Earth's Composition

The Earth consists of a metallic core surrounded by a rocky mantle, which in turn is surrounded by a thin, rocky crust. Much of the crust is covered by an ocean of liquid, salty water. Surrounding it all is the Earth’s atmosphere. Of the rocky and metallic material at or below the surface, only the crust, and in a few locales samples of mantle, are available for direct laboratory study. The composition of most of the Earth’s interior is inferred by indirect means, primarily from the study of meteorites.

Overview

About 4.5 to 4.6 billion years ago, the solar system was formed from a cloud of gas and dust called the solar nebula. The cloud contracted gravitationally, with most of it forming the Sun. The planets and other objects that today orbit the sun formed by condensation and accretion in an equatorial disk that developed around the early protosun. Small, solid grains condensed from the gas as it cooled. As the grains in the equatorial disk orbited the protosun, they collided and stuck together, accreting into small bodies called planetesimals. As the planetesimals collided and grew into protoplanets, their gravitational fields increased, so they swept up more material in the equatorial disk. The innermost planets—Mercury, Venus, Earth, and Mars—were formed mainly from dense metals and rocks, while the outer planets—Jupiter, Saturn, Uranus, and Neptune—were formed mostly of gases and volatile ices. During or shortly after Earth’s accretion, differentiation occurred; the denser metals, such as iron and nickel, sank to the core of the early Earth, while the less dense rocky material rose to the outer portions of the planet.

Samples of Earth’s crust are readily available. Geologic processes have brought samples from the upper part of the mantle to Earth’s surface in certain locales. Most of Earth’s interior is inaccessible to direct study, but meteorites offer clues to its composition. Most meteorites are remnants of the earliest period of planetary formation. They are classified into three main groups based on composition: stony meteorites, stony-iron meteorites, and iron meteorites. Stony meteorites comprise the most abundant group and are composed of silica-associated, or lithophile, elements such as those found in Earth’s crustal materials. Stony-iron meteorites are composed of roughly equal parts of rock (typically the mineral olivine) suspended in a matrix of iron. Iron meteorites are composed of iron (about 80 to 90 percent) along with siderophile elements such as nickel.

Iron meteorites are particularly suggestive to scientists when they attempt to model the composition of Earth’s core. The average density of the entire Earth is about 5.5 grams per cubic centimeter, while the average density of crustal rocks is only about 2.7 grams per cubic centimeter for continental crust and 3.0 grams per cubic centimeter for oceanic crust. This simple comparison indicates that the core must be substantially denser than the average for the entire Earth, and the only reasonably abundant element with about the right density is iron.

The core has two parts: a solid inner core, with a radius of 1,300 kilometers and a density of about 12 to 13 grams per cubic centimeter, and a molten outer core, 2,200 kilometers thick, with a density of about 10 grams per cubic centimeter. The inner core is mostly iron and nickel under high pressure (to make it solid), while the molten outer core probably contains, besides iron and nickel, lighter elements such as sulfur, silicon, oxygen, carbon, and hydrogen. As a whole, the core comprises about one-sixth of Earth’s volume and about one-third of Earth’s mass.

Almost all the remaining two-thirds of the Earth’s mass is contained in the mantle, making the mass of the crust, oceans, and atmosphere insignificant in comparison. The mantle is rich in dense, ultramafic rocks such as peridotite, composed mostly of the minerals olivine and pyroxene.

Shortly after (or perhaps during) the initial condensation of grains and their accretion into planetesimals and protoplanets, the Earth’s thermal history began through the process of radioactive decay. During this early thermal period, radioactive nuclides (atoms of specific isotopes) decayed, producing substantial heat that led to at least partial melting. Much of the heating is attributable to the decay of potassium 40 and short half-life elements such as aluminum 26. After as little as perhaps 100,000 years, the planet separated into the iron-nickel core and magnesium-iron-silicate lower mantle. Over a longer timescale (probably more than ten million years but no more than a few hundred million), the high-volatility compounds (such as lead, mercury, thallium, bismuth, water in hydrated silicates, carbon-based organic compounds, and the noble gases) all migrated to the surface, where the material was outgassed or melted into magmas in a continuous period of crustal reprocessing that lasted for several hundred million years.

Separated into three main layers—the core, mantle, and crust—the Earth is an active body, its internal heat far from exhausted. The complexity of the chemical composition increases with each successive outward layer. This generalized model gives a framework for examining the relationships of earth materials.

Earth’s wide range of pressure and temperature regimes helps explain why several thousand distinct minerals and numerous rock types composed of different combinations of minerals have been recognized in samples of the crust and upper mantle. Sampling a variety of crustal rocks leads to a determination of elemental abundance in the crust. By mass, approximately one-half of the crust is oxygen and approximately one-fourth is silicon. These two elements, plus aluminum, iron, calcium, sodium, potassium, and magnesium, make up more than 99 percent of the Earth’s crust. Silicon and oxygen combine to form the silicon-oxygen tetrahedron, consisting of a single silicon atom surrounded by four oxygen atoms evenly spaced around it three-dimensionally at the corners of a tetrahedron. This silicon-oxygen tetrahedron joined to additional tetrahedra and/or atoms of other elements forms the class of minerals called silicates, by far and away the most common minerals in the crust.

As ultramafic magmas cool, successive minerals crystallize and settle out via reaction series. As the temperature drops in the melt zone, a discontinuous series (a set of discrete reactions) can occur. Magnetite, an oxide of iron and titanium, is the first to settle out, at about 1,400° Celsius (1,700 kelvins). Olivine, a Silicate mineral with a crystal lattice structure of individual silicon-oxygen tetrahedra joined together by other ions (commonly iron and magnesium) and a density between 3.2 and 4.4 grams per cubic centimeter, is the next to crystallize out of the melt. Then comes pyroxene, a silicate mineral with its silicon-oxygen tetrahedra connected in long single chains and a density of 3.2 to 3.6 grams per cubic centimeter. As temperatures in the Magma drop to near 1,000° Celsius (1,300 kelvins), the next to crystallize is amphibole, a silicate mineral with its silicon-oxygen tetrahedra joined in long double chains and a still lower density of 2.9 to 3.2 grams per cubic centimeter. As the cooling progresses, the lattice structures increase in complexity with biotite mica, with its silicon-oxygen tetrahedra joined in planar sheets. Paralleling this discontinuous series of reactions is the continuous reaction series of plagioclase feldspar. It has a full three-dimensional lattice of silicon-oxygen tetrahedra, and it varies continuously from being calcium-rich at high temperatures of crystallization to sodium-rich at lower temperatures. Finally, at still lower temperatures down to about 1,000 kelvins (700 degrees Celsius), come potassium feldspar, muscovite mica, and quartz.

With this information, one can start to hypothesize about how the crust and its ocean basins and continents evolved. The oldest earth materials yet identified are zircon crystals, possibly dating back 4.4 billion years, found in the Jack Hills area of Australia, while the oldest known continental rocks—the Acasta gneiss from the Northwest Territories of Canada—are about 4 billion years old, and they were metamorphosed from earlier igneous rocks. This means that within a few hundred million years after the initial formation of the Earth through condensation, accretion, and differentiation, the first crustal rocks of the Archean eon formed. They probably were composed of olivine, pyroxene, and anorthite (calcium-rich plagioclase feldspar), which crystallized out of basaltic magmas that rose to the surface and cooled and hardened. The early crust, which may have been similar to the anorthosite that makes up much of the ancient highlands on Earth’s moon, formed a sheet that was fractured into pieces and subjected to heating through radioactive decay. Differentiation led to the formation of thicker granitic regions surrounded by the thinner basaltic crust. This was the beginning stage in the development of today’s crust, which consists of two main types: the denser, thinner, mafic or basaltic oceanic crust and the less dense, thicker, felsic or granitic continental crust. The onset of plate tectonics moved the early continental fragments, causing them to collide and weld themselves together into continental shields in episodes of mountain-building, called orogenies.

The Earth’s original inventory of gases appears to have been lost very early in its history, to be replaced with a secondary atmosphere through volcanic outgassing and perhaps impacts of volatile-rich cometary nuclei and carbonaceous chrondrite meteorites. Extensive volcanic activity and high surface temperatures gradually diminished until the hydrosphere (water cycle) was established and oceans appeared.

Life on Earth existed at least 3.5 billion years ago, as evidenced by microfossils similar to modern cyanobacteria (blue-green algae). With the oceans growing in volume and salinity and the development of oxygen-releasing life-forms, Earth’s geochemistry became more complex. By the beginning of the Paleozoic era, about 540 million years ago, the oxygen content of the atmosphere had reached 1 percent of its present level. Life-forms significantly shaped the Earth’s chemical composition. Multicelled animals in the oceans scrubbed carbon dioxide from the atmosphere and locked it up in the carbonate rocks, forming biochemically precipitated limestones. By the latter part of Paleozoic era, about 300 million years ago, coal formed as a result of the first land forests being periodically inundated by ocean transgressions.

Methods of Study

Perhaps no other Earth science is as speculative as that of early Earth history and the geochemical evolution of the Earth. Some of the major challenges confronting Earth scientists are questions about how the Earth’s crust formed and when plate tectonic movement began. It is generally accepted by most Earth scientists that heat flow was substantially greater and hence crustal formation occurred more rapidly in Archean times. Despite the problems of extrapolating back to a time when the first solid rocks were forming, the established models are based on some solid lines of evidence.

In 1873, American geologist James D. Dana made one of the initial advances in the study of the Earth’s internal chemical composition when he suggested that analogies could be drawn from the study of meteorites. Geochemists studying meteorites today have derived radiometric dates of 4.4 to 4.6 billion years for many of them—corresponding to the initial epoch of condensation and accretion in the solar nebula. Because meteorite types approximate the elemental distribution in the Earth, they are valuable samples of what the Earth formed from.

Geophysicists use seismic waves from earthquakes to study the structure of the Earth’s interior. Variations in speed as the waves pass through the Earth, and reflection and refraction of them at internal boundaries, have revealed a differentiated Earth with a very dense metallic core, a less dense rocky mantle, and an even less dense rocky crust “floating” on top. The well-established theory of plate tectonics holds that the crust and upper mantle together form rigid lithospheric plates that are moving, driven by slow convection currents in the mantle.

The drive to study Archean rocks was partly fueled by the United States Apollo missions to the Moon, which returned rocks of comparable age from the lunar surface. Interest in Archean crustal evolution was further aroused by the discovery of Archean lavas called komatiites around greenstone belts (which are agglomerations of Archean basaltic, andesitic, and rhyolitic volcanics, along with their sediments derived by weathering and erosion). Komatiites are ultramafic lavas that formed at temperatures greater than about 1,100 degrees Celsius (1,400 kelvins) and may be fragments of the first crust. Work by field geologists in regions with exposed Archean rocks found successively older granitic rocks—3.8 billion years in western Greenland, 3.9 billion years in Antarctica, and 4 billion years in Canada’s Northwest Territories. Even older detrital zircons with radiometric ages between 3.8 and 4.4 billion years were discovered in somewhat younger sedimentary rocks in western Australia. The zircon find is significant because it sets an approximate birth date for early continental crust, as zircon is a reasonably common though minor constituent of granitic igneous rocks. The Australian zircons probably formed in early continental igneous rocks and then were eroded, transported, and deposited with other sediments in the sandstones in which they were found.

Geochemists have refined their study of these ancient rocks with more sophisticated methods to determine isotope ratios in them. Instruments common in geochemical laboratories today use X-ray diffraction and gamma-ray spectral analysis to determine which isotopes are present. Isotope ratios in rocks are of particular interest to geochemists because they provide clues as to chemical cycles in nature. The equilibria of these cycles, as indicated by the isotope ratios, offer insights into volcanic, oceanic, biological, and atmospheric cycles and conditions in the past.

Context

Perhaps no other area of scientific study is as intriguing and controversial as that of the origin and evolution of the Earth. Geochemists and geophysicists have been at the forefront of the quest to understand the Earth’s present geology in terms of its past. Before the 1960s, little was known of the Earth’s history during early Precambrian times. This lack is significant when one considers that the Precambrian comprises about eight-ninths of the geologic timescale.

It is likely that improved techniques used to analyze rocks and minerals in the laboratory will continue to provide a better understanding of the formation of the Earth’s crustal materials and the evolution of moving lithospheric plates. Radiometric dating and isotope analysis will help unravel the relationships between the greenstone belts and granulite-gneiss associations that typify Archean formations on all continents.

Studying features and materials on other solar system bodies will also lead to a better understanding of the early Earth and its evolution. Similarities and differences in Earth’s early history are expected to be revealed by future space probes to the Moon, Mars, Venus, Mercury, and asteroids. For example, in December 2015 it was reported that China's Chang'e 3 lunar rover had discovered a new kind of basaltic rock on the moon that could have an eventual impact on scientists' understanding of Earth's composition. Also in 2015, the National Aeronautics and Space Administration (NASA) confirmed from data gathered by their Mars Reconnaissance Orbiter (MRO) that there is flowing water on Mars, which could implicate similarities between Earth and Mars. In 2018, the European Space Agency’s Mars Express mission discovered evidence that liquid water existence under the southern Marian ice cap. Four years later, scientists found new evidence suggesting the presence of water, indicating Mars, like Earth, must be geothermally active.

Bibliography

Anderson, Gina. "NASA Confirms Evidence that Liquid Water Flows on Today's Mars." National Aeronautics and Space Administration, 20 Nov. 2017, www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars, Accessed 20 Jan. 2023.

Gregor, C. Bryan, et al. Chemical Cycles in the Evolution of the Earth. New York: Wiley, 1988. Print.

Hartmann, William K. Moons and Planets. 5th ed. Belmont.: Thomson, 2005. Print.

Khan, Amina. "On the Moon, China's Chang'e 3 Yutu Rover Finds a New Type of Basaltic Rock." Los Angeles Times. Tribune, 23 Dec. 2015. Web. 28 Dec. 2015.

Kroner, A., G. N. Hanson, and A. M. Goodwin, eds. Archaean Geochemistry: The Origin and Evolution of the Archaean Continental Crust. Berlin: Springer, 1984. Print.

Lea, Robert. "New Evidence for Liquid Water on Mars Suggests the Planet Is Geothermally Active ." Space.com, 30 Sept. 2022, www.space.com/mars-liquid-water-south-pole-subglacial. Accessed 20 Jan. 2023.

Levin, Harold L. The Earth Through Time. 11th ed. Wiley, 2016.

Tarbuck, Edward J., Frederick K. Lutgens, Dennis Tasa, and Scott Linneman. Earth: An Introduction to Physical Geology. 13th ed. Pearson, 2019.