull - the pulling force exerted by an oceanic plate subducting into the mantle due to its own weight
slate - a fine-grained foliated metamorphic rock which breaks into flat sheets
Snowball Earth - periods of time when glaciers were so extensive they covered most of Earth
Sonoma Orogeny - an Early Triassic (about 250–230 million years ago) mountain‑building event along the western margin of North America
Sonoman Orogeny - another episode of mountain-building on the western side of North America caused by the accretion of island arcs and other fragments of continental crust occurring during the Carboniferous and Permian periods
strain – the measure of change in a rock’s shape or volume in response to applied stress
stramatolite - layered, sedimentary rock structures formed by photosynthetic cyanobacteria trapping sediment in shallow water
strata – distinct layers of sedimentary rock or volcanic deposits that can be distinguished by composition, texture, or other features
stratigraphic column - a diagram that shows a vertical sequence of rock layers in the order they were deposited representing the rock units, their thicknesses, lithologies (rock types), and sometimes fossils or other features to illustrate the geologic history of an area
stratigraphic column - a diagram that shows a vertical sequence of rock layers in the order they were deposited representing the rock units, their thicknesses, lithologies (rock types), and sometimes fossils or other features to illustrate the geologic history of an area
stratigraphy – the branch of geology concerned with the description, correlation, and interpretation of layered rocks, including their age relationships and depositional environments
stress – the force applied to a rock per unit area, which can cause deformation
striations - scratches or grooves that result when a glacier takes a larger particle of rock and drags it against the bedrock, gouging out a scratch mark that shows the direction the glacier was flowing
strike – the compass direction of the line formed by the intersection of a rock layer or planar feature with a horizontal surface
strike-slip fault – a fault in which the main movement is horizontal and parallel to the fault’s strike, caused by shear stress
stromatolites - layered sedimentary structures formed by the growth of bacteria or algae
subduction - process by which dense oceanic lithosphere dives down beneath another tectonic plate
submarine fan – a large, fan-shaped body of sediment deposited on the deep ocean floor by turbidity currents, usually at the base of continental slopes
sulfates - minerals consisting of metal ions bonded to a sulfate ion
sulfides - minerals consisting of metal ions bonded with sulfur
supercontinent - a continent composed of a collection of smaller continental masses
supergroup – a lithostratigraphic unit that consists of two or more groups, representing an even larger-scale rock package, useful for describing very thick or regionally extensive stratigraphic successions
synapsids - a lineage of amniotes that includes mammals and their extinct "mammal-like reptile" ancestors, characterized by a specific skull structure with a single opening behind each eye
syncline – a fold in rock layers that is trough-shaped, with the youngest rocks located at the core of the fold and the limbs dipping inward toward the hinge

T

tabular bed - a broad continuous layer of sedimentary material characterized by a consistent thickness and planar bounding surfaces
Taconic Orogeny - an extensive mountain building event 440-470 million years ago caused when subduction pulled a volcanic island arc into Laurentia
tensional stress – a type of stress that pulls rocks apart, stretching and thinning the crust
tetrapods - a vertebrate animal with four limbs
texture - the size, shape, and arrangement of mineral grains, crystals, or fragments within a rock
therapsids - a major clade of tetrapods that gave rise to mammals
theropods - bipedal "lizard-hipped" dinosaurs characterized by hollow bones, three‑toed limbs
thrust fault – a low-angle reverse fault (typically dipping less than 30°) where older rocks are pushed up and over younger rocks due to strong compressional forces
tidal flat – a low-lying coastal area that is periodically flooded and exposed by tides, often composed of mud and sand
tillite - glacial deposits formed from very poorly sorted sediment; glacial diamictite
Tippecanoe Sequence - A group of North American sedimentary formations formed during marine transgressions in the Ordovician, Silurian, and Devonian periods
tonalite-trondhjemite-granodiorite (TTG) - rocks similar to granite that have been metamorphosed and are part of the Acasta Gneiss Complex
trace fossil – evidence of an organism’s activity such as footprints, burrows, feeding marks, or feces
transform boundary - plate boundary type at which the two plates slide past one another
transgression – a geologic process in which the sea level rises or land level drops, resulting in progressively deeper-water sediments being deposited over shallower-water sediments
travertine - form of limestone deposited by mineral springs
turbidite – a sedimentary deposit formed by an underwater landslide, typically showing graded bedding with coarser material at the base and finer material at the top

U

ultramafic - a composition in which there are very low (~40%) concentrations of silica; resultant rocks are typically very dark in color
unconformity - a gap in the geologic record that represents an interruption in the deposition of sedimentary layers
uniformitarianism – the principle that the geologic processes we see shaping the Earth today (such as erosion, sedimentation, and volcanism) have operated in the same way throughout Earth’s history
uniformitarianism - the recognition that the processes operating on Earth today must have been operating on Earth in the past as well

V

vertebrate - an animal with a backbone, or spinal column, and an internal skeleton
vestigial structures - anatomical features or behaviors that persist in an organism but have lost most or all of their original function through evolution
volcanic - igneous rock formed as molten material cools on the surface
volcanic island arc - a chain of volcanic islands formed on an oceanic plate as a result of the subduction of oceanic lithosphere

W

Walther’s Law – a principle stating that the vertical order of sedimentary facies in a stratigraphic sequence reflects the lateral changes that existed at the time of deposition, facies that are found adjacent to one another in space will succeed one another vertically in the rock record
weathering - breakdown of materials due to exposure to the atmosphere and water

X

xenolith – a fragment of rock enclosed within another, usually a piece of country rock trapped within an igneous intrusion such that the fragment is older than what it is trapped in

Y

yield point – the limit beyond which a rock will no longer deform elastically; permanent deformation (ductile or brittle) occurs once stress passes this point

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