Geology- Midterm 2

• Physical Reservoirs

  Atmosphere, Ocean, Earth's crust
• Biological Reservoir

  plants, animals, Microbes
• Light oxygen and light water

  • water with light oxygen isotopes are more prone to evaporation, light water preferentially stored on land, heavy water stored on oceans
• What is the Precambrian?

  • time prior to Phanerozoic
  • Haden, Archean, Proterozoic
  • last for more than 4 billion years
• What is the origin of the Solar System?

  • solar nebula- dense rotational cloud
  • planets formed near time of sun's formation
  • planets far from the sun are formed from volatile elements
  • planets close to the sun are rocky
• what are meteorites?

  • stony meteorites- rocky composition
  • iron meteorites- metallic composition
  • stony- iron meteorite- mixture of rocky and metallic (used to understand what the core of the planet might have looked like)
• what is the origin of the earth?

  • earth materials differentiated- dense at the center, less dense silicates rose to surface, cooled to form crust
  • meteorite impacts increased concentrations of some elements in upper earth
  • crust, mantle, liquid iron outer core
  • solid iron inner core
• what is the origin of the moon?

  • moon formed by impact
  • mantle of impacting body
  • proportions of Fe and Mg differ from earths mantle
• what are cratons?

  • stable core of modern continents
  • precambrian
• what is the Precambrian shield?

  • part on the craton exposed
  • ex- Canadian shield exposed by glaciation
• when did the fist continental crust form

  • archean
  • high heat flow
  • continent small
• what was earths early Archean atmosphere and oceans like?

  • atmosphere- degrassing from volcanic emissions, CH4 and NH3 abundant, almost no oxygen, no photosynthesis
  • oceans- volcanic emissions cooled, condensed, salt carried to sea by rivers and introduced at ridges
• what is heat flow like through time?

  • decreased through time
  • indicates abundant hot spots, small lithospheric framgents
• what is the origin of continents?

  • first crust is basalt
  • felsic differentiated- formed nuclei of continental crusts, Iceland
  • greenstone belts in Pilbara shield, weakly metamorphosed, abundant chlorite, nested in high grade felsic metamorphic rocks
  • greenstone belts contain igneous rocks, volcanics contain pillow basalt (under water extrusion) and deep water sedimentary rocks (graywackes, mudstone, iron formations)
• where are some of the oldest rocks from?

  • western Australia- zircon crystals in slightly metamorphosed sandstone
  • 4.1-4.2 billion years
  • weathered from felsic rocks
  • canadian shield- 3.8-4.0 billion years
• what are greenstone belts?

  • deep water sediments accreted to continent
  • marine sediments form wedge between continental masses
• what is Archean life?

  • earth is best suited known planet
  • western Australia organic compounds
  • carbon isotopes in rocks (southern Greenland may indicate life was present 3.85 billion years ago)
  • South Africa cherts contain possible mold of prokaryotic cell
  • pillow basalts
  • fossil prokaryotes, the rocks in which the fossils were found came from a vein that may have been produced hydrothermally
  • stromatolites, biomarkers for cyanobacteria
  • mid ocean ridges- high heat chemosynthetic organisms
  • hydrogen oxidation, sulfur reduction, methane production
• what is the miller and urey experiement?

  • produced amino acids found in protiens
  • modeled primitive atmoshepre
• what is the atmosphere and BIF's formation?

  • atmospheric oxygen, early Archean very low concentrations
  • sinks for oxygen- reduced iron and sulfur, take up oxygen as soon as it was produced
  • the evidence is that pyrite transported but not oxidized, uraninite
• how are the BIF formation rocks related to the evolution of the Archean atmosphere?

  • the atmosphere was deficient in free oxygen so that little oxygen was dissolved in seawater
  • as photosynthesizing organisms increased in abundance free oxygen release as a metabolic waste product into the oceans causing the precipitation of iron oxides along with silica generating BIFs
  • a likely source of the iron and silica was submarine volcanism at or near spreading ridges
  • high quantities of dissolved minerals are discharged at submarine hydrothermal vents
• what is the style of crustal evolution?

  • Archean crust forming processes generated granite-gneiss complexes, greenstone belts that were shaped into cratons
  • these same rock associations continued to form during Proterozoic but they did so at a considerably reduced rate
  • many Archean rocks have been metamorphosed although their degree of metamorphism varies and some are completely unaltered
  • vast exposures of Proterozoic rocks show little or no effects of metamorphism in many areas they are separated from Archean rocks by a profound unconformity
• how is the Proterozoic characterized?

  • by a plate tectonic style essentially the same as the present
  • by widespread rock assemblages that are rare or absent in the Archean- red beds, shallow water carbonates
  • by important evolution of the atmosphere and biosphere
• what are the Proterozoic continents?

  • Archean protocontinents assembled during collisions resulting in micronutrients
  • Proterozoic crust accreted forming much larger landmasses: continents and super continents
  • Proterozoic accretion probably took place more rapidly than today because earth possessed more radiogenic heat
• what are the three Paleoproterozoic orogeny?

  • passive margin
  • flysch deposits
  • molasse deposits
• what is a passive margin?

  • thick deposit of quartz sandstone and carbonate platform
  • transitional mudstone
• what is a flysch deposits?

  • turbidites and other deep marine sedimentary rocks signal the active margin
• what is a molasse deposit?

  • marginal marine and continental deposits
• what are the Proterozoic supercontinents?

  • existed at the end of the Paleozoic
  • there were also at least two Precambrian supercontinents
  • the super continent Pangaea, Nuna
  • Nuna- supercontinent that contained all of earths major landmasses, not universally accepted
  • Rodinia- super continent, United Laurentia to other land masses, Fully assembled 1 ga, created pacific ocean, created failed rifts in western Laurentia
• what is the belt supergroup?

  • thick sequence of sediment formed in failed rift
  • 0.9-1.5 Ga
  • accumulated in shallow water during rapid subsidence
• what is the Mesoproterozoic orogeny

  • the only Mesoproterozoic event in Laurentia was the Grenville orogeny in the eastern part of the continent
  • Grenville tocks are well exposed in the present day northern Appalachian Mountains as well as in eastern Canada, Greenland, and Scandinavia
• what is the supercontinent Pannotia?

  • Rodina's separate pieces reassembled and formed another more controversial supercontinent known as Pannotia, about 600-550 million years ago
  • fragmentation was underway again by the beginning of the Phanerozoic giving rise to the continental configuration that existed at the oneset of the Phanerozoic
• what do the super continents look like? - supercontinents are large landmasses that are formed when two or more continents collide

  
• what are the characteristics of ancient glaciation?

  • very few times of widespread glacial activity have occurred during Earth History
  • the most recent one during the Pliocene Pleistocene, 3 million to 11,000 ya, is certainly the best known but we also have evidence for upper carboniferous- lower Permian glaciation
  • Ordovician (ash Gillian) glaciation
  • and at least two more major Proterozoic glaciation
  • we cant calculate how old the ice is, we rely on other evidence
• what are some glacial evidence?

  • tillites with extensive geographic distribution- look like extremely poorly sorted rocks
  • glacial are good at transporting rocks, they are full of material, because they are continuously slowly moving
  • drop stones
  • glacial features such as distinctive glacial striations and polished rock
  • geochemical evidence
• what are the characteristics of the 1st snowball earth?

  • the occurrence of tillites of about the same age, 2.3 billion, suggest an early proterozoic glacial event
  • Gowanda tillites in Michigan, Wyoming and Quebec in north America in Norway Europe also in australis and south Africa
• What are the characteristics of the later Proterozoic glaciation 2nd snowball Earth?

  • tillites and other glacial features dating from between 800 and 600 million years ago are found on all continents except Antarctica
  • glaciation was not continuous during this entire time but shows several episodes, numbered between two and seven (mostly recognized 2 and 4)
  • the most extensive glaciation in Earths history
• what are the characteristics of the carbon isotopes in Neoproterozoic carbonates

  • drop in carbon-13 just before glaciation can be explained as a drop in biological productivity as iced formed over the oceans at high latitudes
  • iced covered the oceans entirely, biological productivity would have ceased
• what are stromatolites?

  • layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria (commonly known as glue- green algae)
  • oldest is about 3.5 billion years
• what is the Paleoproterozoic life?

  • prokaryotic forms
  • gunflint flora 1.9-2 Ga
  • lake superior
  • algae
  • multicellular protists
  • algal ribbons wound into loose coils
• what is the evolution of eukaryotes?

  • union of 2 prokaryotic cells
  • mitochondrion
  • allows cells to derive energy from their food by respirations
  • evolved from 1 prokaryotic cell
  • chloroplast
  • site of photosynthesis
  • protozoan consumed, retained cyanobacterial cell