Archean, Cratons, Creation, Evolution, Giant Impact Hypothesis, Hadean, Iron Catastrophe, Late Heavy Bombardment, Mantle Convection, Primordial Atmosphere, Primordial Life, Runaway Accretion, Solar Nebula Hypothesis, Subduction
And God said,
Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. [Genesis 1:6]
4,5 billion years ago
The age of Heavens and Earth
According to science, the standard model to explain the formation of our Solar System, including Earth, is “The Solar Nebula Hypothesis”. In this model, the Solar system formed from a large, rotating cloud of interstellar dust and gas, composed of hydrogen and helium, created shortly after the Big Bang and some heavier elements ejected by supernovae.
About 4,5 billion years ago, the nebula began a contraction triggered by the shockwave of a nearby supernova, causing the nebula to rotate. As the cloud began to accelerate, its angular momentum, gravity and inertia flattened it into a protoplanetary disk, perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center. The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri type star ignited and then soon evolved into our actual Sun.
Meanwhile, in the outer part of the nebula, gravity caused matter to condense around density perturbations and dust particles, the rest of the protoplanetary disk beginning separating into rings. In a process known as “Runaway Accretion”, larger fragments of dust and debris then successively clumped together to form planets while the solar wind of the newly formed Sun cleared out most of the material in the original protoplanetary disk that had not already condensed into larger bodies. This is in this manner that Earth originally formed about 4,54 billion years ago to be largely completed within the next 10 to 20 million years.
The first day: The Hadean [4,54 billion years ago]
The first eon in Earth’s history, the Hadean, begins with the Earth’s formation 4,54 billion years ago to be followed by the Archean eon that started 3,8 billion years ago.
From 4,54 billion years ago, the proto-Earth kept growing by accretion until its interior was hot enough to melt the heavy siderophile metals that could be found on its surface. It is only ten million years after that the then proto-Earth evolved into the Earth that we now know and began to form. Having higher densities than the silicates, the heavy siderophile metals of the proto-Earth started to sink in the so coined “Iron Catastrophe” process. Between 2 and 4 billion years ago This so called “Iron Catastrophe” eventually resulted in the separation of a primitive mantle of the Earth and the formation, from what was previously an entirely molten core, of its metallic core, thus producing its layered structure and setting up the creation of the Earth’s magnetic field. From there, volcanic outgassing created the Earth’s “Primordial Atmosphere” that contained almost no oxygen, much of the Earth being then still molten because of its extreme volcanism and frequent collisions with other cosmic bodies, one of these collision being responsible for tilting the Earth at an angle and the formation of the Moon, Earth’s only natural satellite. During the Apollo program, rocks from the Moon’s surface were brought to Earth, radiometric dating of these rocks revealing the Moon being about 4,48 to 4,53 billion years old.
According to the “Giant Impact Hypothesis”, in a period called the “Late Heavy Bombardment” that began about 4,1 billion years ago to conclude around 300 million years later. The Moon originated after a cosmic body the size of Mars struck the proto-Earth at the end of the Hadean. This collision releasing about 100 million times more energy than the impact of the asteroid that caused the extinction of the dinosaurs 65 to 66 million years ago, this was enough to vaporize some of the Earth’s outer layers and melt part of the Earth’s mantle and the impactor, a portion of the Earth mantle material being then ejected into orbit around the Earth. The resulting orbital ejecta then condensed into a single new cosmic body within a period of only a couple of weeks. The moon, the Earth’s only natural satellite, was born.
Even though, due to the large heat flow and geothermal gradient of the Earth, that at the end of the Hadean volcanism was still severe, detrital zircon crystals dated to 4,4 billion years ago show evidence of having undergone contacts with liquid water, thus suggesting that the planet already had oceans or seas at that time. Over time, the Earth cooled down even furthermore to form a solid crust allowing even more liquid water to exist on its surface and sustain life.
The second day: The Archean [3,8 billion years ago]
Since its origin, Earth is often described as having had three atmospheres: a first atmosphere, captured from the solar nebula, composed of light elements from the solar nebula, mostly hydrogen and helium; a second atmosphere rich in volatiles greenhouse gases released by volcanoes, after the “Giant Impact” between the Earth and the smaller proto-planet that gave birth to the moon, but still very poor in oxygen; a third atmosphere, rich in oxygen, that emerged about 2.8 billion years ago, when bacteria began to produce oxygen. While it is believed that oceans may have begun forming as early as 4,4 billion years ago, at the beginning of the Archean eon, as the planet cooled, clouds started to form and rain started to fall on Earth to create even more oceans that covered the Earth.
One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, which are the building blocks of life. Recent computer simulations have even shown that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth. The next stage of complexity could have been reached from at least three possible starting points: self-replication, an organism’s ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. In the early Archean, even if the Earth had cooled significantly enough to sustain life, most present life forms could not have survived in its atmosphere still lacking oxygen and the Earth still lacking an ozone layer. Nevertheless, with candidate fossils dated around 3,5 billion years old, it is possible to affirm that “Primordial Life”, already started to evolve after surviving the “Late Heavy Bombardment” period in hydrothermal vents located below the Earth’s surface and this, from as far back as 4,4 billion years ago when there were still no continents and no land.
Earth’s “Mantle Convection”, this process that still drives plate tectonics today, is the result of a heat flow from the Earth’s interior to the Earth’s surface. Such process results in the creation of rigid tectonic plates at mid-oceanic ridges, plates that are subsequently destroyed, by a process called “Subduction”, that happens into the Earth’s mantle subduction zones. During the early Archean, 3 billion years ago, the mantle of the Earth was very much hotter than today, probably around 1600 °C, so the “Mantle Convection” process was much faster than today resulting in a much faster created tectonic plates. During the Hadean and Archean, subduction zones were also more common than today and, therefore, tectonic plates were much smaller than today. The initial crust of the Earth, formed when the Earth’s surface first solidified to later totally disappeared due to the combination of two main factors and phenomena’s: the fast Hadean plate tectonics creation and destruction process and; the intense impacts of cosmic bodies of the “Late Heavy Bombardment” period. However, it is thought that it was basaltic in composition, like today’s oceanic crust, because little crustal differentiation had yet taken place. The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the end of the Hadean, about 4 billion years ago. What is left of these first small continents are called “Cratons”. These pieces of late Hadean and early Archean crust form the cores around which today’s continents grew.
Like the Dalai Lama would say:
“If science proves religion wrong, it is time to change religion.”
To be continued…
Michel Ouellette JMD is a talented keynote and motivational speaker, public affairs & communications Strategist.
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