ch 7-6 geologic time

what are the two ways of dating geological materials (age)

• qualitatively and quantitatively

  • which are older and younger?

what are relative ages based on?

• upon the order of formation and permit determination of older vs younger relationships

  • doesn’t provide specific age

  • number of geological principles to order rocks

what are numerical (absolute) ages?

• actual number of years since an event occurred

  • using radioactive decay of isotopes in rocks

how old is Earth?

• 4.5 billion years old (4.5 Ga)

• eons and eras

one thousand years

• =Ka (kilo-annum)

one million years

• =Ma (mega-annum)

one billion years

• =Ga (giga-annum)

what eon is the vast majority of the Earth’s past in?

• Precambrian

  • Proterozoic

  • Archean

  • hadean

how is Earth’s geologic time scale divided into?

• Eons

  • eras

    • periods

      • epochs

• each division is smaller and smaller

• more recent we have more rocks to create more categories

What major events occurred during the Hadean Eon?

• Formation of Earth and Moon, accretion of planetesimals, differentiation of Earth, formation of a molten iron core, and development of the first crust.

What does it mean that Earth differentiated during the Hadean Eon?

• Dense materials such as iron sank toward the center, forming the core, while lighter materials rose toward the surface.

How did Earth's first atmosphere and oceans form?

• Volcanic degassing released gases (N₂, CO₂, SO₂, H₂O), leading to the formation of the atmosphere and oceans by about 3.8 billion years ago.

high heat flow during Hadean Eon (4.5-4.0 Ga)

• radioactive decay, bombardment by meteorites

Earth differentiated

• molten iron core developed

• end of Hadean: crust

rocks mostly not preserved

• some minerals (zircon) as old as 4.4 Ga

• oldest known rocks on Earth 4.03 Ga (NWT)

what did volcanic gegassing led to?

• development of oceans by 3.7 Ga

• development of atmosphere

  • N2, CO2, SO,2, H2O-rich

  • denser than present day, not oxygenated

The Acasta Gneiss

• oldest rocks found in the Northwest territories

• gives us clues about what life on Earth was like at that time

• Isotopic age dating as 4.03 billion years ago (formed when Earth was 5y/o)

• rocks define boundary between Hadean and Archean eons

What major developments occurred during the Archean Eon 4.0 to 2.5 Ga?

• Formation of continental nuclei, development of oceans, appearance of the first life, and the establishment of Earth's magnetic field ~3.5 Ga.

What is the earliest evidence of life during the Archean Eon?

• Stromatolites produced by cyanobacteria, dating to at least 3.5 billion years ago.

  • led to oxygenated atmosphere

• may have begun in Hadean

How did cyanobacteria change Earth's atmosphere?

• Through photosynthesis, they released oxygen, eventually leading to an oxygenated atmosphere.

well developed hydrosphere during Archean

• large erosive rives

• weathering of rocks made oceans salty

• atmosphere: outgassing rom volcanoes → reducing atmosphere

distribution of rocks

superior province of Manitoba

• most significant deposits of Gold, Nickle, and Cooper, and potential to host diamonds

What major atmospheric change occurred during the Proterozoic Eon (2.3 to 0.541 Ga)?

• The atmosphere became increasingly oxygenated.

• stabilization of continental platforms

What type of organisms became abundant by the end of the Proterozoic Eon?

• Multicellular organisms.

  • evolution from bacteria

  • level of O2 rose significantly

What were the Ediacaran Fauna?

• The earliest known soft-bodied multicellular animals that lived near the end of the Proterozoic Eon.

Ediacaran Fauna fossils

• earliest known, soft complex, multicellular

• first animals, soft bodied

• multicellular

• required oxygen

• precursors to animals with skeletons

• first evidence of simple ecological interactions

• pioneers of complex life

What are the major characteristics of the Phanerozoic Eon 541 (Ma to present)?

• Modern plate tectonics, modern atmosphere, diverse life, and the transition of life from the oceans onto land.

What major biological transition occurred during the Phanerozoic Eon?

• Life expanded from marine environments onto land.

How does the atmosphere of the Phanerozoic compare to today?

• It is essentially the modern oxygen-rich atmosphere.

hydrosphere during Phanerozoic eon

• shallow seas through much of time

atmosphere during Phanerozoic eon

• The atmosphere contained abundant oxygen and supported diverse plant and animal life.

biosphere during Phanerozoic eon

• diversity of life

• transition from marine to terrestrial

what are eons divided into?

• eras

eras of the Phanerozoic

• Paleozoic (“old life”)

  • ←——— >90% of species extinct

• Mesozoic (“middle life”)

  • ←——— ~75% of species extinct

• Cenozoic (“new life”)

when did the “Great Dying” happen?

• Paleozoic era

  • believed to be from massive volcanic events that caused rapid global warming, ocean acidification, and widespread monomial → ocean depletion of O2

  • 90-96% of marine species died

  • ~70% of land vertebrate species died

what era did the dinosaurs die?

• Mesozoic era

  • asteroid impact in Mexico that caused dust and ash to block the atmosphere for months to years

    • halted photosynthesis and causes collapse of food chain

period are divided into?

• epochs

  • smaller time frame

  • more rock preserved to make distinguishing between time period