Geologic Time. The geologic time scale is based upon rock and fossil evidence. It is broken into the following divisions from largest to smallest: eon,

Geologic TimeGeologic Time

The The geologic time scale geologic time scale is based is based upon rock and fossil evidence. It upon rock and fossil evidence. It

is broken into the following is broken into the following divisions from largest to divisions from largest to

smallest: eon, era, period, and smallest: eon, era, period, and epoch. An interactive timeline epoch. An interactive timeline shows how long geologic time shows how long geologic time

really is.really is.

Charles Darwin’s Theory of Charles Darwin’s Theory of EvolutionEvolution describes the describes the process of change that process of change that

produces new life forms over produces new life forms over time.time.

Evolution is driven by the Evolution is driven by the theory of theory of natural selectionnatural selection, , the idea that the organisms the idea that the organisms

that survive to produce that survive to produce offspring are those that have offspring are those that have inherited the most favorable inherited the most favorable

traits for surviving in a traits for surviving in a particular environment.particular environment.

These cactus plants show These cactus plants show convergent evolution. The one convergent evolution. The one on the left is from Arizona, the on the left is from Arizona, the one on the right is from Africa.one on the right is from Africa.

A A fossilfossil is evidence of is evidence of earlier life that has been earlier life that has been

preserved in rock.preserved in rock.

A A fossilfossil can be original can be original remains (not very remains (not very

common), replaced common), replaced remains, molds/casts, remains, molds/casts,

trace fossils, and trace fossils, and carbonaceous films.carbonaceous films.

Fossils aid in relative Fossils aid in relative dating of rocks.dating of rocks.

Relative DatingRelative Dating is the is the process of placing geologic process of placing geologic

events in the process in events in the process in which they occurred.which they occurred.

The Grand Canyon is one of The Grand Canyon is one of the best places for observing the best places for observing

relative time.relative time.

There are certain principles There are certain principles for dating rock layers.for dating rock layers.

The The principle of superpositionprinciple of superposition states that the oldest rock layer states that the oldest rock layer

will be at the bottom and the will be at the bottom and the youngest at the top. Which youngest at the top. Which

picture shows the oldest rocks?picture shows the oldest rocks?

The The principle of original horizontalityprinciple of original horizontality says that sedimentary rocks are says that sedimentary rocks are

first deposited horizontally.first deposited horizontally.

Rocks can be folded, tilted, or Rocks can be folded, tilted, or faulted after they are deposited faulted after they are deposited

horizontally.horizontally.

Rocks can also be intruded by Rocks can also be intruded by magma which later cools to form magma which later cools to form

rock.rock.

The The Principle of Cross Cutting Principle of Cross Cutting RelationshipsRelationships says that an says that an igneous intrusion is always igneous intrusion is always younger than the rock it has younger than the rock it has

intruded or cut across.intruded or cut across.

Number this diagram from oldest Number this diagram from oldest to youngest. 1 is always oldest to youngest. 1 is always oldest

(the first to form).(the first to form).

Observe this animation to Observe this animation to help you visualize geologic help you visualize geologic

processes. Notice the processes. Notice the unconformityunconformity, a gap in the , a gap in the sedimentary rock record.sedimentary rock record.

An An angular unconformityangular unconformity occurs when younger, occurs when younger, flat strata are deposited on top of the older strata flat strata are deposited on top of the older strata

that have been tilted at an angle.that have been tilted at an angle.

A A disconformitydisconformity occurs when younger, flat strata are occurs when younger, flat strata are deposited on top of the older flat strata. The older flat deposited on top of the older flat strata. The older flat strata is uplifted and eroded. The layers are then re-strata is uplifted and eroded. The layers are then re-submerged under water and a second instance of submerged under water and a second instance of

deposition occurs on top of the unconformity.deposition occurs on top of the unconformity.

A A nonconformitynonconformity occurs when sedimentary layers are occurs when sedimentary layers are deposited on top of igneous or metamorphic rock.deposited on top of igneous or metamorphic rock.

CorrelationCorrelation is the matching of is the matching of rock layers from one area to rock layers from one area to

another. Correlated strata will another. Correlated strata will have the same age.have the same age.

Methods of CorrelationMethods of Correlation•Walking the outcrop

•Matching Rock Characteristics

•Using Index Fossils

•Climate indicating fossils

•Matching Key Beds

•Stratigraphic Matching

Index FossilsIndex Fossils are useful to date layers because are useful to date layers because each layer contains fossils unlike those in the each layer contains fossils unlike those in the

layer above or below.layer above or below.

Where is The Grand Staircase?Where is The Grand Staircase?

What is The Grand Staircase?What is The Grand Staircase?

Pictures of The Grand StaircasePictures of The Grand Staircase

Use the following images to give you a sense of Use the following images to give you a sense of what the Grand Staircase and the Colorado what the Grand Staircase and the Colorado

Plateau Look LikePlateau Look Likefrom South to Northfrom South to North

Sedona, ArizonaSedona, Arizona

Grand Canyon with San Francisco Peaks in the Grand Canyon with San Francisco Peaks in the BackgroundBackground

Zion National ParkZion National Park

Bryce Canyon National ParkBryce Canyon National Park

Relative dating helps us to Relative dating helps us to figure out the relative ages of figure out the relative ages of rocks, but it does not help us rocks, but it does not help us to figure out the absolute age to figure out the absolute age of the rocks. So, what do we of the rocks. So, what do we

do?do?

We use We use Absolute DatingAbsolute Dating to to put numbers on our dates.put numbers on our dates.

Trees can be used to record time. We can Trees can be used to record time. We can use tree ring history (dendrochronology) to use tree ring history (dendrochronology) to put an actual date on a historic occurrence put an actual date on a historic occurrence

and to gauge past climates.and to gauge past climates.

Varves, annual deposits of Varves, annual deposits of sediments, can be used for sediments, can be used for geologic dating purposes. geologic dating purposes.

Sediments deposited in glacial Sediments deposited in glacial lakes vary with the seasons. lakes vary with the seasons. Thick, light colored, sandy Thick, light colored, sandy

layers are deposited in spring layers are deposited in spring and summer when runoff of and summer when runoff of

water from a glacier is greater. water from a glacier is greater. Thin, dark colored, clay layers Thin, dark colored, clay layers

are deposited in winter. are deposited in winter. Counting annual glacial varves Counting annual glacial varves can help us to date items up to can help us to date items up to

15,000 years old. 15,000 years old.

Absolute dating can also be Absolute dating can also be based upon the concept of based upon the concept of

radioactivity of chemical radioactivity of chemical isotopes. This is also known isotopes. This is also known

as radiometric dating.as radiometric dating.

Radioactive decayRadioactive decay is based is based upon the conversion from upon the conversion from

one isotope to another, but one isotope to another, but what is an isotope?what is an isotope?

Remember from chemistry Remember from chemistry that an isotope is any that an isotope is any

element with more neutrons element with more neutrons than protons.than protons.

Radioactive isotopes are Radioactive isotopes are unstable and want to achieve unstable and want to achieve stability. To do this, they give stability. To do this, they give

off radiation until they are off radiation until they are stable. This usually involves stable. This usually involves changing from one element changing from one element

to another.to another.

The The parent isotopeparent isotope is the is the original element and is original element and is unstable. The unstable. The daughter daughter isotopeisotope is the product of is the product of

decay and is stable.decay and is stable.

There are three types of radioactive decayThere are three types of radioactive decay

Alpha Decay: Atomic # decreases by 2 and atomic mass decreases by 4 because 2 protons and 2 neutrons are expelled from the nucleus of the atom. Remember that the proton determines the atomic # and protons + neutrons determines the atomic mass.

There are three types of radioactive decayThere are three types of radioactive decay

Beta Decay: Atomic # increases by 1 and atomic mass does not change because an electron is expelled from a neutron, thereby turning the neutron into a proton. Remember that the proton determines the atomic # of an atom.

There are three types of radioactive decayThere are three types of radioactive decay

Electron Capture: Atomic # decreases by 1 and atomic mass does not change because an electron is added to a proton, causing a neutron to form. Remember that the proton determines the atomic # of an atom.

Radioactive decay will Radioactive decay will continue until the resulting continue until the resulting

atom is no longer radioactive. atom is no longer radioactive. In other words, a stable In other words, a stable

isotope is formed.isotope is formed.

Half life is the time it takes for Half life is the time it takes for half of the atoms of unstable half of the atoms of unstable parent isotope to decay to parent isotope to decay to stable daughter isotope.stable daughter isotope.

Graph of Half LifeGraph of Half LifeGraph of M&M Half Lives

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In In Radiometric DatingRadiometric Dating, , scientists use radioactivity scientists use radioactivity

and half-lives of elements to and half-lives of elements to measure absolute time.measure absolute time.

Scientists measure the Scientists measure the amounts of parent and amounts of parent and

daughter isotope in a rock to daughter isotope in a rock to find its age.find its age.

Isotopes Used in Radiometric Dating.Isotopes Used in Radiometric Dating.

Parent Isotope

Decay System

Daughter Isotope

Half-Life (years)

Effective Range (years)

Possible Materials for dating

Carbon-14 Beta Nitrogen-14 5730 100-70,000 Once-living matter (wood, charcoal, bone)

Uranium-238 Alpha and Beta

Lead-206 4.5 Billion > 10 Million Uranium-bearing minerals (zircon)

Rubidium-87 Beta Strontium-87 47 Billion > 10 Million Micas, feldspars, metamorphic rocks

Potassium-40

Beta capture Argon-40 1.3 Billion > 50,000 Micas, amphiboles, feldspars, volc. rocks

Example 1: If a scientist finds Example 1: If a scientist finds 100% parent isotope in a rock, 100% parent isotope in a rock,

how old is the rock if the half life how old is the rock if the half life of the parent isotope is 2 million of the parent isotope is 2 million

years? Look at the following years? Look at the following graph, find where the line graph, find where the line

intersects 100%, then read the intersects 100%, then read the value for the number of half lives value for the number of half lives

on the x axis.on the x axis.

Graph of Half LifeGraph of Half LifeGraph of M&M Half Lives

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Age of Rock = # of half lives Age of Rock = # of half lives XX(times) half life of isotope(times) half life of isotope

Age of Rock = 0 Age of Rock = 0 XX 2,000,000 = 0 years2,000,000 = 0 years

Example 2: If a scientist finds Example 2: If a scientist finds 25% parent isotope in a rock, 25% parent isotope in a rock,

how old is the rock if the half life how old is the rock if the half life of the parent isotope is 4 million of the parent isotope is 4 million

years? Look at the following years? Look at the following graph, find where the line graph, find where the line

intersects 25%, then read the intersects 25%, then read the value for the number of half lives value for the number of half lives

on the x axis.on the x axis.

Graph of Half LifeGraph of Half LifeGraph of M&M Half Lives

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Age of Rock = # of half lives Age of Rock = # of half lives XX(times) half life of isotope(times) half life of isotope

Age of Rock = 2 Age of Rock = 2 XX 4,000,000 = 8,000,000 years!4,000,000 = 8,000,000 years!