This document provides information about the formation and composition of planet Earth and the solar system. It discusses: 1) How Earth is uniquely suited to support life due to factors like its distance from the sun, size, atmosphere, and rotation. 2) The processes that led to the formation of elements, including nuclear fusion and radioactive decay. Hydrogen is the most abundant element in the universe. 3) How the Earth differentiated into layers with different compositions - a crust, mantle, and core. The crust also differentiates into continental and oceanic crust with different thicknesses, densities, and compositions.

1. Geochemistry
CHAPTER ONE
Planet Earth in the solar
system
Compiled by Belisuma H.

2. Earth
• Is the only planet which is certainly support life. Also
called Heaven planet.
• Is fortunate planet (how)? b/c:
I. Its distance from the sun makes the earth unique by
having water in a liquid form. This is b/c of the average
To is 14oc.
II. Its size: has enough gravitational force to hold the
atmosphere.
III. Composition of the atmosphere: O2, N2, CO2, & water
vapour.
IV. Daily rotation: 24hrs is balanced timing b/c it provides
optimum periods of light & darkness.
• One year takes 365 Earth days.
• Earth is close enough to the Sun to stay warm and yet far
enough away to keep cool.
• There is no wind on the earth moon.

3. Formation of the elements
 Nuclear fusion: two atomic nuclei fuse or bond, is additive,
resulting in the creation of different elements by the addition
of protons to an atomic nucleus. It involves the release of
huge amounts of energy and it is the source of the Sun’s
power.
 Radioactive decay: the spontaneous emission of particles or
energy over a given period of time and is one of the ways
where one element can become another by the expulsion of
protons. It takes place inside Earth.
 The most abundant element in the universe is hydrogen, an
element formed by electron orbiting around a single proton.
• The sun itself, which makes up 98% of the total mass of the
solar system, consists of 71%H,27%He,and 2% of heavier
elements.

4. Abundance of elements
• The abundance of a chemical element measures
how relatively common (or rare) the element is, or
how much of the element is present in a given
environment by comparison to all other elements.
•
• Abundance may be variously measured by the
mass-fraction (the same as weight fraction), or
mole-fraction (fraction of atoms by numerical
count, or sometimes fraction of molecules in
gases), or by volume-fraction.
• In the universe as a whole, and in the atmospheres
of gas-giant planets such as Jupiter, the mass-
fraction abundances of hydrogen and helium are
about 74% and 23-25% respectively.

5. • However, since hydrogen is diatomic while
helium is not, in the conditions of Jupiter's outer
atmosphere, the molecular mole-fraction (fraction
of total gas molecules, or fraction of atmosphere
by volume) of hydrogen in the outer atmosphere
of Jupiter is about 86%, and for helium, 13%.
Abundance of elements in the Universe
Ten most common elements in the Milky Way
Galaxy, estimated spectroscopically.

6. z Element Mass fraction in ppm
1 Hydrogen 739,000
2 Helium 240,000
8 Oxygen 10,400
6 Carbon 4,600
10 Neon 1,340
26 Iron 1,090
7 Nitrogen 960
14 Silicon 650
12 Magnesium 580
16 Sulfur 440

7. Estimated abundances of the chemical elements in the Solar system.
•Hydrogen and helium are most common, from the Big Bang.
•The next three elements (Li, Be, B) are rare because they are poorly
synthesized in the Big Bang and also in stars.
•The two general trends in the remaining stellar-produced elements are:
•(1) an alternation of abundance in elements as they have even or odd
atomic numbers, and
•(2) a general decrease in abundance, as elements become heavier.

8. Chronology of planetary evolution
The time frame of the Solar System's formation
has been determined using radiometric dating.
Scientists estimate that the Solar System is
4.6 billion years old. The oldest known mineral
grains on Earth are approximately 4.4 billion years
old.
To estimate the age of the Solar System, scientists
use meteorite an age of (4.6 by).

9. The Solar System: origin and chemical
and evolution of solar system
• Our solar system consist of the Sun and the planets : Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
• The nine major planets including our earth and their moons are revolving
around the sun-anticlockwise rotation.
 The first thing to notice is that the solar system is mostly empty space.
 The planets are very small compared to the space between them. Even
the dots on the diagrams above are too big to be in proper scale with
respect to the sizes of the orbits.

10.  Its includes: the satellites of the planets; numerous comets,
asteroids and meteoroids; and the interplanetary medium.
 The sun is the richest source of electromagnetic energy
(mostly in the form of heat and light) in the solar system.
 The whole solar system, together with the local stars visible
on a clear night, orbits the center of our home galaxy, a
spiral disk of 200 billion stars we call the Milky Way.
 The Sun contains 99.85% of all the matter in the solar
system.
 The planets, which condensed out of the same disk of
material that formed the Sun, contain only 0.135% of the
mass of the solar system.

11. Different hypotheses
• Based on observational facts cosmologists have
developed classes of hypothesis which try to explain
the origin of the earth. Some of them are:
1.Big Bang Theory
• It theorizes that a large quantity of nothing decided to
pack tightly together, and then explode outward into
hydrogen and helium.
• This gas is said to have flowed outward through
frictionless space (“frictionless,” so the out flowing
gas cannot stop or slow down) to eventually form
stars, galaxies, planets, and moons.

12. 2. Steady state hypothesis
• There is no change in the universe.
The origin of Solar System: is equivalent with the
origin of the Earth.
There are various hypothesis which explain the
origin of the solar system.
These are grouped into two as:-
A. Gradualistic Theories:
State that both the sun and the planets are formed
together.
B. Catastrophic Theories (Collision theory)
They are not formed at the same time.

13. 3.The nebular hypothesis.
 State that there was a
slowly rotating H-
clouds called Nebula.
 The gravitational force
contracts the H-cloud.
 B/c of contraction
accelerate the rotation
as a result it has disk
shape.
 B/c of gravitational
force the matter is
drifted toward the
center.
 the center is denser.
 the mass is
accumulated into a

14. • Nuclear fusion occurs when H2 is changed to He.
• During this energy is released as light and heat energy from
the proto –sun.
• The disk gradually cooled while it spun off various solid
compounds condensed out of the gas
– forming small grains by clumped together
– small chunks (Planetsimals) coalesced by the action of
gravity planets.
Some Evidences supporting This Theory
1. The planets revolve around the sun in the same direction.
2. All the equatorial section of the planets are on the same
plane (Co-planar)
3. The sun contains >99% of the mass of the Solar system. but
its momentum is <1% and the planets are vise versa.
4. Except Venus & Uranus all rotates counter clockwise.
Draw back of this theory:
1. There is rarer chance of condensed.
2. The chance of collision of the stars is such a very huge universe
is extremely small.

15. Differentiation of the Earth: bulk
composition of the Earth and chemical
composition of crust, mantle and core
 Early Earth was composed of Si, Fe, Mg, O2, Al, and smaller
amounts of other chemical elements.
 When the Earth under went heating, this homogeneous composition
disappeared, the result was differentiated planet, consisting of
concentric layers of differing composition and density.
 Differentiation also responsible for the emission of gasses from the
interior of the Earth that eventually led to formation of the oceans
and the atmosphere.
 The Earth’s atmosphere and hydrosphere are developed from
degassing (loss of gaseous elements such as carbon, hydrogen, and
oxygen).

16. Quiz
1.Why only planet earth support life?(2%)
2.List processes that lead(form) elements(1%)
3.Differentiate oceanic crust from continental
crust(2%)

17. • At present abundant gases are released during
volcanic eruptions and these are mainly composed
of water (77%), CO2 (12%), SO2 (7%), and N2
(3%) with minor amounts of H2, CO, S, Cl, and
Ar.
Geochemical composition of the Earth
Earth, being a dynamic system, it is continuously
undergoing change in
– Mechanical (plate tectonic-strength, elasticity, and,
viscosity).
– Chemical properties.

18. This properties do depend on
– Pressure
– Temperature
– State of stress
– Nature of the chemistry of the material.
• Chemical composition and mechanical properties
vary widely within the Earth.
• The Earth can be divided into two main parts:
1) Atmosphere: measured from the surface of the Earth upwards
to 150 km (any thing above this called space).
2) Solid Earth: measured from the surface of the Earth down
wards to the core.

19. • The internal structure of the earth consists of three parts:
– Crust, mantle, and core.
• Division between the crust and mantle is called the
Mohorovicic discontinuity (Moho).
• The boundary between the crust and mantle marks a
significant change in chemical composition.
• Two main hypotheses for what the Moho represents:
1) phase transition (from a gabbroic lower crust to an eclogitic
upper mantle).
2) chemical discontinuity (the widely accepted theory for the
Moho is that it represents a chemical change from intermediate
and mafic crustal rocks to an ultramafic mantle).

20. Chemical subdivision of the Earth
• The Earth consists of three concentric layers:
– Core, mantle, and crust
• This orderly divisions results from density differences
between the layers as a function of variations in:
– compositions,
– temperature,
– pressure.
A) Crust
is much thinner under oceans than under continents.
makes up only 0.5% of the Earth’s total mass.

21.  can be divided into two main parts,
 continental
 oceanic.
 Both differ in thickness, density, and compositions.
1) Continental crust
• Averages 35 km in thickness, but it ranges up to 60 km
under mountain ranges and up to 70-80 km in regions
of crustal duplication (Tibet, Himalayas).
• It has average density of 2.7 g/cm3
• Commonly referred to sialic (meaning that contains
considerable Si and Al).

22. • The oldest rocks preserved on the planet are found on
continents (except meteorites, which are often older).
• Continental crust can be divided into:
a) Stable continental regions (e.g., Precambrian cratons,
platforms of undeformed sediments on crystalline
basements)
• Thickness:35-45 km
• Density: 2.69 -2.74 gm/cm3 upper crust; 3.0 -3.25 gm/cm3
lower crust.
• Composition:
– upper crust (Sial): felsic igneous and metamorphic rocks.
» Average composition will be close to that of either a
mafic granodiorite or quartz diorite
– lower crust (Sima): anhydrous (no water)- quartz
andesite or andesite; hydrous-amphibolite or diorite.

23. b) Tectonically active regions (e.g. the Andes,
Himalayas)
• Thickness:55-70 km
• Composition: varies from region to region but may consist
of either mafic, amphibolites or ultramafic, mafic and felsic-
intermediate.
2) Oceanic crust
• It is thin (5-10 km; it is only 7 km thick in the deep ocean
basins)
• Denser than continental crust (3.0 g/cm3)
• Commonly referred to Sima (meaning that contains
considerable Si and Mg).
• Produced wherever hot mantle materials comes into contact
the surface of solid earth (produced at spreading centers).

24. • Consists of three layers of increasing velocity (P-wave
velocities) downwards
1. Layer 1: 1.6 to 2.5 km/s
2. Layer 2: 3.4 to 6.2 km/s
3. Layer 3: 6.4 to 7.0 km/s
– These difference reflect the differing origins of each
layer and their mechanical states.
• Sea water circulates into the cooling oceanic crust to depths
of 3 km, driven by the heat of the magma intrusion.
• The circulation is shown by the metamorphism and
serpentinization of the basalts and gabbros

25. • Oceanic crust can be divided into:
a) Ocean basins: water depth exceeds 4 km, layered and very uniform
and consists of three distinctive layers.
1. Layer 1: thickness of <1 km; deep water sediments in
various stages of lithifications (turning into rock),
foraminiferal ooze, chert, and mudstone.
2. Layer 2: thickness of 1.6-2 km; basalts and dolerites with
pillow lavas, dykes and sills (volcanic extrusive and
intrusive rocks)
3. Layer 3: thickness of 3-5.7 km; the main crustal layer;
dolerite, gabbro, and amphibolites (intrusive mafic rocks)
b) Mid-ocean ridges: in mid-ocean ridges (such as the Mid-Atlantic
Ridge),
1. Layer 1: absent
2. Layer 2: crops out the surface and is thicker than normal
3. Layer 3: thinner than normal and passes transitionally into
the upper mantle.

26. c) Island arcs: the structure beneath island arcs (such as the
Indonesian Arc) is very complex and the composition of the
crust in these region is very heterogeneous.
– The most rock types are in surface exposures are volcanic
andesites (from explosive volcanoes) and deep-sea
sediments.
B) Mantle
• is a thick layer between the Earth’s crust and core.
• comprises about 83% of the Earth’s volume.
• extends from 70 or 35 km to 2,900 km of depth.
• is less dense than the core (3.3-5.7 g/cm3).
• is thought to be composed largely of iron-rich silicate
minerals.

27. • subdivided into
1. upper mantle
2. transition zone
3. lower mantle
1) Upper Mantle
• is measured from the base of the crust down to 400 km.
• accounts for 10 % of the Earth's total mass.
• density 3.25-3.40 gm/cm3
• composition:
– plagioclase peridotite (e.g. olivine + pyroxene) (< 30 km
depth),
– spinel peridotite (30 km - 70 km depth),
– garnet peridotite (> 70 km depth)

28. • In tectonically active regions, eclogite (amphibole
+ garnet + Cpx) is a major component.
• composed of two main zones:
– an upper peridotite zone and
– an underlying primitive mantle or pyrolite zone
2) Transition Zone
• 400 - 1000 km below the Earth's surface.
• 17% of the Earth's total mass.
• the top of the transition zone is marked by phase
transformation
– olivine to a proto-spinel
– pyroxene to a garnet-structure

29. • there are a number of irregular seismic velocity changes.
– 680 km which is marked by the breakdown of olivine
into its constituent oxide components of periclase
(MgO) and stishovite (SiO2).
3) Lower Mantle
• 1000 - 2900 km below the Earth's surface.
• 41% of the Earth's total mass.
• most likely consists of mixed oxides of pyrolite
composition but with increased iron content.

30. C) Core
• is believed to be composed primarily of a nickel-iron alloy (along
with abundant platinum-group elements).
• is divided into two types –
1. liquid outer core (2900 - 5000 km)
2. solid inner core (5000 - 6370 km)
• has density of 10 to 13 g/cm3
• occupies about 16% of the Earth’s total volume.
• is believed to have a chemical composition similar to that of iron
meteorites (Fe- 90.6%; Ni- 7.9%; Co- 0.5% etc)
• There is a sharp but not very smooth boundary between mantle and
core known as Gutenberg discontinuity.