Figure 17. 1: The geologic time scale Figure 17.1: The geologic time scale. Some of the major geologic and biologic events are indicated along the right side. Fig. 17-1, p. 394

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Figure 18.1: If all geologic time were represented by a 24-hour day, the Precambrian would be more than 21 hours long and constitute more than 88% of all geologic time. Fig. 18-1, p. 418

Atmospheric Development Primordial Atmosphere of Earth Hydrogen and Helium Ammonia and methane Thought to have escaped into space from Earth’s hot surface. Second, dense atmosphere developed Mostly water vapor (80%), carbon dioxide (10%), nitrogen (~5 %) Source: volcanic activity and steam vents; also extraterrestrial impacts (comets)

Atmospheric Development Atmospheric Transformation Water vapor evaporated, condensed into clouds and extensive, long-lived rain fell for thousands of years—water vapor decreased Carbon dioxide dissolved into the newly-formed oceans and was locked up into carbonate (limestone) rocks—CaCO3—carbon dioxide decreased Nitrogen was then enriched by the relative decreases of water vapor and carbon dioxide.

Atmospheric Development No free oxygen Photodissociation--Sunlight is thought to have split water vapor into hydrogen and oxygen, hydrogen escaped, oxygen left behind (2-3 by ago) Algal stromatolites enriched the atmosphere in oxygen, which also allowed for the formation of ozone (O3). Ozone prevented most of the UV rays from the Sun reaching the Earth’s surface by absorbing the rays. O2 levels rose from 1-10% during the Proterozoic Eon. Present levels of O2 were probably not achieved until ~400 Ma. “…but it was actually the steady evolution of unicellular organisms that produced oxygen and brought about the change in the atmosphere's makeup.” How Weather Works by Robert Lamb http://science.howstuffworks.com/weather1.htm http://www.ux1.eiu.edu/~cfjps/1400/atmos_origin.html

Geologic Principles Principle of Uniformitarianism (the geological processes observed today are the same as in the past and in the future). Principle of Superposition (each sedimentary layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it) Principle of Original Horizontality (sediments are deposited as essentially horizontal beds)

Alternating beds of limestone and shale Figure 7.10a: Bedding or stratification is shown in these rocks by alternating layers of shale and sandstone. Alternating beds of limestone and shale Fig. 7-10a, p. 156

Uniformitarianism The present is the key to the past. Attributed to Scottish geologist James Hutton (1726-1797) Great geologist, lousy writer Principle of uniformitarianism: The present-day processes have operated throughout geologic time and will be the same processes to operate in the future. Charles Lyell (1830) wrote Principles of Geology and more eloquently wrote about uniformitarianism.

Uniformitarianism Given enough time… A stream could carve a valley

Uniformitarianism Given enough time… Ice could erode rock

Figure 1.8: This glacier in Alaska has retreated in recent years so that the lowest part of the glacial ice (the terminus) is now backing up through the valley that the glacier had earlier carved. Although some glaciers around the world are advancing, the majority are clearly in retreat, apparently the result of warmer temperatures. Fig. 1-8, p. 12

Figure 1.19a: Glaciation during the most recent Ice Age carved this “Garden Wall” in Glacier National Park. Now there are no glaciers in this valley. Climate has changed throughout Earth’s history. Fig. 1-19a, p. 22

Figure 1.19b: A large, continent-size glacier carved these wide grooves in the rock of Kennebunk, Maine. Not only has the glacier retreated out of Maine, but it is also gone from central Canada, where it originally formed during the Ice Age. Fig. 1-19b, p. 22

Uniformitarianism Given enough time… Wind can sculpt rock

Uniformitarianism Given enough time… Weathering could take down a mountain. A little at a time for a long time can have a huge effect.

Geologic Time

Geologic Time Earth is 4.6 billion years old (as old as the formation of the solar system) To a geologist, recent geologic events are those that occurred within the last million years. The Earth goes through cycles that are much longer in duration than our human perspective of time.

Assessing the Age of the Earth Geologists placed relative dates on exposed rock formations based on similarities and differences in rock composition and the preserved biota. relative positions of these rock formations Geologists later placed an absolute dates on rock using radiometric dating techniques to confirm relative ages.

Figure 1. 18: The geologic time scale Figure 1.18: The geologic time scale. Numbers to the right of the columns are ages in millions of years before the present. Fig. 1-18, p. 21

Geologic Time Scale Ages in millions of years Today’s Geologic Date: Recent Epoch Quaternary Period Cenozoic Era Phanerozoic Eon Figure 1.18: The geologic time scale. Numbers to the right of the columns are ages in millions of years before the present. The Earth is currently experiencing an interglacial episode Fig. 17-1, p. 394