Relative and Absolute Dating

By: Keri Holland

- What is Relative Dating?Relative Dating is determining whether

an object or event is older or younger than other objects or events.

Relative Dating

-What is it?

Superposition is a principle that states that younger rocks are lie above older rocks in undisturbed sequences.

- “Younger over older” is a good phrase to remember this principle.

Principle of Superposition

- The geologic column is ideal sequences of rock that contains all the known fossils and rock formations on the earth arranged from youngest to oldest.- Scientists rely on the geologic column to interpret rock sequences.

- When geologists aren’t sure about the age of a rock sequence they are studying, they gather information about the sequence and compare it to the geologic column.

Geologic Column

-Missing rock layers create gaps in rock-layer sequences called unconformities.

-Unconformities are a surface that represents a missing part of the geologic column.-They also represent missing time. Time that was NOT recorded in layers of rock.

Missing Evidence

1. DISCONFORMITIES

2.NONCONFORMITIES

3. ANGULAR UNCONFORMITIES

3 Types of Unconformities

-Disconformities exists where part of a sequence of parallel rock layers is missing. While often hard to see, a disconformity is the most common type of unconformity.

Disconformity

Disconformities

- Nonconformities exists where sedimentary rock layers lie on top of an eroded surface of non-layered igneous or metamorphic rock.

Nonconformity

NONCONFORMITIES

ANGULAR UNCONFORMITIES- Angular unconformities exists between

horizontal rock layers and rock layers that are tilted or folded. The tilted or folded layers were eroded before horizontal layers formed above them.

Angular

Unconformity

Constructing the Geologic Column -Here you will see three rock sequences (a, b, & c) from three

different locations. Some rock layers appear in more than one sequence.

Geologists construct the geologic column by piecing together different rock sequences from all over the world.

THE PRINCIPLE OF UNIFORMATIONUniformation: a principle that states that the

same geologic process shaping the earth today has been at work throughout earth’s history.

“The present is the key to the past” is a phrase that best summarizes uniformation.

Catastrophism: a principle that states that all geologic change occurs suddenly.

Absolute Dating: A Measure of Time- Absolute Dating: a process of establishing

the age of an object, such as a fossil or rock layer, by determining the number of years it has existed.

Radioactive Decay-What is it?

-a process in which radioactive isotopes tend to break down into stable isotopes or other elements.-isotopes: atoms of the same element that have the same number of protons but have different numbers of neutrons.- To determine the absolute age of fossils and rocks, scientists most often analyze radioactive isotopes.

Dating RocksHow Does it Work?

- Consider a stream of molten lava flowing out of a volcano. As long as the lava s in liquid form, the daughter material that is already present and the parent material are free to mix and move around. But eventually the lava cools and becomes solid igneous rock. When this happens, the parent and daughter materials often end up in different minerals. Scientists compare the amount of new daughter material with the amount of parent material that remains. The same daughter material there is, the older the rock is.

- Radiometric Dating: determining the absolute age of a sample based on the ratio of parent material to daughter material.

- Half-Life: the time it takes for ONE-HALF of a radioactive sample to decay.

Types of Radiometric Dating1. Uranium-Lead Method2. Potassium-Argon Method3. Carbon-14 Method

Uranium-Lead Method- Uranium-238 is a radioactive isotope that

eventually decays to lead-206. the half-life of uranium-238 is 4.5 billion years. The older the rock is, the more daughter material(lead-206) there will be in the rock. Uranium-lead dating can be used for rocks MORE THAN 10 MILLION years old. Younger rocks do not contain enough daughter material to be accurately measured by this method.

Potassium-Argon MethodPotassium-40 has a half-life of 1.3 BILLION YEARS, and it eventually decays to argon and calcium. Geologists measure argon as the daughter material for radiometric decay. This method is mainly used to date rocks OLDER THAN 10,000 YEARS.

Carbon-14 Method The element carbon is normally found in three forms, the

stable isotopes carbon-12 and carbon-13 and the radioactive isotope carbon-14. these carbon isotopes combine with oxygen to form carbon-dioxide, which is taken in by plants during photosynthesis. As long as the plant is alive, new carbon-dioxide with a constant carbon-14 to carbon-12 is continually taken in. animals that eat the plants contain the same ratio of carbon isotopes.

Once a plant or animal dies, however, no new carbon is taken in. the amount of carbon-14 begins to decrease as the plant or animal decays. This decrease can be measured in a laboratory. Because the half-life of cabon-14 is only 5,730 years. This dating method is used for dating things that lived within the last 50,000 years.

FOSSILIZED ORGANISMS1. Fossils in Rocks2. Mineral Replacement3. Fossils in Amber4. Mummification5. Frozen Fossils6. Fossils in Tar

Using Fossils to Date RocksGeologists sometimes use index fossils to date rocks while in the field.Index fossils: fossils of organisms that lived during a relatively short, well-defined time span.

The Geologic Time Scale

L A B ! ! !

Get a half-life!-After observing the process , complete the

chart.

Parent Left Half-life in Years Age in Years

1/2 15,000 30,000½ 1.95 billion 3.9 billion

¼ 10,000 40,000½ 2.4 million 4.8 million