Historical Geology – Exam 1 Master Review

Comprehensive Q&A flashcards covering foundational terms, principles, and examples in historical geology, mineralogy, petrology, stratigraphy, geologic time, dating methods, and paleontology—designed for efficient exam review.

What does historical geology study?

Earth’s origin, changes through deep time, and the evolution of life and landscapes.

Name four sub-disciplines within historical geology.

Paleontology, stratigraphy, geochronology, sedimentology.

Give three everyday applications of historical geology.

Understanding natural hazards, locating resources, interpreting past climate change.

Define a topographic map.

A map that shows Earth’s surface relief using contour lines of equal elevation.

What is a contour line?

A line on a map connecting points of identical elevation.

What is a geologic cross-section?

A side-view diagram that displays subsurface rock layers and structures.

How is a geologic map interpreted?

By matching rock units, faults, and folds to reconstruct geologic history in plan view.

State the steps of the scientific method.

Observation, question, hypothesis, testing/experimentation, analysis, conclusion.

Define hypothesis in science.

A testable, educated prediction based on observations.

Define a scientific theory.

A well-supported explanation of natural phenomena, validated by extensive evidence.

What is a scientific law?

A universally consistent relationship in nature, often stated mathematically (e.g., gravity).

Approximate age of Earth?

About 4.6 billion years.

Explain the concept of deep time.

Recognition of Earth’s vast geologic timescale spanning billions of years.

What is plate tectonics?

Theory that Earth’s lithosphere is broken into moving plates causing quakes, volcanoes, and mountains.

Define mineral.

Naturally occurring, inorganic solid with definite chemical composition and crystalline structure.

List six physical properties used to identify minerals.

Luster, streak, hardness, cleavage, fracture, density (plus color/crystal habit).

Which two elements dominate Earth’s crust?

Oxygen and silicon.

Most abundant mineral class in the crust?

Silicates.

Name the three main rock groups.

Igneous, sedimentary, metamorphic.

How do igneous rocks form?

By cooling and solidification of magma or lava.

How do sedimentary rocks form?

By compaction and cementation (lithification) of sediments.

How do metamorphic rocks form?

From alteration of pre-existing rock by heat, pressure, and/or fluids.

What is the rock cycle?

Continuous processes that transform rocks between igneous, sedimentary, and metamorphic forms.

Define felsic rock.

Igneous rock rich in feldspar and silica; generally light in color (e.g., granite).

Define mafic rock.

Igneous rock rich in magnesium and iron; dark-colored (e.g., basalt).

What does foliated mean in metamorphic rocks?

Texture where minerals align into layers or bands due to pressure.

Explain fractional crystallization.

Removal of early-formed crystals from magma, changing the composition of remaining melt.

Describe vesicular texture.

Igneous texture containing gas bubbles (e.g., scoria, pumice).

What is viscosity in volcanology?

Resistance of magma or lava to flow.

Define depositional basin.

A low area where sediments accumulate, commonly created by crustal subsidence.

Give two tectonic settings that create basins.

Rift zones and foreland basins at subduction zones.

What is a craton?

An ancient, stable core of continental lithosphere.

Differentiate shield and platform.

Shield = exposed part of craton; Platform = craton covered by younger sediments.

Define depositional environment.

Specific setting where sediments are laid down (e.g., beach, delta, deep marine).

What is compositional maturity in sediment?

Degree to which unstable minerals are removed; high quartz content = mature.

Define textural maturity.

Extent of sorting, rounding, and sphericity of grains; well-rounded and well-sorted = mature.

Give three common sedimentary structures.

Cross-bedding, graded bedding, mud cracks.

Explain a transgressive sequence.

Rising sea level shifts deeper-water facies landward (shale over siltstone over sandstone).

Explain a regressive sequence.

Falling sea level causes shallower facies to prograde seaward (sandstone over siltstone over shale/limestone).

Define active margin.

Plate boundary at a continent’s edge with frequent tectonic activity.

What is uniformitarianism?

Principle that present geologic processes operated similarly in the past.

Why is Siccar Point famous?

Displays an angular unconformity that demonstrated Earth’s deep time to James Hutton.

Name four pioneers of historical geology.

James Hutton, Charles Lyell, Nicholas Steno, William Smith.

State Steno’s principle of superposition.

In undeformed strata, oldest layers lie at the bottom, youngest at the top.

Steno’s principle of original horizontality?

Sediments are initially deposited in nearly horizontal layers.

Define faunal succession.

Fossil species succeed each other in a recognizable order, enabling correlation.

List the four eons from oldest to youngest.

Hadean, Archean, Proterozoic, Phanerozoic.

List the three Phanerozoic eras (oldest to youngest).

Paleozoic, Mesozoic, Cenozoic.

What ended the Cretaceous period?

A mass extinction that wiped out non-avian dinosaurs ~66 Ma.

Define relative age dating.

Determining the chronological order of events without exact numeric ages.

Give six principles used in relative age dating.

Superposition, original horizontality, cross-cutting relationships, inclusions, lateral continuity, faunal succession.

What is an unconformity?

A surface representing a gap in the geologic record due to erosion or non-deposition.

Differentiate angular, disconformity, and nonconformity.

Angular: tilted layers beneath flat ones; Disconformity: parallel layers with erosion; Nonconformity: sed. rocks over igneous/metamorphic basement.

Define absolute age dating.

Determining the numeric age of rocks/events, typically via radioactive isotopes.

Who discovered radioactivity?

Henri Becquerel.

Role of Marie Curie in geochronology?

Isolated radioactive elements and pioneered study of radioactivity.

Define parent isotope.

Original unstable radioactive atom that decays.

Define daughter isotope.

Stable product formed from radioactive decay.

What is half-life?

Time required for half of a radioactive sample to decay.

Name a mineral commonly used in U-Pb dating.

Zircon.

Why did Lord Kelvin underestimate Earth’s age?

He ignored internal heat from radioactive decay when estimating cooling rate.

What is radiocarbon dating useful for?

Dating organic material up to ~50,000 years old using carbon-14.

Define fossil.

Preserved remains, impressions, or traces of past life.

Differentiate body fossil and trace fossil.

Body fossil = actual remains (bones, shells); Trace fossil = evidence of activity (tracks, burrows).

Describe permineralization.

Minerals precipitate in pore spaces of organic remains, preserving detail.

What is a mold fossil?

Impression left in sediment after an organism decays or dissolves.

Define cast fossil.

3-D replica formed when sediment fills a mold and solidifies.

Evidence supporting evolution (name four).

Fossil record, homologous structures, genetic data, observed natural selection.

Define natural selection.

Process where individuals with advantageous traits reproduce more successfully.

Who formulated natural selection?

Charles Darwin.

Give an example of a transitional fossil.

Archaeopteryx – links dinosaurs and birds.

What is a vestigial structure?

Reduced or nonfunctional body part inherited from ancestors (e.g., human tailbone).

State the principle of lateral continuity.

Sedimentary layers extend outward until they pinch out or meet a barrier.

Explain cross-cutting relationships.

Geologic feature that cuts another is younger than the feature it cuts.

What are inclusions?

Fragments enclosed in a rock that are older than the host rock.

Define dike (geologic).

Sheet-like igneous intrusion that cuts across pre-existing layers.

What is an index fossil?

Widely distributed fossil that lived during a short time span, useful for correlation.

Name two absolute dating methods besides radiometric.

Tree-ring (dendrochronology) and ice-core layer counting.

Describe decompression melting.

Melting caused by pressure drop as hot mantle rocks rise (e.g., mid-ocean ridges).

Differentiate intrusive and extrusive igneous rocks.

Intrusive cool slowly below ground (coarse-grained); Extrusive cool quickly at surface (fine-grained).

Define phaneritic texture.

Coarse-grained igneous texture where crystals are visible to the naked eye.

Define aphanitic texture.

Fine-grained igneous texture where crystals are too small to see.

What is porphyritic texture?

Large crystals (phenocrysts) embedded in a finer groundmass.

Define pegmatite.

Extremely coarse-grained igneous rock formed from late-stage magma rich in volatiles.

What does ultramafic mean?

Igneous composition extremely rich in Mg and Fe (e.g., peridotite).

Explain magmatic differentiation.

Process creating multiple magma types from a single parent through fractionation and assimilation.

What is lithification?

Compaction and cementation turning sediment into sedimentary rock.

Define sandstone.

Clastic sedimentary rock composed mainly of sand-sized grains.

Define conglomerate.

Clastic sedimentary rock with rounded gravel-sized clasts.

Define breccia (sedimentary).

Rock with angular gravel-sized clasts; minimal transport from source.

What rock forms from calcite-rich marine organisms?

Limestone (or chalk for microscopic organisms).

Define evaporite.

Sedimentary rock formed by precipitation of minerals during evaporation (e.g., halite, gypsum).

What is graded bedding?

a sedimentary structure characterized by a gradual change in grain size within a single layer of rock, with coarser particles at the bottom and finer particles at the top

Describe cross-bedding formation.

Inclined layers formed by migration of ripples or dunes under wind/water currents.

Define mud cracks.

Polygonal cracks formed when wet, clay-rich sediment dries and contracts.

What is an alluvial fan?

Fan-shaped sediment deposit where a high-gradient stream exits mountains into a basin.

Define delta environment.

Area where a river enters standing water, depositing layered sediments.

What are turbidites?

Deep-marine deposits from submarine density currents, showing graded bedding.

Define graywacke.

Dark, poorly sorted sandstone containing angular grains in a clay matrix.

What does eolian refer to?

Wind-blown desert environment producing well-sorted sand dunes.