bedrockUnbroken rock found by digging through the soil. The loose cover of soil and broken rock above it is called regolith. The top of the bedrock is called rockhead.
erosionThe hauling off of rock that weathering has loosened. Weathering breaks rock down where it stands, with no movement. Flowing water and wind are two of the things that then carry the pieces off.
faultA crack along which blocks of rock have slipped. Sudden slips on faults cause most earthquakes. Some faults creep slowly instead.
foldA wave-shaped bend squeezed into layered rock. Folds range from tiny crinkles to bends the size of mountains. The layers were flat when they were laid down.
fossilEvidence of ancient life found in a rock layer. Fossils let geologists match layers of the same age in places far apart. Nearly all are found in sedimentary rock.
geologistA scientist who maps, samples and dates rocks. Geologists work in the field and in the laboratory. Many work in mining and energy, and others watch for earthquakes, eruptions and landslides.
Show all 36 terms (30 more)
Geology basics, continued
geologyThe science of the solid Earth and its history. The name joins Greek words for earth and study. Geologists also study the rocks of other planets and moons.
magmaThe underground melt that feeds volcanoes. It can hold crystals and gas bubbles as well as liquid. Every igneous rock began as magma.
mineralA natural solid whose atoms sit in a set pattern. Quartz and calcite are two common ones. Each kind has its own chemical makeup and its own crystal structure.
outcropA spot where rock pokes out through soil and plants. Outcrops are most common on steep hillsides, mountain ridges and river banks.
sedimentLoose sand, silt or mud before it hardens to rock. Sand and silt are sediment. Buried deep enough, they can become sandstone and siltstone.
strataBands of rock, each one a layer of old sediment. One layer is a stratum. Each is set off from its neighbors by a visible surface called a bedding plane.
Rock layers and time 12
core sampleA rod of rock drilled out with a hollow bit. Cores are also taken from lake beds, the sea floor and ice. Their layers record how climate has changed.
eonA division of time made up of several eras. There are four: the Hadean, the Archean, the Proterozoic and the Phanerozoic, which is the one we live in.
epochA division of a period, such as the Holocene. There are 37 defined epochs. The Holocene is the one we live in.
eraA division of an eon, such as the Mesozoic. Paleozoic, Mesozoic and Cenozoic are the three eras of the Phanerozoic eon.
formationA rock unit distinct enough to trace on a map. A formation is usually named after a place near where it was first described. Its thickness can vary widely.
geologic time scaleEarth's history laid out as a chart of named spans. It covers about 4.54 billion years. Its divisions are settled by international committees.
periodA unit such as the Jurassic; an era holds several. The Mesozoic era is divided into the Triassic, Jurassic and Cretaceous periods. The period is the basic unit of the time scale.
radiometric datingFinding a rock's age in years from decayed atoms. It compares how much of a radioactive element is left with how much of its decay product has built up. Uranium turning to lead is one such clock.
relative datingPlacing rocks in order of age without numbers. Fossils help match the layers of one place with those of another. It was the only kind of dating until the early 1900s.
superpositionThe rule that lower layers were laid down first. It holds wherever the layers have not been overturned. Nicolas Steno first proposed it in 1669.
unconformityAn old eroded surface buried under younger rock. It marks a span of time that left no rock behind. James Hutton studied a famous one at Siccar Point in Scotland in 1788.
uniformitarianismExplaining old rocks by processes still at work. Geologists sum it up by calling the present the key to the past. Charles Lyell made the idea widely known.
Folds, intrusions and maps 12
anticlineAn arch-shaped fold, oldest rock at its core. The sides of the fold slope away from its crest. About 80 percent of the petroleum found so far was trapped under anticlines.
aquiferWater-bearing rock that can supply a well. The water fills pores and cracks in the rock. An aquifer is not an underground lake or river.
batholithA giant mass of rock that cooled deep underground. By definition it covers more than 100 square kilometers. Half Dome in Yosemite National Park is part of the Sierra Nevada Batholith.
dikeA wall of once-molten rock cutting across layers. It forms when magma fills a crack and hardens. Near Shiprock in New Mexico a dike stands up where softer rock around it has worn away.
dipThe angle a rock layer slopes down from level. It is measured at right angles to the strike, with a tool called a clinometer.
geologic mapA map colored to show which rock lies where. Each color or symbol stands for one rock unit. Other symbols mark faults, folds and the tilt of the beds.
jointA crack in rock with no slipping along it. Joints seldom come alone. Parallel joints at even spacing make up what is called a joint set.
karstLand shaped by water dissolving the rock beneath. It forms on limestone and other rocks that water can dissolve. Its marks are sinkholes, caves and drainage that runs underground.
sillOnce-molten rock that spread flat between layers. It lies parallel to the beds it pushed between. The Palisades Sill, in New Jersey and New York, is a well-known example.
strikeThe compass direction a tilted layer runs in. It is read by holding a compass level against the rock. It is the direction of a level line drawn across the tilted surface.
synclineA trough-shaped fold, youngest rock at its core. It is an anticline turned upside down. A large one that carries smaller folds is called a synclinorium.
veinMinerals left by hot water in a crack in rock. The minerals come out of solution and grow as crystals on the walls of the crack. White quartz veins show up clearly in dark rock.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Geology, Geologist, Mineral, Stratum, Fault (geology), Erosion, Fossil, Magma, Sediment, Bedrock, Outcrop, Fold (geology), Law of superposition, Principle of original horizontality, Cross-cutting relationships, Unconformity, Hutton's Unconformity, Relative dating, Radiometric dating, Geologic time scale, Mesozoic, Holocene, Uniformitarianism, Geological formation, Core sample, Anticline, Syncline, Joint (geology), Dike (geology), Sill (geology), Igneous intrusion, Batholith, Vein (geology), Karst, Aquifer, Geological map and Strike and dip. Geology, mineral, strata, fault, erosion, fossil, magma, sediment, bedrock, outcrop, fold, law of superposition, unconformity, relative dating, radiometric age, period, uniformitarianism, formation, anticline, syncline, joint, dike, sill, batholith, karst, aquifer, strike, dip, and normal, reverse and strike-slip fault were checked a second time against the Glossary of Geologic Terms of the U.S. National Park Service (https://www.nps.gov/subjects/geology/gri-glossary-of-geologic-terms.htm). The four eons, the three eras of the Phanerozoic and their dates are from Geologic Time Scale of the National Park Service (https://www.nps.gov/subjects/geology/time-scale.htm). Superposition, original horizontality and the two kinds of dating were also read in Geologic Time (https://pubs.usgs.gov/gip/geotime/) and Fossils, Rocks, and Time (https://pubs.usgs.gov/gip/fossils/) of the U.S. Geological Survey, and the water in an aquifer in its Water Science School (https://www.usgs.gov/special-topics/water-science-school/science/aquifers-and-groundwater). That fossils are found mostly in sedimentary rock and are used to match layers is from Fossils of the British Geological Survey (https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/fossils/). The fact after each meaning is from the same pages. Licence: own work
How it works
This geology vocabulary worksheet lines up mineral, strata, fault, erosion, fossil, magma, sediment and five more terms beside their meanings in another order, with an answer key as the last page. It has no map or diagram to label.
Read the full guide
Rock layers and time
Set opens on Geology basics. The second set has superposition, unconformity, relative dating, radiometric dating, geologic time scale, eon, era, period, epoch, uniformitarianism, formation and core sample.
Folds, intrusions and maps
The third set has anticline, syncline, joint, dike, sill, batholith, vein, karst, aquifer, geologic map, strike and dip.
The three kinds of rock and the tests for minerals are on the rock cycle vocabulary worksheet.
Questions and answers
What are the three types of faults?
Normal, reverse and strike-slip. In a normal fault the block above the break moves down. In a reverse fault it moves up. In a strike-slip fault the two sides slide past each other sideways.
What are the principles of relative dating?
Three are taught first. Lower layers are older. Layers are laid down flat, so tilted ones were moved later. A fault or a dike that cuts through rock is younger than the rock it cuts.
How long are eons, eras, periods and epochs?
They have no fixed length. The Phanerozoic eon has lasted about 540 million years so far and the Mesozoic era lasted about 186 million, while the Holocene epoch began only about 11,700 years ago.