The Hadean Eon – early earth and formation of the core
Most geologists refer to the Earth’s first 700 million years as the Hadean Eon.
Differentiation of the Earth’s minerals
The solar system was formed from the condensation of rubble in rotational orbit around the early Sun. In its early stages, before its orbital path was swept clean, the earth was bombarded by rocks circulating through space. From these impacts, the young planet received a huge amount of kinetic energy, which was transformed into heat. The surface of the young Earth was so hot that rocks fused. Eventually, enough of the rubble was agglomerated into relatively widely-spaced chunks that collisions among them became less frequent, although they are still not at an end.
The principal elements left behind in the mantle are as follows:
|Element||Approx. percentage by weight1)Source: http://www.indiana.edu/~geol105/1425chap5.htm|
The first 500 or so million years of the Earth’s existence are often referred to as the Hadean Eon. The Earth then was indeed like Hell, with rocks striking from space; live volcanoes spewing lava, rocks and gases; ultra-high temperatures; and an atmosphere, once there was one, lacking the oxygen necessary for life as we know it (but there are other kinds).
Elements which had originated in exploding stars included large quantities of iron and nickel. As these elements were denser than the others, they were pulled down by gravity through the earth’s other matter, like sand falling through water, causing them to coalesce in the center of the new planet. Lighter material such as silicon was left closer to the surface. This process of differentiation was finished within, probably, 50 million years. It left the earth with a hot iron-rich core, whose existence is essential to the its subsequent history, as will be seen. The core’s temperature is due not only to its tremendous pressure but also to heat generated by radioactive decay of core isotopes.
Formation of the Moon — Giant impact hypothesis
One collision was especially violent – and fruitful. Most scientists now accept the giant-impact hypothesis, according to which another body, called Theia2)In Greek mythology, Theia was a Titaness and was mother of Helios (the Sun), Selene (the Moon) and Eos (the Dawn)., crashed into the Earth at an oblique angle and brought about formation of the Moon.
The impact of Theia and Earth occurred somewhere between 30 and 100 My after Earth’s formation and was so violent that the two bodies merged together, sharing their matter. Then about 1% of the total mass was vaporized from the combined mantle and thrown off into a disk of matter orbiting the Earth, just as earlier in the formation of the solar system. In similar fashion, these particles collided and accreted and became, probably, two orbiting moonlets of the same composition as the Earth. The moonlets crashed together about 5 My later to form the Moon. Around 4 Gya in a sequence of events known as the Late Heavy Bombardment, the Moon was struck by a huge number of asteroids, leaving it pock-marked with the craters we see today.
The two bodies have similar composition probably because Earth and Theia did, indicating that they were formed close to each other. The giant-impact hypothesis explains the similar but not quite equal ratios of oxygen isotopes on Earth and Moon. It also explains the Moon’s rotation in its orbit, keeping the same side toward the Earth. The impact has also been evoked to explain the inclination of the Earth’s rotational axis relative to its orbital plane.
Originally, the Moon was much closer to the Earth than It is now and the Earth’s orbital rotation was much faster. So a day was only five hours long, but the Moon took 84 hours to go around the Earth. Gravitational tides resulting from the pull of the two bodies on each other brought about a slowing of the rotation of the Earth and an increase in distance between the two bodies. In this way, the Earth’s rotational angular momentum has decreased as the Moon’s orbital angular momentum has increased, thus conserving total angular momentum. Mirrors left on the Moon by astronauts have allowed us to observe that the Moon is still receding. Both ancient tidal sediments and coral layers indicate that the day was once significantly shorter. The giant-impact hypothesis seems to offer the best explanation for the Moon’s formation, but study continues.
Continued differentiation and formation of minerals
The Earth-Theia impact essentially forced a restart of the Earth’s differentiation and the Earth once again became molten.
As the earth slowly cooled, crystals of olivine (magnesium silicate) were formed. Being heavier than the surrounding magma, they sank into it and continued growing into larger crystals, eventually forming the green rock dunite. Then pyroxene, a chain silicate3)We will see what that means in the next paragraphs., was formed and mixed with the olivine to form greenish-black peridotite, starting about 4.5 Gya and continuing for hundreds of millions of years. As it rose to the surface and cooled in turn, it hardened and became denser, so when it cracked and broke, it also sank, pushing more magma to the surface to cool and continue the process as the mantle slowly became solid.
Although peridotite is still the principal component of the Earth’s upper mantle, it is rare on the surface of the Earth.
The next, less dense rock to form at the surface was basalt – and it has stayed. Basalt exists in many different forms, but two minerals are essential: plagioclase feldspar and the same pyroxene that exists in peridotite. Basalt magma is produced by partial melting of peridotite in the mantle. Being lighter than the rest of the mantle, it rises to the surface and accumulates in cracks and pockets and finally exits through volcanoes or rifts. Since basalt is lighter than the rest of the mantle, it can pile up into volcanic mountains.
The above figure, with its black basalt “sea” and volcanic cone of lighter rock (rhyolite), gives an idea of how Earth may once have looked. But this Icelandic landscape is the result of volcanic activity known as the “Krafla Fires” which occurred from 1975 to 1984. The Earth still has some seismic tricks in its mantle.
On cooling, thick basaltic lava flows may crack so as to form vertical columns, often hexagonal in cross section.
Formation of oceans and first continents
Zircon, or zirconium silicate, is an extremely hard mineral often used as a gemstone. It can last for a very long time, it may contain uranium, which means it can be dated, and it contains oxygen. High proportions of the heavy-oxygen isotope, 18O, indicate that crystals were probably formed in water. Such is the case for the world’s oldest known zircon, dated to 4.4 Gya, which suggests to many scientists that there was already water on the Earth at that time. Continents had not yet formed, but convection in the mantle gave rise to volcanic heat rising in certain places, called hot spots, which ejected heat and matter through volcanoes at the surface. The Earth would have been covered with an ocean studded with such volcanic islands.
About those convection currents: They were not circulating air, but circulating rock, and on a huge scale. As more basalt sank into the mantle and more granite rose to the surface, the increasing amount of granite coalesced into larger islands, perhaps sometimes under the persuasion of asteroid collisions, and grew into floating plates. Any remaining basalt was lower than the granite plates and so found itself on the floor of the ocean where it is predominantly found still today. By 3 Gya, plate tectonics had started.4)I admit to having some trouble reconciling the view of Earth as one vast, island-dotted ocean from 4.4 Gya with the view of a surface of falling basalt and rising granite up until 3 Gya.
Evidence for the evidence of sediments at least 4 Gya indicates clearly that large bodies of water existed by then. Water brought about the next major step in differentiation of the Earth’s minerals. Surface basalt heated by underlying magma but cooled by water melted at lower temperatures than before. Just as peridotite melting isolated basalt, so did melting basalt isolate silicon. Minerals with high silicon content were even lighter than basalt and so floated to the surface to form the first granite. Granite is composed of four constituents: quartz, two different feldspars and either pyroxene or mica. So we have seen the following steps in differentiation of the Earth’s minerals:
- Settling of heavy iron to the core, leaving lighter elements behind in the mantle.
- Melting magma makes olivine which forms dunite and sinks into the mantle.
- Remaining olivine combines with pyroxene to make peridotite, which forms a crust before it too cracks and sinks into the mantle.
- Melting peridotite produces basalt, which floats to the surface.
- Melting basalt sinks produces silicon-rich granite, which floats on the basalt mantle.
Earlier, the outer gas planets had their composition determined when solar wind blew the lighter gases outward from the inner rocky planets. Much later, continuing differentiation would separate molecules by forming cell membranes – leading to life. It’s all about ordering driven by energy with much heat loss so that total entropy increases.
Age of the Earth
Because of the Earth’s continued volcanic activity, no rocks from the Earth’s beginnings exist. Nevertheless, radioactive dating of the oldest rocks gives an age greater than 3.8 Gy.5)The principal source for all these figures is an excellent review paper, “The scientific age of the Earth”, on-line at http://www.talkorigins.org/faqs/dalrymple/scientific_age_earth.html. Another excellent source is the US Geological Survey at http://pubs.usgs.gov/gip/geotime/age.html. Zircon crystals from western Australia have been found with ages up to 4.3 Gy.
At the same time the Earth was forming from accretion of dust around the Sun, meteorites called chondrites were being formed from the same material. Different methods of radioactive dating indicate that these meteorites are at least 4.6 Gy of age.
Rocks brought back from NASA’s Apollo mission to the Moon have been dated to between 4.5 and 4.6 Gya.
The best measurements of the Earth’s age come from assuming that isotopes of lead (Pb) were in equal ratios in the solar nebula and therefore in planetesimals and meteorites at the time of the planets’ formation. From this, a calculation based on various rocks and meteorites results in
Age of Earth: 4.54 Gy ± 1%.6)U.S. Geological Survey, “Age of the Earth”, http://pubs.usgs.gov/gip/geotime/age.html.
Now that geological hell on earth is over, on to the Archean Eon and the rise of life on Earth.
Notes [ + ]
|2.||↑||In Greek mythology, Theia was a Titaness and was mother of Helios (the Sun), Selene (the Moon) and Eos (the Dawn).|
|3.||↑||We will see what that means in the next paragraphs.|
|4.||↑||I admit to having some trouble reconciling the view of Earth as one vast, island-dotted ocean from 4.4 Gya with the view of a surface of falling basalt and rising granite up until 3 Gya.|
|5.||↑||The principal source for all these figures is an excellent review paper, “The scientific age of the Earth”, on-line at http://www.talkorigins.org/faqs/dalrymple/scientific_age_earth.html. Another excellent source is the US Geological Survey at http://pubs.usgs.gov/gip/geotime/age.html.|
|6.||↑||U.S. Geological Survey, “Age of the Earth”, http://pubs.usgs.gov/gip/geotime/age.html.|