1. Timing the Geologic Record
Clocks in Rocks
Timing the Geologic Record

2. Standards
Examine the scientific processes and logic used in investigations of past events
Know that some atomic nuclei can change, including:
Spontaneous decay
Half-life of isotopes
Fission
Fusion

3. Standards continued
Understand the changes in Earth’s past and the investigative methods used to determine geologic time, including:
Rock sequences, relative dating, fossil correlation & radiometric dating
Geologic time scales, historic changes in life forms, & the evidence for absolute ages (e.g., radiometric methods)
Describe the uses of radioactivity (e.g., radiometric dating)

4. Deep Time
One of the greatest geological discoveries was the immensity of time (how very, very long the Earth has been in existence).
When we talk about time on the scale of billions of years, we are dealing with a concept called deep time – a passage of time so immense that it’s hard for us to grasp.

5. Timescales of Geologic Processes
Range from seconds: meteorite impacts, volcanic explosions, earthquakes
To tens of millions of years: life cycle of oceanic lithosphere
To billions of years: tectonic evolution of continents

6. Timescales of Geologic Processes
We can directly measure short-term processes.
The historical record gives us geologic data for hundreds to thousands of years.
For longer timescales, we must rely on the geologic record – the information preserved in rocks

7. Relative vs. Absolute Ages
There are two ways to date rocks:
Relative dating – tells whether something is older or younger than something else (ages of events are placed in order of occurrence).
No exact date is identified.
ex: Crystal is the 2nd child in a family with 3 children.
2. Absolute dating – gives an actual age of something. An exact date is identified.
ex: Crystal is 16 years old.

8. Using Relative Time to Determine Age
1st came about from the study of fossils - an artifact of life preserved in the geologic record.
Study of fossils led to science of stratigraphy – the study of strata (layers) in rocks.
A set of strata that is chronologically ordered is called a stratigraphic succession.
Stratigraphic principles are used to relatively date strata (i.e., put them in chronological order)

9. Principles of Stratigraphy
Principle of original horizontality
– sediments are deposited under the influence of gravity as nearly horizontal beds.
If we find folded or faulted strata, we know that the layers were deformed after deposition.

10. Principles of Stratigraphy
2. Principle of superposition
– each sedimentary layer in an undisturbed sequence is younger than the one beneath it & older than the one above it.
Strata can be ordered in time from oldest (lowest layer) to youngest (highest layer).

12. In these pictures, which layers of rock are oldest?

13. Principles of Stratigraphy
3. Principle of faunal succession – layers of sedimentary rocks in an outcrop contain fossils in a definite chronological sequence (life changed through time & this change is recorded by the fossils).
The same sequence of fossils can be found in rocks at other locations, & so strata in one location can be matched to the same strata in another location & be determined to be the same age.
i.e., strata with the same fossils are the same age.

14. Principle of Faunal Succession

15. Principles of Stratigraphy
Other disturbances in the layering of sedimentary rocks can provide clues to help determine relative age.
4. Principle of cross-cutting relationships - an igneous intrusion or fault is always younger than the rock it cuts across (the rock has to already exist for the fault or intrusion to cut through it).

16. Cross-Cutting Relationships: Magma

17. Cross-Cutting Relationships: Faults

18. Formations
Outcrops of rock (areas where rock is exposed) are made up of formations – groups of rock layers that can be identified throughout a region by their physical properties.
Ex: the Madera Limestone is a formation that can be found through much of the southwest.
Formations can be one rock type or inter-layered beds of different rock types.

19. Unconformities
In putting together stratigraphic successions of a region, places are often found where formations are missing (remember the great unconformity in the Sandias).
The missing rock was either never deposited &/or was eroded away.
The surface between the two formations where the rock is missing is called an unconformity.

20. Unconformities
There are three types of unconformities that are classified according to rocks above and below them.
Disconformity – upper layers of rock overlie an erosional surface on undeformed, still-horizontal lower layers of rock.
All layers of rock are horizontal, but there is rock missing between the layers.

21. Disconformity

22. Unconformities
2. Nonconformity – the upper layers overlie igneous or metamorphic rock.
Ex: the Madera Limestone overlies the Sandia Granite. Granite is an igneous rock, so the Great Unconformity is a nonconformity.

23. Nonconformity in the Grand Canyon
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