Proterozoic (midterm 2) Flashcards
What time frame was the Proterozoic Eon?
2.5 Ga - 542 Ma
What are some characteristics of the rocks/fossils in the Proterozoic Eon?
- many Proterozoic rocks are exposed and not metamorphosed
- proterozoic fossils are uncommon
- fewer greenstone belts and granite-gneiss complexes
- passive continental margins
- many banded iron formations
List some differences between Archean and Proterozoic plate tectonics
Archean:
- “Archean-style” tectonics - unstable
- metamorphosed and complexly deformed rocks
- fast crustal movement
- rare sedimentary rocks (minor component in GSB and GGC) - most rocks are igneous
- high rate of volcanism
Proterozoic:
- “modern-style” plate tectonics - stable permanent crust
- many undeformed and unmetamorphosed rock successions
- slower crustal movement
- widespread sedimentary assemblages
- lower rate of volcanism
What are some characteristics of plate tectonics specific to Proterozoic time?
- commonly separated from Archean rocks by an unconformity
- abundant stromatolites
- quartzite-carbonate-shale assemblages
- fluvial sands, continental red beds
- deep marine sediments
What is Laurentia? What areas does it make up?
Early continent that makes up the core of North America
- also makes up parts of Greenland, Scotland, and possibly Baltic Shield and Scandinavia
- lots of exposures in North America
What is basement rock?
Crystalline (igneous and metamorphic) rocks that lie beneath sedimentary rocks, mostly all Archean and Proterozoic
Underneath all the younger sedimentary rocks on top
What is an orogen?
A part of Earth’s crust that was or is being deformed during an orogeny (mountain building event) > can form belts
When/where did the growth of Laurentia begin?
2-1.8 billion years ago
(in the Paleoproterozoic era - 2.5-1.6 Ga)
Most of it started from the Wopmay orogen
(Archean cratons coming together at orogens - grows along its southern and eastern margins)
*beginning of the north american continent
List the steps of the Wilson Cycle
- Uplift
- Divergence (spreading - creates an ocean basin)
- Passive continental margin (no significant plate bounday/tectonics)
- Convergence (subduction)
- Convergence (collision) and uplift
- Convergence and uplift
What is the Wilson Cycle?
A model that describes the opening and closing of ocean basins and the subduction and divergence of tectonic plates during the assembly and disassembly of supercontinents
*check textbook definition about this perhaps
Which parts of Laurentia showed up 1.8-1.6 billion years ago (in the Paleoproterozoic)
Yavapai province
Mazatzal province
Which parts of Laurentia showed up 1.3-1 billion years ago (in the Mesoproterozoic)
Grenville province
Granite-Rhyolite province
Llano province
What is the significance of the rocks of the Grenville orogeny? (when did they occur, where are they exposed, what did they maybe result in?)
Grenville Orogeny occurred on the eastern boundary of Laurentia from 1-1.3 Ga
Grenville rocks (metamorphic) exposed in the modern northern Appalachian Mountains, eastern Canada, Greenland, and Scandinavia
Grenville belt may have resulted in the closure of an ocean basin (end of a Wilson cycle) that assembed the supercontinent Rodinia, which persisted into the Neoproterozoic
At the end of the Grenville Orogeny?? - about 75% of present-day North America existed
In the Wilson cycle, there is a change from _______-_______ compact to _______-______ compact
continent-ocean
continent-continent
What timeframe does the Rodinia supercontinent span?
1 Ga - 542 Ma
middle to end of Proterozoic eon
(actually broke up around 750 Ma)
What are continents?
- formed by cratons
- has a shield and platform
- not just land above sea level, edges can be submerged
- granitic crust, thicker than ocean crust
What are supercontinents?
Two or more continents that have joined
- all or most of Earth’s land mass > except oceanic islands
ex. Pangea (300-200 Ma)
- older supercontinents existed
Modern-style plate tectonics were occurring by the ___________
Paleoproterozoic
What are Ophiolites? What is the ideal ophiolite sequence?
Ophiolites are one feature used to recognize ancient convergent plate boundaries (Neoarchean & Paleoproterozoic)
> during subduction, pieces of oceanic lithosphere accrete onto the edge of the continent
Ideal Sequence:
- deep-sea sediments (sandstone, black shale, chert)
- oceanic crust (pillow lava and sheet lava, sheeted dikes, massive gabbro, layered gabbro)
- upper mantle peridotite
What is one of the oldest known complete ophiolites?
The Jormua Mafic-Ultramfic Complex Finland
1.96 Ga
What are the Proterozoic Supercontinents?
What are the two POSSIBLE Archean supercontinents?
Nuna 1.8 Ga (Paleoproterozoic)
Rodinia 1.3-1 Ga (Mesoproterozoic)
Possible Archean Supercontinents:
Vaalbara 3.1 Ga
Kenorland 2.7 Ga
- would’ve been small; accretion and underplating just beginning
What is the supercontinent cycle?
Assembly, fragmentation, reassembly
Explain the Supercontinent Rodinia and what happened to it
Rodinia is the oldest documented supercontinent
- assembled 1-1.3 Ga and began fragmenting about 750 Ma
- continental rifting (divergent plate boundary)
Separate pieces of Rodinia reassembled into the supercontinent Pannotia about 650 Ma > Pannotia fragmented about 550 Ma
What is the most recent glaciation event in Earth’s history
When were there glaciation events before this?
Pleistocene (ice age) spanning from 2.6 Ma - 11,700 years ago
(repeated glacial periods - ice sheets covered a large part of North America 5 different times during this period)
Paleozoic glaciers (after proterozoic)
Proterozoic glaciation
*overall very few widespread glaciation events in Earth’s history
What is the evidence for glaciation events?
- Tillite
- Varves
- Striated, polished bedrock
- extensive geographic distribution
Describe the Neoproterozoic glaciers - what theory did this lead to?
Neoproterozoic (final era of the proterozoic eon) glaciers : 900-600 mya
- on all continents except Antarctica > Tillites and varved mudstones
- episodic, not continuous
- most extensive in Earth’s history
*if there were widespread glacial deposits everywhere did glaciers cover all the land? were seas frozen? snowball earth?
What would some of the triggers be of a snowball earth?
- the near equatorial location of most continents
- high rates of weathering > consumed CO2 in atmosphere leads to glaciers
- glaciers reflect solar radiation > leads to more glaciers
*look over diagram/steps in pictures
How would the snowball earth have ended?
- volcanoes adding CH4 & CO2 to atmosphere > warms atmosphere (greenhouse gas effect)
- movement of continents to higher latitudes > slowed weathering rates
What are theories for how life would’ve survived through a snowball earth?
- it was thin glacial ice?
- hydrothermal vents?
- slushy with some open water?
*the retreat of glaciers was one of the causes of the explosion of life that followed
Explain the general forms of life that existed during the Proterozoic
- abundant bacteria and archaea
- cyanobacteria become more abundant (widespread stromatolites)
- appearance of eukaryotes > sexual reproduction speeds up evolution
What are the subgroups of prokaryotic and eukaryotic cells
Prokaryotic:
- Monera
- Archaea
Eukaryotic:
- Protista
- Fungi
- Plantae
- Animalia
What is Symbiosis and Endosymbiosis? When is this thought to have happened?
1.5 Ga (but there is no fossil evidence for this)
Symbiosis: Relationship between 2+ organisms where both benefit, sometimes to the point where they cannot live separately
Endosymbiosis: One symbiont lives within another
What is the oldest known eukaryote?
The Bangiomorpha
- sexually reproducing red algae
- Somerset Island, Canada
1.2 Ga
What are multi-celled organisms and their advantages? When are they thought to have started existing?
Organisms that have cells performing specialized tasks (ex. reproduction and respiration)
Advantages: Larger size, efficiency, longer life
They existed in Neoproterozoic, likely earlier
- no fossil record for how they came to exist
- colonial organisms likely intermediate step
What were the Edicaran Fauna?
Edicaran Fauna is the name that we give collectively to fossils of this type found in this time period (635-545 Ma)
The first animals
- soft bodied
- widespread (found in every continent except antarctica), but fossils not common
First evidence in the fossil record of hard parts being found - no skeletons yet but they had Chitinous carapace (similar to insect exoskeleton)
- some have spots/small areas of calcium-carbonate (CaCO3, limestone)
- environment = form in shallow seas
- first evidence of simple ecological interactions (This evidence is why we see a shift between this time period and the next - start seeing organisms with these types of skeletons)
The Ediacaran Environment is thought to represent what 3 modern groups?
Phylum Cnidaria
- jellyfish and sea pens
Phylum Annelida
- Segmented worms
Phylum Arthropoda
- primitive versions of insects, spiders, crabs and others
- very limited hard parts
In the Proterozoic context, what are dikes?
Sheet-like intrusions of magma that cut through existing rock layers
What is the basic difference between prokaryotes and eurkaryotes?
Prokaryotes: single celled, fairly simple, reproduce asexually
Eukaryotes: a bunch of other internal structures, reproduce sexually, increases evolution because of genetic variation
