Part A
The Hadean & Archean Eons
Earth's early history is contained in the Hadean and Archean Eons. Originally, the Hadean Eon was not officially recognized, but it is now a widely used term that represents the earliest portion of Earth's history. The Hadean had also been referred to as the "rock-less" eon, as no rocks older than Archean age were known. We know certain events must have occurred during the Hadean Eon, like formation of Earth's internal structure, creation of the Moon, etc. A number of recent rocks and mineral discoveries now provide us with our first direct glimpse of this formerly "rock-less" eon. And as Earth passed its 1 billionth birthday in the Archean Eon, primitive life forms evolved and plate tectonic activity began its relentless reshaping of Earth's surface and its young continents.
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Figure 8-2. Comprised of both the Earth and Moon, the Terran system formed very early in Earth's history. As seen from the Galileo spacecraft in 1990 (left). The earliest known life forms were extremophile organisms like the halobacteria in the salt-encrusted brine pools of Seales Lake, California (right). |
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Geologic Time
The largest units of time are the Eons, of which there are four: Hadean, Archean, Proterozoic, and Phanerozoic (see Figur 8-3). Each of these can be subdivided into smaller Eras, like the Paleozoic, Mesozoic, and Cenozoic Eras of the Phanerzoic Eon. In turn, Eras can be split into smaller Periods and Epochs. Remember, a = annum = year, Ka = kilo-annum = thousand years, Ma = mega-annum = million years, Ga = giga-annum = billion years. If helpful, see the discussion of geologic time in Lab 3.
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Figure 8-3. A wheel of showing the Hadean, Archean, Proterozoic, and Phanerozoic Eons. Included are the Paleozoic, Mesozoic, and Cenozoic Eras of the Phanerozoic Eon. |
The International Commission on Stratigraphy (ICS) has the primary objective of precisely defining the units of the International Geologic Time Scale. A recent version is shown in Figure 8-4.
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Figure 8-4. The official geologic time chart compiled by the International Commission on Stratigraphy. Click HERE for a printable PDF version. |
* IMPORTANT *
Using the PDF link above, print a hard-copy version of this chart for reference.
Refer to the ICS geologic time scale in Figure 8-4 to answer Quiz Me! question A02.
A simplified geologic time scale constructed from the 2020/03 ICS time scale is shown in Figure 8-5. It is not drawn to scale as its focus is including all of the Periods of the Phanerozoic Eon and all of the Epochs of the Cenozoic Era.
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Figure 8-5. Geologic time chart compiled from the 2020/03 ICS version. |
* IMPORTANT *
Using the PDF link above, print a hard-copy version of this chart for use in this lab.
Refer to the simplified geologic time scale in Figure 8-5 to answer Quiz Me! question A03.
Earth: The Early Years
Earth's early history is contained in the Hadean and Archean Eons. The Hadean Eon represents the earliest portion of Earth's history and has been referred to as the "rockless" eon that existed prior to the Archean Eon. We know certain events must occurred during the Hadean Eon, like formation of Earth's internal structure (i.e., differentiation of the core and mantle), creation of the Moon, etc. A number discoveries of rocks older than 3,800 Ma have been recently made that now provide us with our first direct glimpse of this formerly "rockless" eon. As Earth passed its 1 billionth birthday in the Archean Eon, primitive life forms first appeared. The first continents were formed and plate tectonic activity began its relentless reshaping of Earth's surface.
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Figure 8-6. Interstellar clouds and the formation of stars and planetary systems. At left, NGC 6357, a diffuse nebula roughly 8,000 light-years distant in the constellation Scorpius (courtesy of ESO/Very Large Telescope). At right, an artistic concept of a protoplanetary disk (solar nebula) like the onfrom which our Solar System formed (courtesy of NASA/JPL-Caltech). |
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Refer to the Wikipedia's Nebular hypothesis web page (https://en.wikipedia.org/wiki/Nebular_hypothesis) to answer Quiz Me! questions A04 through A06.
Refer to the Wikipedia's Hadean web page (https://en.wikipedia.org/wiki/Hadean) to answer Quiz Me! questions A07 and A08.
Refer to the Wikipedia's Archean web page (https://en.wikipedia.org/wiki/Archean) to answer Quiz Me! questions A09 and A10.