Deep Time - the geologic time scale

In fact, many geologists find that it is easier to think about numbers (ages) like millions of years relative to the total age of the Earth or the length of geologic time. To understand the time since the Earth's inception, we need to put it into perspective that we can understand. To do this we often use analogies for the length of time rather than the absolute value of that time. Here are a few examples in which significant events in Earth history are plotted relative to some known (and easily comprehended) quantity or object:

• The roll of toilet paper: Imagine that all of geologic time is represented by a 1,000 sheet roll of toilet paper (Scott tissue works well) - each sheet is 4,500,000 years. Unroll the toilet paper and mark off significant events in the history of the Earth. (This can be a delicate operation but if the paper rips, tape can be used to fix it.)


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• Trip across the country: Imagine that all 4.5 billion years of geologic time are represented by a roadtrip from Washington, DC, to Seattle - about 4500 km. Mark out specific time periods in geologic time along your route across the country.

• The football field: Imagine that all 4.5 billion years of geologic time are represented by the length of a football field. Mark out specific time periods in geologic time along its length.

• The calendar year: Imagine that the deep time of Earth's history is compressed into one year. Calculate (to the fraction of a second) when significant events in geologic history happened. Assume that January 1 is the formation of Earth and milliseconds before midnight on December 31 is present day.

• The yard/meter stick: Imagine that geologic time is represented by the length of a yard, or better yet a meter, stick. Make a scale model of the geologic time scale to within fractions of inches or centimeters.

• The beer glass: Imagine that geologic time is represented in the volume of a beer glass (it works well to imagine Guinness in a 20 ounce imperial pint - each ounce represents 230,000,000 years). Calculate the volume (this can also be done by %) of beer that represents time before present to significant geologic events.

• The clothes line: Stretch out a clothes line of known length. Have students calculate the where they should place a clothespin on the clothesline for a specific geologic event.

• Length of the hallway: With a measuring tape, have students measure the hallway outside the classroom as the geologic time scale with one end representing present day and the other end, Earth's birth. Give each student a geologic event (with its age) and have them stand at the appropriate distance from "present day".

Relative time/age

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• My mother's birth
• My maternal grandmother meets my maternal grandfather
• My father meets my mother
• My mother's birth
• I graduate from high school
• My maternal grandmother's birth

With a few clues, could you reasonably put them in chronological order? How did you do it? There are certain conditions that must be true in order for a sequential event to occur (e.g., my grandmother must be born and know my grandfather before my mother can be born). The same is true for geologic events - there is a well understood set of principles that govern the deposition of sediments that can help a geologist tell a story about the history of the rocks.

Numerical time/age

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• My maternal grandmother's birth - June 20, 1918
• My maternal grandmother meets my maternal grandfather - September, 1936
• My mother's birth - May 30, 1944
• My father meets my mother - October, 1964
• My birth - May 6, 1970
• I graduate from high school - June, 1988

Although your "relative" list should be in exactly the same order as mine, the "age" of the events will not be the same. This is the concept of numerical age. You can put "relatives" in chronological order just by knowing their relationship to one another; however, when you have done that, you have no idea what the numerical age of any of those relatives is.

Students can relate to the making of a peanut butter and jelly sandwich - this is a great way to help students understand the sedimentary principles listed above:

When I make a PB and J sandwich, I can say that:

• one piece of bread was there for me to spread the PB (or J) on (principle of superposition);

• the bread had to be (relatively) flat for me to spread the peanut butter on it (principle of original horizontality);
• the peanut butter probably covered the entire piece of bread (principle of original continuity);
• if I slice the sandwich, all those layers (PB, J and bread) had to be there for me to cut through them (principle of cross cutting relationships;
• if there is jelly in my peanut butter, I put the jelly on first - and vice versa (principle of inclusions).

• Some grains of sand in Australia have been dated at 4.2-4.4 billion years - this suggests that there were rocks around at that time even if they are no longer there,
• Rocks collected from the Moon have yielded ages as old as 4.6 billion years old,
• Nearly every meteorite that has the same composition as the Earth has an age of 4.6 billion years.

• That would be like polling the University and having 90% of the students have exactly the same birthday! How likely is that?