Relative TimeRelative Time

Dr. David SteerDr. David Steer

IntroductionIntroduction

Relative TimeRelative Time - How do scientists - How do scientists determine the sequence of determine the sequence of geological events? geological events?

Absolute TimeAbsolute Time - How do geologists - How do geologists figure out the ages of rocks, and figure out the ages of rocks, and hence the age of Earth? hence the age of Earth?

How do you normally How do you normally remember the order of events?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

1 2 3

4 5 6

Three principles used to determine relative order of geological events

Principle of Superposition Principle of Cross-cutting Relationships Principle of Original Horizontality

SuperpositionSuperpositionLayers (beds) at the bottom of a sequence of sedimentary rocks are oldest

Rocks at the top are youngest

Oldest

Youngest

Which layer is oldest?Which layer is oldest?

1) Layer 12) Layer 23) Layer 34) All same

Layer 1

Layer 3

Layer 2

From http://www.geo.cornell.edu/geology/classes/geol326/photos.html

Cross-cutting Cross-cutting RelationshipsRelationships

Older rocks may be cut by younger Older rocks may be cut by younger geological featuresgeological features Might be an igneous intrusionMight be an igneous intrusion Might be filling of an eroded featureMight be filling of an eroded feature

If I use a pen to poke a hole in the stackWhat is the youngest event?

What does the pen do to the other units?

Cross-cutting Cross-cutting RelationshipsRelationships

Layered rockformed first

Igneousrock cuts

earlier layering

Which rock is the Which rock is the youngest?youngest?

A

B

C D

1) A2) B3) C4) D

Which statement is false?Which statement is false?

A

B

C D

1) D crosscuts B2) D crosscuts C3) C crosscuts D4) B was deposited

before A

Now tilt and bend the stack

Original horizontalityOriginal horizontality

Sedimentary rocks are deposited Sedimentary rocks are deposited in horizontal layersin horizontal layers

Tilted or folded layers indicate Tilted or folded layers indicate deformation deformation followingfollowing deposition deposition

How do we match How do we match sequences across large sequences across large areas?areas?

Correlation Correlation used to determine used to determine relative ages of rocks across relative ages of rocks across continentscontinents

Correlation Correlation matches fossils and matches fossils and physical characteristicsphysical characteristics in in sedimentary rockssedimentary rocks

Index FossilsIndex Fossils

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

Which statement is true?Which statement is true?1) Layer 1 is older than layer 32) Layer 2 is the same age as layer 33) Layer 3 is younger than layer 24) Layer 2 is older than layer 1

Layer 1 Layer 2

Layer 3

From http://www.geo.cornell.edu/geology/classes/geol326/photos.html

Place these units in order from oldest to youngest

Draw this cross sectionDraw 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 ExamplesStudent Examples