relative time dr. david steer introduction relative time - how do scientists determine the sequence...
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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?
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 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
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
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.