Earth history

The key to understanding present-day climate change lies in interpreting the geologic past. Present in the rocks of the earth’s crust is a continuous record of change. Scientists use the evidence found in those rocks to help understand current Earth processes.

Background

Earth is one of eight major planets and a host of minor planets that circle a fairly average, middle-aged, main-sequence star. It formed from the accretion of residual material from the gravitational collapse of the solar nebula that produced the Sun. Based on their specific distances from the Sun, the planets formed into groups of similar chemical compositions, sizes, and densities. The four smaller inner planets are of higher density and are composed of a mixture of rock and metal. In contrast, the next four planets are massive gas giants with lower densities and larger sizes. The final grouping consists of small, low-density, icy worlds. Unlike the other inner planets, Earth has remained a geologically active planet and has undergone continuous change since its formation. It is a planet dominated by the presence of liquid water. Throughout its long history, Earth has evolved from a lifeless world into one that is populated by an uncountable number of species.

Early History of the Earth

Based on analyses of moon rocks and meteorites, Earth is believed to be about 4.5 billion years old. Many scientists believe that the earth originated as a cold, undifferentiated body that internally heated up from the energy released through giant impacts, radioactive isotope decay, and the mass of the earth itself. Once the appropriate melting temperatures were reached, heavy metallic elements sank to the planet’s center of gravity, as lighter elements were displaced upward toward the surface. This process may have taken place in as little as 50 million years. It is fairly certain that the present core-mantle-crust structure was in place by 4 billion years ago.

Earth’s core consists of both solid and liquid metal, presumably of nickel-iron composition. The outer, liquid metallic core revolves around the inner, solid metallic core and, in the process, generates an electric current. This electric current is responsible for the earth’s magnetic field. Sandwiched between the core and the crust is the mantle, a region of high-density iron- and magnesium-rich silicate rock material. Depending upon specific temperatures and pressures, this material can behave either as a solid or as a liquid.

Scientists closely link the history of the earth to that of the moon. The earth and moon are geologically similar in many ways, but there are significant differences in their respective elemental abundances, and lunar specimens exhibit an apparent lack of certain volatiles. One origin theory suggests that the moon is the product of a huge collision between a Mars-sized object and the primordial Earth. The resulting debris from this impact later accreted and formed the moon. This theory mainly draws its support from computer impact simulations and a great many assumptions, but it lacks the physical evidence necessary to support it.

Finding a common theory of origin for the earth and moon has proven to be quite difficult. In fact, when comparing size, density, and internal structure, the moon seems to have more in common with Mars than it does with Earth. What is certain is that the moon has a definite influence on the earth’s environment. The moon’s daily tidal effects on the world’s oceans are obvious, but the moon also has a gravitational influence on the earth’s axial rotation. Without the gravitational pull of the moon, the earth’s axial tilt would fall outside its normal range of between 21° and 25° and literally fall over. Without the moon “holding the earth in place,” the equator of today could easily become the polar regions of tomorrow.

Evolution of the Atmosphere and Oceans

The primitive earth did not have the same atmosphere that it does today. Many scientists believe that Earth’s original atmosphere may have been a very dense, hot mixture of ammonia and methane. This is consistent with the conditions present during Earth’s proto-planetary stage that probably dissipated during the sun’s T-Tauri phase. The first “permanent” atmosphere formed as a product of volcanic degassing. Through this process, gradually became the dominant gas, along with a significant amount of water vapor. As Earth cooled, water vapor condensed and fell as rain. This rain later filled the lowlands and created the first primitive oceans. Chemical reactions occurring in seawater slowly began to extract CO₂ from the atmosphere and form large amounts of carbonate rocks. Somehow, during the first 500 million years of Earth history, life crossed over the threshold between being a collection of complex organic molecules and living organisms. With lifeforms such as blue-green at work, the CO₂-rich atmosphere slowly transformed into a nitrogen and oxygen-rich atmosphere that could support animal life. This transition is clearly marked in Earth’s geological record when previously dark, iron-bearing sediments turned red from oxidation. It is shortly after this geological benchmark that the first marine animals appear in the fossil record.

Supercontinents and Continental Drift

The formation of supercontinents and continental drift is essentially tied to the internal mechanisms of Earth’s upper mantle. There, convection cells provide the energy necessary to split apart the crust into both large and small sections that can move relative to one another. The direct evidence for the effects of this convective energy is the large number of volcanoes and earthquakes that occur along plate boundaries. No one is certain how long plate tectonics has been a part of Earth history, but it is certainly responsible for the continent-ocean basin relationship, which forms the present crust.

In the geologic past, supercontinents have existed only to be broken apart and distributed across the face of the earth. This movement is continuous, and the formation of supercontinents appears to be inevitable, as is their eventual breakup. The formation of the world’s great mountain chains is the direct result of colliding continents transforming marine sediment into hard rock. New crustal rock is created by volcanic activity at mid-oceanic ridges that pushes plates apart from one another. Such plate movement can also carry older, crustal rock to its destruction in an oceanic trench or be welded together into a new continental mass.

The History of Life

There is strong evidence to support the hypothesis that life has existed on Earth for more than 4 billion years. Initially, single-celled life-forms dominated the planet, and they gradually evolved into more complex forms. Scientists theorize that life could have originated on Earth in two possible ways: either as an indigenous form, created from the organic compounds and conditions present in the primordial Earth, or from organic compounds or even living bacteria that were transported to Earth by comets or meteorites. This later “panspermia” theory suggests that organic compounds and living bacteria may have come to Earth from space and served as the seeds for life.

Regardless of its origin, life has flourished on Earth for billions of years and has adapted to an ever-changing variety of environmental conditions. For over 2.5 billion years, cyanobacteria were the dominant lifeforms and were responsible for the gradual buildup of an oxygen-rich atmosphere. In the Cambrian geological period, there was an explosion in the diversity of marine animal life. Various species seem to have come and gone as if they were the products of some biological experiment to see which could survive best. The survivors continued to evolve into more complex organisms that gradually found their way to land.

It seems that the end of one species' dominance and the beginning of another’s is usually marked by a dramatic change in global climate. The best evidence to support such a theory occurs at the Cretaceous-Tertiary boundary, approximately 65 to 70 million years ago. An iridium-rich layer of sediment, attributed to the impact of an asteroid, marks the boundary between the two geological periods. Dinosaur fossils are found below this layer but are notably missing from the layers above. Scientists interpret this as evidence for that led to the mass extinction of a large number of species. An event such as this is not limited to the Cretaceous-Tertiary periods, but may also be responsible for several other mass extinctions.

Ice Ages and Interglacial Periods

There are many scientific theories that suggest Earth is usually a cold planet covered by vast amounts of ice. Periodically, warm periods emerge that last for several thousand years and eventually phase back into long ice ages. It is during these warm periods that land animals flourish and perhaps even speed up their evolutionary processes. There are no certainties as to when ice ages begin or end. Global climate change is the underlying reason, but what initiates this change? Scientists have suggested such possibilities as changes in the earth’s axial tilt, increased volcanic activity blocking incoming sunlight, shifts in the world’s ocean currents, impact debris from collisions with comets or asteroids, and the effects of human pollution. No one is certain which of these possibilities holds the answer, and the truth is probably found in some combination of factors. What is certain is that ice ages and their interglacial warm periods are features inherent to planet Earth, and they will continue to occur with or without human influence.

Context

Astronomers have presented evidence to support the existence of hundreds of other planets orbiting distant and even exotic stars. These planets range from massive gas giants to a few with nearly earthlike masses. All evidence tends to indicate, however, that Earth is a very unusual planet. It has evolved from a hot gaseous world to one that is dominated by liquid water. Its atmosphere changed from one that was poisonous to animal life to one that is oxygen-rich and supports uncountable species of life, both on land and in the oceans. In the geological past, the processes of may have been beneficial to the evolution of life, while at other times they may have caused mass extinctions. Perhaps the one constant throughout Earth’s history has been that change is inevitable. Present-day humans are the result of the ever-changing conditions of Earth’s surface. Adaptation to climatic changes means survival for some species, while others that cannot change face extinction. Unique to this period in Earth history is the fact that a particular species, human beings, possesses the ability to have either a positive or a negative effect on the environment. One path may lead to a better world, while the other may lead to extinction.

Key Concepts

• accretion: the final process of planetary formation, in which smaller objects are pulled into the nascent planet
• continents: large sections of an earthlike planet’s surface with high elevations and lower-density rocks
• core: the central part of a planetary body, usually composed of high-density metal alloys
• crust: the outer portion of an earthlike planet, consisting of lower-density silicate minerals and rocks
• magnetic field: the flow patterns of magnetism that surround a celestial body as a result of an electro-dynamo effect
• mantle: the middle section of an earthlike planet, consisting of higher-density silicate minerals and rocks
• ocean basins: large sections of an earthlike planet’s surface with low elevations and higher-density rocks
• planetary nebula: the gas and dust that is required to form a star and its family of planets
• plate tectonics: the mechanisms responsible for creating movement in large sections of a planet’s surface
• radiometric dating: an analytical technique using radioactive isotopes to determine the age of rocks and minerals

Bibliography

Canup, R. M., and K. Righter, eds. Origin of the Earth and Moon. University of Arizona Press, 2000.

Hartmann, William K. Moons and Planets. 5th ed. Brooks/Cole-Thomson Learning, 2005.

Knoll, Andrew H. A Brief History of Earth: Four Billion Years in Eight Chapters. Custom House, 2021.

Lee, Robert. "How Earth's Climate Changes Naturally (and Why Things Are Different Now)." Quanta Magazine, 21 July 2020, www.quantamagazine.org/how-earths-climate-changes-naturally-and-why-things-are-different-now-20200721/. Accessed 8 Oct. 2024.

McSween, Harry Y., Jr. Meteorites and Their Parent Planets. 2nd ed. Cambridge UP, 1999.

Russell, Michael. “First Life” American Scientist 94, no. 1 (2006): 32-39.

Schubert, Gerald, Donald L. Turcotte, and Peter Olsen. Mantle Convection in the Earth and Planets. Cambridge UP, 2001.