How Old is the Earth Anyway? Unlocking the ancient Earth

Relative Time Scale Is based on the rock and fossil evidence found in the crust of the Earth The relative age of the rock is due to position. Tells you what happened first, second, third, etc. Sedimentary rocks are especially useful because: They are most likely to contain fossils They are the most common and widespread rock type

Rules for the Relative Dating of Sedimentary Rocks – The present is the key to the past (uniformitarianism)

Principle of Original Horizontality Sediments (that make up Sedimentary rocks) will settle in horizontal layers, unless they have undergone deformation

Law of Superposition The oldest layers are found below the younger layers

Principle of Crosscutting Relationships Any rock that cuts existing rock is younger than the rock that it cuts.

Fossil Succession Fossils represent the remains of once living organisms Most fossils are the remains of extinct organisms The kinds of fossils found in rocks of different ages differ because life on Earth has changed over time

Other Tools: Index Fossils Attributes of an Index Fossil: Fossils of life that existed during limited periods of geologic time. Widespread distribution Any fossil found in a rock is the same age as the rock it was found in. They are used as guides to date the age of the rock in which they are preserved.

Other tools: Unconformity A surface between two rock layers that represents a gap in the rock record.

Types of unconformities Angular Unconformity Disconformities

Angular unconformity:

Disconformities:

Crosscutting Relationships:

Absolute Age- Radiometric Dating Is used to determine the absolute age of earth materials Determines how old a rock is in years. It is based on the natural radioactive decay of radioactive elements found in igneous rocks.

A little background: An atom is made up of protons, neutrons and electrons An element is an atom with a specific number protons. The number of neutrons and electrons in an element can change, but if you change the number of protons in an element, you change the element.

Diagram: The protons and neutrons are found in the nucleus of the atom The electrons are in orbit around the nucleus.

Radioactive decay Most atoms are stable; that is, the number of protons and neutrons never changes. In some atoms, the number of protons and neutrons makes the atom unstable. The unstable atom emits its excess energy and may eject one or more subatomic particles or it will split into two atoms of smaller elements in order to become more stable. This is radioactive decay.

Half-Life As the element loses particles from the nucleus it changes or splits from the radioactive PARENT element to a stable DAUGHTER element. This rate of decay is measured in Half-lives Half-life is the amount of time it takes ½ (50%) of the parent element to decay to the daughter element Radiometric dating is based on the ratio of the Parent element to the Daughter element

Rate of Decay: Half-life Parent Daughter When formed 100% (1) 0 (0) In % and (fraction) Daughter When formed 100% (1) 0 (0) 1 half-life 50% (1/2) 2 half-lives 25% (1/4) 75% (3/4) 3 half-lives 12.5% (1/8) 87.5% (7/8) 4 half-lives 6.25% (1/16) 93.75% (15/16) 5 half-lives 3.125% (1/32) 96.875% (31/32)

Different elements have different half-lives: (in your ESRT add U235) Parent Isotope Daughter Half-life of parent (years) Useful range (years) Carbon 14 Nitrogen 14 5,730 100-30,000 Potassium 40 Argon 40 1.3 billion 100,000 – 4.5 billion Rubidium 87 Strontium 87 47 billion 10 million – 4.5 billion Uranium 238 Lead 206 4.5 billion 10 – 4.6 billion Uranium 235 Lead 207 710 million

To solve half-life problems: Find the number of years for that element’s half-life Find the number of half-lives that have occurred (from table) Multiply the element’s half-life by the number of half-lives that have passed