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Geologic Time

Mr. Litaker

Objectives State the principle of uniformitarianism.

Explain how the law of superposition can be used to determine the relative age of rocks.

Apply the law of crosscutting relationships to determine the relative age of rocks.

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Uniformitarianism In the late 1700’s, James Hutton

published his Theory of the Earth. Hutton put forth the idea of…

UNIFORMITARIANISM: UNIFORMITARIANISM: the same the same forces which change Earth’s forces which change Earth’s surface now, such as volcanism surface now, such as volcanism and erosion, were at work in the and erosion, were at work in the pastpast

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Earth’s Age Hutton’s ideas raised serious

questions about Earth’s age. Until that time, most scientists thought Earth was only 6,000 years old, and all Earth’s features had formed at the same time.

Geologists now estimate that Earth is about 4.6 billion years old

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Relative Dating Principle of Original Horizontality: Principle of Original Horizontality:

sedimentary rocks left undisturbed will sedimentary rocks left undisturbed will remain in horizontal layersremain in horizontal layers

Law of Superposition: Law of Superposition: in in sedimentary rock, each layer is older sedimentary rock, each layer is older than the one above it and younger than the one above it and younger than the one below it.than the one below it.

Relative DatingRelative Dating: tells us the sequence : tells us the sequence

of events, NOT how long ago they of events, NOT how long ago they occurred. What came 1occurred. What came 1stst, 2, 2ndnd, 3, 3rdrd… …

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Cross-Cutting Relationships Principle of Cross-Cutting

Relationships: when a fault cuts through a layer of rock, the fault is younger than the rocks affected

The rock layers must have been there 1st in order to fragment or break, so intrusions (faults) must be younger than the layer they intrude upon

Relative Dating Video

Absolute Dating Methods Absolute Age: numeric age of an Absolute Age: numeric age of an

object, as established by a object, as established by a processprocess, such as radiometric dating

1. Rates of Erosion1. Rates of Erosion: scientists use this method to estimate absolute age only practical for geologic features

which formed within the past 10,000 to 20,000 years

less dependable for older features because rates of erosion can vary

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Absolute Dating Methods 2. Rates of Deposition2. Rates of Deposition: estimate

of absolute age by calculating the rate of sediment deposition. not always accurate because not all

sediment is deposited at the same rate

provides only an estimate of absolute age.

3. Radiometric Dating: 3. Radiometric Dating: comparing the remaining comparing the remaining percentage of an element’s nuclei percentage of an element’s nuclei with one which has not decayedwith one which has not decayed

Nuclei of the same element emit Nuclei of the same element emit particles at a constant rateparticles at a constant rate

Half-Life: time required for half of Half-Life: time required for half of an element’s nuclei to decayan element’s nuclei to decay

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Absolute Dating Methods

Absolute Dating Methods Scientists measure the

concentration of an original element’s nuclei against one which has decayed in a rock, and use the known decay rate of that element to determine the absolute age of the rock.

Radiometric Dating Video

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4. Carbon Dating Atoms of the same element which Atoms of the same element which

have different numbers of have different numbers of neutrons are called isotopesneutrons are called isotopes

All living organisms contain All living organisms contain carbon-12 and carbon-14 isotope.carbon-12 and carbon-14 isotope.

So, living organisms contain So, living organisms contain carbon atoms with 12 neutrons carbon atoms with 12 neutrons and other carbon atoms with 14 and other carbon atoms with 14 neutronsneutrons

Carbon Dating Once a plant or animal dies, the Once a plant or animal dies, the

ratio of 14C to 12C begins to ratio of 14C to 12C begins to change.change.

To find the age of a sample of To find the age of a sample of non-living organic material, non-living organic material, scientists take the ratio of 14C to scientists take the ratio of 14C to 12C and then compare this with 12C and then compare this with the ratio of 14C to 12 C known to the ratio of 14C to 12 C known to exist in a living organism.exist in a living organism.

Carbon Dating Videog

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The Geologic

Time Scale