Earth’s HistoryThe Rock Record

Topic 1 – Telling Time

• Kinds of Time– 2 types

1. Relative time – places an event in order by comparing it with other events

– Example: Sedimentary Rock layers» A – Oldest (Lower)» B – Middle Age» C – Youngest (Upper)

– Actual age not known, only relative!

•Relative time – place in a sequence

Topic 1 – Telling Time2. Absolute time – specific age

–Example: Sedimentary Rock»Rock Layer B deposited 30 Ma (mega anna = million years ago)»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

Two things told with absolute time

1. Actual age

2. Length of time between events

-- used to determine rate of geologic process

So... What’s the story?

List observations

Do not write down interpretations

Finding Age with Relative Time

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

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Finding Age with Relative Time

http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page03.cfm

Finding Age with Relative Time

• What happened here?• Originally layers deposited

horizontally• Then they were tilted

USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, Californiahttp://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm.

Finding Age with Relative Time• Law of Cross-cutting

relationships – igneous rocks are younger than rocks that they intrude into

Cross-cutting

MOST recent

Finding Age with Relative Time

• Law of (fragmented) Inclusion – Rock fragments found in another

rock must be older than the rock it is found in•Examples

– pebbles in a conglomerate– Sand grains in a sandstone

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

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

Finding Age with Relative Time

• Unconformity – break in the rock record– Eroded surface

then 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

Geologic Time

______________________

________________________

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

250 Ma

600 Ma

TIMING: RELATIVE vs. ABSOLUTETIMING: RELATIVE vs. ABSOLUTE

M. d’Alessio, 2004

Murder Mystery?

How is this picture explained using relative dating?

List observations From your observations make interpretations

Law of SuperpositionPrinciples of: Original Horizontality Cross-Cutting Rel. Inclusions Unconformities

Topic 9 – Measuring Absolute Time

http://home.earthlink.net/~colorado_hiking/1_Hiking_topics/pics/bristleconepineT.jpg

4,862 – Prometheus

4,767 -- Methuselah

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

• Tree rings can be used to measure specific dates– 1 ring = 1 year– Width of ring correlates to

temperature and rainfall– Wide ring = high temperature

and rain– Oldest tree is the bristle cone

pine

Radioactive Elements & Absolute Time

• Radioactive decay used to date much farther back in time

• Some elements naturally “decay” in the nucleus– Radioactive decay occurs until a

stable element is formed

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

Topic 12 – Half-Life

• Half-life = rate of decay–Time needed for ½ radioactive items 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

Fossil Preservation

• 1. Original Remains• Frozen• Resin (Amber)

– Sticky substance from evergreens– Usually insects

• Tar Pits– LaBrea

Fossil Preservation

• Replaced Remains• Hard parts replaced by minerals• Usually done by groundwater

– Ex: petrified Wood

• Mold or Cast• Mold – shape of where fossil was• Cast – new material fills mold &

hardens– Ex: Shells

Fossil Preservation• Trace Fossils• Evidence of life other than

remains – trails– Footprints– Burrows– copralites

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

Topic 5 – Fossils as Evidence for Evolution• Oldest rocks with fossils show

simple life forms (only)• As time goes by, some simpler

life forms become more complex

• This process of change is called:EVOLUTION

• Theory of Evolution: Scientific explanation for the past and present diversity of lifehttp://www.thezreview.co.uk/posters/posterimages/e/evolution1.jpg

Topic 5 – Fossils as Evidence for Evolution

• Charles Darwin – “The Origin of Species”, 1859

• - theory states that species evolve slowly over geologic time

• Modern science - species remain fairly constant for millenia, then abruptly change within a relatively short time, often less than 1 million years

http://earth.ast.smith.edu/courses/ast215/darwin.jpg

Topic 6 – Index Fossils and Key Beds

• Index fossils (guide fossils)• Lived for a “short geologic”

time period in a lot of places• Allow dating of rock layers

with relative age3 Characteristics1. Easily recognized (unique)2. Found over large geographic area

– Continental drift theory evidence

3. 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

Topic 6 – Index Fossils and 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

Topic 7 – Rock Correlation

• Correlation: matching of rock layers from one area to another

• Index Fossils• Key Beds

• Allow correlating over great distances

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

Topic 8 – Other Uses for fossils

1. Determine relative age of rocks

2. Correlate rock layers3. Indicate past climate

• Ex: coal only from warm, swampy areas

4. Oil exploration• Microfossils – seen only with

microscope• Help correlate layers of rock to oil

rich layers in other places

Topic 10 – Varves

• Varve• Any sediment that

shows a yearly cycle

• Sedimentary layers resemble tree rings

• Can date ~15,000 years ago– Often associated

with lakes annual turnover

Geologic Timetable (p. 600-601)• Eon – greatest 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 (named for specific occurrence)

• Epoch – subdivision of a period• Age – subdivision of an epoch

– All changes based on significant changes in the fossil record

• The greater the changes, the larger division

• Fossil – ANY evidence of earlier life or environments preserved in rock record

Geologic Timetable

• The geologic time scale can be represented in many ways

Geologic Timetable

Geologic TimetableERA Attributes

Archean (Beginning Earth)Began ~4.6 Ga

Beginning of Earth, almost no fossils

Proterozoic (Earliest Life)

Began 2.5 Ga

Simple plants & animals (worms)No life on land

Paleozoic (Ancient Life)Began 570 Ma

Plant & Animal FossilsLand & Ocean

MesozoicBegan 250 Ma

Middle LifeAge of Reptiles (dinosaurs)

Cenozoic (Recent Life)Began 65 Ma

Age of Mammals