Topic VIIGeologic History

How do you determine how old a rock is?How do you determine how old a rock is?

UniformitarianismUniformitarianism: “the present is the key : “the present is the key to the past” (what is happening today—to the past” (what is happening today—uplifting, erosion… has always happened)uplifting, erosion… has always happened)

Relative Age of Rock LayersRelative Age of Rock Layers: comparing : comparing rock layers in an rock layers in an outcropoutcrop—exposed rock —exposed rock (does not tell you an exact age—but can (does not tell you an exact age—but can tell you which rocks are older)tell you which rocks are older)

“Relatives”

Absolute AgeAbsolute Age: finding the actual age of : finding the actual age of the rock (the rock (radioactive datingradioactive dating) )

Six ways to determine the relative age of an Six ways to determine the relative age of an outcrop—“outcrop—“Sandwich RulesSandwich Rules””

1.1. Law of Original Horizontality:Law of Original Horizontality: all all sedimentary rockssedimentary rocks formed in horizontal formed in horizontal layerslayers

2.2. Law of SuperpositionLaw of Superposition: oldest rock is on : oldest rock is on the bottom (in a sedimentary layer that the bottom (in a sedimentary layer that has not been overturned when folded) has not been overturned when folded)

3.3. Igneous Intrusions (inside) / Igneous Intrusions (inside) / Extrusions (outside):Extrusions (outside): are always are always younger than the rocks they are in or onyounger than the rocks they are in or on(toothpick)(toothpick)

Look for Look for contact metamorphism contact metamorphism (burn) (burn) marks (if you see them, the rock layer is marks (if you see them, the rock layer is older than the metamorphism) older than the metamorphism)

4.4. Mineral VeinsMineral Veins are younger than the are younger than the rocks they are inrocks they are in

5.5. Sediments that make up sedimentary Sediments that make up sedimentary rocks are older than the rocks rocks are older than the rocks themselvesthemselves

6.6. Folds, faultsFolds, faults, , and tiltsand tilts are are youngeryounger than than the rocks they are in (cut)the rocks they are in (cut)

Sequence of Geologic Events:Sequence of Geologic Events:

Events animation

But there is a problem….But there is a problem…. UnconformitiesUnconformities: buried erosional surfaces : buried erosional surfaces

(rock layers are destroyed—usually indicated (rock layers are destroyed—usually indicated by a horizontal wavy line in the cross section)by a horizontal wavy line in the cross section)—creates a “Gap in Time” —creates a “Gap in Time”

More unconformities….More unconformities….

More unconformities….More unconformities….

1 More unconformity….1 More unconformity….

What is correlation?What is correlation?CorrelationCorrelation: : matching upmatching up of rock and of rock and

geologic events in two separate areasgeologic events in two separate areas

How can rocks be correlated?How can rocks be correlated?

1.1. Similarity of rock layersSimilarity of rock layers (color, (color, composition)composition)

2.2. Fossils Fossils (remains of prehistoric life): (remains of prehistoric life): seeing if fossils match up in separate seeing if fossils match up in separate rock layers—can tell the organisms rock layers—can tell the organisms environment (does not work for all environment (does not work for all fossils)fossils)

Fossil Correlation

Index FossilsIndex Fossils: fossils that have a wide : fossils that have a wide geographic distribution and a short period geographic distribution and a short period of existence (1 rock layer)of existence (1 rock layer)

What is needed to make a fossil:What is needed to make a fossil:

Hard parts (soft parts decay quickly)Hard parts (soft parts decay quickly)Quick burial (slows decay)Quick burial (slows decay)

3.3. Volcanic AshVolcanic Ash: can be spread throughout : can be spread throughout the world and settle as a thin layer very the world and settle as a thin layer very quickly (very good time marker)quickly (very good time marker)

****Index FossilsIndex Fossils are the best way to correlate are the best way to correlate rock layersrock layers

How can scientists determine the How can scientists determine the absolute age of rock layers?absolute age of rock layers?

Radioactive DatingRadioactive Dating: determining the age : determining the age of something by comparing the amount of of something by comparing the amount of radioactive isotoperadioactive isotope to the stable decay to the stable decay productproduct

meaning…meaning…The INSTANT a rock is created, it contains

100% of some radioactive isotope. As a rock gets older, the radioactive isotope breaks down into a decay product….scientists can measure this breakdown.

But this can also be used to date the But this can also be used to date the remains of once LIVING things…remains of once LIVING things…

All living things on planet Earth are part of the CARBON CYCLE… (think Carbon Dioxide)…meaning over the course of our life, we build-up Carbon in our bodies…the instant we die, we have 100% Carbon

So, how do you do this…So, how do you do this…

Half LifeHalf Life: amount of time it takes for a : amount of time it takes for a radioactive isotope to decay to ½ its radioactive isotope to decay to ½ its original mass (occurs at a original mass (occurs at a predictable ratepredictable rate) )

ESRT Cover

Carbon-14Carbon-14: isotope used to date the : isotope used to date the remains of once living things—people, remains of once living things—people, shirts, mammoths—less than 50,000 years shirts, mammoths—less than 50,000 years old (this is because its half life is only 5700 old (this is because its half life is only 5700 years—very short)years—very short)

If 100 grams of pure carbon-14 (C-14) starts to If 100 grams of pure carbon-14 (C-14) starts to decay, how much C-14 will be left after 11,400 decay, how much C-14 will be left after 11,400 years? years?

The box method…The box method…

• If 100 grams of pure carbon-14 starts to If 100 grams of pure carbon-14 starts to decay, how much nitrogen-14 (N-14) will decay, how much nitrogen-14 (N-14) will be created in 11,400 years? be created in 11,400 years?

**half life never changes for a particular **half life never changes for a particular isotope no matter what happens!isotope no matter what happens!

Fire Crushing

Geologic Time ScaleGeologic Time Scale

Geologic Time ScaleGeologic Time Scale: time-line that : time-line that shows the history of the Earth (created by shows the history of the Earth (created by correlating correlating fossil evidence fossil evidence throughout the throughout the world) world)

Problems with the geologic time scale:Problems with the geologic time scale:

UnconformitiesUnconformities—time scale may be too —time scale may be too shortshort

What does the Geologic Time Scale What does the Geologic Time Scale show about life on Earth?show about life on Earth?

1.1. Simple life forms began firstSimple life forms began first

2.2. EvolutionEvolution: species have changed to : species have changed to increase their chance of survival (this increase their chance of survival (this leads to diversity of life) leads to diversity of life)

3.3. Most life forms have become extinct Most life forms have become extinct