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The key definitions in Senior's guidelines.


The largest of these time units and encompasses the others


Usually tens to hundreds of millions of years in duration


Generally tens of million of years in duration


Generally hundreds of thousands of millions of years in duration


One of the major beliefs of how the earth changed-
States that geologic processes occurring today have been occurring since Earth formed.


One of the major beliefs of how the earth changed-
James Ussher's proposed theory that earth is only 6,000 years old.

Original Horizontality

The principle that sedimentary rocks are deposited in horizontal or nearly horizontal layers


The principle that in an undisturbed rock sequence, the oldest rocks are at the bottom and each consecutive layer is younger than the layer beneath

Cross-Cutting Relationship

States that an intrusion is younger than the rock its cut across


Erosional events that took place in the past

Angular Unconformity

Affects several layers and involves tilting

Folded Strata

Layers that are wavy or tilted


Erosional events where at least one of the layers involved is not sedimentary


Erosional events between two layers of sedimentary rock (i.e. limestone, shale)


Layers being lifted upward

Radioactive Decay

The emission of radioactive particles and the resulting change into other isotopes over time

Radiometric Dating

When scientists date an object using radioactive isotopes.


A radioactive isotope

Stable Daughter Element

A nonradioactive isotope

Parent Element

Starting with Uranuim 238 to any isotope besides Lead-206


The basic unit of matter


A tiny particle that has a mass and a positive electric charge.


A particle with approximately the same mass as a proton, but it is electronically neutral.


One of two or more forums of an element with differing numbers of neutrons.

Relative Age

The study of the order in which geologic events occurred


In the correct order in which the geologic events occurred

Absolute Age

Enables scientists to determine the numerical age of rocks and other objects


Always younger than the materials it affects (examples include plutons, dikes, sills, batholiths)


Always older than the material they are included in


Intrusive rocks that form underground under conditions of high heat

Uranium 238

A radioactive unstable parent isotope.
Approx. half life= 4.5 billion years

Lead 206

A non-radioactive stable daughter isotope

Radon 222

An isotope that is formed between Uranium238 and Lead 206. Approx. half life= 3.8 days

Half Life

The process when scientists measure the length of time it takes for one half of the original isotope to decay

Radioactive Decay

The spontaneous loss of particles from some isotopes of some elements which are unstable
- Is constant, unchanged by physical or chemical variables
- Is characteristic for radioactive isotope

Carbon 14

Radioactive unstable isotope
Approx. half life= 5,730 days

Carbon 12

Non-radioactive stable isotope


The process in which materials on or near Earth's surface break down and change.

Erosional Surface

A surface where erosion has occurred in the pass (in a diagram, shown as a squiggly line)

Geologic Time Scale

A record of Earth's history from its origin 4.6 billions years ago to the present


The three eons that make up 90 percent of geologic time

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