first_billion_years - The Global Change Program at the

Earth's Early Years:
Differentiation, Water
and Early Atmosphere
We wish to learn:
What happened to the Earth and Moon during the first few hundred million years?
How did the core and mantle form?
What was is the origin of the atmosphere and ocean?
What is the role of early life?
Jump to: [Differentiation. A Molten Planet] [The Earliest Atmosphere, Oceans and
Continents] [Summary] [Self-Test]
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Differentiation: A Molten Planet
Geologists have deduced that the Earth's solid body is composed of
several layers of varying density (see Figure 1). The Earth's core is
composed of two portions, an inner core of solid iron and an outer
core of molten iron (perhaps with some S). Above the core lies the
mantle, which is made up of dense silicates, and the crust, which is
the outer layer of the solid Earth. The oceans and atmosphere are
the outermost layers. As we have already mentioned, the Earth
must have been largely molten at some early stage in order to have
been differentiated into several zones or layers.
Differentiation is perhaps the most significant event in the history of
the Earth. It led to the formation of the core and the mantle and a
crust with continents. Differentiation initiated the escape of gases
Internal Structure of Earth from the moving interior, which eventually led to the formation of
the atmosphere and oceans.
If the Big Bang led to all the universe, the "Big Burp" of differentiation led to much of the
environment we live in. It occurred in the first few 100's of millions of years, perhaps as a
catastrophic event.
Heating of Early Earth
The earliest Earth was probably an unsorted conglomeration, mostly of silicon compounds, iron
and magnesium oxides, and smaller amounts of all the natural elements. It became increasingly
hotter as the protoplanet grew. Four different effects led to the heating of our planet:
1. Accretion. Impacting bodies bombard the Earth and convert their energy of motion (kinetic
energy) into heat. In recent years we also learned that an early collision with a very large
object was responsible for the "extraction" of the Moon from Earth.
2. Self-compression. As the Earth gets bigger, the extra gravity forces the mass to contract
into a smaller volume, producing heat (just like a bicycle pump gets hot on compression).
3. Differentiation. Conversion of gravitational potential energy to heat during core formation
3. Short-lived radiogenic isotopes. The surrounding material absorbs the energy released in
radioactivity, heating up. Today this is a very slow but steady source of heat. About 20 calories
of heat are generated by 1 cubic centimeter of granite in the course of a million years. It would
take this amount of rock 500 million years to brew a cup of coffee!
Rock is a very poor conductor of heat. So the heat just continued to build up until materials
started to melt.
Iron Melts
Calculations have been carried out to determine what
happened to the early "homogenous" earth before
differentiation. At some point, probably within the
first few hundred million years of Earth, the surface
down to a depth of about 500 km became so hot that
iron (a plentiful element) started to melt. The molten
iron collected and began to sink under its own great
weight. About one third of the primitive planet's
Temperature in the Earth's interior
material sank to the center, and in the upheaval,
at different times in its history compared heating rates increased and most of the planet was
with the melting point of iron.
liquified. There might well have been an early ocean
(Taken from Press and Siever 1992).
of molten rock -- a magma ocean more than 100 km
deep. Figure 2 provides the results of a calculation comparison of predicted Earth temperature
profiles and the melting point curve for iron under the corresponding conditions of pressure.
The Iron Core Sinks
The formation of a molten iron core was the first stage of the differentiation of the Earth, in
which it was converted from a homogenous body, with roughly the same kind of material at all
depths, to a layered body, with a dense iron core, a crust composed of lighter materials with
relatively lower melting points, and between them the mantle.
The melting
of iron leads to the
formation of a
heavy liquid layer.
Drops begin to develop
in later stages and
sink toward the center.
The first stage of differentiation, iron melting led to the onset of a mechanism that speeded up
the differentiation process. This is convective overturn -- a process whereby molten material
may overturn, transfering heat buried deeply within the planet to the outer layers. The hotter
material expands, becomes lighter than the material above it, and floats upward, carrying its
heat to the surface, where the hot matter cools and sinks again. Convective overturn probably
was the mechanism that produced the thin crust, and ultimately determined the different
chemical composition of the crust. The very hot young Earth would also have been very active
tectonically (that is with lots of volcanic activity), leading to outgassing of volatile gases from
within the planet that formed the early atmosphere.
The Figure below compares the elemental abundances for the Earth's crust with the whole
Earth. The data show that the crust has a quite different composition from the rest of the Earth,
with oxygen and silicon being the most abundant species. About 90% of the Earth is made of
the four elements iron, oxygen, silicon and magnesium.
Comparison of relative abundances of elements in (a) the Earth's crust and (b) the whole
Compare the abundance of elements in the crust with the values for the Earth as a whole.
Because most of the iron sank to the core, that element drops to fourth place. Conversely,
silicon, aluminum, calcium, potassium, and sodium are far more abundant in the crust than in
the whole Earth. This uneven distribution of elements has great consequences for life and is
called chemical zonation.
The reason for the different make up is that the elements favored in the crust form light-weight
chemical compounds, which are easily melted. Materials such as these melted early during the
differentiation, rose to the surface by convective overturning and accumulated.
The Earliest Atmosphere, Oceans and Continents
We will describe very briefly here the role that differentiation played in the formation of the
atmosphere, oceans and continents. Later sections of the course will provide more details on
the evolution of these entities.
Early Atmosphere
After loss of the hydrogen, helium and other hydrogen-containing gases from early Earth due to
the Sun's heat, we believe that the primitive Earth was devoid of an atmosphere. The first
atmosphere was formed by outgassing of gases trapped in the interior of the early Earth. This
process of outgassing still goes on today in volcanoes.
Composition of volcanic
gases for three volcanoes
Today, volcanic emissions are coupled with tectonic movements of the Earth's crust, as we will
see in later sections. For the Early Earth, extreme volcanism would have occurred during the
process of differentiation, when massive heating, fluid-like motion in the mantle and crust
occurred. It is likely that the bulk of the atmosphere was derived from degassing early in the
Earth's history. In fact there is strong evidence that the atmosphere today carries the
equivalent of only 1% of the total outgassing. The rest of the outgassed material has been
locked up in various rocks types.
The gases emitted by volcanoes today are shown in in Table 1 and in Figure 5. We expect that
significant quantities of nitrogen N2 and ammonia NH3 would have also been emitted at early
times. As can be seen, water vapor is a dominant emission and became the source of the
present-day oceans. Over geologic time and particularly during the active early phase, vast
quantities of water and carbon dioxide have been vented into the air - quite sufficient to form
the oceans.
Volcanic outgassing
Much of the carbon dioxide, however, has been removed from the atmosphere by dissolution
in water and chemical reactions with calcium (e.g., limestone) and other
elements. This lowered the temperature, allowing for additional rain
contributing to the formation of oceans.
Oxygen in the Atmosphere
Life started to have a major impact on the environment once photosynthetic
organisms evolved. These organisms, blue-green algae (picture of stromatolite,
which is the rock formed by these algae), fed off atmospheric carbon dioxide and converted
much of it into marine sediments consisting of the innumerable shells of sea creatures.
While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started
to produce oxygen. For a long time, the oxygen produced did not build up in the atmosphere,
since it was taken up by rocks, as recorded in Banded Iron Formations (BIFs) and continental
red beds. To this day, the majority of oxygen produced over time is locked up in the ancient
"banded rock" and "red bed" formations. It was not until probably only 1 billion years ago that
the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air.
Once oxygen had been produced, ultraviolet light split the molecules, producing the ozone UV
shield as a by-product. Only at this point did life move out of the oceans and respiration
evolved. We will discuss these issues in greater detail later on in this course.
Early Oceans
The Early atmosphere was probably dominated at first by water
vapor, which, as the temperature dropped, would rain out and form
the oceans. This would have been a deluge of truly global
proportions an resulted in further reduction of CO2. Then the
atmosphere was dominated by nitrogen, but there was certainly no
oxygen in the early atmosphere. The dominance of Banded-Iron
Formations (BIFs; see picture) before 2.5Ga indicates that Fe
occurred in its reduced state (Fe2+). Whereas reduced Fe is much more soluble than oxidized
Fe (Fe3+), it rapidly oxidizes during transport. However, the dissolved O in early oceans reacted
with Fe to form Fe-oxide in BIFs. As soon as sufficient O entered the atmosphere, Fe takes the
oxidized state and is no longer soluble. The first occurrence of redbeds, a sediments that
contains oxidized iron, marks this major transition in Earth's atmosphere. A scenario is
illustrated below.
Cumulative history of O2 by
photosynthesis over geologic time.
The start of free O is likely earlier
than shown.
Early Continents
We have only a very general understanding of how the first continents were formed. In one
theory, lava flowing from the partially molten interior spread over the surface and solidified to
form a thin crust. This crust would have melted and solidified repeatedly, with the lighter
compounds moving to the surface. This is called differentiation. Weathering by rainfall broke up
and altered the rocks. The end result of these processes was a continental land mass, which
would have grown over time. The most popular theory limits the growth of continents to the
first two billion years of the Earth.
Suggested Readings
Press, F., and R. Siever, "Earth", 4th ed., W. H. Freeman, New York, 1986.
Schopf, J. W., (ed.)., "The Earth's Earliest Biosphere", Princeton University Press,
Princeton, New Jersey, 1983.
Emiliani, C., "Planet Earth: Cosmology, Geology, and the Evolution of Life and
Environment", Cambridge, 1992.
Holland, H. D., "The Chemical Evolution of the Atmosphere and Oceans", Princeton
University Press, Princeton, New Jersey, 1984.
Take the Self-Test for this lecture.
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