MEASUREING GEOLOGIC TIME

Ways to Measure Time

There are 2 ways to measure geologic time– Relative time (not very accurate, but easy

to complete)– Absolute time (very accurate, but

extremely expensive)

Relative Time Places events in a sequence. Does not tell actual age of event. Does not tell exact time it occurred.

– Example: You remember visiting Disney world before your 12th birthday but after you got your bicycle on your 9th birthday, that is relative time.

Most geologic work is done using relative time.

Measuring Relative Time Based on how one event, rock, or fossil

is aged to another. judge how old one rock is compared to

another Judge the age of fossils found in rocks. They put all of this together in The

Geologic Time Table

Geologic Time Scale (GTS) Geologic time is divided into units like a

year is divided into months. Divisions are marked by major changes.

– changes to earth’s surface, climate, or extinction of species.

Rocks found in an age range on GTS contain similar fossils.

GTS is based on fossils found in each time period.

Geologic Time Scale Divisions

Eras are long time divisions

– Four Eras• Precambrian Time (because it is so long)

• Paleozoic Era

• Mesozoic Era

• Cenozoic Era

Geologic Time Scale Divisions

Periods & Epochs

– Are smaller divisions of the rock record• Period is characterized by unique fossils• Named for location where the rocks were found

that contained the fossils.

Absolute Time

Identifies actual date of an event. Absolute dates difficult to obtainVery expensive to obtain.More Reliable.More accurate than relative

Measuring Absolute Time Counting tree rings 1 ring per year, tells years

of life + tells about growing season, wet, dry, fire, etc. (4,600 years oldest tree). Gives a close account of number of years tree was alive.

Counting varves layer of sediment in a lake or pond, a yearly picture of the sediment deposited.– This in turn lets you know what the growing

season was like. – 15,000 years oldest found.

Measuring Absolute Time Three kinds of radioactive particles

– Alpha ray = 2 P+ & 2 N° (moderately harmful to humans)

– Beta ray = e- (not harmful to humans)

– Gamma rays = high energy particles neutrinos, quarks, X rays (are harmful.)

Measuring Absolute Time Using decay of radioactive elements is

the most accurate way. – element that gives off radiation from its

nucleus.– Break down of the nuclear particle, after the

element emits the electron and becomes a new element and becomes less stable.

– Half-life the time it takes ½ a parent element to break down into a stable daughter element. Like uranium lead.

Common Radio-dating Forms Radiocarbon dating isotope of carbon has

half-life (hl) of 5700 years. – Useful to about 100,000 yrs in past. (Great for

artifacts) after that most carbon-14 is gone.

Uranium=>lead dating 4.5 billion yr hl.– Too long for any thing but oldest rocks.

Potassium=>argon dating 1.3 billion year hl.– Extremely common & can be used to date rocks as

young a 50,000 years.