interior of earth

Department of Mining Engineering,
UET,Lahore

 Interior of earth ?
 Evidence of interior of earth ?
 Chemical composition of earth ?
Department of Mining
Engineering, UET,Lahore

Interior of Earth Crust

 Mass 1.7 % of earths mass
 Depth from surface :5150-6370 km
(1220km)
 Suspended in molten outer core
 Solidification due to decrease in temperature
and increase in pressure

 Mass 30.8% of earths mass
 Depth 2890-5150 km (2260km)
 Molten material
 Less dense
 Earth magnetic field
 Not pure iron ,some other light elements

 Mass 3% of earth mass
 Depth 2700km-2890 km (200-300km)
 4 % of mantle –crust mass
 Part of mantle sink through it but floating on
outer core due to its low density as compared
to outer core

 49.2% mass of total earths mass
 Depth 650-2890km
 72.9% mantle –crust mass

 Mass 7.5% of earth mass
 Depth 400-650 km
 11.1 % of mantle –crust mass
 Fertile layer : production of basaltic magma
 Conversion into dense material

 10.3 % of earth mass
 Depth 10-400km
 15% of mantle –crust mass
 Asthenosphere

 .099% of earth mass
 0-10 km depth
 0.147% of mantle –crust mass
 A large area for volcanoes

 0.374% of earth mass
 Depth 0-50km
 0.544 % of mantle-crust mass

 Drilling up to 15 KM
 Direct reach is impossible ?
 Indirect and accurate evidences for interior.

 Density
 Meteorites
 Moment of inertia
 Magnetic field
 Volcano
 Seismic waves

 Earth density = 5.5 gram per cubic
centimeter
 Density of crust( continental ) = 2.7 gram per
cubic centimeter
 Density of crust (oceanic part) = 3 gram per
cubic centimeter
 It mean inner rocks are more dense .

 Meteorites: material that falls to Earth falls into
three basic categories
 Chondrites: Undifferentiated material thought to
represent the material of the Solar Nebula.
Contain homogeneously mixed rocky and metalic
substances. Most meteroites fall into this
catagory.
 Stony meteorites: Differentiated meteorites
containing lighter silicate material.
 Iron meteorites: Meteorites consisting of metals,
primarily iron and nickel, usually in interlocking
crystals

 A measure of distribution of mass within an
object that determines the ease with which it
rotates.
 Ice skater
 Mass concentrated in centre ,less moment of
inertia and ease in rotation
 Earth’s moment of inertia 15 times less than
an identical sphere of uniform density.

 Centre consist major element = iron
 Symmetrical magnetic filed .
Early ideas about what caused the compass needle to point toward the north
included some divine attraction to the polestar (North Star), or attraction to large
masses of iron ore in the arctic.
 A more serious hypothesis considered the Earth or some solid layer within the
Earth to be made of iron or other magnetic material forming a permanent magnet.
There are two major problems with this hypothesis. First, it became apparent that
the magnetic field drifts over time; the magnetic poles move. Second, magnetic
minerals only retain a permanent magnetism below their Curie temperature (e.g.,
580°C for magnetite). Most of the Earth's interior is hotter than all known Curie
temperatures and cooler crustal rocks just don't contain enough magnetic content
to account for the magnetic field and crustal magnetization is very heterogeneous
in any case.
 The discovery of the liquid outer core allowed another hypothesis: the
geodynamo. Iron, whether liquid or solid, is a conductor of electricity. Electric
currents would therefore flow in molten iron. Moving a flowing electric current
generates a magnetic field at a right angle to the electric current direction (basic
physics of electromagnetism). The molten outer core convects as a means of
releasing heat. This convective motion would displace the flowing electric currents
thereby generating magnetic fields. The magnetic field is oriented around the axis
of rotation of the Earth because the effects of the Earth's rotation on the moving
fluid (coriolis force).

 A volcano is a rupture on the crust of
a planetary mass object, such as the Earth,
which allows hot lava ,volcanic ash,
and gases to escape from a magma chamber
below the surface.

 Igneous rocks that have cooled from magma
contain lumps of rock of different composition
from the magma itself. These lumps are termed
xenoliths, which means ‘foreign piece of rock’.
The xenoliths are formed when magma rising
from deep levels rips off pieces of the rock which
it passes through (the country rock) and carries
these pieces along with it. Some xenoliths come
from deeper levels within the crust, others come
from the uppermost mantle, down to depths of
about 200 km. The mantle xenoliths show us
that the uppermost mantle is made of a rock
called peridotite.

 Oceanic crust is normally destroyed less than 200
Myr (million years) after formation by subduction. An
ophiolite is the technical term for a piece of ancient
oceanic crust that escaped destruction and was
instead shifted onto a continental plate by natural
tectonic forces. Rock exposures cut through
ophiolites allow us to piece together the structure of
oceanic crust and the uppermost mantle beneath.
The mantle part of ophiolites consists of peridotite,
similar to that brought up in xenoliths.
 The difficulty with using ophiolites to infer mantle
composition is that they have sometimes been heavily
deformed and chemically altered by the tectonic
forces that shifted them onto the continent.

 Non-volcanic passive margins (also known as
rifted margins) are plate boundaries where
continental crust is rigidly attached to oceanic
crust. Non-volcanic passive margins form a class
of passive margins that has been discovered
within the past few decades. At non-volcanic
margins, a transition zone exists between the
continental and oceanic crust in which mantle is
exposed at the seabed. The mantle is made of
peridotite that has undergone major chemical
alteration by interaction with seawater.

 Surface waves = moves along surface
 Body waves divided into P waves & S waves
 P (primary) waves pass through liquid &
solids
 S (secondary) waves pass through solid only.

Vp=
[(c+4/3R)÷p]1/2
Vs =(R/p) ½
C =
Incompressibility
p = density
R= Rigidity of
substance

 Seismic waves travel more quickly through
denser materials and therefore generally
travel more quickly with depth.
 Hot areas slow down seismic waves.
 Seismic waves move more slowly through a
liquid than a solid.
 Partially molten areas may slow down the P
waves and attenuate or weaken S waves.

 Mohorovicic Seismic Discontinuity
beyond 200 km the seismic waves arrive sooner than
expected, forming a break in the travel time vs. distance
curve.
 Mohorovicic (1909) interpreted this to mean that the
seismic waves recorded beyond 200 km from the
earthquake source had passed through a lower layer with
significantly higher seismic velocity.
 This seismic discontinuity is now know as the Moho (much
easier than "Mohorovicic seismic discontinuity") It is the
boundary between the felsic/mafic crust with seismic
velocity around 6 km/sec and the denser ultramafic
mantle with seismic velocity around 8 km/sec. The depth
to the Moho beneath the continents averages around 35
km but ranges from around 20 km to 70 km. The Moho
beneath the oceans is usually about 7 km below the
seafloor.

 Low Velocity Zone
Seismic velocities tend to gradually increase with
depth in the mantle due to the increasing
pressure, and therefore density, with depth.
However, seismic waves recorded at distances
corresponding to depths of around 100 km to
250 km arrive later than expected indicating a
zone of low seismic wave velocity. Furthermore,
while both the P and S waves travel more slowly,
the S waves are attenuated or weakened. This is
interpreted to be a zone that is partially molten,
probably one percent or less (i.e., greater than 99
percent solid). Alternatively, it may simply
represent a zone where the mantle is very close
to its melting point for that depth and pressure
that it is very "soft." Then this represents a zone
of weakness in the upper mantle. This zone is
called the asthenosphere or "weak sphere."

 The asthenosphere separates the strong, solid rock of the
uppermost mantle and crust above from the remainder of the
strong, solid mantle below. The combination of uppermost
mantle and crust above the asthenosphere is called
the lithosphere. The lithosphere is free to move (glide) over the
weak asthenosphere. The tectonic plates are, in fact, lithospheric
plates.
 670 km Seismic Discontinuity
Below the low velocity zone are a couple of seismic
discontinuities at which seismic velocities increase. Theoretical
analyses and laboratory experiments show that at these depths
(pressures) ultramafic silicates will change phase (atomic packing
structure or crystalline structure) from the crystalline structure of
olivine to tighter packing structures. A discontinuity at around
670 km depth is particularly distinct.
 The 670 km discontinuity results from the change of spinel
structure to the perovskite crystalline structure which remains
stable to the base of the mantle. Perovskite (same chemical
formula as olivine) is then the most abundant silicate mineral in
the Earth. The 670 km discontinuity is thought to represents a
major boundary separating a less dense upper mantle from a
more dense lower mantle.

 Gutenberg Seismic Discontinuity / Core-Mantle
Boundary (shadow zone 103 to 143)
Seismic waves recorded at increasing distances
from an earthquake indicate that seismic
velocities gradually increase with depth in the
mantle .However, at arc distances of between
about 103° and 143° no P waves are recorded.
Furthermore, no S waves are record beyond
about 103°. Gutenberg (1914) explained this as
the result of a molten core beginning at a depth
of around 2900 km. Shear waves could not
penetrate this molten layer and P waves would be
severely slowed and refracted (bent).

 Lehman Siesmic Discontinuity / The Inner
Core
Between 143° and 180° from an earthquake
another refraction is recognized (Lehman,
1936) resulting from a sudden increase in P
wave velocities at a depth of 5150 km. This
velocity increase is consistent with a change
from a molten outer core to a solid inner
core.

 Core is made of iron with minor amounts of nickel, and
lies at the center of the earth
Mantle is made of iron-magnesium silicates and surrounds
the core. The mantle makes up the bulk of the earth.
Crust occurs as two distinct types, oceanic crust and
continental crust. Both types of crust are lighter (less
dense) and contain more silica than the mantle.
 Oceanic crust is the crust that underlies most of the areas
we call "oceans" it is thinner, is more dense, and contains
less silica and aluminum and more magnesium and iron
than continental crust. The lack of silica makes it darker
than continental crust.
 Because continental crust is thicker and made of less
dense material than the oceanic crust, it "floats" higher on
the earth.

 Granitic continental crust
 Basaltic oceanic crust
 Magma is mefic (megnasium & iron ) with
light elements such as silicon ,oxygen and
aluminum
 Ultrmefaic (mantle) higher densities than
mefic
 Diatremes = diamond bearing ultramafic
rocks
 Felsic = iron silicates

interior of earth

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