GEOLOGIC HISTORY, FOSSILS, & RELATIVE DATING

Geologic History Uniformitarianism Catastrophism The idea that the same geologic processes that are shaping Earth today have been at work throughout Earth’s history. Catastrophism The principle that states that all geologic change occurs suddenly.

What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know?

ROCKS TELL A STORY Rocks can tell where they were made and when Sedimentary rocks can have fossils in them Rocks can tell when mass extinctions happened STUDY AND COMPARISON OF ROCK LAYERS OR STRATA IN THE 19TH CENTURY LED SCIENTISTS TO BELIEV THAT A CORRELATION EXISTS BETWEEN PLACE AND TYPE OF ROCK

PALEONTOLOGY the study of fossils  remains of ancient life Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like footprints Scientific dating Absolute dating (gives age in years) Radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during) Observation of rock layers Radiometric dating: use the natural radioactivity of certain elements found in rocks to help determine their absolute age- the use of half-lifes to determine the absolute age of a sample. In radioactive dating, scientists calculate the age of a sample based on the remaining radioactive isotopes. Radioactive elments decay into nonradioactive elements at a steady rate which is measured in a unit called half-life.

FOSSILS Traces and preserved remains of ancient life found within rock layers Fossils show: Biodiversity How species have changed over time Correlation between rock layers from around the world Relative ages to particular strata Evidence for the geological time scale Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

RELATIVE DATING & AGE Relative Dating: putting rocks and geological events in correct chronological order Relative Age: how old something is in comparison to something else Geologic Column: an ideal sequence of rock layers based on relative age with the oldest on the bottom HOW? Use of sedimentary rocks Use of fossils Study of strata

ABSOLUTE DATING Examples of how absolute dating is used: Radioactive Decay Half-life: the time that it takes for half of the unstable atoms in a sample to decay p.132 Any method of measuring the age of an event or object in years

Using Half-Lives to Date Fossils – p. 132

INDEX FOSSIL Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Short-lived (found only in a few layers of rock worldwide) Wide distribution (geographic range)

Ex/ INDEX FOSSIL: AMMONITE Ammonite fossils are found worldwide, but they existed for only a very specific period of time this means ammonites are found in very specific layers of rock when an index fossil is found, the age of the rocks it is preserved can be determined Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

What kind of rocks are these fossils in? Which layer is oldest? Which layer is youngest? How do you know?

LAW OF HORIZONTALITY Sediments are originally deposited in horizontal layers Folds or inclines: layers must have been deformed after they were deposited.

LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top (Supai is oldest)

LAW OF SUCCESSION Fossils are found in a predictable sequence Fossils in rock B are older then fossils in rock A

LAW OF INCLUSIONS If a rock body (Rock B) contained fragments of another rock body (Rock A), then Rock B must be younger than the fragments of rock it contained

GEOLOGIC TIME SCALE a series of time intervals that divides Earth’s history Each layer of rock represents specific interval of time Index fossils help determine specific period Time periods divided by specific events like mass extinctions