Earth Differentiation

Earth Differentiation

After the initial formation of protoplanets, remaining gas and dust was gradually blown away by the solar wind during the heat-up phase of the young sun. At that time our earth was probably a barren planet, pockmarked by meteorite bombardment, and may have looked on the surface like the Moon. As time progressed, however, the earth heated up by a combination of three processes:

1) radioactive decay of U, Th, and ⁴⁰K produced a buildup of heat in the Earth interior (probably the most important contributor). The figure at right shows the heat generated by radioactive decay at various times in the past. The heat generation decreased over time because the abundance of radioactive elements diminished due to decay.
2) gravitational compression of the still not fully compacted Earth (gravitational potential energy is converted to heat during compaction)
3) meteorite impacts heated up the planet surface areas via shock waves and impact melts

The gradual heat increase had consequences. Once the melting temperature of iron was reached within the earth, the initially random mixture of dust particles and gases began to unmix and differentiate according to the density of the various materials involved. The melting of iron, also known as the "iron event" among geologists, was a major benchmark in early Earth development. The compositional and thermal stratification of the Earth that it produced are essential for many of the various aspects of Earth dynamics that we observe today (in particular plate tectonics, magnetic field).

The Development of Core, Mantle, and Crust

(1)Early Earth heats up due to radio-
active decay, compression, and
impacts. Over time the temperature of
the planet interior rises towards the
Fe-melting line in diagram at right

(2) This diagram shows a model of earth heating, based on the assumption
that radioactive elements were present (calculating back from present
amounts). Temperature drops towards Earth surface (heat loss to
surrounding space), but the interior heat gradually builds up. The
curves show temperature development and distribution at various points
from onset of heat production. When the temperature curves intersect
the melting curve for iron (about at 600 million years of heating) the first
iron "drops" will begin to form. Because melting depends on a combi-
nation of pressure and temperature, the first drops form at a depth of
less than 500km. With more heating the zone of iron melting becomes
thicker.

coreheatingB.jpg (23913 bytes)

(3) The iron "drops" follow gravity and
accumulate towards the core. Lighter
materials, such as silicate minerals,
migrate upwards in exchange. These
silicate-rich materials may well have
risen to the surface in molten form,
giving rise to an initial magma ocean.

coreanim.gif (6610 bytes)

(4) After the initial segregation into a central iron (+nickel) core and an
outer silicate shell, further differentiation occurred into an inner (solid)
and outer (liquid) core (a pressure effect: solid iron is more densely
packed than liquid iron), the mantel (Fe+Mg silicates) and the crust
(K+Na silicates). Initially large portions of the crust might have been molten
- the so called magma ocean. The latter would have cooled to form a layer
of basaltic crust (such as is present beneath the oceans today).
Continental crust would have formed later. It is probable that the
Earth’s initial crust was remelted several times due to impacts with large
asteroids.

This differentiation caused the heavy metals (iron, nickel and related elements) to be concentrated in the core of the earth, whereas the light elements (oxygen, silicon, aluminum, potassium, sodium, calcium etc.) were enriched in an outer layer of the earth that is now termed the mantle and the crust. Not everything, however, goes simply by density. Uranium and Thorium are very heavy elements, and we should therefore expect them to be enriched in the core. Yet, contrary to expectation they are concentrated in crust and mantle. The reason for this aberration is the circumstance that ion size and chemical affinities of U and Th prevent them from being incorporated in the dense, tight crystal structures that iron assumes at the high pressures encountered in the earth's core. Because they can fit much more easily into the more open crystalline structures of silicate and oxide minerals, they are concentrated in crust and mantle.

After partial melting and differentiation, the Earth would have also allowed the release of gaseous compounds formed and trapped in the interior. Modern volcanoes release gases as magma is brought to the surface. These gasses give us an indication of the composition of the Earth’s earliest atmosphere: water vapor, CO2, CO, N2, H2, and hydrogen chloride. Water vapor would have condensed in the atmosphere and rained down as liquid on the surface, covering the Earth with water.

It is also possible that the Earth has acquired some of its water from comets colliding with the Earth and melting in the upper atmosphere. Recently, some astronomers have argued that as many as 15 million small comets (house-sized and smaller) might be adding water to the atmosphere every year. However, this view is still controversial and concrete evidence for the existence of these comets has not yet been found.

The Earth’s ancient atmosphere was probably highly enriched in CO2 - perhaps as much as 100 times the present amount. This may have been an important way the early Earth was warmed - astronomers theorize that the young sun was only 80% as bright as it is today, which would cause glacial conditions across the globe under our present atmosphere.

As soon as the crust became cool enough not to remelt, convection driven plate tectonics probably began. Initially, because the Earth was much hotter than it was today, more heat would have been flowing up from the mantle. This would have created numerous hot spots and rifts, resulting in many small plates and subduction zones, as well as vigorous plate movement.

Pretty soon after the onset of plate tectonics, the first continents should have formed (we will get into the details of this towards the end of this course). Remelting of oceanic crust combined with water along subduction zones would have caused the formation of the first felsic magmas (rich in silica , K, and Na) and the resulting island arcs. Also, remelting of crust over large hot spots might also have created felsic magmas (such felsic magmas are seen erupting from beneath Iceland today). No matter how it exactly happened, the first continents were probably produced as small land masses that eventually accreted together
as plates were subducted and brought these protocontinents into collision.

One of the characteristic minerals that forms in felsic magmas is zircon, and it is also very resistant to weathering, erosion, and remelting. Zircons as old as 4.2 billion years have been found in Archean sediments from Australia, and the oldest rocks discovered so far (3.8-4.0 billion years old) are metamorphic rock of the Acosta Formation from north-central Canada. This indicates that felsic magmas formed as early as 4.2 billion years ago, only 400 million years after the Earth formed. More recently, even older zircons have been discovered in ancient stream deposits from Australia, pushing the earliest formation of continental crust 200 million years further into the past (read story below).

Tiny zircon crystals found in ancient stream deposits suggest that Earth harbored continents and liquid water remarkably soon after our planet formed.

January 17, 2001 -- Scientists are drawing a portrait of how Earth looked soon after it formed 4.56 billion years ago, based on clues within the oldest mineral grains ever found.

Tiny zircons (zirconium silicate crystals) found in ancient stream deposits indicate that Earth developed continents and water -- perhaps even oceans and environments in which microbial life could emerge -- 4.3 billion to 4.4 billion years ago, remarkably soon after our planet formed.

Below: Where the newly-discovered zircons fit in Earth history. Image by Dan Brennan.

The findings by two research groups, one in Australia and the other in the United States, suggest that "liquid water stabilizes early on Earth-type planets," said geologist Stephen Mojzsis, a member of the NASA Astrobiology Institute's University of Colorado, Boulder, team. "This increases the likelihood of finding life elsewhere in the universe" because conditions conducive to life can evidently develop faster and more easily than once thought.

It also "gives us a new view of the early Earth, where the Earth cooled quickly" after gas and dust in the newborn solar system congealed to form planets, said geologist William Peck, of Colgate University in Hamilton, New York. "There were continents and water really early -- and maybe oceans and life -- all to be obliterated later by meteorites, with almost no record left except these zircons."

Until roughly 3.9 billion years ago, swarms of comets and meteorites whacked the young Earth often enough to occasionally vaporize the surface zones of the oceans and erase any life residing there. The earliest known evidence of microbial life on Earth comes from carbon isotope patterns investigated by Mojzsis and colleagues in 3.85-billion-year-old Greenland sediments.

Now, the zircons from Western Australia demonstrate that continents and water existed 4.3 billion to 4.4 billion years ago. "Life could have had the opportunity to start 400 million years earlier than previously documented," Mojzsis said.

"Life could have arisen many times, only to be smashed, and it only gets a hold once the meteorites taper off," Peck added.

Mojzsis and Peck belong to separate research teams, one that found a 4.4-billion-year-old zircon in 1999 and another team that unearthed a pair of 4.3-billion-year-old zircons last year from the same area of Western Australia's Jack Hills rock formation. Both groups published their studies in the Jan. 11, 2001, issue of the British journal Nature.

The 4.4-billion-year-old zircon is "our earliest record of the earliest crust" on Earth, Peck said. That zircon and the slightly younger zircon grains measure roughly 250 microns wide -- less than one one-hundredth of an inch.

see caption "These zircons have really been through the wringer," said Peck.

Left: The Jack Hills region of Western Australia, where the zircons were discovered. Photo by Simon Wilde.

Their history began sometime after Earth formed, when "liquid water interacted with rocks," he said. That interaction can happen in one of three ways: when water exchanges with minerals in rocks, when crystals grow out of solution in ground water, or when mineral veins are deposited. Exposure to water increased the rocks' normally low ratio of the uncommon isotope oxygen-18 to the more-common isotope oxygen-16, he said.

Later, the rocks were melted underground -- or perhaps during a meteorite bombardment -- and the zircons formed as crystals within molten granite that was cooling to form solid rock.

The zircon-laden granite eventually was thrust upward to form mountains, which later eroded. The granite vanished, but the zircons ultimately came to rest 3 billion years ago in sandy Australian stream sediments. These sediments later hardened into rocks that subsequently were altered by heat and pressure.

Both research teams used instruments called ion microprobes to date and analyze the zircon crystals, which often contain uranium, rare earth elements and other impurities. Uranium decays to lead at a known rate. Uranium-lead ratios in the zircons showed they formed as early as 4.4 billion to 4.3 billion years ago when they crystallized in molten granite.

Below: Microscopic view of a zircon (zirconium silicate) crystal determined to be 4.4 billion years old. Photo by John W. Valley

see caption Continental crust is different than crust that underlies the oceans. Granite is a common rock in continents. And zircons commonly crystallize in granite.

So the zircons indicate granite was present 4.3 billion to 4.4 billion years ago, while the granite means continents existed at that time. Such old granitic rock has not been found; it all has subsequently been eroded away or otherwise recycled. The ancient zircons are surviving vestiges of crustal granite from Earth's early years.

"The fact you have a 4.4-billion-year-old zircon from granite suggests there had to be the rock of the continental crust," said geologist Sam Bowring of the Massachusetts Institute of Technology.

Ion microprobe analysis of rare-earth elements within the zircon crystals also found levels typical of continental rocks, Peck said.

The presence of water on the young Earth was confirmed when both groups analyzed the zircons for oxygen isotopes and found the telltale signature of rocks that have been touched by water: an elevated ratio of oxygen-18 to oxygen-16.

As a result, "we know there was liquid water at some point before 4.4 billion years ago," Peck said. Liquid water had to collect somewhere, raising the possibility of oceans, he added.

He said it also is likely oceans existed because "to make continents, you need to have water."

Peck said that before there were oceans, giant plates of Earth's crust already could have started moving and colliding with each other, causing large blocks of rock to dive downward in a process called subduction. Without oceans, that rock could not have melted to form continental rock like granite, he said.

Below: Outcrop of the type of rock where the zircons were discovered. The hammer shows scale. Photo by Simon Wilde.

see caption Once there were oceans, however, seawater would have reacted with and hydrated lava erupting from undersea volcanoes at the mid-ocean ridges. The lava would then have cooled and formed new seafloor, which later subducted. The water trapped in minerals within the sinking rock lowered its melting point, triggering volcanic eruptions that probably produced island chains made of granitic rocks. It is thought that such "island arcs" ultimately clumped together to form continents.

"Oceans, atmosphere and continents were in place by 4.3 billion years ago," said Mojzsis.

According to Peck, the first oceans might have formed from water brought to Earth by comets or have been emitted during early volcanic eruptions from what became mid-ocean ridges.

The zircons suggest that life could have existed on Earth 4.3 billion years ago, said Mojzsis, because three key factors necessary for life to take hold were present: energy, organic material (from incoming comets and atmospheric reactions) and -- according to the zircons -- liquid water.

Credits: Discovery of the 4.4-billion-year-old zircon was reported by Peck, Simon Wilde at the Curtin Institute of Technology in Australia; John Valley at the University of Wisconsin, Madison; and Colin Graham of the University of Edinburgh in the United Kingdom. Wilde found the 4.4-billion-year-old grain in 1999 while dating zircons from a rock collected in 1984, Peck said. Mojzsis and colleagues say they found a pair of 4.3-billion-year-old zircons last year from the same area of Western Australia's Jack Hills rock formation. Mojzsis worked with geochemist Mark Harrison of the University of California, Los Angeles, and Robert Pidgeon of the Curtin Institute of Technology.