INTERIOR OF THE
EARTH
What do you imagine about the nature of the earth? Do you
imagine it to be a solid ball like cricket ball
or a hollow ball with a thick cover of rocks i.e. lithosphere?
Have you ever seen photographs or images
of a volcanic eruption on the television screen? Can you
recollect the emergence of hot molten lava,
dust, smoke, fire and magma flowing out of the volcanic
crater? The interior of the earth can be
understood only by
indirect evidences as neither any one has nor any one can reach the interior of
the
earth. The configuration of the surface of the earth is
largely a product of the processes operating
in the interior of the earth. Exogenic as well as endogenic
processes are constantly shaping
the landscape. A proper understanding of the physiographic
character of a region remains
incomplete if the effects of endogenic processes are
ignored. Human life is largely influenced
by the physiography of the region. Therefore, it is
necessary that one gets acquainted with
the forces that influence landscape development. To
understand why the earth
shakes or how a tsunami wave is generated, it is necessary
that we know certain details of the
interior of the earth. In the previous chapter, you have
noted that the earth-forming
materials have been distributed in the form of layers from
the crust to the core. It is interesting
to know how scientists have gathered information about these
layers and what are
the characteristics of each of these layers. This is exactly
what this chapter deals with.
INTERIOR OF THE EARTH
SOURCES OF INFORMATION ABOUT THE INTERIOR
The earth’s radius is 6,370 km. No one can reach the centre
of the earth and make
observations or collect samples of the material. Under such
conditions, you may wonder how
scientists tell us about the earth’s interior and the type
of materials that exist at such depths.
Most of our knowledge about the interior of the earth is
largely based on estimates and
inferences. Yet, a part of the information is obtained
through direct observations and
analysis of materials. Direct Sources The most easily
available solid earth material
is surface rock or the rocks we get from mining areas. Gold
mines in South Africa are as deep
as 3 - 4 km. Going beyond this depth is not possible as it
is very hot at this depth. Besides
mining, scientists have taken up a number of projects to
penetrate deeper depths to explore
the conditions in the crustal portions. Scientists world
over are working on two major projects
such as “Deep Ocean Drilling Project” and “Integrated Ocean
Drilling Project”. The
deepest drill at Kola, in Arctic Ocean, has so far reached a
depth of 12 km. This and many
deep drilling projects have provided large volume of
information through the analysis of
materials collected at different depths. Volcanic eruption
forms another source of
obtaining direct information. As and when the molten
material (magma) is thrown onto the
surface of the earth, during volcanic eruption it becomes
available for laboratory analysis.
However, it is difficult to ascertain the depth of the
source of such magma.
Indirect Sources
Analysis of properties of matter indirectly provides
information about the interior. We
know through the mining activity that temperature and
pressure increase with the
increasing distance from the surface towards the interior in
deeper depths. Moreover, it is
also known that the density of the material also increases
with depth. It is possible to find the
rate of change of these characteristics. Knowing the total
thickness of the earth, scientists have
estimated the values of temperature, pressure and the
density of materials at different depths.
The details of these characteristics with reference to each
layer of the interior are
discussed later in this chapter. Another source of information are the
meteors that at times reach the earth. However, it may be
noted that the material that becomes
available for analysis from meteors, is not from the
interior of the earth. The material and the
structure observed in the meteors are similar to that of the
earth. They are solid bodies
developed out of materials same as, or similar to, our
planet. Hence, this becomes yet another
source of information about the interior of the earth.
The other indirect sources include gravitation, magnetic
field, and seismic activity.
The gravitation force (g) is not the same at different
latitudes on the surface. It is greater
near the poles and less at the equator. This is because of
the distance from the centre at the
equator being greater than that at the poles. The gravity
values also differ according to the
mass of material. The uneven distribution of mass of
material within the earth influences
this value. The reading of the gravity at different places
is influenced by many other factors.
These readings differ from the expected values. Such a
difference is called gravity anomaly.
Gravity anomalies give us information about the distribution
of mass of the material in the
crust of the earth. Magnetic surveys also provide
information about the distribution of
magnetic materials in the crustal portion, and thus, provide
information about the
distribution of materials in this part. Seismic activity is
one of the most important sources of
information about the interior of the earth. Hence, we shall discuss it in some detail.
Earthquake
The study of seismic waves provides a complete picture of
the layered interior. An earthquake
in simple words is shaking of the earth. It is a natural
event. It is caused due to release of
energy, which generates waves that travel in all directions.
Why does the earth shake?
The release of energy occurs along a fault. A fault is a
sharp break in the crustal rocks.
Rocks along a fault tend to move in opposite directions. As
the overlying rock strata press
them, the friction locks them together. However, their
tendency to move apart at some point of
time overcomes the friction. As a result, the blocks get
deformed and eventually, they slide
past one another abruptly. This causes a release of energy,
and the energy waves travel
in all directions. The point where the energy is released is
called the focusof an earthquake,
alternatively, it is called the hypocentre. The energy waves
travelling in different directions
reach the surface. The point on the surface, nearest to the
focus, is called epicentre. It is
the first one to experience the waves. It is a point directly
above the focus.
Earthquake Waves
All natural earthquakes take place in the lithosphere. You
will learn about different
layers of the earth later in this chapter. It is sufficient
to note here that the lithosphere refers
to the portion of depth up to 200 km from the surface of the
earth. An instrument called
‘seismograph’ records the waves reaching the surface. A
curve of earthquake waves recorded
on the seismograph is given in Figure 3.1. Note that the
curve shows three distinct sections
each representing different types of wave patterns.
Earthquake waves are basically of two
types — body waves and surface waves. Body waves are
generated due to the release of energy
at the focus and move in all directions travelling through
the body of the earth. Hence, the name
body waves. The body waves interact with the surface rocks
and generate new set of waves
called surface waves. These waves move along the surface.
The velocity of waves changes as
they travel through materials with different densities. The
denser the material, the higher
is the velocity. Their direction also changes as they
reflect or refract when coming across
materials with different densities. There are two types of
body waves. They
are called P and S-waves. P-waves move faster and are the
first to arrive at the surface. These
are also called ‘primary waves’. The P-waves are similar to
sound waves. They travel
through gaseous, liquid and solid materials. S-waves arrive
at the surface with some time
lag. These are called secondary waves. An important fact
about S-waves is that they can
travel only through solid materials. This characteristic of
the S-waves is quite
important. It has helped scientists to understand the
structure of the interior of the
earth. Reflection causes waves to rebound whereas refraction
makes waves move in
different directions. The variations in the direction of
waves are inferred with the help of
their record on seismograph. The surface waves are the last
to report on seismograph.
These waves are more destructive. They cause displacement of
rocks, and hence, the collapse
of structures occurs.
Propagation of Earthquake Waves
Different types of earthquake waves travel in different
manners. As they move or propagate,
they cause vibration in the body of the rocks through which
they pass. P-waves vibrate
parallel to the direction of the wave. This exerts pressure
on the material in the direction of the
propagation. As a result, it creates density differences in
the material leading to stretching
and squeezing of the material. Other three waves vibrate
perpendicular to the direction of
propagation. The direction of vibrations of S-waves is
perpendicular to the wave direction
in the vertical plane. Hence, they create troughs and crests
in the material through which they
pass. Surface waves are considered to be the most damaging
waves.
Emergence of Shadow Zone
Earthquake waves get recorded in seismo-graphs located at
far off locations. However,
there exist some specific areas where the waves are not
reported. Such a zone is called the
‘shadow zone’. The study of different events reveals that
for each earthquake, there exists
an altogether different shadow zone. Figure 3.2 (a) and (b)
show the shadow zones of P and
S-waves. It was observed that seismographs located at any
distance within 105°
from the epicentre, recorded the arrival of both P and S-waves.
However, the seismographs located
beyond 145° from epicentre, record the arrival of P-waves,
but not that of S-waves. Thus, a
zone between 105 ° and 145 ° from epicentre was identified
as the shadow zone for both the types
of waves. The entire zone beyond 105 ° does not receive
S-waves. The shadow zone of S-wave is
much larger than that of the P-waves. The shadow zone of
P-waves appears as a band
around the earth between 105 ° and 145 ° away from the
epicentre. The shadow zone of S-waves
is not only larger in extent but it is also a little over 40
per cent of the earth surface. You can
draw the shadow zone for any earthquake provided you know
the location of the epicentre.
Types of Earthquakes
(i) The most common ones are the tectonic earthquakes. These
are generated due to
sliding of rocks along a fault plane.
(ii) A special class of tectonic earthquake is sometimes
recognised as volcanic
earthquake. However, these are confined to areas of active
volcanoes.
(iii) In the areas of intense mining activity, sometimes the
roofs of underground
mines collapse causing minor tremors. These are called
collapse earthquakes.
(iv) Ground shaking may also occur due to the explosion of
chemical or nuclear
devices. Such tremors are called explosion earthquakes.
(v) The earthquakes that occur in the areas of large
reservoirs are referred to as
reservoir induced earthquakes.
Measuring Earthquakes
The earthquake events are scaled either according to the
magnitude or intensity of the
shock. The magnitude scale is known as the Richter scale.
The magnitude relates to the
energy released during the quake. The magnitude is expressed
in absolute numbers,
0-10. The intensity scale is named after Mercalli, an
Italian seismologist. The intensity
scale takes into account the visible damage caused by the
event. The range of intensity scale
is from 1-12.
EFFECTS OFEARTHQUAKE
Earthquake is a natural hazard. The following are the
immediate hazardous effects of
earthquake:
(i) Ground Shaking
(ii) Differential ground settlement
(iii) Land and mud slides
(iv) Soil liquefaction
(v) Ground lurching
(vi) Avalanches
(vii) Ground displacement
(viii) Floods from dam and levee failures
(ix) Fires
(x) Structural collapse
(xi) Falling objects
(xii) Tsunami
The first six listed above have some bearings upon
landforms, while others may be
considered the effects causing immediate concern to the life
and properties of people in
the region. The effect of tsunami would occur only if the
epicentre of the tremor is below
oceanic waters and the magnitude is sufficiently high.
Tsunamis are waves
generated by the tremors and not an earthquake in itself.
Though the actual quake
activity lasts for a few seconds, its effects are devastating
provided the magnitude of the
quake is more than 5 on the Richter scale.
Frequency of Earthquake Occurrences
The earthquake is a natural hazard. If a tremor of high
magnitude takes place, it can cause
heavy damage to the life and property of people. However,
not all the parts of the globe
necessarily experience major shocks. We shall be discussing
the distribution of earthquakes
and volcanoes with some details in the next chapter. Note
that the quakes of high
magnitude, i.e. 8+ are quite rare; they occur once in 1-2
years whereas those of ‘tiny’ types
occur almost every minute.
STRUCTURE OF THEEARTH
The Crust
It is the outermost solid part of the earth. It is brittle
in nature. The thickness of the crust
varies under the oceanic and continental areas. Oceanic
crust is thinner as compared to the
continental crust. The mean thickness of oceanic crust is 5
km whereas that of the
continental is around 30 km. The continental crust is
thicker in the areas of major mountain
systems. It is as much as 70 km thick in the Himalayan
region. It is made up of heavier rocks having
density of 3 g/cm3. This type of rock found in the
oceanic crust is basalt. The mean density
of material in oceanic crust is 2.7 g/cm3.
The Mantle
The portion of the interior beyond the crust is called the
mantle. The mantle extends from
Moho’s discontinuity to a depth of 2,900 km. The upper
portion of the mantle is called
asthenosphere. The word astheno means weak. It is considered
to be extending upto 400
km. It is the main source of magma that finds its way to the
surface during volcanic
eruptions. It has a density higher than the crust’s (3.4
g/cm3). The crust and the
uppermost part of the mantle are called lithosphere. Its
thickness ranges from 10-200 km.
The lower mantle extends beyond the asthenosphere. It is in
solid state.
The Core
As indicated earlier, the earthquake wave velocities helped
in understanding the
existence of the core of the earth. The core-mantle boundary
is located at the depth of
2,900 km. The outer core is in liquid state while the inner
core is in solid state. The density of
material at the mantle core boundary is around 5 g/cm3 and
at the centre of the earth at 6,300
km, the density value is around 13g/cm3. The core
is made up of very heavy material mostly
constituted by nickel and iron. It is sometimes referred to
as the nife layer.
VOLCANOES AND VOLCANIC LANDFORMS
You may have seen photographs or pictures of volcanoes on a
number of occasions. A volcano
is a place where gases, ashes and/or molten rock material –
lava – escape to the ground. A
volcano is called an active volcano if the materials
mentioned are being released or have
been released out in the recent past. The layer below the
solid crust is mantle. It has higher
density than that of the crust. The mantle contains a weaker
zone called asthenosphere.
It is from this that the molten rock materials find their
way to the surface. The material in
the upper mantle portion is called magma. Once it starts
moving towards the crust or it
reaches the surface, it is referred to as lava The material
that reaches the ground includes
lava flows, pyroclastic debris, volcanic bombs, ash and dust
and gases such as nitrogen
compounds, sulphur compounds and minor amounts of chlorene,
hydrogen and argon.
Volcanoes
Volcanoes are classified on the basis of nature of eruption
and the form developed at the
surface. Major types of volcanoes are as follows:
Shield Volcanoes
Barring the basalt flows, the shield volcanoes are the
largest of all the volcanoes on the earth.
The Hawaiian volcanoes are the most famous examples. These
volcanoes are mostly made
up of basalt, a type of lava that is very fluid when erupted.
For this reason, these volcanoes
are not steep. They become explosive if somehow water gets
into the vent; otherwise,
they are characterised by
low-explosivity. The upcoming lava moves
in the form of a fountain
and throws out the cone at the top of the vent and develops
into cinder cone.
Composite Volcanoes
These volcanoes are characterised by eruptions of cooler and
more viscous lavas
than basalt. These volcanoes often result in explosive
eruptions. Along with lava, large
quantities of pyroclastic material and ashes find their way
to the ground. This material
accumulates in the vicinity of the vent openings leading to
formation of layers, and this makes
the mounts appear as composite volcanoes.
Caldera
These are the most explosive of the earth’s volcanoes. They
are usually so explosive that
when they erupt they tend to collapse on themselves rather
than building any tall
structure. The collapsed depressions are called calderas.
Their explosiveness indicates that
the magma chamber supplying the lava is not only huge but is
also in close vicinity.
Flood Basalt Provinces
These volcanoes outpour highly fluid lava that flows for
long distances. Some parts of the
world are covered by thousands of sq. km of thick basalt
lava flows. There can be a series of
flows with some flows attaining thickness of more than 50 m.
Individual flows may extend
for hundreds of km. The Deccan Trapsfrom India, presently
covering most of the
Maharashtra plateau, are a much larger flood basalt
province. It is believed that initially the
trap formations covered a much larger area than the present.
Mid-Ocean Ridge Volcanoes
These volcanoes occur in the oceanic areas. There is a
system of mid-ocean ridges more
than 70,000 km long that stretches through all the ocean
basins. The central portion of this
ridge experiences frequent eruptions. We shall be discussing
this in detail in the next chapter.
VOLCANIC LANDFORMS
Intrusive Forms
The lava that is released during volcanic eruptions on
cooling develops into igneous
rocks. The cooling may take place either on reaching the
surface or also while the lava is
still in the crustal portion. Depending on the location of
the cooling of the lava, igneous rocks
are classified as volcanic rocks(cooling at the surface) and
plutonic rocks(cooling in the
crust). The lava that cools within the crustal portions
assumes different forms. These forms
are called intrusive forms. Some of the forms are shown in
Figure 3.5.
Batholiths
A large body of magmatic material that cools in the deeper
depth of the crust develops in the
form of large domes. They appear on the surface only after
the denudational processes remove
the overlying materials. They cover large areas, and at
times, assume depth that may be several
km. These are granitic bodies. Batholiths are the cooled
portion of magma chambers.
Lacoliths
These are large dome-shaped intrusive bodies with a level
base and connected by a pipe-like
conduit from below. It resembles the surface volcanic domes
of composite volcano, only
these are located at deeper depths. It can be regarded as
the localised source of lava that
finds its way to the surface. The Karnataka plateau is
spotted with domal hills of granite
rocks. Most of these, now exfoliated, are examples of
lacoliths or batholiths.
Lapolith, Phacolith and Sills As and when the lava moves
upwards, a
portion of the same may tend to move in a horizontal
direction wherever it finds a weak
plane. It may get rested in different forms. In case it
develops into a saucer shape, concave
to the sky body, it is called lapolith. A wavy mass of
intrusive rocks, at times, is found at
the base of synclines or at the top of anticline in folded
igneous country. Such wavy materials
have a definite conduit to source beneath in the form of
magma chambers (subsequently
developed as batholiths). These
are called the phacoliths. The near horizontal bodies of the
intrusive igneous rocks are called sillor sheet, depending
on the thickness of the
material. The thinner ones are called sheet while the thick
horizontal deposits are
called sills.
Dykes
When the lava makes its way through cracks and the fissures
developed in the land, it
solidifies almost perpendicular to the ground. It gets
cooled in the same position to develop a
wall-like structure. Such structures are called dykes. These
are the most commonly found
intrusive forms in the western Maharashtra area. These are
considered the feeders for the eruptions
that led to the development of the Deccan traps.
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