Figure 2: Geological time scale 4 Eons of time

  1. Hadean Eon -- no rocks preserved
    • 3,800 m.y. -- the surface cools; begin continental growth
  2. Archean Eon -- a bunch of small granite 'islands' appear and move about
    • 2,500 m.y. -- good signs of oxygen in the atmosphere
  3. Proterozoic Eon -- Rodinia supercontinent, vast cyclic changes
    • 550 m.y. -- vast surge of new life forms; continents take shape
  4. Phanerozoic Eon
    • Paleozoic Era (550-250 m.y.)
    • Mesozoic Era (250-65 m.y.) -- era of the dinosaurs
      • Triassic Period (250-200 m.y.) -- first dinos & mammals @ 225 m.y.
      • Jurassic Period (200-150 m.y.) -- biggest dino ever - Seismosaurus
      • Cretaceous Period (150-65 m.y.) -- Pangea forms up at the end
    • Cenozoic Era (65 m.y. to modern) -- era of the mammals
      • Paleocene Period (65-56 m.y.)
      • Eocene Period (56-39 m.y.)
      • Oligocene Period (39-23 m.y.)
      • Miocene Period (23-5.5 m.y.)
      • Pliocene Period (5.5-2.4 m.y.)
      • Pleistocene Period (last 2.4 m.y.)
        • Holocene Epoch (last 10,000 years)
          • Today -- hi-rise condos, café lattes, wars, Degas & Picasso, speeding tickets

(one billion years = 1,000 million years. Millions of years is abbreviated m.y.)

The geological record on Earth is most conveniently studied and catalogued as a chronological development of changes. The geological law of superposition rules supreme, that younger layers lay above older ones - though there are some profound exceptions like during mountain-building when crunching-shoving forces move things around and tip them over. We don't even like to mention that there is a big upside-down rock series in the Spring Mountains west of Las Vegas due to big-scale 'thrust faulting', shoving an older rock series on top another. But we have A SINGLE METHOD of determining absolute ages of rocks, by measuring the results of radioactive decay of chemical elements like uranium, thorium, potassium, rubidium, etc. I personally worked in such a lab in Tucson for 10 years and know the process and its various pitfall traps. (Carbon-14 or radiocarbon dating shares the basic process of measuring radioactive decay, but can only be used to age-date dead organic tissues, and not rocks, that are less than ~50,000 years old.) The first age-dates on rocks were made in about 1896 in Britain, and were surprisingly precise, but a major advance came in about 1950 with new high-tech equipment called a mass spectrometer that could measure isotope levels. The technique is too tricky to explain here except to say that what is measured is the ratio between a 'parent' radioisotope (uranium, rubidium, potassium) and its 'daughters' of decay (lead, strontium, argon). The ratio goes into an equation of exponential decay using an extrapolated laboratory-measured rate of decay (half-life), and an age of the rock is determined. This represents the age of formation by cooling of an igneous rock, or the last time of an intense heating of the rock that redistributes isotopes that 'resets' the clock. We now have more than 50,000+ age-dates on rocks around the world (my guess).

The body of combined age-dates of the world's rocks produces a logical sequence of events. There are indeed spurious age-dates that we must contend with. Religious fundamentalists use these anomalies to discredit the entire science as they desire enlightenment, just like the rest of us.

The game is to figure out our planet's history. Go to other sources for details. Based upon stratigraphy and geological events worldwide we divide total Earth history into four major eons of time, thus Figure 2, see above.

Hadean Eon is the time before surface rocks formed when the entire planet became molten due to intense radioactive decay and when the heavy metal core congealed at the center. The solar system seems to have been born in orbit of the Milky Way galaxy's center point, taking about 250 m.y. to go around once - so far, 11 times around. Early in Hadean time was when, by best account, a small stray planet hit Earth, blew a fair chunk of rock debris out, some of which went into orbit, then collected together into the body we call the moon. Why don't we have a name for our moon? It's like calling your dog 'dog', or 'Hey You!' I much prefer the name 'Georgette' or perhaps 'Appleyard.' Anyway, that just had to be a bleak Thursday early in Hadean time. Another early possible cataclysmic event was a minor 'readjustment' of the vigorous young sun called a 'T-tauri' explosion whose solar wind outburst may have blown away the bulk of the atmospheres of the inner planets, explaining why the outer planets have such thick atmospheres. No oceans then - too hot, so all the water that seeped out of the inner Earth after T-tauri time was present as a steam atmosphere along with nitrogen, carbon dioxide, methane, etc. Water vapor got blown out as far as Pluto that has tall mountains of ice. Little or no oxygen on Earth in the early days. At 3,900 m.y. it is guessed that two other planets collided, producing the asteroid belt (with pretty good evidence), but then a bunch of stray debris hit all other planets, causing great welts on the moon and Mars, and on Earth. Most of the moon's craters are thought to have formed at that time. There are some big ones still visible on Earth such as in eastern Quebec Canada, seen easily on Google Earth, and 150+ others. An impact in South Africa called the Vreedefort ring damaged the early crust that then bled out huge volumes of basalt magmas that happened to bring up enough gold to become the site of the world's largest gold mine, and then, tremendous human greed and angst.


During Archean time the planet became covered with an initial thin oceanic basalt crust. It thickened with basalt volcanoes everywhere, and soon pockets of a new kind of magma were produced, a less dense rock called granite that rose and solidified into mushroom-shaped islands. And so were born the cores of continents. Figure 7 shows details of this process - see the figure's write-up. And as the atmospheric temperature dropped below 212° F, the oceans started condensing as shallow freshwater boiling lakes in early valleys. Heat-loving bacterial life showed up very early and lived on the first sandy shores of the island continents, basking in the near-boiling tidal waters. Their fossils, oldest on Earth, are in the coastal Isua Complex of SW Greenland. These bacteria still live - called 'bifs' (banded iron formations) (Figures 37 & 38), blown out of black smoker hot spring vents in the oceanic spreading centers. The fellow who first dove in the Alvin submarine to discover these smoker vents in about 1978 off the southern tip of Baja California, Jack Corliss, was my boss during Biosphere 2 construction.

Then at about 3,500 m.y. ago we find the earliest fossil traces of another life form called stromatolites, with the capability of photosynthesis (Figures 39 & 40), so that levels of their waste oxygen started to increase in the atmosphere. They still live in intertidal zones of warm oceans, shaped like 1-2 foot diameter mushroom-shaped heads in colonies or simple bio-scums. Photosynthesis is an extraordinarily tricky process to have begun by 'chance' atomic encounters, and this process of conversion of sunlight into chemical energy sits as a huge stumbling block towards our understanding of life's origin. I once had a very interesting afternoon coffee at Biosphere 2 (Oracle) with H.J. Morowitz, an expert on the origin of life. Like the rest, he was grappling with the whopping improbability of this reality.

During Proterozoic time the continents grew much larger, to nearly their modern extents. At about 2,500 m.y. ago were the first obvious signs of some oxygen in the atmosphere, marked by the presence of 'redbeds,' which are sandstones with their sand grains stained red-brown by oxidized iron minerals like hematite. Signs of massive continental collisions - the early mountain chains they produced, now worn away into plains like across the Canadian Arctic. The first marine floating algae are recorded at 2,000 m.y. (Figures 34 & 35), and then later, marine burrowing worms (Figures 41 & 42) and jellyfish, and then sponges and many other unrecognizable critters who left fossil traces. A supercontinent called Rodinia came crashing together late in the eon, and finally broke up.

Phanerozoic time saw vigorous continental drift and many temporary arrangements of continental groups. For awhile there were two big clusters, called Laurasia and Gondwanaland. Then all conjoined into Pangea at the end of Paleozoic time, and then it broke apart with parts drifting into modern positions. Figure 5 displays this last breakup dance. During Phanerozoic time came the great bursting forth of multitudes of life in the oceans and on the continents by some extremely mysterious process, something called evolution. We do not understand more than a modicum of that process. The process is hidden behind barriers that we cannot breach, such as the fact that we can find fossil traces of some 0.01% of all organisms that have likely ever lived. Human history is similarly sketchy as we continue to destroy all the libraries. See E.O. Wilson's book, The Diversity of Life, (W.W. Norton, 1992) or Richard Fortey's book Life.

All of the Southwest - the lands south of southern Wyoming and west of Denver - contain only rocks of the Proterozoic and Phanerozoic eons, and nothing older than about 1,800 m.y.. Wyoming and land northward contains Archean-age rocks. There are no rocks on Earth from the Hadean eon, the time after formation when the Earth virtually melted so that all older rocks were recycled. There are Archean rocks on all major continents. Oldest rocks preserved on Earth are found in SW Greenland, NW Canada, NW Australia, central Africa. A great showing of late Archean rocks, granites and gneisses, is the fabulous mountains of Grand Teton National Park of Wyoming, with great hiking trails - take the Jenny Lake trail up the canyon. Gneiss is a layered igneous rock with the minerals of granite with layers due to intense compression.

There is a large-scale issue regarding the age distribution of continental rocks across the planet. Since Archean time we can find many rock sequences through all of time, except for a great scarcity of rocks between about 1,500 and 550 m.y. ago - with only a few exceptions. This time seems to mark a period when the Earth's surface was very quiet, with little or no continental drift, mountain-building, volcanism, or other disruptive process. But surely the production of inner heat continued, which is the driving force of it all. So what happened? We guess that some kind of re-arrangement of internal layering was taking place at a layer boundary buried down about 670 kilometers (420 miles). This time of missing rock record is found on all continents, and is labeled the time slot of the Great Unconformity, glibly called the 'missing time' interval. Time's not missing, its all here, just the rocks are missing. It represents about one-quarter of planetary history - gone - missing - kaput. No preserved rock record, except in a very few places, that happens to include Arizona. Planetary development has not been a steady forward-march of events. Progress is a word for humankind's ego, not planetary events. But change seems inevitable.

Section 3. An active plate boundary (cross-section)

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