Earth History: Relative Time and Absolute Time

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  • Topic: Geology, Geologic time scale, Fossil
  • Pages : 5 (750 words )
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  • Published : January 31, 2013
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Geology 1203 Earth history

Assignment: relative time
Preamble:
Understanding Earth history involves determining the sequence of geologic events which occurred over immense spans of time. The geological time scale that we use today was first developed in the 1800’s and is constantly being revised as our understanding improves. Geologists deal with time in two different ways:

Relative Time - the occurrence of events is determined relative to one another (from oldest to most recent). This can be a local series of events or be applied to global events. The eras and periods of the geological time column were established using relative time (mostly based upon the fossil content).

Absolute Time - The discovery of radiometric dating allowed geologists to assign a numerical age to the formation of certain rocks. Fixing how old they actually were in billions (Ga), millions (Ma) or thousands (Ka) of years before the present. Hence the term “absolute” time. These two ways of looking at time are not unique to geologists. You use them everyday. If you agree to meet a friend “after class” - that is a relative time. The time of day when you actually meet is dependent upon when your class is scheduled and when it is finished. If instead you arrange to meet at 1:00pm -that is absolute time.

Part A: Creating A Personal Time Column
In this exercise you are going to create a time column of your own. Begin by creating a relative time column with key events. Then you will assign years to these events fixing them in absolute time. To give you an idea how to proceed I have provided a template with the starting and ending events you should use. Remember that when ordering events you must start with the oldest on the bottom (when listing vertical) or on the left (when listing horizontally). Example:

You should start with your birth and end with you taking this course. You may include whatever events you wish in between but you must have at least ten more events.

Event

Year

you enrolled in GEOL 1203

2011
Youngest



Oldest
you were born

?

Part B: Applying the Principles of Relative Time
In most cases the correct order of events can be determined without knowing their actual ages simply by establishing that event B occurred before event C, but after event A. The relative order of geologic events can be established by applying the Principles of Relative Time. The Principle of Original Horizontality:

sedimentary rocks are deposited as horizontal or nearly horizontal layers. Any marked deviation from horizontality indicates that some movement or deformation of the Earth’s crust occurred after deposition of the inclined layer.

The Principle of Superposition:
In an ordinary vertical sequence of sedimentary rocks, the layer at the bottom of the sequence is oldest, and successively higher layers are successively younger. The Principle of Crosscutting Relationships:

Geologic features such as faults, unconformities and igneous intrusions, which cut through rocks, must be younger than the rocks through which they cut.
The Principle of Inclusions:
Rocks or rock fragments that are included within another rock layer must be older than the layer in which they are included.
The Principle of Faunal Succession:
Fossils succeed one another in a definite and determinable order.

Example:
The sequence of events for this typical block diagram are listed from oldest to youngest. e

Events

Youngest



d

4. Deposition of strata b c d e
3. Erosion - creating an angular unconformity
2. Tilting or deformation
1. Deposition of strata a
Oldest

c
b

a
unconformity

Apply the principles of relative time to reconstruct the geological history of the following block diagrams.
Note: in block diagrams different types of rocks or strata are illustrated by different patterns or colours. In the following exercises the strata and structures are also indicated by letters. Unconformities are indicated by wavy lines....
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