Piecing together Earth’s history1. Relative DatingA. Using laws of superposition, cross cutting, etc. to

determine a sequence of events (this happened, then that)

B. Index Fossils – constrain (pin down) a time range when a rock layer was deposited.

Fossils of extinct organisms, that were wide spread, and lived during a specific period of time.

Example: Trilobites

Geologic Time & Earth’s Evolution

Relative Dating: Index Fossils

Piecing together Earth’s history2. Absolute DatingRadioactive elements in rocks, like uranium,

potassium, carbon, breakdown at a known rate. If you know how much was in the initial sample … you can determine age. (Isotopic dating)

How Earth was Made, Part 1: http://www.youtube.com/watch?v=-IVBfhbhU-c&feature=related

Ch 23.6: Geologic Time & Earth’s Evolution

Ch 23.6: Geologic Time & Earth’s Evolution

ERAS & PERIODSWhy all the divisions?To manage Earth’s history, geologists have divided Earth’s history into EONS. Eons are divided into ERAS. Eras are divided into PERIODS. Periods are divided into EPOCHS.

You need to know the major developments in terms of life forms, tectonics, atmosphere and climate for each ERA.

Geo. Time is broken up into ERAS (based on life forms and enviro.)1. Cenozoic (now – 65 mya)2. Mesozoic (254 ma)3. Paleozoic (540 ma)4. All time before that =

Precambrian (540 ma – 4.6 billion)

ERAS are NOT EQUAL in TIME.

The Precambrian was the longest ERA. It took Earth a LONG time to be suitable for complex life.

Each era ends with a mass extinction followed by a burst of speciation.

Mass Extinctions: Punctuate E’s History

Between each Era (and some periods)

Mass Extinctions

• = High % of species go extinct

• Species that are better suited to environment are successful, reproduce, and evolve.

• Causes: Meteor impact; Volcanism; Glaciation; anything that can change atmosphere, climate, or environmental conditions.

CH 23.6 Geologic Time and Earth’s Evolution

Complex multicellular life is a “recent” development.

Precambrian

http://www.pbs.org/wgbh/nova/link/hist_01.html

Fossils from this time are not abundant in the rock record because most organisms are “soft bodied” – no skeletons or shells.

Precambrian: 4.5 billion – 540 ma•Earth starts off as MOLTEN and VOLCANIC•Atmosphere = CO2, CH4(methane), H2O= Green house gases •Oceans form and first continents form when Earth cools

• Origin of water = volcanic steam & meteorites (comets)

•1st life = bacteria (single celled) 3.5 bya - use chemosynthesis (not sunlight) for energy. They can withstand extreme conditions.

Precambrian: 4.5 billion – 540 ma•Cyanobacteria (blue-green algae)

• ~ 3 bya – take in CO2 and release O2 (photosynth) PLANET CHANGING!

• Temps. Decrease (take in CO2)• Oxygen makes O3 = OZONE LAYER Shields surface from UV radiation (UV kills cells)O2 atmosphere– more complex organisms to develop!•Simple multicellular organisms in shallow seas•1st Supercontinent (Rhodinia)•MASS EXTINCTION = SNOW BALL EARTH ICE AGE! Closes PRECAMBRIAN

Paleozoic Era

Paleozoic Era: 540 – 245 mya•CAMBRIAN EXPLOSION starts off ERA – “explosion” in the diversity of life forms • Snowball Earth is thawed by millions of years of volcanic activity.•Warm temperatures•LIFE IN SHALLOW SEAS•½ through, life “invades” land (amphibians, land plants, insects) (Thank you Ozone)

Paleozoic Era: 540 – 245 mya•Early GIANT FORESTS/Swamps become our fossil fuels (Buried, but not decomposed plant and animal matter fossil fuels)•Pangaea assembles Interior of continent becomes DRY and COOL(favors reptiles etc)•Ends with…MASS EXTINCTION = “GREAT DYING” Volcanic activity? Lava flows in Siberia for millions of years change climate?

Mesozoic

Mesozoic Era: 245 - 65 mya•AGE of DINOs•WARM and subtropical environment•Virtually ice-free•Pangaea rifts apart•1st Flowering plants; Giant insects; birds•Mammals appear (small)

•Ends with…MASS EXTINCTION = ASTEROID IMPACT = K-T EVENT (+ Lava flows in India)Meteoroid impact in the Gulf of Mexico

Cenozoic

Cenozoic Era: 65 mya - present•AGE of Mammals•ICE AGES – Glaciers advance and retreat

• Ocean currents change due to new continent positions; Mt. Building changes air circulation

•Hominids (human ancestors) – 2 mya; Modern humans – 200,000 yrs•Global Mountain building - affect air circulation drier & cooler conditions•Cooler & Drier Grasslands (not tropical forests)•When glaciers grow …

• Sea level drops • More land is exposed (land bridges)

•Ends with… ?????

What does a mass extinction look like in the Rock Record?

Black layer = carbon rich; carbon from organisms that died abruptly and could not all be decomposed; carbon in organism changes w/heat and pressure to become oil/coal.

White layer = Debris from asteroid/impact

Meteoroid that hit 65 mya caused a Mass Extinction

•75% Species on planet go extinct.•90% of planktonic life in ocean die (plankton supports the oceanic food chain).•Takes 10,000 years to recover.

http://www.lpi.usra.edu/science/kring/epo_web/impact_cratering/Chicxulub/Chicx_title.html

How do we know when Meteoroids have hit Earth in the Past?

Look for … in the rock layer1. Iridium – an element not

common on Earth but common on meteroids (asteroids or comets)

2. Shocked Quartz – heat changes quartz grains

3. Tektites - quartz grains vaporized and cooled so fast they turn in to glass beads.

K-T Boundary: This layer is found on every continent in rocks 65 my old.

K-T ExtinctionWhen: 65 million years ago (End of Cretaceous-

Beginning of Tertiary Periods)Where: Chicxulub – Yucatan Pennisula MexicoWhat: Asteriod or Comet – 10km in diameter

makes a crater 180 km in diameter.Impact Event: 100 MillioMegatons TNT (6 x more

energetic than 1980 Mt. St. Helen’s eruption)Results– Fires; Magnitude 10 Eqs; Tsunamis;

Climate change; Volcanism; Mass extinction (collapse of ocean food chain & bye-bye dinosaurs)

Human Evolution & Hominidshttp://www.talkorigins.org/faqs/homs/species.html

Evolution of Early Humanswww.pbs.org/.../evolution/adaptable-human.html

Cave paintingshttp://www.culture.gouv.fr/culture/arcnat/chauvet/en/

Evolutionary Treehttp://www-tc.pbs.org/wgbh/nova/beta/assets/img/the-adaptable-human/image-05-large.jpg

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Mass Extinctions In Earth's History

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