Relative Time Dr. David Steer

Introduction Relative Time - How do scientists determine the sequence of geological events? Relative Time - How do scientists determine the sequence of geological events? Absolute Time - How do geologists figure out the ages of rocks, and hence the age of Earth? Absolute Time - How do geologists figure out the ages of rocks, and hence the age of Earth?

How do you normally remember the order of events?

Place these events in some logical order D) World Trade Center Destroyed E) Man lands On Moon B) Persian Gulf War C) JFK Assassinated F) Challenger Explodes A) MTV Founded

Oldest to Youngest D) World Trade Center Destroyed E) Man lands On Moon B) Persian Gulf War A) MTV Founded C) JFK Assassinated F) Challenger Explodes

Three principles used to determine relative order of geological events Principle of Superposition Principle of Cross-cutting Relationships Principle of Original Horizontality

Superposition  Layers (beds) at the bottom of a sequence of sedimentary rocks are oldest  Rocks at the top are youngest Oldest Youngest

Which layer is oldest? 1)Layer 1 2)Layer 2 3)Layer 3 4)All same Layer 1 Layer 3 Layer 2 From

Cross-cutting Relationships Older rocks may be cut by younger geological features Older rocks may be cut by younger geological features Might be an igneous intrusion Might be an igneous intrusion Might be filling of an eroded feature Might be filling of an eroded feature

If I use a pen to poke a hole in the stack What is the youngest event? What does the pen do to the other units?

Cross-cutting Relationships Layered rock formed first Igneous rock cuts earlier layering

Which rock is the youngest? A B C D 1)A 2)B 3)C 4)D

Which statement is false? A B C D 1)D crosscuts B 2)D crosscuts C 3)C crosscuts D 4)B was deposited before A

Now tilt and bend the stack

Original horizontality Sedimentary rocks are deposited in horizontal layers Sedimentary rocks are deposited in horizontal layers Tilted or folded layers indicate deformation following deposition Tilted or folded layers indicate deformation following deposition

How do we match sequences across large areas? Correlation used to determine relative ages of rocks across continents Correlation used to determine relative ages of rocks across continents Correlation matches fossils and physical characteristics in sedimentary rocks Correlation matches fossils and physical characteristics in sedimentary rocks

Index Fossils Best correlations use index fossils that represent a short time period but are found across a large region

Which statement is true? 1)Layer 1 is older than layer 3 2)Layer 2 is the same age as layer 3 3)Layer 3 is younger than layer 2 4)Layer 2 is older than layer 1 Layer 1Layer 2 Layer 3 From

Place these units in order from oldest to youngest

Draw this cross section Unit B: Basalt crosscuts all older units and covers a shale unit. Event F: Stream cuts partially into limestone unit. Unit L: Chemical sedimentary rock that was deposited directly on top of low silica extrusive unit. Unit M: Oldest unit is plutonic rock that was partly eroded. Units S: Sandstone that formed over the non-shell bearing limestone rock unit. Unit R: Light colored extrusive rock that completely crosscuts and covers all other units. Unit C: Small grained clastic rock deposited directly over granite.

Student Examples