Historical Geology
INTRODUCTION
WHAT HAPPENED DURING THE EOARCHEAN?
CONTINENTAL FOUNDATIONS—SHIELDS, PLATFORMS AND CRATONS
Archean Rocks
Greenstone Belts
Evolution of Greenstone Belts
¬PERSPECTIVE EarthÕs Oldest Rocks
ARCHEAN PLATE TECTONICS AND THE ORIGIN OF CRATONS
THE ATMOSPHERE AND HYDROSPHERE
How Did the Atmosphere Form and Evolve?
EarthÕs Surface Waters—The Hydrosphere
THE ORIGIN OF LIFE
Experimental Evidence and the Origin of Life
Submarine Hydrothermal Vents and the Origin of Life
The Oldest Known Organisms
ARCHEAN MINERAL RESOURCES
SUMMARY
The following content objectives are presented in Chapter 8:
¬ Precambrian time, which accounts for most of geologic time, is divided into two eons: the Archean and the younger Proterozoic.
¬ The Archean geologic record is difficult to interpret because many of the rocks are metamorphic, deformed, deeply buried, and they contain few fossils.
¬ Each continent has at least one area of exposed Precambrian rocks called a shield and a buried extension of the shield known as a platform. A shield and its platform make up a craton.
¬ All cratons show evidence of deformation, metamorphism, and emplacement of plutons, but they have been remarkably unaffected by these activities since Precambrian time.
¬ The two main associations of Archean rocks are granite-gneiss complexes, which are by far the most common, and greenstone belts that consist of igneous rocks but sedimentary rocks are present primarily in their upper parts.
¬ Greenstone belts likely formed in several tectonic settings, but many appear to have evolved in back-arc marginal basins and in rifts within continents.
¬ Plate tectonics was taking place during the Archean, but plates probably moved faster, and igneous activity was more common then because Earth possessed more heat from radioactive decay.
¬ Gases released during volcanism were responsible for the origin of the hydrosphere and atmosphere, but this early atmosphere so formed had little or no free oxygen.
¬ The oldest known fossil organisms are single-celled bacteria and chemical traces of bacteria-like organisms known as archaea. Bacteria known as blue-green algae produced irregular mats and moundlike structures called stromatolites.
¬ Resources found in Archean rocks include gold, platinum, copper, zinc, and iron.
To exhibit mastery of this chapter, students should be able to
demonstrate comprehension of the following:
¬ the determination of the subdivisions of the Precambrian Era
¬ the events—and evidence—for Earth during the Eoarchean
¬ the formation of the shields and platforms that make up the foundations of continents
¬ the difficulties in interpreting the Archean geological record
¬ the stratigraphic succession and models for the origin of greenstone belts
¬ the features of Archean plate tectonics and crustal evolution
¬ the origin and evolution of the atmosphere and hydrosphere
¬ the experimental evidence and models for the origin of life
¬ the nature of the Archean fossil record
¬ the major types and geologic associations of Archean age ore deposits
1. All geologic time from EarthÕs origin to the beginning of the Phanerozoic Eon is included in the Precambrian. Precambrian also refers to rocks lying stratigraphically below Cambrian-aged rocks.
Figure 8.1 Geologic
Time Represented by a 24-hour Clock
2. The Precambrian is divided into two eons, the Archean and the Proterozoic, each of which has further subdivisions.
3. Rocks from the latter part of the Eoarchean indicate that crust existed then, but very little of it has been preserved.
Figure 8.2 The
Precambrian Geologic Time Scale
Figure
8.3 Earth
As It May Have Appeared Soon After It Formed
4. All continents have an ancient craton made up of an exposed shield and a buried platform. In North America, the Canadian shield is made up of smaller units delineated by their ages and structural trends.
Figure 8.4 Origin of Granitic Continental Crust
Figure 8.5 The Distribution of Precambrian Rocks
Figure 8.6 The Geologic Evolution of North America
5. Archean greenstone belts are linear, syncline-like bodies of rock found within much more extensive granite-gneiss complexes.
Figure 8.7 Archean Rocks in North America
Figure 8.8 Greenstone Belts and Granite-Gneiss Complexes
Figure
8.9 Greenstone
Belts in North America
6. An ideal greenstone belt consists of two lower units of mostly igneous rocks and an upper sedimentary unit. They probably formed in back-arc basins and in intracontinental rifts.
Figure 8.10 Origin of a Greenstone Belt in a Back-Arc Marginal Basin
Figure 8.11 Origin of a Greenstone Belt in an Intracontinental Rift
7. Many geologists are convinced that Archean plate tectonics took place, but plates probably moved faster, and igneous activity was more common then because Earth had more radiogenic heat.
Figure 8.12 Origin
of the Southern Superier Craton
Some geologists argue that data supports modern-style subduction processes in plate tectonics, possibly dating to the Hadean or Eoarchean. Mid-Archean island arc volcanoes, subduction zone related volcanogenic sulfide deposits, and isotopic data from the worldÕs oldest zircons are just some of the data supporting modern subduction processes in the early Precambrian. Cawood, ÒPrecambrian Plate Tectonics: Criteria and Evidence,Ó GSA Today, July 2006 v.16, no.7, p.4-11.
8. Outgassing was probably responsible for the early atmosphere and the hydrosphere. However, the atmosphere so formed lacked free oxygen but contained abundant carbon dioxide and water vapor.
Table 8.1 Composition of EarthÕs Present-Day Atmosphere
Figure 8.13 Outgassing and EarthÕs Early Atmosphere
Figure 8.14 Evolution of the Atmosphere
Figure 8.15 Ratio of Radiogenic Heat Production in the Past Compared to the Present
9. Models for the origin of life by natural processes require an oxygen-deficient atmosphere, the necessary elements for organic molecules, and energy to promote the synthesis of organic molecules.
Figure 8.16 Stanley
MillerÕs Experimental Apparatus
10. The first naturally formed organic molecules were probably monomers, such as
amino acids, that linked together to form more
complex polymers, including nucleic acid and proteins.
Figure
8.17 Experimental
Production of Thermal Proteins and Microspheres
Researchers announced that they have uncovered vials from some of Stanley MillerÕs related experiments. After Miller died in 2007, the Scripps Institution of Oceanography inherited his scientific possessions. Eleven vials were recovered from a second version of the Miller experiment, which revealed better results than the original. The apparatus was similar to the first experiment, except that a steam injector simulated water vapor from early volcanoes. Testing of the vialsÕ contents revealed 22 amino acids, and researchers concluded that volcanoes were important in the origin of life. Earth, December 2008, v 53 n. 12 p.24.
11. RNA molecules may have been the first molecules capable of self-replication. However, the method whereby a reproductive system formed is not known.
12. Submarine hydothermal vents were probably more common during the Archean. Several minerals containing zinc, copper, and iron precipitate around them, and they support communities of organisms. These environments may represent the environments in which life evolved.
Figure 8.18 Submarine
Hydrothermal Vents
13. The only known Archean fossils are of single-celled prokaryotic bacteria such as
cyanobacteria, but chemical compounds in some Archean rocks may indicate the
presence of archaea.
14. Stromatolites that formed by the activities of photosynthesizing bacteria are found in
rocks as much as 3.5 billion years old. .
Figure 8.19 Stromatolites
Figure 8.20 Probable Archean Microfossils
Archean mineral resources include gold, chrome, zinc, copper, and nickel.
Robert Hazon, a geophysicist at Carnegie Institution for Science, studied the long-term growth of EarthÕs mineral complexity. Whereas interstellar dust contains only 12 or so minerals, the accretion of interstellar dust and the formation of protoplanets produced more minerals. Meteorite-size masses reveal about 60 minerals, whereas protoplanets of 200 km or more in diameter have 250 minerals.
The first life on Earth didnÕt significantly increase the number of minerals. About 2.5 billion years ago the mineral number was about 1,500. With the arrival of cyanobacteria, mineral types increased dramatically, and today there are about 4,300 minerals. Science News, December 6, 2008, v. 174 n. 12 p. 10.
Life in the Archean
1. To present a vivid image of the Precambrian world, have the students try to envision a world with nothing living on the land surfaces. (Some of the recent images of the stark Martian landscape may be appropriate to use.) An image of a very stark landscape is easy to imagine; even modern deserts seem lush in comparison. Reinforce this with a discussion of the role that plants play in stabilizing the land. How would the processes of weathering and erosion compare with modern-day rates?
2. Paleontologists deal with an incomplete fossil record and must recognize the limitations of their data base. Should we expect the fossil record of the Archean to be as complete as that of more modern times? What conditions would be necessarily for the preservation of microscopic, single-celled, soft-bodied organisms? The likelihood of these organisms being preserved and avoiding destruction is very slim. Also discuss how some regular structures—the so-called nannobacteria—are currently debated as early organic life forms.
3. Why do researchers look for signs of organic activity, as opposed to fossils of the earliest organisms themselves? What products, or ÒsignsÓ would living organisms leave behind?
1. Archean tectonic processes are poorly understood. What evidence suggests that the intracontinental rift model is valid for the formation of greenstone belts? Does the modern analog of the Sea of Japan make the back-arc marginal basin model more acceptable?
2. Can geologists easily distinguish between early life forms that are preserved in rocks, and regular inorganic structures? What type of evidence and/or features must accompany these structures before we can definitively accept them as fossils of ancient life?
|
abiogenesis anaerobic |
craton granite-gneiss complex |
photosynthesis platform |
|
autotrophic |
greenstone belt |
Polymer |
|
back-arc marginal basin |
heterotrophic |
Precambrian shield |
|
black smoker |
monomer |
prokaryotic cell |
|
Canadian shield |
outgassing |
Stromatolite |
|
continental accretion |
photochemical dissociation |
submarine hydrothermal vent |
|
|
|
|
|
1. b |
5. a |
9. d |
|
2. c |
6. d |
10. b |
|
3. e |
7. c |
|
|
4. a |
8. a |
|
11. Black smokers discharge water
saturated with dissolved minerals. Several minerals containing zinc, copper,
and iron are precipitated around them. Biologically, they support unique
communities of organisms, and it is proposed that the first self-replicating
molecules came into existence near these vents on the seafloor.
12. Continents grow at their margins in a process called continental accretion. This happens as plates collide with island arcs and other plates.
13. In photosynthesis, water
and carbon dioxide are combined to make organic molecules and molecular oxygen
is a byproduct. In photochemical dissociation, UV light breaks up water
molecules into H2 and O2 components. Of the two
processes, photosynthesis is more important in adding O2 to the
atmosphere.
14. Monomers are simple
organic molecules such as amino acids, and polymers are more complex molecules
consisting of monomers linked together in a specific sequence. In MillerÕs and
subsequent experiments, all 20 common amino acid monomers have been produced by
putting energy into a mixture of simple gases thought to mimic the early
atmosphere. Proteinoids, which consist of more than 200 linked amino acids,
were synthesized when dehydrated concentrated amino acids were heated. Monomers
probably formed continuously and by the billions and accumulated in the early
oceans, while polymers may have been formed when monomers were washed up on to
a beach, or a cinder cone. They may have been concentrated by evaporation and
polymerized by heat.
15. Greenstone belts have volcanic rocks, including thick accumulations of pillow lava, in their lower and middle parts; they are topped by sedimentary rocks. They include ultramafic lava flows, which require temperatures far in excess of those found in lava flows today. The rocks typically have synclinal structures and most have been intruded by granitic magma and cut by thrust faults. Greenstone belts likely formed in back-arc marginal basins, as lava flows and their overlying sediments, that opened and closed during the extremely hot Archean.
16. Graywacke is a sandstone with abundant clay and rock fragments, while argillite is a slightly metamorphosed mudrock. Small-scale cross-bedding and graded bedding indicate these rocks represent turbidity current deposition. These rocks are found within the sedimentary rocks of a greenstone belt, usually in the upper unit.
17. Stromatolites represent reefs constructed
by algae. They form and grow as sediment grains are trapped on sticky mats of
photosynthesizing cyanobacteria. They are significant in that they represent
early life forms that photosynthesized.
18. Archean rocks are extremely old. They have been altered and are not in their original state. The stratigraphic relationships that may once have existed are now destroyed. There are no fossils, so Archean rocks canÕt be used as an indicator of relative age or position.
19. The earliest cells depended on an external food source (heterotrophic), existed in an environment without oxygen (anaerobic), and were simple cells lacking a nucleus or other complex structures (prokaryotic).
Apply Your Knowledge
1. The oldest known rocks on Earth, the 3.96-billion-year Acasta Gneiss in Canada and the 3.8-billion-year-old rocks from Montana and Greenland indicate some crust had evolved by Eoarchean time. In addition, some sedimentary rocks in Australia contain 4.4-billion-year-old detrital zircons, so source rocks at least that old must have existed. Plate tectonics operates on Earth, so much of the early crust would have been recycled during subduction.
2. Ultramafic lavas cannot form today because Earth has cooled significantly since the Precambrian. The extremely high temperatures needed to form ultramafic rocks have not been seen on Earth since the Precambrian. If Earth were to get hot enough, ultramafic lavas would form again.
3. The processes of volcanism, seismicity, plate movements, and mountain building are associated with tectonic activity. (Hot spots, representing intraplate tectonism, are associated with stationary mantle plumes.) Plate tectonics is driven by the convection cells in the mantle, although the specific mechanism is debated. Convection cells are produced by temperature differences. The heat of Earth came from gravitational compression, meteorite bombardment, and radiogenic heating. The residual heat from gravitational compression is dissipated, and meteorite bombardment is (fortunately) much reduced now. The process that is still yielding internal heat is radiogenic heating. However, Earth had all its radioactive elements at its formation, and with each half-life, the amount of parent decreases by half. Therefore, eventually, the remaining radioactivity will be insufficient to drive tectonics. Earth produced 3-6 times more heat than it does today
4. Precambrian subdivisions are based on absolute ages, as opposed to the Phanerozoic rocks that are based on stratotypes. The principles of superposition, original horizontality, and inclusions are used to decipher a set of sedimentary rock layers, but most of the Precambrian rocks are deformed and highly metamorphosed. Therefore, relative dating is not of much use in Precambrian age rocks.