Planet Earth: How it Works

1

• Lesson 4
• Earths Internal Structure

2
The proto-Earth energy and heat
Formation of Earth collision of
fragments ? Collision energy heat Radioactive
decay heat
? Heat accumulation production - irradiation
3
Origin of the Planets (from lesson 2)
The nebula hypothesis ? Gravity reshapes the
proto-planet into a sphere The interior separates
into core and mantle
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Earth layering process
Heat accumulation was faster than
irradiation ? Earth began to melt
? Less dense (more buoyant) molten materials
migrated to the surface (Si, Al, Na, K
todays rocks)
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Earth layering process
Denser (less buoyant) molten materials sank
towards the centre (Ni, Fe, other metals rare
on the crust)
? Layers of different composition sorted by
density (proto-Earth was homogeneous)
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Earth layering process
The iron catastrophe
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Proportion of major elements
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The Earths layers
Layers of Earth based on composition ? The
Crust The Mantle The Core
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The Earths layers
The crust ? Relatively thin skin 7-10 km
beneath oceans (oceanic crust) 25-70 km beneath
land (continental crust) Oceanic crust is denser
than continental crust
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The Earths layers the crust
Differences between oceanic e continental crust
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The Earths layers
The mantle ? Almost 3,000 km thick (largest
volume of material from Earth) Density
increases from about 3.5 g/cm3 (top) to about
5.5 g/cm3 (base)
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The Earths layers the mantle
The mantle sublayers
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The Earths layers
The mantle movement ? Most of the mantle is
solid rock (although plastic, or soft) Warm
(less dense) mantle gradually flows upward (15
cm/year) while cooler, denser mantle sinks
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Mantle convection
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The Earths layers
The mantle movement ? The asthenosphere is the
layer of soft but solid rock from the upper
mantle that flows and moves the Earth plates
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The Earths layers the asthenosphere
The asthenospheric mantle and plate tectonics
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The Earths layers
The core ? Outer core (2,900-5,155 km) 11
g/cm3 (composition liquid iron-nickel
alloy) Temperature 7,200-9,000 ºF Inner core
(5,155-6,371 km) 13 g/cm3 (composition solid
iron-nickel alloy) Temperature 9,00011,000 ºF
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Earths Magnetic Field
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Questions?
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Lesson 4 review questions
1. What are the two major sources of heat
accumulation during the formation of the
proto-Earth?
2. The iron catastrophe is
3. Explain why iron, although being the most
abundant element in the composition of our
planet (35), is very rare in the crust (it
represents only 6 of the composition of the
crust)?
4. What are the two most abundant elements on
rocks at the surface of our planet?
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Lesson 4 review questions
5. Which one is denser, the oceanic or the
continental crust?
6. Which one is thicker, the oceanic or the
continental crust?
7. Which of the three Earth layers contains the
largest volume of material?
8. What is the name of the layer of soft but
solid rock from the upper mantle that flows and
moves the Earth plates?
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Lesson 4 review questions
9. Explain the mantle convection (movement
within the mantle).
10. The core is divided in outer core and inner
core. Explain the difference between them in
terms of density, temperature, and physical
state?
11. Temperatures in the inner core can reach
11,000 ºF. How do you explain the fact that,
with such high temperatures, the inner core
remains solid?
12. Explain the behavior of pressure and
temperature as you move from the crust to the
core of our planet.
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Lesson 4 review questions
13. The Earths magnetic field is produced by
14. Why is the Earths magnetic field so
important for life in our planet?
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Where are we???
Module I The Realm of Geology Lesson 1 What is
geology? Module II Earths Origins Lesson 2
Earths Origins Lesson 3 Planets of the Solar
System Lesson 4 Earths Internal Structure
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Where are we???
Module III Plate Tectonics Lesson 5 Wegeners
Continental Drift Lesson 6 Sea-Floor
Spreading Lesson 7 Plate Motion and Plate
Boundaries
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• Lesson 5
• Wegeners Continental Drift

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Plate Tectonics
Theory of Plate Tectonics
? Continental Drift (Alfred Wegener) Sea-floor
Spreading (Harry Hess)
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Plate Tectonics
Developed in the early 1960s ? Triggered a
revolution in the Earth Sciences
1) Unified the study of Earth (Earth
Sciences) ? From palaeontology (fossils) to
seismology (earthquakes) to orogenesis
(mountain building)
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Plate Tectonics
2) Provided explanations to ancient questions
? Why earthquakes/volcanic eruptions occur in
very specific places on Earth?
? How and why great mountain ranges like the
Alps and Himalayas formed?
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Plate Tectonics Origins
The fit of South America and Africa
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Plate Tectonics Origins
The fit of South America and Africa (before the
20th century)
?
Francis Bacon (16th century) Georges de Buffon
(18th century) Alexander von Humboldt (19th
century)
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Plate Tectonics Origins
The fit of South America and Africa
1858 ? maps from Antonio Snider-Pellegrini
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Plate Tectonics Origins
1912 ? Theory of Continental Drift by Alfred
Wegener
?
He proposed the existence of a single continent
Continents have been drifting ever since
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Plate Tectonics Origins
Continental masses moved over time
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Plate Tectonics Origins
1912 ? Theory of Continental Drift
1915 ? Die Entstehung der Kontinente und
Ozeane (The Origin of Continents and Oceans)
German Meteorologist Alfred Lothar Wegener
overwhelming criticism ? The theory was severely
attacked from the start
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
The remarkable fit between Africa South America
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Similar geology between Africa, South America,
Antarctica, India, Australia
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Plate Tectonics Origins
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Similar fossil record
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Evidence of dramatic changes in the climate
Coal deposits in Antarctica
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Evidence of dramatic changes in the climate
Grooves carved by glaciers
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Plate Tectonics Origins
Glacial grooves at Kelleys Island, OH
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Evidence of dramatic changes in the climate
Grooves carved by glaciers
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Plate Tectonics Origins
Evidence for the Continental Drift Theory
?
Evidence of dramatic changes in the climate
Reconciling glacial deposits
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Plate Tectonics Origins
Fatal weakness
? Could not answer the fundamental
question What kind of forces could be strong
enough to move such large masses of solid rock
over such great distances?
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Plate Tectonics Origins
Fatal weakness
Theory of Continental Drift ? Wegeners point
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Plate Tectonics Origins
Fatal weakness
? Most geologists agreed that it was not possible
to move the continental rock masses through the
rigid basalt crust of the ocean basins
The theory was dismissed as being eccentric,
preposterous, and improbable
Wegener died in 1930
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Questions?
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Lesson 5 review questions
1. Plate tectonics triggered a revolution in the
Earth Sciences for two reasons. Describe them.
2. Describe the theory of continental drift
proposed by Alfred Wegener in 1912.
3.Describe one of the pieces of evidence that
Wegener produced to support his idea.
4. Why Wegeners theory of continental drift was
dismissed by the scientific community?
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• Lesson 6
• Sea-floor Spreading

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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
4 major scientific developments
1) Demonstration of the ruggedness and youth of
the ocean floor
? 1925-27 ?The RV Meteor ? Atlantic
bathymetry Central Valley along the crest of the
Mid-Atlantic Ridge ? It is a rift-valley !!!
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
1) Demonstration of the ruggedness of the ocean
floor
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Plate Tectonics after Wegener
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
4 major scientific developments
1) Demonstration of the ruggedness and youth of
the ocean floor
? 1947 ? Seismologists on RV Atlantis The oldest
ocean basin ? 200 million years The oldest
continental rock ? 4 billion years
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
1) Demonstration of the youth of the ocean floor
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
4 major scientific developments
2) confirmation of repeated reversals of the
Earth magnetic field in the
geologic past
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
4 major scientific developments
3) Emergence of the seafloor-spreading theory
and associated recycling of oceanic crust
? 1961 ? mid-ocean ridges mark structurally weak
zones where the ocean floor was being ripped ?
New magma rises through the ridge and new
oceanic crust is created
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Plate Tectonics after Wegener
Sea-floor spreading
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Plate Tectonics after Wegener
Sea-floor spreading
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Plate Tectonics after Wegener
Sea-floor spreading ? New question
? How can new crust be continuously added along
the oceanic ridges without increasing the size
of the Earth?
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Plate Tectonics after Wegener
Sea-floor spreading ? New answer
Harry Hess and Robert Dietz (early 1960s) ? If
the Earth's crust was expanding along the
oceanic ridges, it must be shrinking elsewhere
!!!
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Plate Tectonics after Wegener
Harry Hess and Robert Dietz (early
1960s) ? Subduction zones Whole-mantle
convection
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Plate Tectonics after Wegener
Whole-mantle convection
Convection cells
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Plate Tectonics after Wegener
Convection cells
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Plate Tectonics after Wegener
Whole-mantle convection
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Plate Tectonics after Wegener
Whole-mantle convection answers
1. Why the Earth does not get bigger with sea
floor spreading
2. Why there is so little sediment accumulation
on the ocean floor
3. Why oceanic rocks are much younger than
continental rocks
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Plate Tectonics after Wegener
Theory of Continental Drift ? the follow up
4 major scientific developments
4) Precise documentation that the world's
earthquake and volcanic activity is concentrated
along oceanic trenches and submarine mountain
ranges
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Plate Tectonics after Wegener
Concentration of Earthquakes
1954 ? French seismologist J. P. Rothé
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Plate Tectonics after Wegener
Concentration of Earthquakes
Earthquakes with depth of crustal movement)
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Plate Tectonics after Wegener
Concentration of Earthquakes
Earthquakes with depth 100km (related to
subduction)
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Plate Tectonics after Wegener
Concentration of Earthquakes


100 km
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Plate Tectonics a new theory
? Incorporates the ideas of Continental
Drift and Sea-floor Spreading in a unified model
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Tectonic plates
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Questions?
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Lesson 6 review questions
1. What 4 major scientific developments provided
the mechanisms to support Wegeners theory?
2. Why rocks from oceanic crust are much younger
than rocks from continental crust?
3. Describe the theory of sea-floor spreading.
4. If new land is being created at the mid-ocean
ridges, why the Earth is not getting bigger?
5. What is a subduction zone?
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Lesson 6 review questions
6. Describe the concept of whole-mantle
convection?
7. Most of the worlds earthquake and volcanic
activity occur at very specific places in our
planet. What do these places have in common?
8. Most shallow earthquakes ( concentrated along what type of plate boundary
convergent or divergent?
9. Most deep earthquakes ( 100 km deep) are
concentrated along what type of plate boundary
convergent or divergent?