Origin and evolution of the Earth

The planets of our solar system probably formed from a disc-shaped cloud of hot gases, the remnants of a stellar supernova. Condensing vapours formed solids that coalesced into small bodies (planetesimals), and accretion of these built the dense inner planets (Mercury to Mars). The larger outer planets, being more distant from the sun, are composed of lower-density gases, which condensed at much cooler temperatures.

As the early Earth accreted to something like its present mass some 4.5 billion years ago, it heated up, mainly due to the radioactive decay of unstable isotopes (Box 1.1) and partly by trapping kinetic energy from planetesimal impacts. This heating melted iron and nickel (Ni) and their high densities allowed them to sink to the centre of the planet, forming the core. Subsequent cooling allowed

Box 1.1 Elements, atoms and isotopes

Elements are made from atoms—the smallest particle of an element that can take part in chemical reactions. Atoms have three main components: protons, neutrons and electrons. Protons are positively charged, with a mass similar to that of the hydrogen atom. Neutrons are uncharged and of equal mass to protons. Electrons are about 1/1836 the mass of protons, with a negative charge of equal value to the (positive) charge of protons.

Atoms are electrically neutral because they have an equal number (Z) of protons and electrons. Z is known as the atomic number and it characterizes the chemical properties of the element.

The atomic weight of an atom is defined by its mass number and most of the mass is present in the nucleus.

Mass number

Equation 1 shows that the mass of an element can be changed by altering the number of neutrons. This does not affect the chemical properties of the element (which are determined by Z). Atoms of an element which differ in mass (i.e. N) are called isotopes. For example, all carbon atoms have a Z number of 6, but mass numbers of 12, 13 and 14, written:

12C, 13C, 14C (isotopes of carbon)

In general, when the number of protons and neutrons in the nucleus are almost the same (i.e. differ by one or two), the isotopes are stable. As Z and N numbers become more dissimilar, isotopes tend to be unstable and break down by radioactive decay (usually liberating heat) to a more stable isotope. Unstable isotopes are called radioactive isotopes (see Section 2.8).

Mass number

Fig. 1 Representation of the hydrogen atom. The dots represent the position of the electron with respect to the nucleus. The electron moves in a wave motion. It has no fixed position relative to the nucleus, but the probability of finding the electron at a given radius (the Bohr radius, r) can be calculated; r = 5.3 x 10-5mm for hydrogen.

Fig. 1 Representation of the hydrogen atom. The dots represent the position of the electron with respect to the nucleus. The electron moves in a wave motion. It has no fixed position relative to the nucleus, but the probability of finding the electron at a given radius (the Bohr radius, r) can be calculated; r = 5.3 x 10-5mm for hydrogen.

Atomic number, Z

Fig. 1.1 The cosmic abundance of elements. The relative abundance of elements (vertical axis) is defined as the number of atoms of each element per 106 atoms of silicon and is plotted on a logarithmic scale.

Atomic number, Z

Fig. 1.1 The cosmic abundance of elements. The relative abundance of elements (vertical axis) is defined as the number of atoms of each element per 106 atoms of silicon and is plotted on a logarithmic scale.

solidification of the remaining material into the mantle of MgFeSiO3 composition (Fig. 1.2).

1.3.1 Formation of the crust and atmosphere

The crust, hydrosphere and atmosphere formed mainly by release of materials from within the upper mantle of the early Earth. Today, ocean crust forms at mid-ocean ridges, accompanied by the release of gases and small amounts of water. Similar processes probably accounted for crustal production on the early Earth, forming a shell of rock less than 0.0001% of the volume of the whole planet (Fig. 1.2). The composition of this shell, which makes up the continents and ocean crust, has evolved over time, essentially distilling elements from the mantle by partial melting at about 100 km depth. The average chemical composition of the present crust (Fig. 1.3) shows that oxygen is the most abundant element, combined in various ways with silicon, aluminium (Al) and other elements to form silicate minerals.

2.90 3.3 Rapid increase in density 4.3

Gradual increase in density

Crust

13.6

Crust

2900 d

5150 A

Inner core

Fig. 1.2 Schematic cross-section of the Earth. Silica is concentrated in the crust relative to the mantle. After Raiswell et al. (1980).

Inner core

Depth (km)

""^q}- Siliceous rocks "200 ^ Basic rocks of olivine and pyroxene - Dense Mg and Fe silicates

700 J

Dense oxides? MgO and SiO2

2900 d

5150 A

6371

Solid metal

Fig. 1.2 Schematic cross-section of the Earth. Silica is concentrated in the crust relative to the mantle. After Raiswell et al. (1980).

Various lines of evidence suggest that volatile elements escaped (degassed) from the mantle by volcanic eruptions associated with crust building. Some of these gases were retained to form the atmosphere once surface temperatures were cool enough and gravitational attraction was strong enough. The primitive atmosphere was probably composed of carbon dioxide (CO2) and nitrogen gas (N2) with some hydrogen and water vapour. Evolution towards the modern oxidizing atmosphere did not occur until life began to develop.

1.3.2 The hydrosphere

Water, in its three phases, liquid water, ice and water vapour, is highly abundant at the Earth's surface, having a volume of 1.4 billion km3. Nearly all of this water (>97%) is stored in the oceans, while most of the rest forms the polar ice-caps and glaciers (Table 1.1). Continental freshwaters represent less than 1% of the total volume, and most of this is groundwater. The atmosphere contains comparatively little water (as vapour) (Table 1.1). Collectively, these reservoirs of water are called the hydrosphere.

Fig. 1.3 Percentage of major elements in the Earth's crust.

Table 1.1 Inventory of water at the Earth's surface. After Speidel and Agnew (1982).

Reservoir Volume (106 km3) Percentage of total

Table 1.1 Inventory of water at the Earth's surface. After Speidel and Agnew (1982).

Oceans

1350

97.41

Ice-caps and glaciers

27.5

1.98

Groundwater

8.2

0.59

Inland seas (saline)

0.1

0.007

Freshwater lakes

0.1

0.007

Soil moisture

0.07

0.005

Atmosphere*

0.013

0.001

Rivers

0.0017

0.0001

Biosphere

0.0011

0.00008

Total

1385.9

100

*As liquid equivalent of water vapour.

*As liquid equivalent of water vapour.

The source of water for the formation of the hydrosphere is problematical. Some meteorites contain up to 20% water in bonded hydroxyl (OH) groups, while bombardment of the proto-Earth by comets rich in water vapour is another possible source. Whatever the origin, once the Earth's surface cooled to 100°C, water vapour, degassing from the mantle, was able to condense. Mineralogical evidence suggests water was present on the Earth's surface by 4.4 billion years

Atmosphere (0.013) Vapour transport 0.04

Precipitation Evaporation 0 111

Precipitation Evaporation 0 111

Fig. 1.4 Schematic diagram of the hydrological cycle. Numbers in parentheses are reservoir inventories (106km3). Fluxes are in 106km3yr-1. After Speidel and Agnew (1982).

ago, soon after accretion, and we know from the existence of sedimentary rocks laid down in water that the oceans had formed by at least 3.8 billion years ago.

Very little water vapour escapes from the atmosphere to space because, at about 15 km height, the low temperature causes the vapour to condense and fall to lower levels. It is also thought that very little water degasses from the mantle today. These observations suggest that, after the main phase of degassing, the total volume of water at the Earth's surface changed little over geological time.

Cycling between reservoirs in the hydrosphere is known as the hydrological cycle (shown schematically in Fig. 1.4). Although the volume of water vapour contained in the atmosphere is small, water is constantly moving through this reservoir. Water evaporates from the oceans and land surface and is transported within air masses. Despite a short residence time (see Section 3.3) in the atmosphere, typically 10 days, the average transport distance is about 1000 km. The water vapour is then returned to either the oceans or the continents as snow or rain. Most rain falling on the continents seeps into sediments and porous or fractured rock to form groundwater; the rest flows on the surface as rivers, or re-evaporates to the atmosphere. Since the total mass of water in the hydrosphere is relatively constant over time, evaporation and precipitation must balance for the Earth as a whole, despite locally large differences between wet and arid regions.

The rapid transport of water vapour in the atmosphere is driven by incoming solar radiation. Almost all the radiation that reaches the crust is used to evaporate liquid water to form atmospheric water vapour. The energy used in this transformation, which is then held in the vapour, is called latent heat. Most of the

Fig. 1.5 Variation in relative amounts of solar radiation (energy per unit area) with latitude. Equal amounts of energy A and B are spread over a larger area at higher latitude, resulting in reduced intensity of radiation.

remaining radiation is absorbed into the crust with decreasing efficiency with increasing latitude, mainly because of the Earth's spherical shape. Solar rays hit the Earth's surface at 90 degrees at the equator, but at decreasing angles with increasing latitude, approaching 0 degrees at the poles. Thus, a similar amount of radiation is spread over a larger area at higher latitudes compared with the equator (Fig. 1.5). The variation of incoming radiation with latitude is not balanced by an opposite effect for radiation leaving the Earth, so the result is an overall radiation imbalance. The poles, however, do not get progressively colder and the equator warmer, because heat moves poleward in warm ocean currents and there is poleward movement of warm air and latent heat (water vapour).

1.3.3 The origin of life and evolution of the atmosphere

We do not know which chance events brought about the synthesis of organic molecules or the assembly of metabolizing, self-replicating structures we call organisms, but we can guess at some of the requirements and constraints. In the 1950s there was considerable optimism that the discovery of deoxyribonucleic acid (DNA) and the laboratory synthesis of likely primitive biomolecules from experimental atmospheres rich in methane (CH4) and ammonia (NH3) indicated a clear picture for the origin of life. However, it now seems more likely that the synthesis of biologically important molecules occurred in restricted, specialized environments, such as the surfaces of clay minerals, or in submarine volcanic vents.

Best guesses suggest that life began in the oceans some 4.2-3.8 billion years ago, but there is no fossil record. The oldest known fossils are bacteria, some 3.5 billion years old. In rocks of this age there is fossil evidence of quite advanced metabolisms which utilized solar energy to synthesize organic material. The very earliest of autotrophic (self-feeding) reactions were probably based on sulphur (S), supplied from volcanic vents.

(organic matter)

However, by 3.5 billion years ago photochemical splitting of water, or photosynthesis was happening.

(If you are unfamiliar with chemical reactions and notation, see Chapter 2.)

The production of oxygen during photosynthesis had a profound effect. Initially, the oxygen gas (O2) was rapidly consumed, oxidizing reduced compounds and minerals. However, once the rate of supply exceeded consumption, O2 began to build up in the atmosphere. The primitive biosphere, mortally threatened by its own poisonous byproduct (O2), was forced to adapt to this change. It did so by evolving new biogeochemical metabolisms, those that today support the diversity of life on Earth. Gradually an atmosphere of modern composition evolved (see Table 3.1). In addition, oxygen in the stratosphere (see Chapter 3) underwent photochemical reactions, leading to the formation of ozone (O3), protecting the Earth from ultraviolet radiation. This shield allowed higher organisms to colonize the continental land surfaces.

In recent decades a few scientists have argued that the Earth acts like a single living entity rather than a randomly driven geochemical system. There has been much philosophical debate about this issue, often called the Gaia hypothesis, and more recently, Gaia theory. This view, suggested by James Lovelock, argues that biology controls the habitability of the planet, making the atmosphere, oceans and terrestrial environment comfortable to sustain and develop life. There is little consensus about these Gaian notions, but the ideas of Lovelock and others have stimulated active debate about the role of organisms in mediating geochemical cycles. Many scientists use the term 'biogeochemical cycles', which acknowledges the role of organisms in influencing geochemical systems.

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