DIFFERENTIATION OF EARTH

CONTENTS Introduction Structure of the earth Differentiation of the earth When did differentiation happen? Homogeneous accretion Heterogeneous accretion Process of differentiation Sources of heat to melt earth Layers of the earth. Critical Evidence from the Moon Conclusions References

Introduction Earth is third planet from the sun it is the 5th largest planet in the universe and the largest terrestrial planet in the solar system in terms of diameter, mass and density.

Earth belongs to milky way galaxy, Local group cluster or Virgo super cluster.

Earth is the only place in the universe where life is known to exist and in other planets of the solar system.

The planet earth formed about 4.6 billion years ago in a fully molten state.

Differentiation of the earthThe materials making up the present- day solid

earth are not mixed together in a homogeneous mass. Rather they arranged in a series of concentric layers of diffusing density. The heaviest materials, molten metals, lie at the center of the earth and are overlain by progressively lighter layers of silicate mineralsTwo general theories have been proposed to explain the origin of Core-Mantle- Crust layering;

Homogeneous accretionHeterogeneous accretion

When did differentiation happen?

About 4.5 billion years ago

After beginning of Earth’s accretion at 4.56 billion years ago

Before the formation of the Moon’s oldest known rocks, 4.47 billion years ago

Sources of heat to melt Earth

Frequent and violent impacts There was likely one particularly large impact

Moon aggregated from the ejected debris Earth’s spin axis was tilted

Decay of radioactive elements This heat generation was greater in the past

than today

Homogeneous accretion

Homogenous accretion: Similar elements stick together, creating a solid mass. The heat generated in this process melts the particles. The heavier elements sink to the center because of gravity, creating the Earth's solid core.

Heterogeneous accretion

About 4.6 billion years ago, the Earth formed as particles collected within a giant disc of gas orbiting a star. Once the sun ignited, it blew all of the extra particles away, leaving the solar system. The exact process probably included both homogenous and heterogeneous accretion.

PROCESSES OF DIFFERENTIATION In a liquid or soft solid sphere, denser material sinks to the center and less dense material floats to the top. When rock is partially melted, the melt and the remaining solid generally have different chemistry and density. The melt is usually less dense than the “residue.” The melt is enriched in “incompatible” elements. The residue is enriched in “compatible” elements.

Processes of Differentiation

Early earth heats up due to radioactive decay, compression and impacts. Over time the temperature of the planet interior rises towards the Fe-melting.

Fig : The earth heats up

2) The iron “drops” follows gravity and accumulates 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.

Fig 4: Descending iron drops and rising lighter material

3) 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 mantle (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.

Fig : Differentiated Earth

Layers of the earth.

Contd..Layer Relative

position Density (in grms/cubic cm

Composition

Crust Outermost layer; thinnest under the ocean, thickest under continents; crust and top of the mantle called the lithosphere

Oceanic crust(3-3.3)Continental crust(2.7-3)

Solid rock- mostly silicon and oxygen oceanic- crust basalt;Continental crust- granite

Mantle Middle layer, thickest layer; top portion called the asthenosphere

Mantle(3.3-5.7)

Hot softened rock; contains iron and magnesium

Core Inner layer, consists of two parts- outer core and inner core

Outer core (9.9-12.2)Inner core(12.6-13.0)

Mostly iron and nickel; outer core- slow flowing liquid, inner core, solid

Critical Evidence from the Moon

No direct data are available from the first 600 m.y. of earth history this can only be evaluated from

information available at present on the possible evolution of the other members of the solar

system, particularly the Moon. So the evidences obtained from the moon are critical.

1) After fragments of anorthosite were found in the regolith and identified as originating in the

highlands, it becomes clear that the moon had differentiated early to form a highland shell of

anorthosite gabbroic composition. This is required that an outer shell of the primitive moon

had been melted to yield the observed thickness of the anorthosite shell.

2) The second reason is that the record of lunar events are between 4400 and 3200m.y. this is in

marked contrast with the earth where knowledge of this period is restricted to fragmentary out

crops of rock dating back only to about 3800 m.y.

3) Thirdly, evidence for a completely molten early moon is provided by the lunar gravitational

and magnetic data. Till recently there has been no strong evidence for an iron core in the moon.

ConclusionsThe earth is a part of our Solar System, consisting of the Sun, nine

planets, the asteroid belt, and occasional comets. After the Earth was formed, there were

many planetesimals still in the Solar System. These bombarded the Earth in what is

called the Great Bombardment, which was so intense that the Earth's temperature

increased significantly. In such a state, differentiation occurred, in which the heavier

elements, such as iron and nickel, sunk to the center, and the lighter elements floated to

the top. This led to a layered structure of the Earth. These layers are grouped as follows:  

   

Inner Core: The inner core, about 1200 km thick, consists of mainly iron and nickel in a

molten state due to the intense heat and pressure.

Contd..Outer Core: The outer core is about 2300 km thick, and consists also mainly of nickel

and iron, but in a solid form.

Mantle: The mantle, about 2900 km thick, is rich in lighter elements such as oxygen,

silicon, magnesium, and iron.

Crust: The crust is composed of the lightest elements of the earth, and varies in

thickness from about 10 km beneath the oceans to about 70 km thick beneath the

continents.

The earth heated up by a combination of three processes of Radioactive decay of U. Th

and K, Gravitational compression and Meteorite impacts.

References Don L. Eicher (1980), History of earth, Prtice;Hall, Inc Pp 8-26.

M.W.Mc Elhilly (1979), The earth its origin, structure and evaluation, Academic Press London Ltd. Pp 2-53.

C.M.R Fowler(1990), The solid earth, Cambridge university press,Pp 4-30

Internet Sources:

www.google.com www.Wikipedia.com