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Earth’s History:

Geologic Time

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Determining the Age of Earth Materials

2 Methods of dating earth materials

1. Relative time – places an event by comparing it with

other events

Example: Sedimentary Rock layers - superposition

A – Oldest (Lower)
B – Middle Age
C – Youngest (Upper)

Actual age not known – only age relative to other layers

Relative time – older than or younger than

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Determining the Age of Earth Materials

2. Absolute dating – specific age

Example: Sedimentary Rock

Rock Layer B deposited 30 Mya
B is 10 million years older than C
B is 5 million years younger than A

Rock Layer	Absolute Age
A	35 Ma
B	30 Ma
C	20 Ma

Important things we can learn from absolute dating:

1. Age of material in years

2. Can calculate the length of time between events

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Relative Dating

- Estimating the age of rock layers using geologic principles such as superposition, cross-cutting, and unconformities

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Relative Dating Laws

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Determining Age using Relative Dating

Law of superposition – in a sequence of horizontal sedimentary rocks, the oldest rocks are on the bottom

Originally Horizontal

Younger

Timing

Grand Canyon, Arizona, Copyright Larry Fellows

http://www.earthscienceworld.org/imagebank/search/results.html?ImageID=hmwnq6

Older

There are two principles that we learn from the man on the carpet, and that show up here in the Grand Canyon. When layers of rock are deposited, they usually start out flat (horizontal). This is because they often form under water where individual grains of sediment settle down to the bottom of the ocean, lake, or stream. Settling leads to horizontal layers. In the murder mystery, the carpet is also placed down originally horizontal.

We can also tell about the timing of a sequence of events. The man in the murder mystery is on top of the rug, so he had to fall AFTER the rug was placed on the ground (otherwise, someone would have had to slide the rug underneath the body, which is totally unrealistic). The same is true of layers of sediment. The oldest layers are on the bottom, and younger, newer layers get added on top of what is already there.

Example: S’more bar at a campfire

Plate

Graham Cracker

Chocolate bar

Toasted marshmallow

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Determining Age using Relative Dating

What happened here?
What was the original orientation of these layers?
Why aren’t they still in their original position?

USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California

http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm.

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Determining Age using Relative Dating

Law of cross-cutting relationships – igneous rocks are younger than rocks that they intrude into

Cross-cutting

MOST recent

Sometimes, layers get cut by something else. Here, the reddish-brown layers were deposited flat. Then, the dark layer cut across all the other layers. It is younger, because you can’t cut layers until they have all been deposited. This dark object was originally molten magma that was squeezed into the layers as it fought its way towards the surface. It eventually solidified, forming what you see here.

In the murder mystery, there is an example of cross-cutting relations. Where is it? (Coffee spilled across the papers). The papers were there before the coffee spilled.

Optional: The last geologic event in this picture was the carving of the Grand Canyon by the Colorado river. It cuts right through all the other rocks, so it must have happened AFTER they were deposited.

Magma under pressure squeezes into nearby rock whenever fractures form in the rock. Then the magma slowly cools and becomes rock itself.

This newly formed igneous rock is, of course, younger than the rock it intruded. In the same way, fractures and faults are younger than the rock that they cross.

Faults cannot occur in rock that does not already exist. The rock must be present first, which means it will be older than the fault, when the fault occurs.

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Where can you see cross-cutting rock or faults?

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Determining Age using Relative Dating

Law of Inclusions

Rock fragments found in another rock must be older than the rock they are found in

Examples

- pebbles in a conglomerate

- Sand grains in a sandstone

- chocolate chips in a cookie

http://www.geology.sfasu.edu/rocks/conglomerate02.jpg

http://www.zionnationalpark.com/zioninfo/photos/Sandstone-2.jpg

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Geologic Law of Inclusions

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Determining Age using Relative Dating

Unconformity – break in the rock record

Eroded surface then re-buried
Parts of rock record missing like pages in a book
Gap in geologic time

http://www.bamboo.hc.edu.tw/~sts/course-2003/course/textbook/text05/ch14/images/ch14-032.jpg

Optional (can be skipped for younger kids): Sometimes, layers give an indication of many different events. First, there was a set of layers laid down flat. Then, they got tilted, like the flatirons in a previous photo. Over time, the tips of the layers got eroded down into a new, flat surface. Then, another set of flat layers got deposited on top.

This feature is called an “unconformity” because one set of layers does not fit in with the tilting of the other set of layers (don’t ‘conform’).

Advanced topic: Erosion and deposition don’t happen in the same place at the same time. Either you are building up layers or eroding them away. Because of the erosion that happens during part of the sequence of events leading up to an unconformity, there are no rocks that are deposited during that time. So, if you were to date each layer in this sequence, you would find that there was a big gap where no rocks were deposited. Something is missing! But because there is an unconformity, you know that it must be missing. The missing telephone in the murder mystery is an analog to an unconformity. You know that there had to be something there before, and an event must have removed it. So sometimes the absence of evidence is itself evidence of an event!

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*

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Unconformities

An unconformity is a surface of non-deposition or erosion separating younger and older rock layers. Because no rocks are being preserved during the period of non-deposition or erosion, they represent a break in the record of geologic time. There are three major types:

Nonconformity –
Angular unconformity -
Disconformity -

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Nonconformity

a nonconformity is an unconformity separating older igneous or metamorphic rock from younger overlying sedimentary rock.

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Angular Conformity

An angular unconformity separates tilted or folded rock layers from overlying units that have a different formation.

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Disconformity

A disconformity is an unconformity which separates parallel to sub-parallel sedimentary layers.

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Be able to give the history of this…

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Geologic Time

___________

____________

http://www.astro.lsa.umich.edu/users/cowley/grand33.jpg

250 Ma

600 Ma

Age Determinations: RELATIVE vs. ABSOLUTE

Here is a picture from the Grand Canyon showing different layers and an unconformity. You can tell which rocks are older because the oldest rocks are on the bottom. We call this “relative dating” because it describes the relationship between the ages and positions of different layers.

Another way geologists can tell about a sequence of events is by using radiometric dating. You’ve probably heard of carbon-14 dating, but there are all sorts of elements that radioactively decay over time. Carbon is not very abundant in most rocks, so we can’t use carbon-14 dating. Instead, most rock ages are determined using the decay of Uranium, or sometimes other elements. These radiometric clocks, when properly calibrated, tell us the age in years of a given layer of rock. Oldest layers are still on the bottom, but now we know exactly HOW old.

In the murder mystery, there are two examples of absolute dating. What are they? (Wrist watch on dead body and calendar open on desk). Geologist can read the layers of rock almost as easily as a person could read the pages of a calendar.

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Measuring Absolute Ages

Tree rings can be used to measure absolute ages

1 ring = 1 year
Width of ring correlates to growing conditions
Wider ring = optimal growing conditions
Oldest tree is the bristle cone pine

4,862 – Prometheus

4,767 -- Methuselah

http://www.championtrees.org/champions/articles/AP21010.htm

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Radioactive Elements & Absolute Dating

Radioactive decay is used to determine the age of materials in years
Some elements “decay” naturally

Radioactive decay occurs until a stable element is formed

http://www.pbs.org/wgbh/nova/first/radiocarbonce.html

Radiometric dating uses the abundance of parent and daughter isotopes to determine the age of the material.

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Half-Life

Half-life = rate of radioactive decay

Time needed for 1/2 of the radioactive atoms to decay
Ranges from seconds to billions of years

Radioactive material	Found in	Half-life (years)
C-14	Wood, peat, coal , bones, shells	~5,700
K-40	Mica, feldspars	~1.3 billion
Rb-87	Mica, feldspars	~47 billion
U-235	Many rocks	713 million
U-238	Oldest igneous rocks	4.5 billion

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Using Half-life

The half-life of a radioisotope is the time required for half of the atoms in a given sample to undergo radioactive decay
Half-life is given the symbol t_½
Different radioisotopes have different half-lives
The amount of radioactive isotope remaining can be calculated:

N_t = N_o x (0.5)^{number of half-lives}

Where: � N_t = amount of radioisotope remaining � N_o = original amount of radioisotope � number of half-lives = time ÷ half-life

*

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Using Half-life

Consider strontium-90 which has a half-life of approximately 28 years.

Initially, at time t=0, the sample is 100% strontium-90
After 28 years, only half the original amount of strontium will remain: � ½ x 100% = ____________
After another 28 years, only half of this amount of strontium-90 will remain: � ½ x 50% = _____________
After another 28 years, only half of this amount of strontium will remain: � ½ x 25% = ____________

*

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Using Half-life

Calculate the percentage of strontium-90 remaining after 280 years.

N_t = N_o x (0.5)^{number of half-lives}

N_t = ? % �N_o = 100% �number of half-lives = time ÷ half-life = 280 ÷ 28 =10
N_t = ???

*

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Types of Fossil Preservation

Frozen in ice
Resin (Amber)

Tree sap
commonly insects

Tar Pits

LaBrea

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Fossil Preservation

Replaced Remains
Hard parts such as bones and shells are replaced by minerals
Groundwater carries dissolved minerals in solution. These replace the hard parts.

Ex: petrified Wood

Mold or Cast

Mold – shape of the fossil
Cast – new material fills mold & hardens

Ex: Shells

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Fossil Preservation

Trace Fossils
Other types of evidence of life include:

trails
footprints
burrows

http://www.delargy.com/images/2004_7_Colorado/dinosaur%20footprint.JPG

http://csd.unl.edu/csd-esic/ResourceNotesImages/volume16/page-24.jpg

http://www.scienceviews.com/photo/thumb/SIA0651.jpg

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Index Fossils

Index fossils
Organisms that existed during specific times only.
Allow dating of rock layers with relative age

3 Characteristics

Easily recognized (unique)
Found over large geographic area
Limited in time

Only lived over short period of time, therefore, only found in FEW rock layers

http://www.paleocurrents.com/img/2002_09_13FI/HTML/138-3848_img_std.jpg

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Ammonite Index Fossil in Central Italy (mid-Devonian in age)

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Key Beds

Key Bed
Single rock layer that acts like an index fossil
Easily recognized
Large area

Ex: Ash from volcanic eruptions

Iridium layer from Chicxulub impact (K-T)

http://www.astro.uva.nl/encyclopedie/images/chicxulub.gif

Iridium Layer

Clay with high rare element (Ir) found in Mexico, Italy, Denmark & New Zealand

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Clay layer with iridium spike from Gubbio, Italy

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K-T Boundary

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Rock Correlation

Correlation: matching of rock together from one area to another

Index Fossils
Key Beds

Allow correlating over great distances

Iridium Layer

Clay with high rare element (Ir) found in Mexico, Italy, Denmark & New Zealand

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Geologic Timescale

Eon – longest division of geologic time
Era – 2 or more of these form an eon
Period – basic unit of geologic time in which a single type of rock system is formed
Epoch – subdivision of a period
Age – subdivision of an epoch