Chapter 9 Fossils: Sections 2, 3, 4, 5 in Brief Rusty Sturken Duluth Middle School Sixth Grade Earth Science September, 2010 Personal photo from the American Museum of Natural History July, 2007

9-2 Relative Age & Absolute Age Relative Age- the age of a rock compared to the ages of other rocks Absolute Age- the number of years since a rock formed

9-2 The law of superposition In (undisturbed) horizontal layers of sedimentary rock, the oldest layer is at the bottom. Each higher layer is younger than the layers below it. Youngest Layers Oldest Layers http://www.mysciencebox. org/files/images/Grand%20 Canyon%20NPS.jpg

9-2 Geologic Column Geologic column-The vertical layers of horizontal rock of different ages that have formed over the history of Earth. In the geologic column, the oldest rocks are on the bottom http://www.6000years.org/graphics/geo-column.jpg

9-2 Index Fossils Index fossils are useful to geologists because they help determine the relative ages of the layers of rock where they are found, they can match rock layers of similar age To be an index fossil a fossil must: - be widely distributed (found in many areas) - have existed briefly (a short amount of time)

9-2 The Principle of Original Horizontality The principle of original horizontality says that most sedimentary layers of rock are deposited in a horizontal position

9-3 Radioactive Dating Some elements (radioactive elements) break down over time to form atoms of a different element, this is called radioactive decay Half-life: the time it takes for half of the radioactive atoms in a sample to decay

9-3 Radioactive Dating Geologists use radioactive dating to determine the absolute ages of rocks Potassium-Argon for very old rocks due to long half-life Carbon-14 for dating plant and animal materials (they contain carbon)

9-3 Radioactive Dating/Age of Earth Radioactive dating is usually only used to date igneous rock Radioactive dating shows that the oldest moon rocks are about 4.6 billion years old. Scientists infer that Earth is only a little older than those moon rocks so Earth is about 4.6 billion years old.

9-4 The Geologic Time Scale The geologic time scale is a record of the life forms and geologic events in Earth’s history Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

9-4 The Geologic Time Scale The basic units of the geologic time scale are eras, period, and epochs Cenozoic Era: 65 million years ago to present, “Age of Mammals” we are now living in the Cenozoic Era Mesozoic Era: 245 million years - 65 million years ago sometimes called “Age of Dinosaurs” Paleozoic Era: 245-544 million years ago Paleo- means “ancient” –zoic means “life”

9-5 Earth’s History Earth formed from a mass of dust and gas about 4.6 million years ago The mass extinction at the end of the Paleozoic Era affected plants and animals, on land and in the seas. Scientists do not know what caused the mass extinction but up to 95 percent of life in the oceans disappeared

Geologic History http://files.abovetopsecret.com/images/member/58e5483d1fed.gif