Hadean
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The Hadean Eon
The Hadean Eon of Precambrian Time: 4500 to 3800 million years ago
The name Hadean was coined by geologist Preston Cloud for the pre-Isuan sequence whose record may not be preserved on Earth but is better known from Moon rocks. Consequently, the time sequence and stratigraphy of the Hadean are largely based on lunar events. For example the Nectarian Era is defined by reference to the formation of the Nectaris Basin (southwestern Nearside). The Hadean has no place in the ICS system followed in the rest of Palaeos. The ICS lumps everything earlier than 3600 Mya into the Eoarchean Era of the Archean Eon.
During Hadean time, the Earth and Solar System formed by coagulation and gravitational contraction from a large cloud of gas and dust around the sun, called an accretion disc. The sun formed the nucleus, shrinking in on itself by gravitational compaction until it reached a stage where it ignited with nuclear fusion and give off light and heat. The surrounding particles within this cloud coalesced into planetisimals which then aggregated to form microplanets (rather like modern asteroids). The energy of the collisions between the larger microplanets, as well as interior radioactive and gravitational heating, generated a huge amount of heat, and the Earth and other planets would have been initially molten. The Earth and Moon formed rather late in this process, from a collision between two large bodies -- a mars-sized planetoid and a slightly larger body.
During this period the heavier molten iron sank to the down to become the core, while the lighter rock rose to the surface. The lightest of all became the crust as a sort of "scum" on the surface. There was also an outgassing of volatile molecules such as water, methane, ammonia, hydrogen, nitrogen, and carbon dioxide. An initial steam atmosphere formed of water from comets and hydrated minerals. Rain fell to form a proto-ocean 4.3 to 4.4 billion years ago. All terrestrial planets had a similar process in their early histories.
Once most of the planetisimals were gone the planetary bombardment stopped, and a stable rocky crust was able to formed on the Earth. This is the age of the oldest rocks on earth and also of moon rocks. Atmospheric water condensed into oceans and proto-life formed in the soup of primordial organic molecules, either in the early oceans or in clay or rocks within the crust itself. These stages are considered in detail below.
Image: Nectaris Basin from Lunar Orbiter Imagery.
The Geological Time-Scale of the Hadean
| Eon | Stage(Era) | when began myrs ago | duration myrs |
|---|---|---|---|
| Hadean | Early Imbrian | 3,850 | 50 |
| Nectarian | 3,950 | 100 | |
| Basin-Groups 1-9 | 4,150 | 200 | |
| Cryptic | 4560 | 410 |
The Dynamics of the Hadean[2]
The Hadean or Pregeologic Eon is the time period during which the Earth was transformed from a gaseous cloud into a solid body. In terms of "Year of the Earth," it begins on January 1 and ends about 26 February. The process of solidification is poorly known, however, and the Hadean may have lasted as long as one billion years.
Earth cross-sectionThis is the period during which the Earth's crust [1] was formed. This crust melted and reformed numerous times, because it was continuously broken up by gigantic magma currents that erupted from the depths of the planet, tore the thin crust, and then cooled off on the surface before sinking again into the heart of the Earth.
The details of this slow, destructive process are still uncertain. However, it is thought that the heavy elements, like iron, tended to sink towards the center of the Earth because of their higher density, while the lighter components, particularly the silicates, formed an incandescent ocean of melted rock on the surface. Approximately 500 million years after the birth of the Earth, this incandescent landscape began to cool off. When the temperature fell under 1000� C., the regions of lower temperatures consolidated, become more stable, and initiated the assembly of the future crust.
Development of the Earth's Crust
In principle therefore the Earth was a sphere of melted rock, churned by convective movements between the hot inner layers, while the outer, surface regions were in contact with the cold of surrounding space. The dissipation of heat to space began the cooling of our planet. In the magma ocean blocks began to appear, formed from high melting point minerals. These red hot, but solid slabs were similar (although on a very different scale) to the thin edges of crust that we see forming on the surface of flowing lava. The result is what sees in the imaginative reconstruction to the right. Note that, in those times, the Moon, still burning, was only 16,000 km from the Earth (compared to 384,000 km today), and for that reason occupied a big part of the sky. Truly a nightmare landscape!
But those first fragments of crust must also have been very unstable, easily resorbed by the liquid mass of magma and sucked into the depths. Only, with the further cooling of the planet, might those fragments become numerous and large enough to form a first, thin, solid cover -- that is to say, a true primitive crust. This primordial crust might have developed as a warm expanse of rocks (some hundreds of degrees Celsius), interrupted by numerous large breaks, from which enormous quantities of magma continued to erupt. At this early point in the history of the Solar System, meteoric bombardment was intense, and it would have continually opened new holes in the crust, immediately filled by magma. The scars left by this intense meteoric bombardment, which continued for at least 700 or 800 million years, have been almost totally erased on the Earth by subsequent reworking of the crust. However, the resulting impact craters and lava flows are perfectly preserved on the Moon and on many other bodies in the Solar System whose geological evolution ended long ago.
The Primordial Atmosphere
As a result of the high temperatures at the center of the Earth, and due to volcanic activity, the crust emitted halogen gasses, ammonia, hydrogen, carbon dioxide, methane, water vapor, and other gasses. In the following 100 million years, these gasses accumulated to form the primordial atmosphere. This atmosphere was quite similar to the atmosphere of Titan, one of the larger moons of Saturn. The primordial atmosphere is believed to have reached a pressure of 250 atmospheres and would have been extremely toxic to life as we now know it.
Little by little our planet assumed a more familiar look, with a dense gaseous cloud zone we could call an atmosphere, a liquid zone with oceans, lakes and rivers, or hydrosphere, and a solid zone, or lithosphere with the first outlines of what would one day become continents. Then, under the lithosphere, the mantle and the core differentiated (see below).
Outgassing
The process of cooling and consolidation of the Earth's surface was accompanied, as still occurs in volcanoes, by strong outgassing of new atmosphere, formed essentially from methane (CH4), hydrogen (H2), nitrogen (N2) and water vapor, with smaller amounts of noble gases and carbon dioxide. Most of the hydrogen, the lightest component, escaped into space as also happens today. The other gases and vapors accumulated, including water vapor. The water did not condense at this point, because the temperatures of the crust was still very high. From a petrographic point of view, the primitive crust was similar to basalt, a dark volcanic rock, with less than 53 % SiO2 by weight. This basalt was formed from the material of the mantle, but had a rather different composition. The more ancient blocks found on the Moon, approximately 4.6 billion years old, are in fact just basalts with a high aluminum content. But the composition of the crust must have differed even more from that of the mantle. Disrupted by highly energetic convective movements, the thin lithospheric covering would have been fragmented into numerous small plates in continuous mutual movement, separated and deformed by bands of intense vulcanism. The remelting of part of the crust, analogous to subduction, gradually produced magmas richer in silicates. Thus, around the basalts appeared andesites: fine granular volcanic rocks, whose name derives from the Andes, where several volcanoes are known to form rocks of this type.
Formation of the oceans
At the same time, another important series of events began to unfold that led to the formation of sedimentary rocks through the processes of erosion, drift, and accumulation. These processes began to occur as soon as the surface cooled enough to allow the water cycle to establish itself In fact, the primitive Earth long remained covered in darkness, wrapped in dense burning clouds into which continuously poured water vapor from volcanic emissions. When temperatures finally cooled sufficiently, the clouds began to melt into rain, and the primordial atmosphere produced storms of unimaginable proportions, under which the Earth groaned and flowed. At first, falling on incandescent rock, the rain evaporated, but the evaporation gradually cooled the crust until the water could accumulate in the depressed regions of the Earth's surface, forming the first oceans. On the primordial continents, the first river networks were created, and they transported detritus torn from elevated regions and then deposited on the bottom of the primordial seas. The metamorphism and remelting of the products of the erosion ultimately produced magmas and lava increasingly rich in silicates, and therefore of different composition from the mantle and the primitive crust.
The Birth of granites
From all these processes, such as the remelting of part of the basaltic primitive crust, accompanied by metamorphism and melting of large quantities of sediments, there gradually formed magmas similar in composition to granites, and therefore able to "float" on basalt.
Fragment by fragment, formed in the beginning from island chains similar to modern-day volcanic island arcs, the continental crust was born, and so the external land cover of the planet. This new type of crust had a unique feature of fundamental importance: its low density kept it riding on the surface. Thus it was able to undergo intense transformations, such as mechanical deformation (tectonics) or metamorphism, but remain always in proximity to the surface. While the primitive basaltic crust has probably been permanently lost, geologists have found some traces of those first outlines of continental crust. In 1983, in western Australia, were found the most ancient rocks known to date. These rocks are dated to about 4.2 billion years. Remarkably, they are sandstones. This means that they were derived from the erosion of other rocks, still more ancient!
[1] The Crust: The more external part of the crust, or Lithosphere constitutes the superficial covering of the Earth. Two kinds of crust are easily distinguished by composition, thickness and consistency: continental crust and oceanic crust. Continental crust has a thickness that, in mountainous regions chains may reach 40 kilometers. It is composed mainly of metamorphic rock and igneous blocks enriched in potassium, uranium, thorium and silicon. This forms the diffuse granitic bedrock of 45% of the land surface of the Earth. The oceanic crust has a more modest thickness, on the order of 5-6 kilometers, and is made up of basaltic blocks composed of silicates enriched in aluminum, iron and manganese. It is continuously renewed along mid-ocean ridges.
[2] From Era Precambriana by Prof. Franco Maria Boschetto, translated from the Italian by ATW040312.
