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41 terms

Chapter 21- Geologic Time

The key definitions in Senior's guidelines.
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Eon
The largest of these time units and encompasses the others
Era
Usually tens to hundreds of millions of years in duration
Period
Generally tens of million of years in duration
Epoch
Generally hundreds of thousands of millions of years in duration
Uniformitarianism
One of the major beliefs of how the earth changed-
States that geologic processes occurring today have been occurring since Earth formed.
Catastrophism
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
Superposition
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
Unconformity
Erosional events that took place in the past
Angular Unconformity
Affects several layers and involves tilting
Folded Strata
Layers that are wavy or tilted
Nonconformity
Erosional events where at least one of the layers involved is not sedimentary
Disconformity
Erosional events between two layers of sedimentary rock (i.e. limestone, shale)
Uplifting
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.
Unstable
A radioactive isotope
Stable Daughter Element
A nonradioactive isotope
Parent Element
Starting with Uranuim 238 to any isotope besides Lead-206
Atom
The basic unit of matter
Protons
A tiny particle that has a mass and a positive electric charge.
Neutrons
A particle with approximately the same mass as a proton, but it is electronically neutral.
Isotopes
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
Sequence
In the correct order in which the geologic events occurred
Absolute Age
Enables scientists to determine the numerical age of rocks and other objects
Intrusions
Always younger than the materials it affects (examples include plutons, dikes, sills, batholiths)
Inclusions
Always older than the material they are included in
Plutons
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
Weathering
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
Precambrian
The three eons that make up 90 percent of geologic time