Precambrian (Hadean - Archean) Flashcards
What does the Precambrian actually refer to?
The span of Precambrian?
The time before the emergence of LARGE and VISIBLE animal fossils
Encompasses Earth’s FIRST 4 billion years or so
What are the 3 EONS that make up the Precambrian?
What percentage of Earths total history is Precambrian?
Hadean, Archean, Proterozoic
87%
When does the emergence of large bodied animals happen?
Around 550 m.y.
What marks the appearance of animals?
CAMBRIAN
Difference between before and after the the Precambrian and Cambrian? Regarding fossils
Cambrian = LARGE ANAIMAL FOSSILS
Precambrian = NO LARGE ANIMAL FOSSILS
What is the interval where Early Earth was molten and no rocks were preserved?
Hadean Eon, 4.5-4.0 billion years ago
What is the Archean eon?
Occurs in-between the Earth’s cooling and the first oxygenated atmosphere
What are modern continents built around?
Modern continents are built around stable cores of Archean-Proterozoic rock called CRATONS
What are cratons?
Represent Earth’s earliest continents
What MAKES UP (parts of the world) North America’s craton?
North America’s craton spans the central continent and includes most of Greenland
What makes up the ancient continent Laurentia?
Greenland and North America COMBINED
What was NA referred to at the time of the ancient continents?
Laurentia
What do cratons consist of?
Very old metamorphic rocks (1-4 Billion years old) intruded by igneous rock
MOST ARE FOLDED GNEISS WITH SOME GRANITE
What are unique about cratons compared to other plate boundaries?
Cratons are stable cores, meaning that they are not easily affected by plate tectonic motion
They, however, can have pieces added over time
What does NA’s craton do for the country?
The stable craton of NA represents the US’s flattest portion of the entire country
How and where do continents grow?
Continents grow AROUND cratons through ACCRETION
What is accretion?
The addition of continental landmass due to continental collisions
How did Asia grow as a continent?
Asia grew as continent due to the accretion of India
ABOUT 10 MILLION YEARS AGO
What was accreted to the NA craton? When?
Western North America was accreted to the NA craton during the last 600 million years
PROCCESS ADDED LAND TO NA
What can cratons be divided into?
Can be divided into distinct terranes
What atmosphere was the Earth’s Archean eon?
The atmosphere during the Archean lacked oxygen but did contain abundant amounts of carbon dioxide
What are terranes? What is unique about them?
Terrane is a geologically distinct region of Earth’s crust that behaves as a coherent crustal block
EACH TERRANE IS AN INDEPENDENT LANDMASS UNTIL IT ATTACHES TO ANOTHER TERRANE THROUGH ACCRETION
When (what eon) and how many terranes were formed to eventually make the NA craton?
During the Archean, 6 distinct terranes formed independently, each a landmass
What did cyanobacteria form?
How did these form?
They formed layered stromatolites
Stromatolites form from the upward growth of a microbial mat consisting of sticky bacteria
What happened to iron during the Archean period?
The Archean atmosphere was so low in oxygen that iron did not oxidize (to rust) at Earth’s Surface
What can be found in very old river deposits?
Pyrite (FeS2)
What caused the rise of oxygen?
Emergence of life in the form of photosynthetic cyanobacteria
What are Earth’s oldest fossils?
Archean-age bacteria
Why and how do these microbes (cyanobacteria) form stromatolites that are able to be preserved?
Microbes incorporate sediment into the stromatolite through TRAPPING & BINDING
As new sediment falls on the mat, it becomes TRAPPED and BOUND as the microbes grow upward
Why do microbes grow upward?
They grow upward in order to reach light, photosynthetic cyanobacteria reach for the sky during the day and lay down at night
What are stromatolites made out of? Specifically what rock?
Are made out of limestone (calcite)
BY FAR THE MOST CONSPICUOUS COMPONENT OF THE PRECAMBRIAN ROCK RECORD
Why did oxygen increase during the Archean?
Photosynthesis of photosynthetic cyanobacteria
What’s the formula for photosynthesis?
Water (H20) + Carbon Dioxide (CO2) + Sunlight —-> Food (CH2O) + Oxygen (O2)
What happened to the microbes in the absence of animals during the Archean?
They took over the environment, coating every available surface
The seafloor at this time would’ve been heavily coated in a thick microbial mat
What are wrinkle structures?
The preserved remanent of a microbial mat in a sandy beach environment
THEY ARE A SEDIMENTARY STRUCTURE
