The Origin of the Earth: Part V

Here is the fifth and final installment of my Origin of the Earth paper. Enjoy! For those of you who are just reading this, be sure to check out Parts I-IV below.

The Origin of the Earth
Part V: The Moon, the Magma Ocean, and the Mantle

The Moon and the Magma Ocean:

Since an early magma ocean may have played a key role in core formation, theories about the presence of a large magma ocean on the early Earth are important to understand. Many planetary geologists believe that about 30 million years after the formation of the solar system, a Mars-sized planetesimal collided with a mostly-accreted (~90%) Earth (Abe, 1996; Solomatov, 2000). The impact melted and vaporized a large portion of the upper Earth, and a magma ocean was formed and distributed over Earth’s surface. Additionally, the moon was created when a piece of the Earth was ejected into space by the impactor. Likely, the moon formed from a mixture of Earth and impactor material (Jacobsen, 2005). The magma ocean created by this impact may have taken as long as 100 million years to cool (Solomatov, 2000). A thick proto-atmosphere of either captured solar nebula gases or volatiles released from the Earth and the impactor as a result of the collision may have played a key role in blanketing the magma ocean, allowing radiation to be lost to space more slowly and the ocean to crystallize over a longer time (Abe and Matsui, 1986; Abe, 1997).

The exact depth and extent of this magma ocean is debated, but likely the magma ocean was fairly deep, potentially as deep as the modern core-mantle boundary, and crystallized in two stages (Abe, 1997; Solomatov, 2000). The deep magma ocean likely crystallized in two stages at two different rates (Solomatov, 2000). Interestingly, the magma ocean is believed to have crystallized from the bottom-up. This may seem counterintuitive at first as the Earth is hotter at depth. However, there is also higher pressure at depth. Figure 11 is a pressure and temperature diagram which shows the solidus and liquidus for orthopyroxene, a common mantle mineral, and several mantle adiabats. Because the mantle and, presumably, the ancient magma ocean are convecting, material is assumed to rise quickly without transferring heat—that is, the material rises adiabatically, cooling slightly as pressure decreases but remaining hotter, overall, than the upper mantle/magma ocean. Because the mantle adiabats in the magma ocean cross the solidus of mantle minerals at high pressures (i.e. deeper in the Earth), the magma ocean would have crystallized from the bottom up (Solomatov, 2000). Likely, the bottom part of the magma ocean crystallized in a short time period of about 1000 years (Solomatov, 2000).

Figure 11: Pressure-Temperature diagram for the upper mantle (Solomatov, 2000).

The upper part of the magma ocean, the part above ~28 GPa, crystallized much more slowly over a time period of about 100 million years (Solomatov, 2000). Again, a blanketing proto-atmosphere as well as subsequent impacts which re-melted the upper part of the magma ocean, enabled the magma ocean to exist for this long time period. Figure 12 illustrates the proposed two-layer model of the magma ocean.

Figure 12: Two-Layered Magma Ocean (Solomatov, 2000)

If a deep magma ocean did exist on Earth for this long period, then this magma ocean may have played a significant role in core formation, which is believed to have occurred around the same time. Some researchers believe that an iron-rich layer settled to the bottom of the molten part of the magma ocean (Solomatov, 2000). Researchers have noted that it is difficult for metal-rich iron and sulfide melts to migrate through a static, solid silicate matrix (Rushmer et al., 2000). Immiscible iron and sulfide melts are able to travel through a silicate liquid much more easily (Rushmer et al., 2000). Thus, modeling the segregation of the iron core in a magma ocean is much easier than trying to model the segregation of the iron core in a solid Earth. If an iron-rich layer did form at the base of a molten magma ocean, then Rayleigh-Taylor instabilities would have developed as this iron-rich layer is much denser than the lower magma ocean. Eventually, this instability would have caused the iron melt to travel downwards. The melt may have done this as large diapirs (see Figures 13 and 14) or by percolation in-between crystal grains (Figure 15). In a way, the migration of iron-rich material downward in the Earth can be thought of as reverse volcanism. Volcanic melts are believed to travel upwards through diapirs and percolation in-between mantle crystals. The core may have formed through similar mechanisms, just reversed—denser melts just migrated downward instead of lighter melts migrating upwards.

Figure 13: Rayleigh-Taylor Instability Overturn

Figure 14:Descending iron melts in the magma ocean (Stevenson, 1990)

Figure 15: Melt percolation between crystal boundaries (Rushmer et al., 2000)

There is still much investigate about the characteristics of the magma ocean and how this ocean may have affected the differentiation of the Earth. Potentially, there was more than one magma ocean. There may have been a large magma ocean associated with the formation of the moon and several smaller magma oceans associated with other impacts (Abe, 1997). Thus, scientists must learn about the nature of different types of magma oceans: deep verses shallow, short-lived verses long-lived, and soft (high melt fraction, low viscosity) verses hard (low melt fraction, solid-like viscosity) (Abe, 1997). Scientists also need to learn more about blanketing atmospheres associated with magma oceans, the nature of viscosity and convection in a magma ocean, and the role that crystal size and crystal kinetics in the crystallization of a magma ocean (Abe and Matsui, 1986; Solomatov and Stevenson, 1993).

Differentiation of the Upper Earth:

Since the focus of this paper is the origin and development of the early Earth, only a short summary of the later differentiation of the upper Earth will be presented here. In brief, then, after the formation of the core the upper Earth differentiated into enriched, continental crust and a depleted mantle, which is the source for the oceanic crust (Best, 2003). Enriched means that the crust contains high abundances of elements, such as rare Earth elements, which are incompatible in mantle minerals and preferentially go into melts. Thus, the continental crust is believed to have formed first from the upper Earth and was followed by the formation of the denser oceanic crust. The abundances and patterns of incompatible elements, especially the light rare Earth elements, in oceanic crust suggests that the source for this crust was depleted by the formation of the continental crust (Best, 2006). Also, enriched means that the certain isotopic ratios are high while others are low. For instance, the continental crust has low 143Nd/144Nd and high 87Sr/86Sr ratios relative to oceanic crust (Best, 2003; Dickin, 2006). This reverse isotopic trend can be explained because Sm (the parent of 143Nd) is more compatible than Nd while Rb (the parent of 87Sr) is less compatible than Sr.

There is some debate in geology as to the timescales over which the continental and oceanic crust formed. However, the oldest crustal rocks are the 4 billion year old Acasta gneiss in Canada, so the continental crust was at least partly formed by this time (Valley, 2006). The oldest minerals on Earth are 4.4 billion year old zircons from Australia, and the existence of zircons this old suggests that at least small amounts of Granitic proto-continent existed by that time (Valley, 2006). Determining when oceanic crust first formed on Earth is more difficult as dense oceanic crust is generally subducted and recycled, which means that most of Earth’s oceanic crust is younger than 200 million years. The world’s oldest ophiolite (a section of oceanic crust thrust up over continental crust, usually at a back-arc basin) is the ~3.8 billion year old Isua ophiolite in Greenland, so oceanic crust was formed by at least this time period (Furns et al., 2007).

In addition to the continental and oceanic crust observed on Earth today, recent study of short-lived 142Nd isotopes suggests that there may have been an early protocrust, perhaps formed as a result of the crystallization of the magma ocean (Caro et al., 2005; Boyet and Carlson, 2005). This protocrust is called upon to explain anomalies in the 142Nd/144Nd ratio in Archean rocks (Caro et al., 2005). Supposedly, this protocrust was subducted in the Earth and has never been sampled (Caro et al., 2005; Boyet and Carlson, 2005). However, no one has yet put forth a reasonable physical mechanism by which the subduction of this protocrust could have occurred. Potentially, the observed 142Nd/144Nd anomalies are result of flaws in geochemical models assuming a chondritic starting material for the Earth rather than from a mysterious protocrust that was subducted and has never been sampled.

Summary: The Origin of the Earth in a Nutshell

Our solar system evolved from the solar nebula, which was composed of stardust from extinct stars and thus rich in heavier elements relative to Cosmic abundances. Likely triggered by a shockwave from a nearby exploding supernova, the solar nebula collapsed gravitationally to evolve into the solar system. The solar nebula heated up, began spinning faster, and formed into a disk. Eventually, a proto-sun formed at the dense, hot center of the young solar system. At the same time, gases and dust began to condense in the outer, cooler parts of the solar system. Heavier, more refractory elements condensed closer to the sun, forming the terrestrial planets, while hydrocarbons condensed further from the sun, forming large bodies which were able to capture gases from the solar nebula and develop into gas giant planets. At the furthest reaches of the solar system, icy planets formed from methane, water, and ammonia ice. The Earth is believed to have accreted from chondritic planetesimals about 4.567 billion years ago. Chondrite meteorites come from old, undifferentiated asteroids that have undergone very little alteration or metamorphism. Carbonaceous chondrites are rich in organic material and are the least altered and metamorphosed of the chondrites. Thus, carbonaceous chondrites are often used as the starting material for Earth in geophysical and geochemical models. Earth was mostly accreted by ~10 million years after the formation of the solar system, and there was probably significant accretion of the Earth up to ~100 million years. Towards the end of Earth’s accretion, impacts between large planetesimals may have played a key role Earth’s growth and development. In particular, an impact from a large, Mars-sized impactor ~30 million years after the formation of the solar system may have created the moon and a deep magma ocean on Earth. The formation of the core probably occurred gradually as Earth accreted. However, the final stage of core formation may have been aided by the descent of iron and sulfur rich melts through a molten, silicate magma ocean. Eventually, the magma ocean crystallized, and the upper Earth differentiated into an enriched, continental crust and a depleted mantle, the source for oceanic crust. Potentially, an early proto-crust may have existed early in Earth’s history.

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