Grade: 7
TOPIC: Layers of Earth and Plate Tectonics
Earth is the only planet in the Solar System where life exists. The existence of life on the
Earth is possible due to its three realms—land, water and air.
 The landmass is the solid realm, called the lithosphere. It is the outer solid layer of
the Earth. The Greek word lithos means rocks.
 The water bodies are liquid realm, called the hydrosphere.
 The air around the Earth is the gaseous realm, called the atmosphere.
 The narrow zone of contact between lands, water, air where all forms exist, is called
the biosphere.
Exploring inside the Earth:
The Erath has four layers: Crust, Mantle, Outer core and the Inner core. However, the Earth
is not composed of uniform material. The materials in the inner layers are heavier than that
of the outer layers. The density of the rocks increases slowly towards the centre of the
Earth. The area around the centre may have heavy metals like nickel and iron. The increase
in density is can also be because of increasing pressure of the overlying layers. Thus, the
Earth has a layered structure with varying density of materials.
THE CRUST:
 It the solid outermost layer. The crust together with the upper mantle is called the
lithosphere.
 The crust is a relatively thin layer with an average thickness of about 60km.
 The crust comprises of two parts – the upper part is the continental crust and the
lower part is the oceanic crust.
 Properties of continental and oceanic crust:
Continental
1. Older, thicker, cannot sink.
2. Permanent
3. 70-80km
Oceanic
Younger, lighter, can sink.
Constantly destroyed and replaced.
10-20km.
4. Less dense
More dense.
5. Mainly made with rocks with a Basaltic rocks.
composition similar to granite.
 The main elements of the crust are silica, aluminium and magnesium.
 The outer crust has valuable soils and provides us with most of the minerals.
THE MANTLE:
 Very thick layer and extends up to an average depth of about 2900km.
 The boundary between the crust and the mantle is called the MOHO or the
Mohorovicic discontinuity.
 It is made up of dense and heavy materials such as oxygen, iron and magnesium.
 Temperature: between 900-2200 degree Celsius. The temperature is quite high and
the hot rocks form magma in this layer.
 The pressure of the overlying layers, keeps the lower parts of the crust and the upper
part of the mantle in an almost solid state.
 If cracks appear in the crust, the pressure is released and the molten matter from
inside the Earth tries to reach the surface through volcanic eruptions.
THE CORE:
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The core is divided into – inner and outer core.
The outer core is about 2275km thick.
The core is made up of heavy metals such as nickel and iron.
The temperature of the inner core can be as high as 5000 degree Celsius.
 The outer layer is in the molten state, whereas the inner core is firm and slid due to
high lithospheric pressures.
 The hea
etals a d ple t of i o
a e espo si le fo the Ea th s ag etis .
It can therefore be concluded that the interior of the Earth is made up of the above
mentioned layers. However, how do we verify it?
Most of our knowledge about the interior of the Earth comes from the study of the
behaviour of the seismic waves in the Earth. These are produced naturally during an
Earthquake but they can also be produced artificially through explosions. There are four
kinds of Seismic waves –P waves or the Primary waves, S waves or the Secondary waves,
Love waves( named after a British physicist) and Rayleigh waves( named after a British
physicist). Out of these, P a d S a es p o ide a i u i fo atio a out the Ea th s
interior. We shall study that in detail.
The properties of P and S waves are summarised in the following table:
WAVE TYPE
WAVES TRAVEL IN
FORM OF
PARTCLE MOVEMENT
P WAVES
Compressional
Longitudinal
THE Compressions
S WAVES
or Shear or Transverse
Crests and Troughs
Back and forth
Up and down
SPEED
Fast
Slow
DESTRUCYION CAUSED
Less
Great
MEDIUM OF TRAVEL
Travels through all mediums
Do not travel through fluids
THE PLATE TECTONIC THEORY:
The idea of
ovi g co ti e ts
In 1912, Alfred Wagener, German meteorologist observed that the coastlines of many
adjacent continents were very similar. There were several precursors who had noticed the
similarity of the coastlines of Africa and South America in the 19th century and early 20th
Century, but he was the first person who made a serious attempt. He publicized this idea
of Co ti e tal Drift , in which he proposed:
 All the continents were a part of a single super continent called super continent
Pangaea.
 Pangaea broke apart into many pieces (the present day continents) which drifted
apart in different directions about 200 million years ago.
He used the following observations as evidences to support his theory:
 Almost all adjacent continents fit into each other like a jigsaw puzzle.
 The Appalachian mountain range in North America reappears as the Northern
Highlands in Europe across the Atlantic Ocean.
 Fossils of marsupials (Tropical animals like the kangaroo, opossum and koala) were
found in Antarctica where they could never have survived if Antarctica had been in its
present position throughout earth history.
 The rocks in Brazil and West Africa, India and Madagascar, Australia and Antarctica
match in composition, contained fossils, age, thickness and sequence right from the
age of 4000 million years to 200 million years.
Ho e e , Wage e s h pothesis as e
o i i g to geologists; it failed to explain
the force and the mechanism that caused the continents to drift and so was rejected by
geophysicists.
How do the continents move?
The idea of o i g o ti e ts was again given by Harry Hess in 1962 who noticed the
following occurrences in the Atlantic Ocean near the Mid Atlantic Ridge (MAR):
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The crust at the MAR is very thin.
There is a continuous chain of active volcanoes all along the MAR.
The oceanic crust in the Atlantic Ocean is not more than 200 million years old.
Rocks on both sides of the ridge are symmetrical in composition, age and
magnetic properties.
Hess the efo e postulated that olte
ate ial f o the Ea th s a tle o ti uousl
erupts from the crests of the mid ocean ridges. As the magma cools, it is pushed away from
the flanks of the ridges. This spreading is thought to bring about the migration, or drifting
apart, of the continents. The idea of Hess came to be known as Sea Floor Spreading.
THE PLATE TECTONIC THEORY
Continental Drift + Sea Floor Spreading = the Plate Tectonic Theory
Features of this theory:
 Ea th s ust is ade up of solid lithosphe i plates.
 These plates move over a thin and slippery layer called the Asthenosphere.
 Due to continuous eruption of magma from the mantle at the mid-oceanic ridges,
new oceanic crust is added to the existing crust, which in turn causes the plates to
move apart.
 At the other end, two plates push against each other, and the heavier plate subsides
under the lighter plate. This region is known as the Subduction Zone.
Plate Boundaries:
In simple language, plate boundary is the place where two plates meet.
The above image shows all the 17 plates. Out of these 17 plates, 7 are the most important.
They are:
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African Plate
Antarctic Plate
Indo-Australian Plate
Eurasian Plate
North American Plate
South American Plate
Pacific Plate.
The red arrows show plate boundaries.
What happens at the plate boundaries?
There are three types of plate boundaries.
1. Divergent
2. Convergent
3. Transform
1. Divergent Plate Boundary (or Constructive Plate Boundary):
Diagram:
At constructive margins, such as the Mid Atlantic Ridge, two plates move away from each
other. Molten rock or magma immediately rises to fill any possible gap and forms new
oceanic crust. The Atlantic Ocean is widening by about 3 cm a year which means that the
Americas are moving away from Eurasia and Africa—a process known as the Sea Floor
Spreading.
2. Convergent Plate Boundary (or Destructive Plate Boundary):
Three possibilities:
 Continental-Continental Collision
 Oceanic-Oceanic Collision
 Continental-Oceanic Collision
Continental-Continental Collision:
Diagram:
Collision margins are where two plates consisting of continental crust, move together. As
continental crust can neither sink nor be destroyed, the rocks between them are forced
upwards to form Fold Mountains. The Indian Plate is moving into the Eurasian plate at a
rate of 5 cm a year. The land between them, which was once the bed of the now nonexistent Tethys Sea, has been buckled and pushed upwards to form the Himalayas. This
movement, which is still taking place, accounts for major earthquakes.
Oceanic – Oceanic Collision:
Diagram:
Trench: a long, narrow, deep depression
in the ocean bed, typically one running
parallel to a plate boundary.
1. Island Arc: a curved chain of volcanic
islands located at a tectonic plate
margin, typically with a deep ocean
trench on the convex side.
Two plates consisting of Oceanic crust, forms trenches. E.g. the Marina Trench.
Continental-Oceanic Collision:
Diagram:
(Buoyant: able or tending to keep afloat or rise to the top)
Destructive margins occur where plates consisting of oceanic crust move towards plates of
continental crust. In the South Pacific Ocean, the Nazca Plate (Oceanic) is moving toward
South American Plate (Continental). Where they meet, the Nazca Plate is forced
downwards to form a subduction zone and an associated deep-sea trench (Peru-Chile
Trench). The increase in pressure, as the plate is forced downwards, can trigger severe
earthquakes. As the oceanic crust continues to descend, it melts, partly due to the heat
resulting from friction caused by contact with the South American Plate and partly due to
increase in temperature as it re-enters the mantle. Some of the newly formed magma,
being lighter than the mantle, rises to the surface to form volcanoes and a long chain of fold
mountains (the Andes).
3. Transform Fault (or Conservative Margins):
Diagram:
Conservative Margins are found where two plates slide past one another. As crust is neither
being formed nor being destroyed at this plate boundary, new landforms are not created
and there is no volcanic activity. However, earth uakes a o u if t o plates sti k . This is
the situation in California, where the San Andreas Fault marks the junction of the Pacific
and North American Plates. The American Plate moves slowly than, and at a slight angle
into, the Pacific Plate. Instead of plates slipping evenly past each other, they tend to stick—
like a machine without oil. When sufficient pressure builds up, one plate is jerked forward
sending shockwaves to the surface. These shockwaves caused an earthquake in San
Francisco in 1906. Similar shockwaves caused another earthquake in 1989.
Earthquakes:
A Ea th uake is a shaki g of the Ea th. It is aused
sudde
o e e ts i the Ea th s
crust due to the release of energy as rocks suddenly break or shift under stress.
Important Terminologies:
• Seismic Waves: Energy stored in the strained rock, is released as powerful vibrations.
These vibrations, tremors or shock waves are called seismic waves. They travel
outward in all directions.
• Seismic Focus: The place below the Ea th s su fa e he e a Ea th uake o igi ates.
• Epicenter: Poi t o Ea th s su fa e di e tl a o e the seis i fo us.
Where do Earthquakes occur?
• Along the plate edges
• Along the Faults.
What are Faults?
A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks
to move relative to each other. This movement may occur rapidly, in the form of an
earthquake.
Types of Faults:
1. Normal Fault:
A normal fault drops rock on one side of the fault down relative to the other
side. The following figure shows the fault and arrows indicate movement. The block
farthest to the right that is shaped kind of like a foot? That's the foot wall and the
block on the other side of the fault is resting or hanging on top of the foot wall block?
That's the hanging wall.
2. Reverse Fault:
Along a reverse fault one rocky block is pushed up relative to rock on the other side.
3. Strike-slip Fault:
Strike-slip faults have a different type of movement than normal and reverse faults.
The rocky blocks on either side of strike-slip faults, on the other hand, scrape along
side-by-side. You can see in the illustration that the movement is horizontal and the
rock layers beneath the surface haven't been moved up or down on either side of the
fault. Carefully notice from the illustration that pure strike-slip faults do not
produce fault scarps.
In real-life faulting is not such a simple picture! Usually faults do not have purely up-anddown or side-by-side movement as we described above. It's much more common to have
some combination of fault movements occurring together. For example, along California's
famous San Andreas strike-slip fault system, about 95% of the movement is strike-slip,
but about 5% of the movement is reverse faulting in some areas!
Measuring Earthquakes:
The vibrations produced by earthquakes are detected, recorded, and measured by
instruments call seismographs. The zigzag line made by a seismograph, called a
"seismogram," reflects the changing intensity of the vibrations by responding to the motion
of the ground surface beneath the instrument. From the data expressed in seismograms,
scientists can determine the time, the epicentre, the depth, and the type of faulting of an
earthquake and can estimate how much energy was released.
The severity of an earthquake can be expressed in several ways. The magnitude of an
earthquake usually is expressed by the Richter scale. It is named after the American
seismologist Charles Francis Richter. The scale is logarithmic so that a recording of 7, for
example, indicates a disturbance with ground motion 10 times as large as a recording of 6.
A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a
Richter value of 6 or more are commonly considered major; great earthquakes have
magnitude of 8 or more on the Richter scale.
The Mercalli Scale expresses the intensity of an earthquake's effects in a given locality in
values ranging from I to XII. This scale was introduced by an Italian seismologist Giuseppe
Mercalli.
Volcanoes:
A mountain or hill, typically conical, having a crater or vent through which lava, rock
fragments, hot vapour, and gas are or have been erupted from the earth's crust. The name
"volcano" has its origin from the name of Vulcan, a god of fire in Roman mythology.
Volcanoes and Plate Tectonics:
Volcanoes are formed at locations where tectonic plate boundaries converge, diverge and
over hotspots.
Structure of a Volcano:

Magma Chamber – A magma chamber is a large underground pool of molten rock
sitti g u de eath the Ea th s
ust. This
ag a is less de se tha the su ou di g
mantle and so it seeps up to the surface through cracks and flaws in the crust. When it
reaches the surface, it results in a volcanic eruption.

Lava – Lava is the rock expelled from a volcano during an eruption. When it first comes
out, the lava can have a temperature higher than 700 degrees C. It then flows downhill
from the eruption point until it cools and hardens.

Main Vent – A ol a o s
ai
e t is the poi t i the Ea th s
ust
he e hot
ag a
has reached the surface. The familiar cone-shaped volcano builds up as ash, rock and
lava ejected during eruptions fall back to Earth around the vent.

Crater – A volcanic crater is a circular depression in the ground caused by volcanic
a ti it . A ol a o s e ts a e lo ated at the otto
of the ate .

Pyroclastic Flow – Pyroclastic flows are fast moving currents of hot gas and rock with
travel downhill from a volcano, reaching speeds of 700 km/hour. The gas can reach
temperatures of more than 1,000 degrees C, and is one of the greatest dangers from
volcanic eruptions.

Ash Cloud – Volcanic ash consists of small pieces of pulverized rock and glass created
during volcanic eruptions. These fragments are so small, and heated to such a
temperature that they can be carried in the air for many kilometres.

Volcanic Bombs – Volcanic bombs are chunks of lava blasted into the air which solidify
before they reach the ground. Some bombs can be extremely large, measuring 5-6
meters in diameter and landing more than 500 meters from the volcanic vent.

Secondary Vent – On large volcanoes, magma will reach the surface through several
different vents, and not just the main vent. This can form cones, eject lava, and cause
destruction.

Secondary Cone – Secondary cones build up around secondary vents on larger
volcanoes.
Classification of Volcanoes:
1. Active Volcanoes: are those that have erupted in recent periods. E.g.
Stromboli near Sicily, Izalco, in El Salvador.
2. Dormant Volcanoes: are those that have erupted only in the early periods of
history and are now quite. E.g., Mount Fuji of Japan, Mount Kilimanjaro of
Africa.
3. Extinct Volcanoes: are volcanoes that have not erupted in the historical
period. E.g. mt. Aconcagua in South America, Mt. Elbrus in Europe.
Shapes of Volcanoes:
Cinder Cone Volcanoes:
Cinder cone volcanoes (also called scoria cones) are the most common type of volcano and
are the symmetrical cone shaped volcanoes we typically think of. Volcanic cones are built
up by the accumulation of the ejected lava. When the lava that flows very slowly and builds
a conical shaped mountain with steep slopes. These volcanoes are the most picturesque
but they also represent most devastating kind of volcanic activity. Such volcanoes erupt
with great force and blows out an entire cone composed of lava and ash. E.g. Mt. Fuji,
Japan.
Fire Fountain:
Shield/Dome Volcanoes:
With some volcanoes the overflow of lava takes place quite and generally covers a wide
area. The great outpourings of lava gradually build up a wide, gently sloping and enormous
thick shield. Mauna Loa and Kilauea of Hawaii are examples.
Fissure Volcanoes:
Although volcanic activity from a central cone is the most familiar, the largest amount of
ol a i
ate ial that o es out of the Ea th s i te io is e t uded f o ho izo tal a ks
called the fissures.
Vast continental areas of Africa and other continents are made up of such lava flows. The
Deccan Plateau of India, Northern Ireland and Iceland are made up of such eruptions.
Composite or Stratovolcanoes:
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many
layers (strata) of hardened lava, pumice, and volcanic ash. The lava flowing from
stratovolcanoes typically cools and hardens before spreading far due to high viscosity.
Stratovolcanoes are sometimes called "composite volcanoes" because of their composite
layered structure built up from sequential outpourings of eruptive materials. They are
among the most common types of volcanoes, in contrast to the less common shield
volcanoes. E.g. Mt. St. Helens.
Note: The exact shape of a volcano is mainly determined by the
composition of lava that flows out i.e. the kind of lava and ash it
produces.
1. If the lava is very viscous i.e. thick and sticky then it flows
very slowly and solidifies close to the vent forming high
cones. E.g. Mt Fuji, Japan.
2. If the lava is very fluid, then it spreads very quickly and
the co e is al ost flat, for s a shield or a do e.
Measuring Volcanoes
Scientists measure the size of a volcanic explosion according to the Volcanic Explosovity
Index (VEI). This grades explosion on a scale of 0-8, 0 being a non explosive eruption where
8 represents largest eruption. There have so far been no known eruptions with VEI 8.
Greatest Volcanic Eruption: Tambora.
Influence of Volcanic Eruptions on Man and Environment:
Destructive:
1. Loss of Life
2. Damage to property
Constructive:
1.
2.
3.
4.
Fertile soil formed as a result of weathering of volcanic rocks.
Source of precious stones.
Hot springs are of medicinal and therapeutic value.
Geo thermal energy.