Geology Definitions and Concepts
Types of Rocks
Igneous Rock
Rock formed when molten material cools and solidifies.
- Intrusive igneous rock – Forms when magma cools slowly beneath Earth’s surface. Slow cooling allows larger crystals to develop.
Examples: granite, diorite, gabbro. - Extrusive igneous rock – Forms when lava cools rapidly at or near Earth’s surface. Crystals are usually very small or may not be visible.
Examples: basalt, rhyolite, obsidian.
Sedimentary Rock
Rock formed from sediments that accumulate and become compacted and cemented together, or from minerals that precipitate from water.
- Clastic sedimentary rock – Made from fragments of previously existing rocks.
Examples: sandstone, shale, conglomerate. - Chemical sedimentary rock – Forms when dissolved minerals precipitate from water.
Examples: rock salt, gypsum. - Biochemical or organic sedimentary rock – Forms partly from the remains of organisms.
Examples: some limestones and coal.
Metamorphic Rock
Rock that has been changed by heat, pressure, fluids, or a combination of these without completely melting.
- Foliated metamorphic rock – Minerals become aligned into layers, bands, or planes because of pressure.
Examples: slate, phyllite, schist, gneiss. - Non-foliated metamorphic rock – Does not show obvious mineral alignment or layering.
Examples: marble, quartzite.
Basic Rock and Mineral Terms
Magma
Molten or partially molten rock beneath Earth’s surface.
Lava
Magma that has reached Earth’s surface.
Sediment
Loose particles of rock, minerals, or organic material that can be transported and deposited by water, wind, ice, or gravity.
Weathering
The physical or chemical breakdown of rock at or near Earth’s surface.
Erosion
The removal and transportation of weathered rock or sediment.
Deposition
The settling and accumulation of sediment after it has been transported.
Lithification
The process that turns loose sediment into sedimentary rock, usually through compaction and cementation.
Metamorphism
The alteration of existing rock by heat, pressure, or chemically active fluids without completely melting the rock.
Mineral
A naturally occurring, inorganic solid with a relatively definite chemical composition and an ordered crystal structure.
Rock
A naturally occurring solid mass made of one or more minerals, mineraloids, or organic materials.
Basic Geological Structures and Formations
Bed or Stratum
A distinct layer of sediment or sedimentary rock.
Bedding
The arrangement of sedimentary rocks into layers.
Formation
A recognizable and mappable body of rock with characteristics that distinguish it from surrounding rock units. In formal geology, a formation is an important unit used to describe and map rock sequences.
Outcrop
A place where bedrock is naturally or artificially exposed at Earth’s surface.
Bedrock
Solid rock beneath soil, sediment, or other loose surface material.
Fault
A fracture in Earth’s crust along which movement has occurred.
- Normal fault – The hanging wall moves downward relative to the footwall, usually because the crust is being stretched.
- Reverse fault – The hanging wall moves upward relative to the footwall, usually because the crust is being compressed.
- Thrust fault – A low-angle reverse fault.
- Strike-slip fault – Rock masses move mainly horizontally past one another.
Fold
A bend or curve in originally flat or layered rocks caused mainly by deformation.
- Anticline – An upward-arching fold. Older rocks are generally found toward the center.
- Syncline – A downward-curving fold. Younger rocks are generally found toward the center.
- Monocline – A step-like bend in otherwise relatively horizontal rock layers.
Joint
A fracture in rock where little or no movement has occurred.
Unconformity
A surface representing a gap in the geological record caused by erosion or a period when sediment was not deposited.
- Angular unconformity – Younger layers lie across older tilted or folded layers.
- Disconformity – An erosional or non-depositional surface between roughly parallel sedimentary layers.
- Nonconformity – Sedimentary rocks lie above older igneous or metamorphic rocks.
Contact
The boundary between two different rock units.
Intrusion
A body of igneous rock formed when magma enters existing rock and solidifies underground.
Dike
A sheet-like igneous intrusion that cuts across existing rock layers or structures.
Sill
A sheet-like igneous intrusion that generally runs parallel to existing rock layers.
Pluton
A large body of intrusive igneous rock that crystallized underground.
Batholith
A very large intrusive igneous body, commonly composed of granite or similar rocks.
Vein
A fracture or opening in rock that has been filled with minerals deposited from fluids.
Common Landforms and Depositional Features
Alluvial Deposit
Sediment deposited by flowing water, especially rivers and streams.
Alluvial Fan
A fan-shaped deposit of sediment formed where a stream leaves a steep valley and enters flatter terrain.
Floodplain
A relatively flat area beside a river that is periodically flooded and covered by sediment.
Delta
A deposit of sediment that forms where a river enters a lake, sea, or other relatively still body of water.
Terrace
A relatively flat, elevated surface along a valley that often represents an older river or floodplain level.
Talus
An accumulation of broken rock fragments at the base of a cliff or steep slope.
Glacial Moraine
A ridge or accumulation of sediment deposited directly by a glacier.
Karst
A landscape developed mainly through the dissolution of soluble rocks such as limestone. Karst areas commonly contain sinkholes, caves, and underground drainage.
Sinkhole
A depression or opening in the ground caused by collapse or dissolution of underlying rock, particularly limestone.
Larger Geological Features
Basin
A broad area in which rock layers dip toward the center. Basins often accumulate thick sequences of sediment.
Dome
A geological structure in which rock layers arch upward and dip away from the center.
Mountain Belt
A large region of mountains produced by tectonic processes such as continental collision, faulting, and volcanic activity.
Rift
A region where Earth’s crust is being stretched and pulled apart.
Rift Valley
A long depression formed as blocks of crust drop downward between faults during extension.
Orogeny
A major episode of mountain building caused by tectonic activity.
Craton
An ancient, relatively stable portion of continental crust.
Terrane
A block of crust with a geological history that differs from neighboring rocks and was later joined to another landmass through plate tectonic processes.
Physiographic Province
A geographic region characterized by similar landforms, underlying geology, and geological history.
Fall Line
A boundary between an upland region of harder rocks and a lower coastal plain of softer sediments. Rivers commonly develop rapids or waterfalls where they cross this boundary.
Useful Geologic Time Terms
Relative Dating
Determining whether a rock or geological event is older or younger than another without assigning an exact numerical age.
Absolute or Numerical Dating
Determining an approximate age in years, commonly using radioactive isotopes.
Principle of Superposition
In an undisturbed sequence of sedimentary rocks, the oldest layers are generally at the bottom and the youngest are at the top.
Original Horizontality
Sediments are generally deposited in approximately horizontal layers. Tilted or folded layers were usually deformed after deposition.
Cross-Cutting Relationships
A fault, intrusion, or other feature that cuts through another rock unit is younger than the rock it cuts.
Index Fossil
A fossil from an organism that lived during a relatively short period of geological time and was geographically widespread, making it useful for correlating and dating rock layers.