Chapter 4 Geologic Time

Flashcards covering key vocabulary terms related to Geologic Time, the history of Earth's age determination, fundamental principles of relative dating, types of unconformities, and absolute dating using radioactivity.

Sir Thomas Browne (17th Century)

Believed Earth's history was short, 'but 5 days older than ourselves.'

James Ussher (17th Century)

Calculated Earth's creation to be on Sunday, October 23, 4004 BC.

Nicolaus Steno (1669)

Observed 'solids within solids' in rocks, identifying loose grains entombed in rock and 'glossopetrae' (tongue stones) as animal remains.

James Hutton

Known as the 'Father of Geology,' he proposed uniformitarianism and the concept 'The present is the key to the past.'

Uniformitarianism

The principle that Earth processes we observe today (e.g., erosion) also occurred in the past and over long periods.

Charles Lyell

Published 'Principles of Geology' in 1830, illustrating the idea of uniformitarianism and was a friend of Charles Darwin.

Original Horizontality

Sediments are usually laid down in nearly horizontal layers, so tilted or folded layers were originally flat.

Superposition

In undisturbed sedimentary sequences, the oldest layers must be on the bottom, and layers get younger from bottom to top.

Cross-cutting Relationships

Features (like intrusions or faults) that cut across a rock body must be younger than the rock body they cut.

Law of Inclusions

Clasts (fragments) contained within a rock body must be older than the rock body itself.

Unconformity

A discontinuity in the rock record representing a time of erosion or non-deposition; a gap in time.

Angular Unconformity

An unconformity where the contact separates overlying younger layers from eroded, tilted, or folded older layers.

Disconformity

An unconformity where the contact representing missing rock layers separates beds that are parallel to each other.

Nonconformity

An unconformity where an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock, usually representing a large gap in the geologic record.

What unconformities mean

Represent a long period of non-deposition and/or erosion, indicating the sediment surface was above sea level due to sea level drop or ground uplift.

Relative Age Dating

Allows scientists to recognize discrete geological events and order them in time without assigning specific numerical dates.

Numerical Dating (Absolute Dating)

Allows scientists to determine a numerical age for geological events, typically using radiometric methods.

Radioactivity

The propensity of certain atomic nuclei to naturally disintegrate and emit energy and/or subatomic particles.

Proton

A subatomic particle with an atomic mass of 1 and a positive (+) charge, defining the element.

Neutron

A subatomic particle with an atomic mass of 1 and no charge.

Electron

A subatomic particle with negligible mass and a negative (-) charge.

Atomic Number

The number of protons in an atom, which determines the element.

Atomic Mass

The sum of the number of protons and neutrons in an atom's nucleus.

Isotopes

Atoms of the same element (same number of protons) but with a different number of neutrons.

Radioactive Isotopes

Unstable isotopes whose nuclei decay, producing daughter products and releasing energy as heat.

Parent Isotope

The original unstable radioactive isotope that undergoes decay.

Daughter Product/Isotope

The stable (or sometimes radioactive) element formed from the decay of a parent isotope.

Half-Life

The fixed and constant time it takes for half of the parent atoms in a radioactive sample to decay to daughter atoms.

Dating Accomplishment

Determining the age of a sample by measuring the parent-daughter ratio with a mass spectrometer and knowing the half-life of the parent isotope, ideally in igneous rocks.

Age of Earth

Determined by radiometric dating of meteorites, which formed at the same time as our planet, revealing an age of approximately 4.6 billion years.

Geologic Time Scale

A chronological system of relative and absolute dating techniques that categorizes Earth's history into eons, eras, periods, and epochs.