We learn:
|
 |
| Internal Structure of Earth |
Earth's solid body is composed of several layers of varying density (see Figure). 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.
Differentiation in the first few 100's of millions of years led to the formation of the core and the mantle and a crust, and initiated the escape of gases from the moving interior that eventually led to the formation of the atmosphere and oceans.
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!
Iron Melts
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 material sank to the center, and in the upheaval, heating rates increased and most of the planet was liquified. There might well have been an early ocean of molten rock -- a magma ocean more than 100 km deep. 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 Figure below compares the elemental
abundances for the Earth's crust with the whole Earth, showing that
the crust has a quite different composition from the rest of the Earth,
with abundant oxygen and silicon. About 90% of the
Earth is made of the four elements iron, oxygen, silicon and magnesium.
|
|
The Earliest Atmosphere, Oceans and Continents
After loss of the hydrogen, helium and other hydrogen-containing gases from early Earth due to the Sun's radiation, primitive Earth was devoid of an atmosphere. The first atmosphere was formed by outgassing of gases trapped in the interior of the early Earth, which still goes on today in volcanoes.For the Early Earth, extreme volcanism occurred during differentiation, when massive heating and fluid-like motion in the mantle occurred. It is likely that the bulk of the atmosphere was derived from degassing early in the Earth's history. The gases emitted by volcanoes today are in Table 1 and in Figure.
 |
Composition of volcanic
gases for three volcanoes |
|
|
Volcanic outgassing |
Oxygen in the Atmosphere
Stromatolite and Banded-iron Formation (BIF)
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 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; picture) 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.
 |
Cumulative history of O2 by photosynthesis over geologic time. The start of free O is likely earlier than shown. |
Early Continents
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.
All materials © the Regents of the University of Michigan unless noted otherwise.