Geology: Dating, Time Scale, and Fossils

What is relative dating in geology?

The process of determining the relative chronological order of geological events without knowing their absolute numerical ages.

What is absolute dating in geology?

The process of determining an approximate numerical age of a rock or fossil, often using radiometric methods.

What is the Principle of Superposition?

In undeformed sedimentary rock layers, older layers are at the bottom and younger layers are at the top.

What is the Principle of Original Horizontality?

Sedimentary layers are deposited in horizontal or nearly horizontal sheets due to gravity.

What is the Principle of Lateral Continuity?

Sedimentary layers extend outward in all directions until they thin out or terminate against the edge of a depositional basin.

What is the Principle of Cross-Cutting Relationships?

A fault or intrusive igneous body is younger than the rock layers it cuts through.

What are inclusions in geology?

Pieces of one rock contained within another; the rock containing the inclusions must be younger than the inclusions themselves.

What is the Principle of Faunal Succession?

Fossil organisms succeed one another in a definite and determinable order, allowing rock layers to be correlated globally.

What is radioactive decay?

The spontaneous breakdown of an unstable atomic nucleus into a more stable form, releasing radiation.

What is a parent isotope in radiometric dating?

The unstable radioactive isotope that decays over time into a daughter product.

What is a daughter product in radiometric dating?

The stable isotope resulting from the radioactive decay of a parent isotope.

What is a half-life in radiometric dating?

The time required for one-half of the atoms of a radioactive parent isotope to decay into its daughter product.

What are examples of radioactive isotopes used in dating?

Uranium-Lead, Potassium-Argon, and Rubidium-Strontium.

What is Carbon-14 dating used for?

Dating organic materials up to about 50,000 to 60,000 years old.

What is an unconformity in geology?

A surface of erosion or non-deposition that represents a significant gap in the geologic record.

What are the primary causes of unconformities?

Tectonic uplift, erosion, and subsequent subsidence or renewed deposition.

What is an angular unconformity?

An unconformity where tilted or folded sedimentary rocks are overlain by younger, more flat-lying strata.

What is a disconformity?

An unconformity between parallel layers of sedimentary rocks representing a period of erosion or non-deposition.

What is a nonconformity?

An unconformity where younger sedimentary rocks rest directly upon older eroded igneous or metamorphic rocks.

What is an eon in the geologic time scale?

The largest division of geologic time, comprising multiple eras.

What is an era in the geologic time scale?

A major division of geological time, being a subdivision of an eon, characterized by major changes in life forms.

What is a period in the geologic time scale?

A subdivision of an era, characterized by specific fossil types and geological events.

What is an epoch in the geologic time scale?

A finer subdivision of a geological period.

What is the Hadean eon?

The earliest eon in Earth's history, marked by Earth's formation and a molten surface.

What is the Archean eon?

The eon when Earth's crust cooled, continents formed, and the earliest single-celled life emerged.

What is the Proterozoic eon?

The eon preceding the Phanerozoic, marked by the buildup of atmospheric oxygen and multicellular life.

What is the Phanerozoic eon?

The current eon, spanning from 541 million years ago to the present, characterized by abundant complex life.

What is the Paleozoic era?

An era of the Phanerozoic eon known for the explosion of marine life and the colonization of land.

What is the Mesozoic era?

The era known as the Age of Reptiles, dominated by dinosaurs and ending with a mass extinction.

What is the Cenozoic era?

The current era, known as the Age of Mammals, following the extinction of the non-avian dinosaurs.

What is a fossil?

The preserved remains or traces of ancient organisms from a past geologic age.

Why are fossils important in geology?

They provide evidence of past life, climate, and environments, and help correlate rock layers.

What are body fossils?

Fossils consisting of the preserved hard or soft parts of an organism, such as bones, shells, or teeth.

What are trace fossils?

Evidence of biological activity rather than the organism's body, such as footprints, burrows, and coprolites.

What are fossil imprints?

Impressions left by an organism in soft sediment, such as leaf or feather impressions.

What are index fossils?

Fossils of widely distributed organisms that lived during a short geologic time span, used to date rock layers.

What are the characteristics of a good index fossil?

Widespread geographical distribution, short temporal range, and abundant preservation.

What is fossil correlation?

Using matching fossils in different rock layers to establish their relative ages and equivalence across distant locations.

What is a half-life?

length of time required for half of the radioactive atoms in a sample to decay