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?
- 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 - mostly igneous
- high rate of volcanism
Proterozoic:
- “modern-style” plate tectonics - stable permanent crust
- many undeformed and unmetamorphosed rock successions
- slower crustal movement
- widespread sedimentary rocks
- lower rate of volcanism
What is Laurentia?
Early continent that makes up the core of North America
What is basement rock?
Igneous and metamorphic rocks that lie beneath sedimentary rocks, mostly all Archean and Proterozoic
What is an orogen?
A part of Earth’s crust that is deformed during an orogeny (mountain building event)
When/where did the growth of Laurentia begin?
2-1.8 billion years ago - in the Paleoproterozoic
Most of it started from the Wopmay orogen (western north america and grew along southern and eastern margins)
List the steps of the Wilson Cycle
- Uplift
- Divergence (spreading - creates an ocean basin)
- Passive continental margin
- 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 tectonic plate activity during the assembly and disassembly of supercontinents
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 1.3-1 Ga
Grenville metamorphic rocks exposed in the modern Appalachian Mountains, eastern Canada, Greenland, and Scandinavia
Grenville belt may have resulted in the closure of an ocean basin that assembed the supercontinent Rodinia
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?
about 1 Ga - 542 Ma
middle to end of Proterozoic eon
Describe what makes up and forms continents?
- formed by cratons > has a shield and platform
- granitic crust, thicker than ocean crust
What are supercontinents?
Two or more continents that have joined
- all or most of Earth’s land mass
ex. Pangea (300-200 Ma)
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 (top to bottom):
- deep-sea sediments
- oceanic crust
- upper mantle
What are the Proterozoic Supercontinents?
Nuna 1.8 Ga
Rodinia 1.3-1 Ga
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)
Pieces of Rodinia reassembled into the supercontinent Pannotia about 650 Ma > Pannotia soon fragmented after that
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 (end of the proterozoic eon) glaciers :
- on all continents except Antarctica > Tillites and varved mudstones
- episodic, not continuous
- most extensive in Earth’s history
- because they were so extensive it led to the snowball earth theory
What would some of the triggers be of a snowball earth?
- continents close to the equator
- high rates of weathering > consumed C02 in atmosphere leads to glaciers
- glaciers reflect solar radiation > leads to more glaciers
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
