1. Earth and Space Sciences Unit 4
Geologic Time
Earth and Space Sciences Unit 4

2. Relative Age The chronologic sequence of geologic events
Relative age is derived from stratigraphy,
the study of rock layers and how those layers are formed

3. Four Basic Principles of Stratigraphy.
Principle of original horizontality
Strata, or sedimentary rock layers, are horizontal when they are deposited as water-laid sediment
Principle of stratigraphic superposition
Strata accumulate from the oldest on the bottom to the youngest on the top
Principle of cross-cutting relationships
A rock stratum is always older than any geologic feature that cuts across it
Principle of lateral continuity
Rock strata extend outward horizontally; they may thin or pinch out at their farthest edges, but they generally do not terminate abruptly unless cut by a younger rock unit

4. Numerical Age and Unconformities
The exact number of years of a geologic feature
Sequence of strata in any particular location is not necessarily continuous in time.
Called an unconformity
An unconformity is a break or gap in the normal stratigraphic sequence.
It usually marks a period during which sedimentation ceased and erosion removed some of the previously laid strata.
The three common types
Nonconformities
angular unconformties
disconformities

5. Correlation
The establishment of the time equivalence of strata in different places.
Fossil assemblages, usually consisting of hard shells, bones, and wood, have been the primary key to correlation of strata across long distances.
The principle of faunal and floral successions is the stratigraphic ordering of fossil assemblages.

6. Geologic Column
A composite diagram that shows the succession of all known strata, arranged in chronological order of formation, based on fossils and other age criteria.

7. Geologic Time Scale
Divided into several different units of time, called eons, eras, periods, and epochs.
The majority of Earth’s history is divided into three eons, in which fossils are very rare or nonexistent.
Hadean, Archean, and Proterozoic
The fourth and most recent eon, the Phanerozoic, is the only eon in which fossils are abundant

8. Phanerozoic Eon
Very dramatic changes in fossil assemblages occur between the three eras of the Phanerozoic eon—the Paleozoic, Mesozoic, and Cenozoic—which were separated by major extinction events.
Paleozoic Era was a time of unique diversity
known as the “Cambrian Explosion.”
Rocks formed before the Cambrian Period cannot be differentiated by the fossil record;
these rocks are considered to be Precambrian.

9. Radioactivity and Dating
Process in which an element spontaneously transforms itself into another isotope of the same element, or into a different element, through the release of particles and heat energy.
The radioactive decay of isotopes of chemical elements provides a basis for radiometric dating, which gives values for the numerical ages (age in years) of rock
Radioactive decay is not influenced by chemical processes or by heat and high pressure in Earth so it is an extremely accurate gauge of numerical age.

10. Radiometric Dating
Based on the principle that in any sample containing a radioactive isotope, half of its atoms of that isotope will change to daughter atoms in a specific length of time, called the half-life.
atom decay during a unit of time is always the same
Radioactive isotopes with a long half-life, such as uranium, are most useful for dating rocks.
Carbon-14, which has a much shorter half-life, is most useful for dating organic materials of relatively recent origin (less than 70,000 years).

11. Magnetic Polarity
Radiometric dating is primarily useful for igneous rocks
A complementary technique called magnetic polarity dating works for sedimentary rocks
Magnetic polarity dating involves paleomagnetism
the study of reversals in Earth’s magnetic field. As yet, magnetic reversals, or periods of time in which Earth’s magnetic polarity reverses itself, are not fully understood.

12. Age Most of Earth’s history took place in Precambrian time
The oldest Earth rocks discovered are about 4.0 billion years old

13. Older Rocks?
Earth is not a good place to look for the oldest rocks in the solar system.
Earth’s surface has been subjected to a lot of geologic activity.
This has reset some radiometric clocks and destroyed the earliest fragments of the crust.
Samples from the Moon and from meteorites indicate that the solar system formed about 4.56 billion years ago