Geological Time and Evolution

Welcome to Poorna Pal's 'Earth Revealed' telecourse (Geol 101: Physical Geology; section #7695) at the Glendale Community College

Home | Quizzes  |  Module 1 | Module 2 | Module 3 | Module 4 | Module 5           05/05/15
 

Help available 'Online'

 

Help available at this site
 
 

You may need to download the Adobe Acrobat Reader to view these files
     
 

Click on the image on the right to browse the online edition of USGS publication "Geological Time"
 
 
 
 
  • Textbook Chapter:   8 (Time and Geology)

    This chapter will help you develop a sense of the vast amounts of time over which the geological processes have been at work. The following three aspects of these studies
    are covered here:

    • 'Relative' versus 'Absolute' Dating of Rocks

    • Correlation or the determination of age relationships between geographically widely separated rock units

    • The Geological Time Scale

  
 
 
 
 
 
 

(based on the chapter introduction)

    

 

Poorna's Web Outline

Geologic Time and Evolution

 

Source:

 http://www.talkorigins.org/faqs/faq-age-of-earth.html#dal01

Estimating the Earth's Age

Lead-ratio (or the Pb207-Pb206 isochron) method provides the most direct means to estimate the Earth's age. As Pb207 comes from radioactive decay of U235 and Pb206 from that of U238, the plot of these two lead isotopes (relative to the non-radiogenic Pb204 or Pb208) should be linear if the solar system formed from a common pool of matter that was uniformly distributed in terms of the Pb-isotope ratios. The older the sample, the higher the uranium-to-lead ratio and Pb206/Pb204 and Pb207/Pb204 values will be. The slope of the straight-line fit to 5 meteorite and one terrestrial data thus yields an estimate of ~4.55 Ga. This is also the result from dating of meteorite samples by Rb-Sr and other radiometric methods.

 

Geological Time and the Evolution of Life

 

  • Life and the Geological Scale of Time

It is not clear whether life intrinsically evolved on the earth or, having originated elsewhere, proliferated on the earth after the first oceans appeared ~4 Ga ago. Based on the earliest evidence of life, the 3.7-4 Ga old stromatolites, the first 500-1000 Ma of earths history appears to have been altogether barren.

Based on the fossil evidence, we divide the geological time into the following:

  • Phanerozoic: the most recent 570 Ma of earth's history with a well-preserved record of life, comprising 

    • Paleozoic: the 245-570 Ma era of primitive life-forms (vertebrate life evolved in the early part of this era, with Devonian as the age of fish);

    • Mesozoic: the 65-245 Ma era that began with the evolution of dinosaurs as also mammals but was dominated by the dinosaurs; and

    • Cenozoic: the most recent 65 Ma of earth's history that began with the extinction of dinosaurs and has been dominated by the mammals.

  • Precambrian: the period since the earth's evolution, ~4.5 Ga ago, until the dawn of the Phanerozoic (circa 570 Ma), and therefore named Precambrian, it is divided into:

    • Proterozoic: an era of primitive life-forms that is usually divided into the

      • late (570-1250 Ma before the present)

      • middle (1.25-1.9 Ga before the present); and

      • early (1.9-2.5 Ga before the present); and

    • Azoic or Archean: the earliest 2-2.5 Ga of earth's history.

  • Gradualism, Punctuated Equilibrium and Mass Extinctions:

    • Evolution of life over the geological times has followed three strands:

      • evolution of new species,
        e.g., the end-Permian appearance of dinosaurs and mammals,

      • extinction of some existing species,
        e.g., the end-Cretaceous extinction of dinosaurs, and

      • proliferation of some existing species,
        e.g., the Cenozoic domination of mammals.

    • Darwinian evolutionary model sought gradual morphological changes, leading to the evolution of new species, as would result from adaptation to the environmental change. But, compared to this gradualism, the observed fossil record displays sudden appearance of new species following periods of prolonged morphological statis. The Eldredge-Gould model of punctuated equilibrium (i.e., new species appear suddenly when, under environmental stress, portions of the gene pool of some existing species undergo rapid speciation) overcomes this problem.

See, for instance, Punctuated Equilibrium at Twenty: A Paleontological Perspective by Donald Prothero (Skeptic  vol. 1, no. 3, Fall 1992, pp. 38-47): http://www.skeptic.com/01.3.prothero-punc-eq.html and Score One for Punk Eek: The fitful evolution of bacteria supports a controversial theory by John Horgan (Scientific American, July 21, 1996): http://www.sciam.com/article.cfm?chanID=sa004&articleID=000DFABC-A1BF-1C76-9B81809EC588EF21

  • Mass Extinction Events:

    Instances of sudden mass extinction events too exist. For instance, the end-Cretaceous dinosaur extinction, ~65 Ma ago, was also when 75% of the species disappeared and, at the end of the Paleozoic, ~245 Ma ago, an estimated 90% of all the species became extinct. Indeed, as the graph alongside shows, such events have recurred with a 25-30 Ma cyclicity that matches those in the records of bolide impacts as also volcanism. Hence the controversy about extraterrestrial catastrophism versus terrestrial cataclysms as the source of the environmental trauma that triggered these extinction events.

 
  • Want to visit the Grand Canyon?

Click on this profile of the Colorado plateau, shown on the left, to learn about the geology of the Grand Canyon of Colorado river.

Note: Rotating globe at the top left corner of this page is from http://adinet.net/Globe_HTML/frames/scientific/f2p3a7_78kb2.htm

This site was last updated 05/05/15