Reference Book: Fundamentals of Physical Geology, Sreepat Jain. The slides were prepared keeping focus on FY BSc Geology students.

1. Geology and Plate tectonics
Mr. Chinmay U. Dongare
Assistant Professor (Temp.)
Department of Geology, The Maharaja Sayajirao University of Baroda,
Vadodara-02
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2. What is Geology and what do Geologists do?
• Geology deals with the study of earth, its materials,
processes that affect them, the products formed and the
earth’s history since its birth i.e. 4.54 billion years ago.
• Also, it involves the study of ocean floor, and the
interior of the earth.
• Geologists: Investigates composition of earth’s
material, locates minerals and their exploitation,
studies geological phenomenon like Earthquakes and
Volcanoes to predict and minimize its damaging
effects.
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3. • It is thus, the science of earth, dealing with its
composition, structure, and geological history;
studying the origins, properties, and
compositions of both rocks and minerals.
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Credits: Academic Comics

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Geology
Physical
Geology
Geomorphology
Structural
Geology
Petrology Mineralogy
Economic
Geology

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Physical Geology
The subject deals with the physical forces and processes
that bring about changes in the earth’s crust or to the
surface of the earth on account of their prolonged
existence and action.
Physical Geology is broadly divided into two branches—
those dealing with the internal dynamics of the earth
(Endogenous Geology) and those with external dynamics
(Exogenous Geology).

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Those processes that occur on the outer fringes of the
earth come into the preview of Exogenous Geology
and include the formation and development of seas,
rivers, streams, landforms (underground water),
erosion, weathering, and all sedimentary and allied
processes.
The movement of earth’s crust, earthquakes, and volcanic
eruptions, etc. (i.e., the endogenous processes) form the
preview of this branch.

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Earth
Atmosphere Lithosphere Biosphere Hydrosphere
It is the land part of the earth (litho = stone) and includes
all the solid materials constituting earth, from surface
downward.
Both crust and upper mantle make up the Lithosphere
which extends ~80–100 km below.
 Ocean = Few km Continental lithosphere = 120 km

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Lithosphere is fragmented into
massive plates that fit around the
globe like pieces in a jig saw puzzle.
 These plates move independently
and slowly, relative to one another
and slide on top of a somewhat fluid
(plastic) part of the mantle called the
Asthenosphere.
 Movement of plates causes
earthquakes.
(Credits: Sreepat Jain (Fundamentals of Physical Geology))

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Internal Structure of Earth
 The Core, Mantle, and Crust constitute the three main layers
of earth.
 Compositionally (chemically), the outer thin crust is mostly
silicate (SiO2-based) and mantle, the layer below is
accompanied by metal oxides (such as MgO, FeO, Al2O3,
CaO, and Na2O) in mineral composition.
 Mantle is the largest by volume, making up almost 87 % of
the earth.
 The core makes up 35 % of the total mass of the earth and is
probably made of almost pure iron, perhaps even in a single
crystal form.

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(Credits: Sreepat Jain (Fundamentals of Physical Geology))

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Only for Information

16. Continental Drift Theory
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• In the early 1900s a German explorer and scientist
(Alfred Wegener) proposed the continental drift theory.
• He proposed that there was once a single “super
continent” called Pangaea.
• He believed that the continent floated on the oceanic
crust (like an iceberg in the ocean).
• Most scientist rejected his theory due to lack of
evidence.

17. Evidence of continental drift
• Continents fit together like a jigsaw puzzle.
• Fossils match across oceans.
• Rock types and mountain ranges match across
oceans.
• Climate Evidence (Glacial Deposits).

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19. • Ocean floor moves like a conveyor belt carrying
continents with it.
• New ocean floor forms along cracks in the ocean
crust as molten material erupts from the mantle
spreading out and pushing older rocks to the
sides of the crack. New ocean floor is continually
added by the process of sea-floor spreading.
Sea-Floor Spreading

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Sea floor spreading + Continental drift theory =
Plate Tectonics Theory
Plate tectonics has enabled us to predict geologic events and explain almost
all aspects of what the earth experiences.
The theory explains why mountains, earthquakes, and volcanoes occur, and
where they do, the ages of deformational events, the ages and shapes of
continents and ocean basins, as well as other aspects of the earth.
 It is a holistic theory and explains crustal movements.
According to this theory, the earth is made of Plates, composed of
lithosphere (~100 km thick) that ‘‘floats’’ on the ductile Asthenosphere.
The plates behave as rigid bodies with some ability to flex, but deformation
occurs mainly along plate boundaries.

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Plate Boundaries
 The plates move in three ways—toward each
other, away from each other, and sliding past each
other.
 These are accordingly classified into three types:
1. Convergent – Towards each other
2. Divergent – Away from Each other
3. Transform – Sliding Past each other

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These occur where oceanic lithosphere is
pushed back into the Mantle.
 These are marked by Oceanic trenches and
Subduction zones.
Three types of Convergent boundaries are:
1. Ocean-Ocean Convergence
2. Ocean – Continent Convergence
3. Continent – Continent Convergence
Convergent Plate Boundary

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 When two oceanic lithosphere plates collide, one
subducts beneath the other resulting in the formation
of an oceanic trench on the seafloor.
 The sinking plate becomes a Subduction zone.
An Island arc is produced where subduction occurs.
E.g. Islands of Tonga, Mariana and Phillipines in
western pacific.

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25. Ocean – Continent Convergence
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 When oceanic lithosphere runs into a plate with continental
lithosphere, the oceanic plate is subducted beneath the
continental, due to its higher density.
 As the denser plate subducts, it heats up.
 As it does so, dehydration reactions release water into the
overlying mantle asthenosphere, causing a reduction in the
melting temperature and facilitating the production of andesitic
magma.
 These magma rise to the surface and form a volcanic arc
(magmatic arc) parallel to the trench.
 The magma that is created by this convergence forms a
magmatic arc.

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Continent– Continent Convergence
 Both plates have continental lithosphere; these collide with one
another, and their margin becomes a continental collision
margin.
 They are characterized by fold-thrust mountain belts that
develop along the zone of collision.
 Neither of the continental plates are subducted below as they
are too buoyant.
 The Himalaya Mountain chain and the Tibetan Plateau are
examples of this kind of margin.
 Both were formed when the Indian plate collided with the
Eurasian plate.

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Divergent Plate Boundary
 At these boundaries, the plates move away from each other.
 This can occur in the middle of the ocean or in the middle of a
continent.
 Those occurring at oceanic ridges, the plates move away from the
ridge in opposite directions thereby creating new oceanic
lithosphere by erupting basaltic magma.
 When occurring in the middle of a continent, the divergent
boundary is marked by rifting, basaltic volcanism, and uplift.
 Features: MOR, Rift valleys and fissure volcanoes.

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Transform boundaries occur where two plates slide
past one another horizontally.
 Earthquakes are shallow like the ones noted in the
famous San Andreas Fault (California, USA).
 Conservative boundary.
Transform Plate Boundary

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