The Layers of The Earth

The Layers of The Earth

The earth consists of four layers. Starting from the

surface of the earth, the layers are the crust, mantle,

outer core, and inner core. These layers are different

in composition and thickness.

Earth's Crust

Earth's crust is the outer layer of the earth. All living

beings live on the earth's crust. The crust occupies

less than 1% of the earth's total volume.

Composition:

The composition of the crust is not uniform everywhere.

The upper part of the crust consists of felsic rocks

(for example granite, and pumice) which are igneous

rocks composed of lighter elements (such as silicon,

oxygen, aluminium, sodium, and potassium). On the

other hand, the lower part of the crust consists of mafic

rocks (for example olivine, and pyroxene) made of

silicate or igneous rocks rich in magnesium and iron .

The felsic rocks are lighter in colour while the mafic

rocks are dark in colour. 

Types of crust:

There are two types of crust:

• Continental crust: The continental crust is

    thicker than the oceanic crust. The thickness

    can vary from 30-50 kilometres. It is made of less

    dense felsic rocks. The continental crust is older

    and has an average age of 2 billion years.

• Oceanic crust: The oceanic crust is made of

    denser mafic rocks. It is thinner than the continental

    crust. The thickness can vary from 5-10 kilometres.

    The oceanic crust is younger due to subduction and

    continuous reformation of the plates (see Earthquake).

The earth's crust lies on the mantle. The boundary of

crust and mantle is called Mohorovičić discontinuity

(Moho) (see Seismic waves). The earth's crust and upper part of the mantle

together form a layer called lithosphere. The lithosphere

splits into large irregular-shaped plates floating on the

lower part of the mantle (asthenosphere). These plates

are called tectonic plates (see Earthquakes). 

Earth's Mantle

Earth's mantle lies between the crust and outer core.

Its thickness is up to 2900 kilometres and occupies

84% of earth's volume. Mantle behaves like solid in

small time-scale, but moves like a viscous fluid (like

liquid caramel) on long time-scale. Earth's mantle can

be divided into 3 layers: upper mantle, transition zone,

lower mantle.

• Upper mantle: The upper mantle starts beneath

    the crust and extends to a depth about 660 kilometres.

    The top part is called the mantle lithosphere which

    is rigid and the lower part is called asthenosphere.

    The asthenosphere is partially molten and moves due

    to convection current and it is more ductile than the

    lithosphere and lower mantle. The convection current

    in the upper mantle occurs due to the variation in

    temperature and composition of the layer. The

    temperature of this layer ranges from 500K ( 227 °C

    or 440 °F) at the boundary of the crust to 1200K

    (930 °C or 1700 °F) at the boundary of the lower mantle.

Composition: The primary minerals found in the

upper mantle are the silicate minerals rich in

magnesium and iron. The upper mantle is composed

of olivine, pyroxene, and garnet.

Pressure: The pressure in the upper mantle is so

high that it prevents the melting of the materials

because we know that the melting point increases

with pressure. The maximum pressure here is 24 Gpa.

Transition zone: This region is located between the

upper mantle and the lower mantle. It lies at a distance

of 410 kilometres to 660 kilometres beneath the earth's

surface.

Composition: Olivine is the primary component of the

mantle. At a depth of 410 kilometres olivine transforms into a denser mineral called wadsleyite. Around 520 kilometres the wadsleyite changes to ringwoodite. Near 660 kilometres, ringwoodite changes into two new denser minerals, bridgmanite, and periclase. As the pressure and temperature increase with depth, these transformations happen. Some scientists believe that as the density increases with depth, the transition zone prevents the subducted tectonic plate from moving further into the mantle.

Water: The important aspect of mantle transition is that it

contains water in the hydroxide form. The hydroxide is an

ion of hydrogen and oxygen with a negative charge. The

hydroxide ions are trapped in crystalline form in the

mineral rocks ringwoodite and wadsleyite. Near the

bottom of the transition zone, the ringwoodite and

wadsleyite transform due to high temperature and

pressure, as a result, the crystalline structure has

broken and the hydroxide melts and separates. These

melted particles move upward to the minerals that can

hold them and keep the balance of water in the

transition zone.

• Lower Mantle: The lower mantle extends from

    660-2900 kilometres below the earth's surface. The

    lower mantle can be divided into three layers,

    uppermost (660-770 kilometres), mid-layer(770-2700

    kilometres), and D layer (2700-2900 kilometres). The

    temperature ranges from 1960K (1690 °C; 3070 °F) at the

    upper part to 2630K (2,360 °C; 4,270 °F) near the bottom

    at 2700 kilometres.

Composition: The major minerals found in lower

mantle are bridgmanite (also known as silicate

perovskite consisting of magnesium, iron and silicon), ferropericlase (consisting of magnesium and iron

oxides) and calcium silicate perovskite.

Pressure: The pressure of the lower mantle varies

from 24-127 Gpa. The heat softens the rocks but the

high pressure keeps the lower mantle solid. This layer

is less ductile than the upper mantle and transition zone.

D Layer: The D-layer is divided into two layers D'

(D prime) and D'' (D double prime). The D' layer is

1800 kilometres thick and D'' is  200 kilometres thick.

The D'' layer is the core-mantle boundary. This portion

is not spherical but dome-shaped. The D'' layer

shows unpredictable movements. The iron of the

outer core influences the formation of diapirs. 

Here the mobile and ductile materials are forced

into brittle overlying rocks.The iron diapirs release

heat and energy and move upward to the lower

mantle and transition zone, and maybe even erupt

as a mantle plume.

Mantle Convection: The mantle convection current

transfers heat from the inner planet to the outer layer

and causes tectonic plate movements.

Earth's Core

Earth's core occupies 15% of the total earth's volume.

The core is divided into two layers: outer core and inner

core.

Outer Core: The outer core of the earth is 2400 kilometres

thick and its boundary starts from 2890 kilometres below

the earth's surface and ends at 5150 Kilometres. The

temperature of the outer part of the outer core is about

3000K-4000K (2,700–4,200 °C; 4,900–7,600 °F) and it is

about 4,000–8,000 K (3,700–7,700 °C; 6,700–14,000 °F)

near the inner core. The temperature is influenced by the

temperature of the inner core and mantle, and keeps this

layer in liquid form. It is composed of iron and nickel. The

convection of the liquid of the outer layer creates the earth's

magnetic field, which protects our planet from harmful solar

winds. The pressure varies from 135-330 Gpa.

Inner Core: The inner core is located at the centre of our

planet below the upper core with a radius of 1220 kilometres

which is about 20% of the earth's radius. The primary

components of this part are iron and nickel. The temperature

of the core is nearly 5,700 K (5,430 °C; 9,800 °F) which is

comparable to the temperature of the sun.

Pressure: The pressure of the surface of the core is 330 Gpa.

Despite the high temperature this layer is solid because of

its immense pressure.