Geologic History. Geologic Time Scale Chart of Earths history showing events, time units, & ages...

GeologicGeologicHistoryHistory

Geologic Time ScaleGeologic Time Scale


Chart of Earth’s history showing events, time units, & ages

Era: the four largest subdivisions of the geologic time scale – Precambrian, Paleozoic, Mesozoic, Cenozoic

Period: subdivisions of eras on the geologic time scale

Epoch: subdivisions of periods on the geologic time scale – ONLY in the Cenozoic

Traces from our pastTraces from our past

• Index Fossils: a fossil of a species that existed briefly and was widespread geographically, used in determining the relative age of rock layers (i.e. trilobites)

• Fossils and Ancient Environments: fossils can be used to determine what the environment of an area was like long ago.

Traces from our pastTraces from our past

Fossils: the remains or traces of a once-living organism preserved in rock.

How fossils form:– organisms have a better chance of being

preserved if they have hard parts (bones, shells, teeth, or wood)

– usually found in sedimentary rocks

Petrified RemainsPetrified Remains

Petrified Remains – plant or animal remains that have been turned to rock; this happens when minerals carried in groundwater replace the original material.

Petrified WoodPetrified Wood

Carbonaceous FilmCarbonaceous Film

• Carbonaceous Film – a fossil impression in a rock, consisting only of a thin carbon deposit

Mold and CastMold and Cast

Mold – a cavity in a rock that has the shape of a fossil that was trapped there; water dissolved the fossil away, leaving its imprint

Mold and CastMold and Cast

Cast – type of fossil formed when an earlier fossil in rock is dissolved away, leaving behind the impression of that fossil, and new sediments or minerals enter the mold

Original RemainsOriginal Remains

• Original Remains – sometimes the actual organism or parts of organisms are found (mosquitoes in amber, wooly mammoth frozen in ice)

Trace FossilTrace Fossil

• Trace Fossil – footprints, worm holes, burrows & other traces of animal activity preserved in rock

Trace FossilTrace Fossil

Age Dating TechniquesAge Dating Techniques

Relative Dating: determining the order of events and the relative agesrelative ages of rocks by examining the positions of rocks in layers

Age Dating TechniquesAge Dating TechniquesRelative Dating – another good diagram

UniformitarianismUniformitarianism

• In 1795, James Hutton described a ‘new In 1795, James Hutton described a ‘new concept’ stating that: concept’ stating that: – The geologic processes now at work were The geologic processes now at work were

also active in the past.also active in the past.– The present physical features of Earth were The present physical features of Earth were

formed by these same process, at work over formed by these same process, at work over very long periods of timevery long periods of time

• ““the present is the key to the past”the present is the key to the past”

Relative DatingRelative Dating

• Law of Original Horizontality: sediments are deposited in a horizontal fashion


• Law of Superposition: in layers of undisturbed rock, the oldest are on the bottom, and rocks become younger toward the top

Law of SuperpositionLaw of Superposition

Relative datingRelative dating

• Law of Cross-Cutting Relationships: igneous rock intrusions or faults are younger than the rocks that have been intruded or faulted

Relative DatingRelative Dating• Law of Included Fragments: pieces of

one rock found in another rock must be older than the rock in which they are found


• Correlation: the matching of rock layers from one area to another to establish a relative date


Unconformity: one or more missing layers in a sequence of rocks, this is the result of gaps in the time/rock record

• angular unconformity – tilted rock layers meet horizontal rock layers, this indicates layers are missing and there is a gap in the time record

• disconformity – the top rock layer is eroded before the next layer can be deposited causing a gap in the time record

Angular UnconformityAngular Unconformity

DisconformityDisconformity


• Absolute Dating: determining the age of rocks using the radioactive decay of atoms.

• Radiometric Dating: an Absolute Dating method that uses the rate of decay of radioactive isotopes in rocks.


• Radioactive Decay: the decay of an atom of one element to form another element, occurring when an alpha particle or beta particle is expelled from the original atom (Ex. parent atom (K) decays to daughter product (Ar))

• Half-Life: the time it takes for half of the parent atoms of an isotope in an object to decay into the daughter product (K-Ar = 1.3 billion years)

Age Dating TechniquesAge Dating TechniquesTree rings – counting the number of rings on a

cross section of a tree

Radio-Carbon dating – the radioactive carbon isotope used for dating organic material:

Carbon 14 decays to Carbon 12

The ‘half life’ of Carbon 14 is 5700 years…

Half life – how long it takes for half of the material to decay

Precambrian TimePrecambrian Time

• from 4.6bya to 545mya• makes up ~90% of Earth’s history• very little is known about fossils from this period• cyanobacteria appeared around 3.5 bya

– responsible for oxygen and

indirectly for ozone in

the atmosphere• invertebrates developed

at end of era

Paleozoic EraPaleozoic Era::545mya to 245mya• began when animals

developed hard parts that could be fossilized

• life moved from ocean to land

• end signified by mass extinction probably caused by formation of Pangaea

• Early - “Age of the Invertebrates”

• Late – “Age of the Plants”

Mesozoic EraMesozoic Era• 245 mya to 66 mya• Laurasia &

Gondwanaland developed from the break-up of Pangaea

• Reptilian eggs developed a hard shell

• Dinosaurs evolved during the Triassic and dominated during the Jurassic and Cretaceous periods

Mesozoic Era (cont.)Mesozoic Era (cont.)• Mammals appeared

during the Triassic• Birds appeared during

the Jurassic• Angiosperms

appeared during the Cretaceous

• End signified by mass extinction

• “Age of the Reptiles” (Dinosaurs)

Cenozoic EraCenozoic Era

• 66mya to present

• Increased tectonics created the Alps, Himalayas, and the Appalachians• Global temperature drop

• Mammals evolved and

dominated• Homo sapiens emerged

500,000yrs. ago• “Age of the Mammals”

EvolutionEvolution• Organic Evolution: gradual change in life-

forms through time• Species: group of organisms similar to

each other and that typically reproduce only with each other.

• Natural Selection: natural process by which some organisms survive and reproduce because they have traits favorable to survival in an environment, while others die out because they lack those traits

EvolutionEvolution

• Endangered: describes a species that has a small number living and thus in danger of dying out

• Habitat: any place where organisms live, grow, and interact

General Related Facts – General Related Facts – (the amazing impact humans have had)(the amazing impact humans have had)

• Four years ago 19,836sq. km. of Amazon rainforest was destroyed

• More than 80% of all deforestation of the Amazon has occurred since 1980

• More than ¼ of all pharmaceuticals come from rainforest plants

General Related Facts – General Related Facts – (the amazing impact humans have had)(the amazing impact humans have had)

only 22% of Earth original forest coverage remains…..– Western Europe lost 98% of its primary

forests; – Asia 94%; – Africa 92%; – N. America 66%; – S. America 54%

Geologic History. Geologic Time Scale Chart of Earths history showing events, time units, & ages...

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