GEOLOGIC HISTORY, FOSSILS, & RELATIVE DATING

Geologic History  Uniformitarianism  The idea that the same geologic processes that are shaping Earth today have been at work throughout Earth’s history.  Uniformitarianism  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  Rocks can tell where they were made and when  Sedimentary rocks can have fossils in them  Rocks can tell when mass extinctions happened

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 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

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 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

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  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

 Any method of measuring the age of an event or object in years  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 ABSOLUTE DATING

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) 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  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

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

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

LAW OF HORIZONTALITY Sediments are originally deposited in horizontal layers  The Cross Cutting Law  The Cross Cutting Law:  Folds or inclines: layers must have been formed after they were deposited.  The Cross Cutting Law  The Cross Cutting Law:  Folds or inclines: layers must have been formed after they were deposited.

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