FORMING FOSSILS

IN THE BEGINNING

How old is Earth?

Earth is currently believed to be about 4.54 billion years old, but that number came after centuries of debate. In 1779, French naturalist Comte de Georges Louis Leclerc Buffon (1707-1788) caused a stir when he announced 75,000 years had gone by since Creation, the first time anyone had suggested that the planet was older than the biblical reference of 6,000 years. By 1830, Scottish geologist Charles Lyell (1797-1875) proposed that Earth must be several hundred million years old based on erosion rates; in 1844, British physicist William Thomson, later first baron of Largs (Lord) Kelvin, (1824-1907), determined that Earth was 100 million years old, based on his studies of the planets temperature. In 1907, American chemist and physicist Bertram Boltwood (1870-1927) used a radioactive dating technique to determine that a specific mineral was 4.1 billion years old (although later on, with a better knowledge of radioactivity, the mineral was found to be only 265 million years old). Using different adaptations of Boltwoods methods on terrestrial, lunar, and meteorite (space rock that falls to the surface of Earth) material, scientists now estimate that Earth is between 4.54 and 4.567 billion years old.

How old is the oldest rock and mineral found on Earth?

The oldest rock discovered on Earth, the Acasta gneisses found in the tundra in northwestern Canada near the Great Slave Lake, is about 4.03 billion years old. The oldest minerals yet found are 4.404 billion years old and were found in Western Australia. The minerals zircon crystals eroded from their original rock, and then were deposited in younger rock.

Cases released by erupting volcanoes, such as carbon dioxide, nitrogen, and water vapor, did a great deal during Earths early history to make the atmosphere life-sustaining for plants and animals (iStock).

What caused the early Earths water and atmosphere to form?

No one really knows how the oceans filled with water. One theory is that volcanoes released enough water vapor to allow the oceans waters to condense. Another theory states that comets bombarded Earth just after the formation of the solar system, bringing enough water to eventually fill the oceans.

The origin of Earths atmosphere is also debated, but not as intensely. In this case, it is more likely that some of the atmosphere originated from gases that were part of the solar nebula, gases brought by comets, and those produced from volcanic activity. Earth probably would have had a thicker atmosphere, too, but the young, active Suns heat boiled away the lighter materials elements that are still found today around the gas giant planets Jupiter, Saturn, Uranus, and Neptune.

What gases began to accumulate after Earths crust finally solidified?

As Earths crust solidified, gases began pouring out of fissures and volcanoes, accumulating in the forming atmosphere. These same gases still emanate from modern volcanoes, and include carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), nitrogen (N2), and hydrogen chloride (HC1).

As these gases interacted in the atmosphere, they combined to form hydrogen cyanide (HCN), methane (CH4), ammonia (NH4), and many other compounds. This atmosphere would be lethal to most present day life-forms. Fortunately for life on

Lilies are grown in a greenhouse in Almere, the Netherlands. Just as this structure allows tropical plants to grow in a cold climate, the natural greenhouse effect created by Earths atmosphere warms our planet (iStock).

Earth, over the next two to three billion years the atmosphere continued to change until it reached close to its present composition.

How did oxygen form on early Earth?

The early atmosphere was composed mainly of water vapor, carbon dioxide and monoxide, nitrogen, hydrogen, and other gases released by volcanoes. By about 4.3 billion years ago, the atmosphere contained no oxygen and about 54 percent carbon dioxide.

About 2.2 billion years ago, plants in the oceans began to produce oxygen by photosynthesis, which involved taking in carbon dioxide. By two billion years ago, there was one percent oxygen in the atmosphere, and plants and carbonate rocks caused carbon dioxide levels to decline to only four percent. By about 600 million years ago, atmospheric oxygen continued to increase as volcanoes and climate changes buried a great deal of plant material plants that would have absorbed oxygen from the atmosphere if they had decomposed in the open. Today, our planets atmosphere levels measure 21 percent oxygen, 78 percent nitrogen, and only 0.036 percent carbon dioxide.

What is the greenhouse effect?

The greenhouse effect, as its name implies, describes a warming phenomenon. In a greenhouse structure, closed glass windows cause heat to become trapped inside.

The greenhouse effect functions in a similar manner, but on a planetary scale. In general, it occurs when the planets atmosphere allows heat from the Sun to enter but refuses to let it leave.

Without this greenhouse effect on Earth, life as we know it would not exist. On our planet, solar radiation passes through the atmosphere and strikes the surface.

As it is reflected back toward space, some solar radiation is trapped by atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, and water vapor, resulting in the gradual increase of Earths temperatures. The rest of the radiation escapes back into space. Without this heat, life as we know it would be impossible, Earth would be about 100 degrees cooler, and the oceans would freeze.

What is ozone and how did it benefit the early Earth?

Ozone (O3) compared to the oxygen (O2) we breathe usually refers to a blanket of gas found between 9 and 25 miles (15 and 40 kilometers) in the layer of Earths atmosphere called the stratosphere. The so-called ozone layer is produced by the interaction of the Suns radiation with certain air molecules. The blue-tinged ozone gas is also found in the lower atmosphere. While beneficial in the stratosphere, ozone forms photochemical smog at ground level. This smog is a secondary pollutant produced by the photochemical reactions of certain air pollutants, usually from industrial activities and cars.

The stratospheres ozone layer is important to all life on the planet because it protects organisms from the Suns damaging ultraviolet radiation. Scientists believe that about two billion years ago, oxygen was being produced by shallow water marine plants. This sudden geologically speaking outpouring of oxygen helped to build up the ozone layer. As the oxygen levels increased, ocean animals began to evolve.

The formation of the ozone layer in the upper atmosphere eariy in Earths history created a radiation boundary that protects life on the planet. Today, scientists are concerned about the hole in the ozone that has appeared over the South Pole, as seen in this 1987 satellite image (National Oceanographic and Atmospheric Administration).

Once the protective ozone layer was in place in the atmosphere, it allowed the marine plants and animals to spread onto land, safe from the Suns radiation.

 
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