Geologic Time   By examining layers of sedimentary rock, geologists developed a time scale for dividing up earth history. Earlier in the 20th century, radiometric-dating techniques allowed scientists to put absolute dates on divisions in the geologic time scale. In this segment, we will learn how geologists: determine the relative ages of rock units, how radiometric techniques can be used to date some rocks.

Geological Time: Absolute & Relative Dating Notes

Geological Background: Rocks Igneous rocks are formed by volcanic processes Often by cooling of molten rocks (at surface or within Earth’s interior) They are important for radiometric dating (see K-Ar dating)

Geological Background: Rocks Sedimentary rocks are formed from deposition of sediments into layers Deposited by wind, water, gravity into layers Sediments formed by erosion of other rocks Layers are gradually hardened over time Sedimentary rocks are important because fossils are found in these rocks

Geological Background: Rocks Metamorphic rocks have “morphed” into another kind of rock Formerly sedimentary or igneous Changed by heat and pressure Not particularly important in paleontology But pretty!

How do geologists determine how old rocks are? Relative dating Determine whether the rock is older or younger than other rocks

Law of Superposition Geologists try to determine the order in which events have happened during Earth’s history. They rely on rocks and fossils to help them in their investigation. The process of determining whether an event or object is older or younger than other events or objects is called relative dating.

Law of Superposition As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older. Superposition is a principle that states that younger rocks lie above older rocks, if the layers have not been disturbed.

Law of Superposition Disturbing Forces Not all rock sequences are arranged with the oldest layers on the bottom and the youngest layers on top. Some rock sequences have been disturbed by forces within the Earth. These forces can push other rocks into a sequence, tilt or fold rock layers, and break sequences into moveable parts.

Disturbed Rock Layers Geologists often find features that cut across existing layers of rock. Geologists use the relationships between rock layers and the features that cross them to assign relative ages to the features and the layers. The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them.

Relative Age Dating

How do geologists determine how old rocks are? Absolute dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years *Not all rocks can be dated absolutely, so combinations of techniques are used.

Radioactive Decay Absolute dating is any method of measuring the age of an event or object in years. To determine the absolute ages of fossils and rocks, scientists analyze isotopes of radioactive elements. Atoms of the same element that have the same number of protons but different numbers of neutrons are called isotopes.

Radioactive Decay, continued Most isotopes are stable, meaning that they stay in their original form. Other isotopes are unstable. Scientists call unstable isotopes radioactive.

Radioactive Decay, continued Radioactive isotopes tend to break down into stable isotopes of the same or other elements in a process called radioactive decay.

Radioactive Decay, continued Because radioactive decay occurs at a steady rate, scientists can use the relative amounts of stable and unstable isotopes present in an object to determine the object’s age. Dating Rocks—How Does It Work? In radioactive decay, an unstable radioactive isotope of one element breaks down into a stable isotope. The stable isotope may be of the same element or of a different element.

Radioactive Decay, continued The unstable radioactive isotope is called the parent isotope. The stable isotope produced by the radioactive decay of the parent isotope is called the daughter isotope.

Radioactive Decay, continued The rate of radioactive decay is constant, so scientists can compare the amount of parent material with the amount of daughter material to date rock. The more 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 is called radiometric dating. If you know the rate of decay for a radioactive element in a rock, you can figure out the absolute age of the rock.

Radiometric Dating, continued A half-life is the time needed for half of a sample of a radioactive substance to undergo radioactive decay. After every half-life, the amount of parent material decrease by one-half.

Half Life Is the time it takes for one half of the parent material to become daughter material. 1 HL= 50% parent 50% daughter 2 HL= 25% parent 75% daughter 3 HL= 12.5% parent 87.5% daughter 4 HL= 6.25% parent 93.75% daughter So if the HL of an isotope is 10,000 years how old is the object if 4 HL have occurred? 40, 000 years old

If the initial sample mass is 1 kg after 5 half lives how much parent material is there and how much daughter material is there?

Step 1. 1kg divided by 2 is. 5 kg Step 2. 5 kg divided by 2 is 0 Step 1. 1kg divided by 2 is .5 kg Step 2. 0.5 kg divided by 2 is 0.25 kg parent material and 0.75 daughter material. Step 3. 0.25kg divided by 2 is 0.125 kg parent material and 0.875 kg daughter material. Step 4. 0.125kg divided by 2 is 0.0625 kg parent material and 0.9375 kg daughter material. Step 5. 0.0625 kg divided by 2 is 0.03125 kg parent material and 0.96875 kg daughter material.

Types of Radiometric Dating Scientists use different radiometric-dating methods based on the estimated age of an object. There are four radiometric-dating techniques. Potassium-Argon Method Potassium-40 has a half-life of 1.3 billion years, and it decays leaving a daughter material of argon. This method is used mainly to date rocks older than 100,000 years.  

Types of Radiometric Dating, continued Uranium-Lead Method Uranium-238 is a radioactive isotope with a half-life of 4.5 billion years. Uranium-238 decays in a series of steps to lead-206. The uranium-lead method can be used to date rocks more than 10 million years old.

Types of Radiometric Dating, continued Rubidium-Strontium Method The unstable parent isotope rubidium-87 forms a stable daughter isotope strontium-87. The half-life of rubidium-87 is 49 billion years. This method is used for rocks older than 10 million years.

Types of Radiometric Dating, continued Carbon-14 Method Carbon is normally found in three forms, the stable isotopes carbon-12 and carbon-13, and the radioactive isotope carbon-14. Living plants and animals contain a constant ratio of carbon-14 to carbon-12. Once a plant or animal dies, no new carbon is taken in. The amount of carbon-14 begins to decrease as the plant or animal decays.

Types of Radiometric Dating, continued The half-life of carbon-14 is 5,730 years. The carbon-14 method of radiometric dating is used mainly for dating things that lived within the last 50,000 years.