Clocks in Rocks: Timing the Geologic Record

These flashcards cover essential concepts in the study of geological time scales, dating methods, and principles of stratigraphy.

Geologic Time Scale

A system of chronological dating that relates geological strata to time.

Relative Age

The age of a rock or fossil in comparison to other rocks or fossils.

Absolute Age

The numeric age of a geological feature or event, often expressed in years.

Isotopic Dating

A method of dating geological materials by measuring the ratios of isotopes within them.

Principle of Original Horizontality

Layers of sediment are originally deposited horizontally under the action of gravity.

Law of Superposition

In an undeformed sequence of sedimentary rocks, the oldest rocks are at the bottom.

Faunal Succession

The principle that fossil organisms succeed one another in a definite, recognizable order.

Unconformity

A surface of contact between two bodies of sedimentary rock that has different ages.

Disconformity

An unconformity where layers above and below the surface are parallel.

Angular Unconformity

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers.

Cross-Cutting Relationships

The principle that an igneous intrusion or fault is younger than the rocks it cuts through.

Half-Life

The time required for half of a sample of a radioactive substance to decay.

Parent Isotope

The original radioactive isotope before decay.

Daughter Isotope

The product of the decay of a parent isotope.

Uranium-Lead Dating

A radiometric dating method using the decay of uranium isotopes to lead.

Potassium-Argon Dating

A radiometric dating method used to determine the age of rocks by measuring the ratio of potassium to argon.

Rubidium-Strontium Dating

A radiometric dating method that uses the decay of rubidium to strontium.

Carbon-14 Dating

A method of dating organic materials using the radioactive decay of carbon-14.

Paleozoic Era

An era in geologic time that lasted from about 541 to 252 million years ago.

Mesozoic Era

An era in geologic time approximately 252 to 66 million years ago, known for the dominance of dinosaurs.

Cenozoic Era

The current geological era, beginning 66 million years ago, marked by the dominance of mammals and birds.

Hadean Eon

The earliest eon in Earth's history, from about 4.6 to 4.0 billion years ago.

Archean Eon

The eon of Earth's history from about 4.0 to 2.5 billion years ago.

Proterozoic Eon

The eon spanning from 2.5 billion to 541 million years ago.

Phanerozoic Eon

The current eon, which began 541 million years ago and is characterized by the presence of abundant animal and plant life.

Radiometric Dating

A technique used to date materials such as rocks or carbon, based on the known decay rates of radioactive isotopes.

Geologic Column

A representation of the vertical sequence of rock layers.

Sequence Stratigraphy

The branch of geology that deals with the stratigraphic sequences of sedimentary rocks.

Chemical Stratigraphy

A stratigraphic technique that uses the chemical properties of rocks to correlate layers.

Paleomagnetic Stratigraphy

A method of determining the age of rocks based on the record of Earth's magnetic field in sediments.

Mass Extinction

A rapid increase in extinction rates of a large number of species in a short period, often linked to significant geological or environmental changes.

Precambrian Period

The span of time in Earth's history before the Cambrian period, encompassing about 4 billion years.

Tectonic Forces

Forces that shape the Earth's structure through movements of the lithosphere.

Erosion

The process by which rock, soil, and other surface materials are worn away and transported.

Sedimentary Rock

Rock formed by the accumulation and compaction of mineral and organic particles.

Fossils

Preserved remains or traces of ancient organisms found in sedimentary rocks.

Geologic Processes

Natural processes that shape and change the Earth's crust over time.

Stratigraphic Record

The observable layers of rock and sediment that provide a record of geological history.