DM 5113, MSDM, DU

1. Internal Structure of The Earth
INTERNAL STRUCTURE OF THE EARTH
The immense amount of heat energy released from gravitational energy and from the decay of
radioactive elementsmeltedthe entireplanet,and it is still cooling off today. Denser materials like
iron(Fe) sankintothe core of the Earth, while lightersilicates(Si),otheroxygen(O) compounds, and
water rose near the surface.
The earth isdividedinto four main layers: the inner core, outer core, mantle, and crust. The core is
composedmostlyof iron(Fe) andisso hotthat the outer core is molten,with about10% sulphur(S).
The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in
the mantle,whichiscomposedof iron(Fe),magnesium(Mg),aluminum(Al),silicon(Si), and oxygen
(O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a
plastic manner.The crust ismuch thinnerthanany of the otherlayers, and is composed of the least
dense potassium(K),calcium(Ca) andsodium(Na) aluminum-silicate minerals. Beingrelativelycold,
the crust is rocky and brittle, so it can fracture in earthquakes.
Major Structural unit of the earth
Three major structural units/ layers of the earth
1. CRUST
2. MANTLE
3. CORE
These structures are divided on the basis of seismic waves (P and S wave velocities)
 Outer most and thinnest layer
 Its relatively cool and consist of hard rocks
 Oceanic crust is about 5-10 km thick, basaltic composition dominated by silica and
magnesium (SiMa)
 Continental crust is about 20-40 km thick but under mountain it can be 70 Km thick,
composition is granitic
( Silica and Al dominantly).
Mantle
The mantle lies directly below the crust.
It is almost 2900 kilometers thick and makes up 80 percent of the Earth’s volume.
Chemical composition may be similar throughout the mantle
Temperature and pressure increase with depth resulting strength of mantle rock to vary with
depth, and

2. create layering within the mantle.
The upper part of the mantle consists of two layers
1) Lithosphere
2) Asthenosphere
Core
Core is the innermost of the Earth’s layers.
Outer core: 2900 to 5150 km (liquid)
Inner core: 5150 to 6370 km ( solid)
Over all it is a sphere with a radius of about 3470 kilometers and is composed largely of iron and
nickel and have a density about 12 x 103
Kg/m3
In the boundary of outer core P- waves is marked by an abrupt reduction in the velocity and bent
inwardsandproducingshadowzone aswell bythe disappearance of S- waves, it may be concluded
that it is in liquid state (molten) because of the high temperature in that region.
Nearits center,the core’stemperature is about 6000ºC, as hot as the Sun’s surface. The pressure is
greater than 1 million times that of the Earth’s atmosphere at sea level.
Thisextreme pressure overwhelmsthe temperature effectandcompressesthe innercore to a solid.
Lithosphere
Outerpart of the Earth includingboththe uppermostmantle andthe crust,make upthe lithosphere
its mechanical behavior is similar to that of the crust.
The lithosphere can be as thin as 10 kilometers where tectonic plates separate. The lithosphere is
about 75 kilometers thick beneath ocean and 125 kilometers under the continents.
A tectonic (or lithospheric) plate is a segment of the lithosphere.
Asthenosphere
The asthenosphere extends from the base of the lithosphere to a depth of about 350 kilometers.
Increasingtemperature withdepthgradually,asa resultsmall degreesof partial melting,possiblyas
much as 10% in regions of high heat flow. This partial melt is an important source of magma and a
lubricant to ease the tectonic movements of the lithospheric plate.
This change in rock properties occurs over a vertical distance of only a few kilometers.
This zone also called as low velocity zone where the velocities of s wave is decrease.
So the asthenosphere flows slowly, perhaps at a rate of a few centimeters per year (Lithospheric
plates glide slowly over the asthenosphere like sheets of ice drifting across a pond )

3. At the base of the asthenosphere, increasing pressure causes the mantle to become mechanically
stronger, and it remains so all the way down to the core.
Interior of the Earth
Physical Layering
Because of variations in temperature and in pressure, the materials inside the earth vary in their
physical properties with depth.
Inner Core is the central part of the iron-nickel core. It is a solid iron sphere. The reason that the
iron is solid is that the pressure at the center of the earth is significantly higher than the pressure
above,while the temperature isonlyslightlyhigher. While higher temperature would tend to melt
materials, higher pressures tend to create solids.
Outer Core constitutes the remainder of the iron-nickel core and is liquid. It is liquid because the
pressure is lower.
Mesosphere: The majority of the mantle from the core-mantle boundary is solid and is called the
mesosphere.
Asthenosphere: Nearer to the surface of the earth the temperature is still relatively high but the
pressure is greatly reduced. This creates a situation where the mantle is partially melted. The
asthenosphere is a plastic solid in that it flows over time.
Lithosphere:Above the asthenosphere,the temperature beginstodropmore rapidly. Thiscreates a
layer of cool, rigid rock called the lithosphere. The lithosphere includes the uppermost part of the
mantle andit alsoincludesall of the crust.That is,the crust is the upperpart of the lithosphere, and
the upper mantle is the lower part of the lithosphere.
Internal Structure of the Earth
The Divisions of Inner Space
Size of the Earth
Radius = 6370 km
Diameter = 12,740 km
Determining the Earth's Internal Structure
Earth has a layered structure. The boundaries between the layers are called discontinuities.
The layeredstructure isdeterminedfromstudiesof how seismicwavesbehaveas they pass through
the Earth. P- and S-wave travel times depend on properties of rock materials through which they
pass. Differences in travel times correspond to differences in rock properties.

4. Seismicwave velocity depends on the density and elasticity of rock. Seismic waves travel faster in
denser elastic rocks.
Speedof seismicwavesincreaseswithdepth(pressure, density and elasticity increase downward).
It can be seen in the paths of the P- and S-waves as they travel through the Earth in the diagram
below.
SHADOW ZONES
Refraction of seismic waves as they travel through the Earth
Refraction of seismic waves as they travel through the Earth
Note the curvedwave paths indicating gradual increases in density and seismic wave velocity with
depth. Also note the sharp refraction (bending of waves) at the discontinuities or boundaries
between layers.
Note the shadow zones. There is a large S-wave shadow zone (labelled " No direct S-waves" )
extending across the side of the globe opposite from the epicenter (from 105o
). S-waves cannot
travel through the molten (liquid) outer core.
There isa smallerP-wave shadowzone,seenonbothsides(gray shading), from 105o
to 140o
. The P-
wave shadow zone makes a ring around the globe.
Major layers of the Earth were detected before 1950.
Fine detailswere delineatedin1960's by observingthe behaviorof seismic waves generated during
nuclear testing.
The Earth's Internal Layered Structure
and Composition
Crust
Continental Crust (averages about 35 km thick; 60 km in mountain ranges; diagram shows range of
20-70 km)
Granitic composition
Oceanic Crust (5 - 12 km thick; diagram shows 7-10 km average)
Basaltic composition
Thin layer of unconsolidated sediment covers basaltic igneous rock.
Oceanic crust has layered structure (ophiolite complex) consisting of the following:
Pillow basalts (basalts that erupted sub-aqueously)
" Sheeted dikes " - interconnected basaltic dikes
Gabbro (coarse grained equivalent of basalt; cooled slowly)
The Earth's Internal Layered Structure and Composition

5. Mantle (2885 km thick)
Composition: peridotite (Mg Fe silicates), kimberlite (contains diamonds), eclogite - based on
studiesof rockfrom mantle broughtupby volcanoes,fromdensitycalculations, and composition of
stony meteorites.
Lithosphere - outermost 100 km of Earth . Consists of the crust plus the outermost part of the
mantle which is solid. Divided into tectonic or lithospheric plates that cover surface of Earth .
Asthenosphere - lowvelocityzone at100 - 250 km depthin Earth (seismicwave velocitydecreases).
Rocks are at or near melting point. Magmas generated here. Solid that flows (rheid); plastic
behavior.
Convection in this layer moves tectonic plates.
Less is known about the mantle below the astheno-sphere.
The Earth's Internal Layered Structure and Composition
Outer core (2250 km thick)
S-waves cannot pass through outer core, therefore we know the outer core is liquid (molten).
Composition: Molten Fe (85%) with some Ni, based on studies of composition of meteorites.
Core may also contain lighter elements such as Si, S, C, or O.
Convection in liquid outer core plus spin of solid inner core generates Earth's magnetic field.
Magnetic field is also evidence for a dominantly iron core.
The Earth's Internal Layered Structure and Composition
Inner core (1220 km radius)
Solid Fe (85%) with some Ni - based on studies of meteorites
VELOSITY AND DENSITY VARIATION WITHIN THE EARTH
Summary of the layers and discontinuities within the Earth
AVERAGE COMPOSITION OF THE CRUST
Elemental abundances in the crust (% wt ).