Republic of the Philippines

Mountain Province State Polytechnic College

Bontoc, Mountain Province

THE EARTH’S STRUCTURE, PLATE TECTONICS AND

MAGNETIZATION

Module 3 of 5 Modules

Geology for Civil Engineers

Joebeth T. Papat
Email: joebethpapat1@gmail.com
Messenger: Joebeth Tobe Papat
CP #: 09457701962

Engineering Department

Second Semester, S. Y. 2020-2021

INTRODUCTION

Module three of these course focuses on the internal structure of the earth and
its relation to plate tectonics and magnetization.

This module is good for 3 weeks; thus, activities herein are also expected to be
finished within that period. All learning activities herein are intended for all male and
female students regardless of religious affiliation and cultural background. Send your
outputs through email, or you may send it through courier.

LEARNING OUTCOMES:
At the end of the module, you should be able to:
1. discuss the plate movements;
2. explain why there is a continental drift; and
3. differentiate the three types of plate boundary.

PRETEST
Before you finally proceed to the lesson, try to write the correct labels for the
figure below. Write your answers corresponding each item below the figure.

1.

2.

3.

4.

1.
2.
3.
4.

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 LESSON 1: EARTHS STRUCTURE, PLATE TECTONICS AND MAGNETIZATION
Objectives
At the end of the lesson, you should be able to:
1. label the main layers of the earth correctly;
2. compare the different plate boundaries; and
3. explain the role of the earth’s magnetic field in relation to plate tectonic
theory.

LET’S ENGAGE

Are you familiar with continental drift? What can you understand from this? How
is this related to tectonic plates?

The word, tectonic, refers to the deformation of the crust as a consequence of

plate interaction. The surface expression of mantle convection.

These plates shift over time, largely driven by convection in the mantle, which
leads to rearrangement of the continents. Mapping of the seafloor and examining the
magnetization of rocks ultimately led to the acceptance of plate tectonics as a legitimate
theory.

LET’S TALK ABOUT IT

To understand the plate tectonics, let us first investigate the earth’s internal
structure so we can relate why there is continental drift.

A. Earth’s Structure
The Earth is made up of 3 main layers: Core, Mantle and Crust

Figure 3.1 Internal structure of the earth

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 The Crust is the upper thermal boundary layer associated with mantle
convection.
Earth’s Crust is cold and brittle. It is a thin layer, 0.4% of Earth’s mass and 1%
of its volume.

The Earth’s crust is made of continental Crust and oceanic crust.

Continental Crust Oceanic Crust

• thick (10-70km) • thin (~7 km)
• buoyant (less dense than oceanic • dense (sinks/subducts under
crust) continental crust)
• mostly old • young (Atlantic 200 my)
• Primarily granitic type rock (Na, K, Al, • Primarily basaltic (Fe, Mg, Ca, low
SiO2) SiO2)
• 40 km thick on average • 7 km thick
• Relatively light, 2.7 g/cm3 • Relatively dense, 2.9 g/cm3

Cool, solid crust and upper (rigid) mantle “float” and move over hotter, deformable
lower mantle.

Earthquakes and seismic waves inform us about the interior structure of a planet
(see figure 3.2).

Figure 3.2 Internal structure of the earth

Source: http://www.geo.brown.edu/research/Milliken/GEOLO810_files

The crust is rigid; ocean is ‘basaltic’, continents ‘granitic’; extends to 5-70 km

depth. The mantle is solid but ductile (can flow); Mg/Fe-rich; extends to 2,900 km depth.
The core has a solid inner region and liquid outer region; made of Fe, Ni.

The lithosphere consists of the crust and the upper part of the mantle. It is the
lithosphere that takes part in plate tectonics, where plates sink into the lower
asthenosphere (also part of the mantle).

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Figure 3.3 The Lithosphere
Source: http://www.geo.brown.edu/research/Milliken/GEOLO810_files
B. Plate tectonics

The Earth’s crust is composed of a number of plates, most of which do not follow
the shapes of the continents. It is divided into plates which are moved in various
directions. This plate motion causes them to collide, pull apart, or scrape against each
other. Each type of interaction causes a characteristic set of Earth structures or
“tectonic” features.

The word, tectonic, refers to the deformation of the crust as a consequence of

plate interaction. The surface expression of mantle convection.

These plates shift over time, largely driven by convection in the mantle, which
leads to rearrangement of the continents. Mapping of the seafloor and examining the
magnetization of rocks ultimately led to the acceptance of plate tectonics as a legitimate
theory.

Figure 3.4 Plates shift over time which leads to rearrangement of the continents.
Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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World plate has major and minor plates as shown in the figure below.

Major plates – Pacific, African, Eurasian, North American, Antarctic, South American,
Australian

Minor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Cocos, Scotia, Juan de
Fuca

Figure 3.5 Map of the world plate

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures
Plate Composition
Plates are made of rigid lithosphere. The lithosphere is made up of the crust and
the upper part of the mantle (see figure below).

Figure 3.6 Earth’s structure and the lithosphere

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 Lithosphere and asthenosphere show more detailed description of earth’s layered
structure according to mechanical behavior of rocks, which ranges from rigid to
deformable.

a. Lithosphere: it is a rigid layer shell that includes upper mantle and crust (here
is where plate tectonics work), and a cool layer.

b. Asthenosphere: this layer is below lithosphere, part of the mantle, weak and
deformable (ductile, deforms as plates move), partial melting of material
happens here and it is a hotter layer.

Figure 3.7 The lithosphere and asthenosphere

Craton, is an old and stable part of the continental lithosphere.

Having often survived cycles of merging and rifting of continents, cratons are
generally found in the interiors of tectonic plates.

They have a thick crust and deep lithospheric roots that extend as much as
several hundred km into the mantle.

The term craton is used to distinguish the stable portion of the continental crust
from regions that are more geologically active and unstable.

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 Figure 3.8 Craton
Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Cratons can be described as Shields, Precambrian crystalline rock that crops

out at the surface and Platforms, in which the basement rock is overlaid by younger
sediments and sedimentary rock. The age of these rocks is in all cases greater than 540
million years, and radiometric age dating has revealed some that are as old as 2 to 3
billion years

Figure 3.9 Shields and platforms

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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Plate movement
“Plates” of lithosphere are moved around by the underlying hot mantle convection
cells as shown in the figures below.

Figure 3. 10 The plate movement

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Figure 3.11 The plates movement

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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Plate Boundaries
Sites of significant geologic activity includes earthquakes, volcanism, orogenesis.

There are three types of plate boundary

• Divergent
• Convergent
• Transform

Figure 3.12 Three types of plate boundary

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

1. Divergent Boundaries
This causes spreading of ridges as shown in the figure below. As plates move
apart, new material is erupted to fill the gap

Figure 3.13 Spreading ridge

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Iceland: is an example of continental rifting

Iceland has a divergent plate boundary running through its middle

Figure 3.14 Iceland continental rifting

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 Figures below is a modern divergence: An East African Rift System.

Figure 3.15 East African Rift System

2. Convergent Boundaries
There are three styles of
convergent plate boundaries
– Continent-continent
collision
– Continent-oceanic crust
collision
– Ocean-ocean collision

Figure 3.16 Three convergent

boundaries
Source:
http://www.geo.hunter.cnuy.edu/~fbuo
n/GEOL_231/Lectures

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Continent-Continent Collision: Forms mountains, e.g. European Alps, Himalayas

Figure 3.17 Continent-Continent Collision

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Figure 3. 18 Himalayas
Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Continent-Oceanic Crust Collision: Called SUBDUCTION

Figure 3.19 Continent-Oceanic Crust Collision

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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During subduction,
• Oceanic lithosphere subducts
underneath the continental
lithosphere
• Oceanic lithosphere heats
and dehydrates as it subsides
• The melt rises resulting in
volcanism
E.g. The Andes

Figure 3.20 The Andes

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Ocean-Ocean Plate Collision

When two oceanic plates collide, the older denser slab will sink back into the
mantle forming a subduction zone.

The subducting plate is bent downward to form a very deep depression in the
ocean floor called a trench. Trench systems occur for both continent-ocean and ocean-
ocean boundaries.

The world’s deepest parts of the ocean are found along trenches. E.g. The Mariana
Trench is 11 km deep!

Island arcs form, continents Collide, and crust recycles at convergent plate
boundaries.

The formation of an island arc along a trench as two oceanic plates converge. The
volcanic islands form as masses of magma reach the seafloor. The Japanese islands
were formed in this way.

Figure 3.21 Formation of an island arc along a trench as two oceanic plates converge
Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 Motion of the plates:
✓ Rates: average 5 cm/year
✓ Mid-Atlantic Ridge = 2.5 – 3.0 cm/yr.
✓ East-Pacific Rise = 8.0 – 13.0 cm/yr.

Figure 3.22 Ocean-Ocean, Aleutian Islands, Alaska

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

3. Transform Boundaries: Where plates slide past each other. For example, is the san
Andreas transform fault.

Figure 3.23 View of the San Andreas transform fault

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 The view of the earth plate is shown in the following figure.

Figure 3.24 The earth plates

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

Volcanoes and Plate Tectonics

Volcanism is mostly focused at plate margins.

Volcanoes are formed by: Subduction, Rifting and Hotspots (see figure 3.25
below).

Figure 3.25 Volcanoes formation

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 Figure 3.26 Pacific Ring of Fire
Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

What are Hotspot Volcanoes?

Hot mantle plumes breaching the surface in the middle of a tectonic plate. ‘Hot
Spots’ appear to be caused by stable, deep-seated plumes and give rise to volcanic
chains. The plume remains still, but the plate rides over it and makes new volcanoes.
The Hawaiian island chain are examples of hotspot volcanoes.

Figure 3.27 Hawaiian island chain

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

The tectonic plate

moves over a fixed
hotspot forming a chain
of volcanoes.

The volcanoes get

younger from one end to
the other

Figure 3.28 Formation of chain of volcanoes.

Source: http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures

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 The origin of the plumes for hot spots is still a debate, but a potential theory is:
- heat is transferred across a boundary (outer core-lower mantle)
- plumes may be linked to the return of crust to the core-mantle boundary
- it is the accumulation of subducted material that gives rise to plumes & hot
spots

Some subducted slabs may reach the core-mantle

boundary. Hotspots may be sourced from the core-
mantle boundary. Importantly, convection in the liquid
outer core is believed to cause the ‘dynamo’.

Figure 3.29 Origin of Plumes for hot spots

Source: http://www.geo.brown.edu/research/Milliken/GEOLO810_files

Plate Tectonics Summary

• The Earth is made up of 3 main layers (core, mantle, crust)
• On the surface of the Earth are tectonic plates that slowly move around
the globe
• Plates are made of crust and upper mantle (lithosphere)
• There are 2 types of plate
• There are 3 types of plate boundaries
• Volcanoes and Earthquakes are closely linked to the margins of the
tectonic plates

C. Magnetization

The Earth’s magnetic field protects us from the harmful radiation of the solar
wind. The solar wind consists of energetic particles radiated by the Sun, and protection
from such radiation has surely played an important role in the evolution of life as we
know it.

But what causes the magnetic field?

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 The ‘dynamo theory’ postulates that a rotating, convecting, and electrically
conducting fluid can sustain such a magnetic field. In the case of Earth this can be
linked to the outer core (see figure 3.30).

Figure 3.30 Cartoon of magnetic field (left) and computer model of Earth’s field (right)
Source: http://www.geo.brown.edu/research/Milliken/GEOLO810_files

The Earth essentially acts as a giant magnet with magnetic field lines connecting
the two magnetic poles. These poles reverse over time and can be recorded in rocks,
which is one way the theory of plate tectonics was finally established.

By examining the strength and orientation of magnetization in old rocks (ancient

crust) we can learn a lot about the internal dynamics and evolution of planets. If a planet
does not exhibit magnetized crust then we can infer that a dynamo did not exist.

Figure 3.31 Poles reverse over time and can be recorded in rocks
Source: http://www.geo.brown.edu/research/Milliken/GEOLO810_files

ITS YOUR TURN

Activity 3.1.
Do the following as instructed in each item. Use a separate bond paper for your
answers.
1. Explain how plate tectonic is related to volcanic formation. 5 points
2. Compare and contrast the styles of convergent plate boundaries. 5 points
3. What causes continental drift? 5 points
4. Investigating the internal structure of the earth and the plate tectonics is
important in the field of engineering. Can you cite examples or situations below
how this lesson impacts your learning as an engineering student? 10 points

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Rubric:
Percentage Points Criteria
40% 2 4 8 Response is clear and directly answer
each part of the question
40% 2 4 8 Ideas are organized and relates answers
to concepts discussed
20% 1 2 4 Minimal errors in grammar and spelling
Total= 100% 5 10 20

POST ASSESSMENT
Alternate- response test. Read and understand the statements carefully. Determine
whether each statement is true or false. Write R if it is true, otherwise W, on the space
provided at the right side of this paper.
Items Ans.
1. The Earth’s magnetic field protects us from the harmful radiation of the
solar wind
2. The magnetic poles reverse over time and can be recorded in rocks.
3. The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.
4. Plates are made of crust and upper mantle
5. Volcanoes are formed only by subduction and hotspots.
6. Subduction zone is formed when subducting plate is bent downward to
form a very deep depression in the ocean floor.
7. The Earth’s crust is made of continental Crust and not oceanic crust.
8. There are three types of plate boundary. This are divergent, continental
and transform.
9. Lithosphere is a rigid layer shell that includes core and crust where plate
tectonics work.
10. The plates shifting over time leads to rearrangement of the continents.
11. The asthenosphere is located below lithosphere.
12. Oceanic crust is denser hat continental crust.
13. Mountains can be formed at convergent plate boundaries.
14. The lithosphere is the earth’s rigid surface layer.
15. The lithosphere is the earth’s hot malleable layer located beneath the
asthenosphere.

REFERENCES
Dhakal, S. (2016). Fundamentals of geology. https://www.researchgate.net/publication

Geology 11 lecture notes. https://web.viu.ca/earle/geol111/lecture-notes

Introduction to basic geologic principles.

http://www.geo.brown.edu/research/Milliken/GEOLO810_files/PlanetaryGeolo
BackgroundMaterial.pdf

Montgomery, C. W. (1989). Fundamentals of geology. Wm. C. Brown Publishers.

Tectonic landforms.
http://www.geo.hunter.cnuy.edu/~fbuon/GEOL_231/Lectures/TEctonoc%250
Landforms.pdf

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