Key ideas – Earth History
We have learned to interpret events deep in the past that help us comprehend our present Earth, solar system, and Universe.
Evidence about the Earth’s history comes mostly from rocks, and evidence about Space comes from the electromagnetic energy reaching our planet radiated or reflected from planets, stars, and other parts of the Universe.
The “rock record”, mostly in the form of fossils and radioactive elements, is used to interpret the Earth’s past.
Uniformitarianism states that the processes shaping our planet in the past are similar to those active at the present time.
The Law of Original Horizontality states that sedimentary rock layers begin as flat-lying layers, so tilting, folding, or faulting are evidence of crustal deformation.
The Law of Superposition states that older rock is found under younger rock; exceptions to this law include folding, faulting, intrusions, and inclusions.
Folds, faults, and intrusions are always younger than the rocks in which they are found; inclusions are always older.
If rock layers are overturned by folding or faulting, then the oldest layers will be above younger layers.
Fossils are the remains of once-living organisms. Fossils are only preserved under favorable conditions (especially rapid burial and hard parts). Most organisms did not leave fossils.
Even though the “fossil record” is very incomplete, what we do have demonstrates a pattern of organic evolution through time that includes patterns of change and mass extinction events.
Index fossils can be used to determine the “relative age” of rock layers. Index fossil organisms are widespread geographically, but existed for only relativey short periods of the past.
The best way to correlate rock layers is by using index fossils.
Fossils are usually found in sedimentary rocks. Igneous and metamorphic rock rarely contain fossils because they were destroyed by heat and pressure.
Fossils tell us about the environment of the Earth at the time the fossil formed. For example, fossils of marine organisms found on land, especially high on mountains, indicates that what became a continent was once part of an ancient sea.
The climate that existed millions of years ago can be determined by studying the fossils of that time.
Radioactive decay can be used to tell the absolute age of rock layers of fossils. There will always be some uncertainty or error in determining the absolute age, but we can get some information about how many thousands, millions or billions of years ago and organism lived or a rock formed.
Different radioactive isotopes are useful for different time periods, depending on their “half-life” or decay rate. (ESRT p. 1)
Carbon 14 can only be used for fairly recent events because its half-life is only 5,700 years. This isotope works best with organic (living) remains less than 50,000 years old.
Uranium-238, Rubidium-87, and Potassium-40 are examples of radioisotopes used to determine the age of very old rocks because their half-lives are in billions of years.
Layers of volcanic ash can serve as good time markers because they were rapidly distributed over a large area, and sometimes contain datable radioisotopes.
The geologic time scale is divided into eras, periods, and epochs of varying length, based upon fossil evidence and radioisotopes. Divisions often indicate significant events, such as the appearance of new lifeforms and/or mass extinctions of older lifeforms. (ESRT pp. 8-9)
The age of the Earth is approximately 4.6 billion years old. Using a scale of 1 mm = 1 million years, a time line of the Earth would be about 4.6 meters long.
Relatively early in its history, Earth began to form an atmosphere through outgassing of water vapor, carbon dioxide, methane, and other gases from the interior.
The evolution of lifeforms, especially photosynthetic bacteria and plants who released oxygen, dramatically changed the atmosphere into the nitrogen- and oxygen-rich mixture now existing.
The oceans also formed as a result of release of water vapor from the interior, followed by precipitation that slowly filled the lower lying parts of the surface.
Tectonic plate motions have changed the distribution of land and sea throughout geologic time. These changes have affected global climate and patterns of life.
Evidence about the history of the Universe comes from studying light and other forms of radiation from stars. Interpretation of cosmic background radiation and a red-shift (Doppler effect) in the light form distant galaxies indicates the Universe is still expanding.
The Universe is estimated to be more than 10 billion years old. The currently accepted value is about 13 billion.
Earth and the rest of our solar system is estimated to have formed about 5 to 4.6 billion years ago from a giant cloud of gas and debris (nebula) derived from the destruction of earlier stars in supernova explosions.
Meteorites found on Earth provide additional information about the early history of the solar system and formation of our planet.
*****See also related concepts in the Astronomy unit.
