Chapter 8 Sections 2 & 3 The Relative Age of Rocks Radioactive Dating

Two ways to express the age of rocks and fossils 1.Relative age = its age compared to the age of other rocks 2.Absolute age = the number of years since the rock has formed Table talk: Describe how the age of each family member could be given as a relative age or an absolute age.

Law of Superposition In horizontal layers of sedimentary rock, the oldest layer is at the bottom.

How Scientists Determine Relative Age 1. Clues from igneous rocks Extrusion = lava that hardens on Earth’s surface Intrusion = lava that hardens below Earth’s surface Table Talk: An extrusion is always younger than the rocks below it. Why?

2. Clues from FaultsFault = a break in Earth’s crust A fault is always younger than the rock it cuts through

3. Gaps in the geologic record Some layers are incomplete. They may have eroded, exposing older layers beneath. Scientists study the layers and patterns of erosion to help determine relative age.

Using Index Fossils to Date Rocks Index fossils are: widely distributed of a type of organism that existed only briefly They help tell the relative ages of the rock layers in which they occur code cfp-2042 (active art/index fossils) Ammonites --- index fossils --- found near Greece. Their dark coloring indicates that they came from very deep in the ocean.

TABLE TALK: If you found the trilobite fossil (left) in a rock, how old might that rock be? (Use the geologic time scale for help.)

Section 8.3 Radioactive Dating Scientists can also determine absolute age through radioactive dating All matter, including rocks is made up of atoms When all atoms of a particular material are the same, this is called an element Some elements exist in forms that are unstable Radioactive decay is when the atoms of one element break down to form atoms of another element The rate of decay is called that elements “half life”

How radioactive dating works Scientists measure the amount of a radioactive element (parent) in a rock Then they measure the amount of the stable element into which the radioactive element decays (daughter) By calculating the parent- daughter ratio, scientists can determine the age of the rock

See Brain Pop “Carbon Dating”