Earths Interior and Origin Introduction the question

Earth's Interior and Origin
Introduction: the questions to be answered
• We live on the surface of a sphere (with a 4000-
mile radius) made mostly of rock that revolves
around the sun.
• How did scientists determine what is inside the
Earth even though we can’t directly sample
much of it?
• More specifically, there are a number of
questions to be answered.
1) Is the Earth made from the same material
from its surface to its center (e.g., like a
solid rubber ball)? Or is it made from layers
of different composition, as baseball is?
2) If the Earth has layers, what are those
layers made from chemically and how do we

know that?
3) Is the entire earth solid, or is there evidence
that materials found within Earth are in
different physical states (i.e., liquid and
gas)?
Here are the answers:
The Earth has both different chemical and physical
zones within it
• The Earth has zones in it that are made from
different chemicals (i.e., chemical zones).
• The Earth also has zones in it that differ in
their physical state (i.e., physical zones).
I. Chemical Zones
• Evidence from earthquake (seismic) waves

indicates that the earth has three different
chemical zones within it.
• The crust is an outer thin layer. It varies in
thickness from 7 to 30 miles.
• The mantle, a middle zone, is far thicker,
starting at a depth of around 20 miles (the base
of the crust) and going to a depth of 1800 miles.
• The core the large center zone. It extends from
the base of the mantle (1800 miles below the
surface) to the Earth’s center at about 4000 miles
below the surface.
How do we know these chemical zones exist?
• Seismic waves bounce back (reflect) to the
surface (like an echo off of a canyon wall)
when they encounter the boundary between
different materials:
• If the velocity of the earthquake wave is
known (and it is), then the time it takes for
the wave to travel from the earthquake’s

focus (origin point) to the boundary and
back to the surface indicates how far the
boundary is from the surface (i.e., its depth).
• The calculation is a simple one: velocity x
time = distance (e.g., miles/hour x hours
travelling = miles travelled)
The evidence from seismic waves that tells
scientists:
1) That there are two boundaries in the Earth
that separate it into three different zones, the
crust, mantle, and core.
2) How thick each of the three zones is.
3) That these zones are very different
chemically from each other because the
reflection of waves from the boundaries
between them is very strong.
How do scientists determine the composition of
each of these zones?
A. The chemical composition of the crust
• There are two different types of crust.
• Crust that underlies the continents is

thicker than that the underlies the ocean
floor.
• Continental crust also has a different
composition than oceanic crust:
• Continental crust is made mostly from
granite and granite-like rocks.
• Oceanic crust is made mostly of
basalt.
How do we know the chemical composition
of the continental and oceanic crust?
• Direct sampling of continental crust –
easy
• Direct sampling of ocean crust –
doable
B. The chemical composition of the mantle

• Humans have never been able to drill
through the Earth’s crust to directly
sample pieces of the mantle (although
three different countries attempted to).
• The technological challenges proved to be
too great.
• But scientists have, however, put together
indirect evidence of different types to
reliably determine the mantle’s chemical
composition.
1) Mantle Xenoliths
• When melted rock rises towards the
surface, it sometimes breaks off solid
pieces of the surrounding rock.
• These pieces later appear as rock
fragments included in the igneous rock
(i.e., the rock formed by cooling and
solidification of the melt) called
xenoliths.
• Sometimes mantle xenoliths are found.

• Mantle xenoliths found in this way are
typically made from a roughly 50-50
mixture of two minerals, olivine and
pyroxene. (See the photo above.)
• Because these xenoliths represent
pieces of mantle brought up through
volcanoes, it is reasonable to assume
that the mantle is made up of olivine
and pyroxene.
2) Ophiolite Complexes
• Mountain-sized body that is made of
several different rock layers.
• Typically found in mountain ranges.

• Compare this to the modern ocean
floor.
• What can we assume about the
composition of the mantle from
ophiolites and why?
• Because the evidence from mantle
xenoliths and ophiolite complexes
both indicate that the mantle is
made from olivine and pyroxene,
scientists are confident that is the
mantle’s composition
3) Seismic Velocities
• Seismic waves travel at different
velocities through different materials
• Scientists can therefore test the
theory that the mantle is made from
olivine and pyroxene in lab
experiments.

4. An analogy to meteorites
• The sun and the planets of our solar
system are believed to have formed at
the same time (4.55 Billion years ago)
from the same material.
• The planets closest to the sun
(Mercury, Venus, and Mars) resemble
the Earth.
• Those farther away (Jupiter, Saturn
etc.) are mostly big spheres of frozen
gases.
• The asteroid belt is a zone between
Jupiter and Mars.
• It contains rock debris believed to be
formed when Earth-like planets
collided with each other.
• Many meteorites originate in the
asteroid belt and provide a clue to the
Earth’s interior.

• Many meteorites are basaltic, like the
Earth’s oceanic crust.
• Many meteorites are made from
olivine and pyroxene, consistent with
the previously discussed evidence for
the composition of the mantle.
C. The chemical composition of the core
• No known geologic process has brought a
sample of the core form its location at the
Earth’s center to the surface (unlike for
the mantle).
• Our knowledge of the core is all from
monitoring its effects
1) Gravity as a Clue to the Core’s
Composition
Gravity is the natural force of attraction
that exists between all matter.
• The more mass something has, the
greater its force of attraction it has.
• A large object has more mass than a
smaller object made of the same
material, so it exerts a greater force

of gravity.
• The Earth, because it is so big, is the
main force of gravity in our
environment. (That’s why things
fall towards the ground.)
• But objects of the same size can also
differ in their gravitational pull.
• For example, gold is denser (i.e., has
more mass for its size) than
Styrofoam, so it exhibits a greater
force of gravity than does a piece of
Styrofoam the same size.
The Earth’s gravity:
• The Earth’s force of gravity can be
measured, providing a clue to its
interior.
• That measurement allows us to
calculate the overall mass of the
Earth. (You can loosely think of the
mass of the Earth as its weight.)

• I’ll leave the details of that
calculation to a course in physics.
How to determine the core’s mass:
• The total mass of the earth is the
sum of the mass of its crust, mantle
and core.
Earth’s total mass = mass of
the crust + mass of the mantle
+ mass of the core.
• Based on the evidence that I described
previously, scientists already know
three terms of the equation, shown in
red.
• Knowing three of the four terms
allows us to calculate the only
unknown, the mass of the core (in
black).
• The mass of the core is an important
clue to what the core is made from
because we also know the size of the
core (based on seismic reflections).
• Dividing the mass of the core that we

just calculated by the volume of the
core gives us the core’s density
(mass/volume = density).
• The density of a substance is an
inherent property. For example, a
block of Styrofoam weighs less than a
block of gold the same size (i.e., gold
has a greater density than Styrofoam).
• The density of the core as calculated
by the method above is more like that
of metals, rather than the rocks that
make up the mantle and the core.
But which metal is it? How would
you narrow your search?
• Using your (correct) reasoning
method, scientists believe that the
core is made from iron with a little
nickel mixed
2) Magnetism as a Clue to the Core’s
Composition
• Magnetism is an attractive force of
nature that exists between some
objects (e.g., a magnet and your steel
refrigerator).

• Magnetism can also be created by
electrical currents.
• The Earth has a magnetic force field
that surrounds it. That’s why compass
needles point to the North Pole.
• But the Earth’s magnetic field cannot
be due to magnetic material buried in
its interior.
• Electrical currents must be responsible
for the Earth’s magnetic field.
• But the rock in the crust and mantle
conduct electricity poorly.
• The core must be made of an electrical
conductor or else the Earth would not
have a magnetic field.
• That is consistent with the evidence
for the core’s composition based on
measuring its gravity.

II. Physical Zones of the Earth
• The Earth is not only divided into chemical
zones (i.e., the crust, mantle and core, which
have different chemical compositions from
each other).
• It also has zones within it that differ in their
physical state (i.e., physical zones).
• The chemical and physical zones are
superimposed on each other.
• The following diagram illustrates the chemical
zones on the left half and the physical zones
on the right half.
Physical zones in the core:
• The inner core (dark green) is solid iron.
• The outer core (lighter green) is liquid (i.e.,
melted) iron.
Physical zones in the crust and mantle:

The lithosphere: It’s made of the entire crust
fused to a small portion of the upper
mantle; extends to a depth of about 100
miles; and is rigid and brittle.
The asthenosphere: It extends from the
bottom of the lithosphere (at 100 miles
deep) to a depth of about 250 miles; made
entirely from semi-solid mantle material;
and can bend and flow.
The solid lower mantle (or mesosphere): It
extends from a depth of 250 miles to
1,800 miles deep (i.e., making it far
thicker than the asthenosphere), and is a
rigid and brittle solid.
.
How do scientists know that these physical
zones exist?
• The short answer is evidence from seismic
waves.
• Detecting the zones in the core:
P-waves arrive at the opposite side of the
Earth, S-waves do not.
P-waves energy reflects off of the

boundary between the outer liquid core
and the inner solid core.
Detecting the zones in the crust and mantle:
Both P-waves and S-waves move more
slowly through the asthenosphere.
1
Continental Drift
Introduction
• Continental drift, a theory postulating that
continents move across the surface of the globe the
same way boats drift through water.
• Alfred Wegner was the first to systematically
develop comprehensive evidence to support the
theory (in 1912 – 1929).
• Here’s his original figure showing his
reconstruction of drifting

• His story has many facets, including those that
interest psychologists, sociologists, historians etc.
2
Alfred Wegener’s evidence for continental drift
1) The jigsaw puzzle fit of the continents
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2) The alignment of mountain belts
3) Climate anomalies
For example:
• Coal beds in Arctic islands
• Evidence of past continental glaciation in
Africa and South America
• Evaporites in temperate regions
4

4) The alignment of climate zones
5) Evidence from glacial striations
• When glaciers slide downhill over land they
leave striations in the rock caused by pebbles
in the glacier that scratch the rock.
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• Striations indicate the direction of the glacier’s
movement.
• The arrows on the map show the direction that
the glacier moved over land.
Why is the direction of glacial movement
in South America hard to explain if it
were always located where it is today?
How does a belief in continental drift
solve that problem?

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6) Fossil evidence
• Fossils are remnants of preexisting life
• Fossils of a freshwater dinosaur in Africa
looked identical to freshwater dinosaur fossils
in Africa.
• This helped support Wegener’s case.
Time for a Survey
Were you convinced by Alfred Wegener’s
evidence?
If so, imagine yourself in Wegener’s place
when you read the following views of him
and his theory by some prominent
scientists of his time:
• A paleontologist named E. W. Berry savaged the
continental drift theory as “a selective search . . .
ending in a state of auto-intoxication in which the
subjective idea becomes an objective fact,” a
critique that accused Wegener of indulging in both
onanism and mystification.
• The American geologist Bailey Willis called
continental drift a “fairy tale” that “encumbers the
literature and befogs the mind of fellow students.”

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• Philip Lake, a British geologist, accused Wegener
of “not seeking truth . . . blind to every fact and
argument that tells against it.”
• Rollin Chamberlin of the University of Chicago
questioned whether one could take Wegener
seriously and still call oneself a scientist. “If we
are to believe Wegener's hypothesis,” he averred,
“we must forget everything which has been
learned in the last seventy years and start all over
again.”
But let’s pretend you were a scientist in the US in
the period 1912 to 1955 and you merely stated that
Wegener’s ideas had some value. Here’s how one
author described how the scientific community in
the US would treat you:
“In the U.S. the reaction was particularly
strong; for an American geologist to express
sympathy for the idea of continental drift was
for him to risk his career”
(Alfred Wegener and the Hypothesis of Continental Drift
Author(s): by A. Hallam, Scientific American , Vol. 232,

1975, pp. 88-97).
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So, if the evidence that I presented above convinced
you that Wegener was correct, and you were living
in his time, you would have been
a) viewed as delusional and
b) had a hard time achieving a successful
career in science.
The broader context of Wegener’s story
• Many lessons can be learned from the story of how
Wegener developed his theory, how the scientific
community viewed it, and how Wegener was
treated.
• To be sure, Wegener’s critics were right about
some things: Wegener was wrong on major points
(e.g., how continents drifted).
• But the manner in which Wegener and his theory
were treated reveals a lot, not only about how the

scientific world operates, but also about human
nature in general.
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Here are key points that you’ll find in accounts of
Wegener’s story:
• Some scientists were blinded by the prestige of
Wegener’s critics. Many people accepted
arguments from leaders in the field based more on
their reputations as leaders than on the evidence.
• The prestigious critics themselves were blinded by
their own reputations. They were so enamored of
their own theories, they failed to consider
reasonable alternatives.
• There was an element of mob action in this. People
were flocking to the side of prestigious critics. Who
wants to be left behind as an outsider? Or who
wants their careers threatened?
• And as many mobs typically do, this mob resorted
to name-calling. That behavior scares people from
freely expressing themselves and impedes progress
in understanding any issue.

• Wegener’s ideas were dismissed partly because he
was viewed as an outsider. He was a generalist
trained in meteorology. People with expertise in
other fields, like geology, didn’t welcome the ideas
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of someone they regarded as an untrained outsider.
So those ideas did not get a fair hearing.
Wegener’s experience is echoed in today’s world.
• For example, the link below describes an example
of a scientist (Roger Pielke Jr.) who was subjected
to a campaign against him by politicians,
journalists and academics.
https://www.dropbox.com/s/u8bq7cwz2vi27mn/My
%20Unhappy%20Life%20as%20a%20Climate%20H
eretic%20-%20WSJ.pdf?dl=0
• Although he believes “climate change is real and
that human emissions of greenhouse gases risk
justifying action,” he was called a climate denier.
• Why? Because he questioned the conclusion of
research studies linking global warming to
hurricane activity.

• The charge against him – climate denier – is as
powerful a weapon today as being labeled a drift-
believer was in Wegener’s day. Both charges could
ruin careers.
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Take-away lessons for you from Wegener’s story
• Cede to the authority of evidence and logic. Don’t
believe someone just because they hold an
important or powerful position.
• Check your own assumptions frequently: maybe
your ideas don’t hold up to scrutiny.
• Avoid group-think: it’s easy to go along with the
crowd. But sometimes they’re wrong and a feeling
of wanting to belong trumps your own sound
thinking.
• Be open to others’ views, even if you strongly
object to them. We can learn much from the free
exchange of ideas. Rather than blocking speakers
from presenting their thoughts, or shouting them
down, hear them out. Maybe they have something
useful to say. And if you think not, present a more

persuasive counter-argument.
• Be tolerant: name-calling drives away people who
might have good ideas.
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Evidence found in the 1950s and 1960s bolsters
the theory of drift
1) Paleomagnetism (meaning old magnetism)
• The Earth has a magnetic field that is generated
by electrical currents in its core.
• North and South Pole should always be
approximately lined up with the Earth’s spinning
axis.
• Certain rocks (e.g., igneous rock), get slightly
magnetized in the direction of the poles when
they form. This provides a record of the Earth’s
magnetism over time.

• Paleomagnetic evidence gathered in the 1950s
seem to show:
a) The poles moved to different latitudes over
time. (This is called apparent polar
wandering.)
b) There were two North and South Poles
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• But the way the magnetic field is created means
that neither of these things is possible.
• The theory of continental drift solves that
mystery.
2) Radiometric age dating
• Radiometric dating techniques were developed
for practical use in the late 1950s and early
1960s.

• A 1967 study using these techniques helped
confirm the continental drift theory.
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1
Plate Tectonics
Introduction
• Wegener was wrong about the mechanism for
continental drift
• Clues from the ocean floor (from the 1940s
through 1960s) eventually led to the discovery of
the mechanism (plate tectonics).
• Over about 5 years, the scientific community
moved from ridiculing Wegener, to recognizing
his theory as correct.

Clues from the ocean floor
1) High heat flow at ocean ridges
2
2) Age and distribution of ocean sediment
The Evidence
3
3. Magnetic stripes on the ocean floor
• The Earth’s magnetic field periodically reverses
polarity.
• The field weakens over hundreds or thousands of
years and then rebuilds strength.

• Sometimes the field is rebuilt with the polarity
reversed compared to today’s status.
• If so, a compass needle would then point to the
South Pole, rather than the North Pole.
4
• Today’s orientation is called normal
magnetization. The opposite orientation is called
reverse magnetization.
• The magnetization of the rock that makes up the
ocean floor provides as a valuable clue to the
mechanism of plate tectonics.
*****************************************************
*******************************
The Evidence

5
The three pieces of ocean floor evidence that I
described above (heat flow, sediment distribution,
and magnetic stripes) proved key in helping
scientists not only to understand how continents
drifted, but also how many other large-scale
features of the Earth were created.
That overarching theory, which I detail below, is
called ‘plate tectonics.”
Convection: the mechanism behind continental
drift
• In the 1960s, a number of scientists, using evidence
from the ocean floor, finally developed a plausible
theory of how continents drifted.
• That theory, called plate tectonics, also explained
many other features of the Earth.
• The driving force for continental drift the other
features is convection in the mantle.
Convection is the movement of heat that is due to the
movement of material.
• Hot air rising is an example of convection. The hot
air (because it is less dense than cold air) will rise
and carry the heat it contains with it.

6
• This type of heat transfer typically creates
circulation (motion) in liquids and gases (e.g., in a
heated pot of soup.
• Winds are another example of convection.
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A demonstration of convection, and defining a
convection cell
• Here is a video that shows convection in action:
https://www.youtube.com/watch?v=b0Na_1xyodI
• It shows a reservoir filled with cooking oil
containing sprigs of the spice thyme (in order to
make circulation patterns in the oil more visible).
• Below is a still photo from the video that shows
you the circulation pattern that develops.
• The geometric pattern shown above by the arrows

outlines how the convecting fluid is moving. That
movement forms a geometric pattern called a
convection cell.
8
Convection in the asthenosphere: the driving force
• If you recall from the lesson about the Earth’s
interior, the asthenosphere is the physical zone
immediately below the lithosphere.
• A series of convection cells operating in the
asthenosphere move plates of lithosphere around
the surface, as shown in this generalized diagram.
• This is analogous to how simmering soup moves
pepper flakes around on its surface.
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How plate tectonics explains features at the
Earth’s surface

Convection cells operating in the asthenosphere
explain many of the features of the Earth’s surface, as
the following examples demonstrate.
I. Features that occur as the result of convection
under continental lithosphere
Overview of convection under continental crust
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Detailed view of convection under continental crust
• The features that develop
1) Normal faults
2) A rift valley
3) Volcanoes

4) Earthquakes
In short, the theory of plate tectonics explains how
rift zones form and why they are associated with
normal faults, volcanoes, and earthquakes.
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II. Continents drifting apart and an ocean floor
forming between them
A. The process
12
B. An Example
13

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III. Features that occur as the result of convection
under ocean lithosphere
15
• The features that occur include those that helped
scientists recognize convection was the driving
force for moving plates of lithosphere.
1) High heat flow at ocean ridges
The evidence
The explanation
16

2) Age and distribution of ocean sediment
The evidence
The explanation
17
3) Magnetic stripes on the ocean floor
The evidence
The explanation
This video demonstrates the process:
https://www.youtube.com/watch?v=YIAXiE8RedA
(The box in the lower left of the screen shows you the
change in the Earth’s polarity. The bands that form at
the ridge axis (above the rising melted rock, in red)

reflect the polarity at the time of their formation.)
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IV. Features that occur at ocean trenches
• Recall from the earthquake discussion that
trenches are long narrow valleys on the ocean floor.
• Some parallel continental coastlines. Some parallel
island arcs (an arc-shaped chain of islands)
• Sea floor created at the trenches is pushed back
into the mantle at trenches
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1) Features of “ocean-continent” trenches:
The Cascade Mountains are an example

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2) Features of “ocean-ocean” trenches:
In the case of the Lesser Antilles, these features form
because the Atlantic Ocean seafloor is being pushed
under the Caribbean seafloor.
21
V. How continents drift
• Although Wegener was right in his belief that
continents drifted, his model for how they drifted
was not plausible.
• The plate tectonic model, in contrast, provides a
plausible explanation for how continents drift.

1
The Effect of Geologic Processes on Biologic
Evolution
Introduction
• Many geologic processes affect large regions of the
Earth.
• Some of these processes have affected the biologic
evolution of animal and plant species …
• …and can do so in the future.
Biologic evolution: the change in inherited traits in
animal and plant species that occurs over the course
of many generations.
Here’s how it works
• Physical characteristics vary among members of
any species, including humans.
• Note, for example, the range in adult heights,
hair curliness, skin color etc.

2
• To demonstrate such variations, I’ll use a
“histogram”, a plot that shows how many data
points fall within each category across a range.
• This histogram shows the number of adults at
each height for both men and women:
• Notice the distribution pattern for both men and
women: there are many height values clustered
centrally, forming a peak.
• The center of the peak in each case represents the
average height.
• The curves taper off to the sides because the
farther a measured height is from average, the
less likely it is to occur.
• This pattern is called a “normal distribution” or
“normal curve” or “bell curve.”
3

• The normal curve for physical traits develops as
a result of random genetic mutations.
• Physical characteristics are largely encoded in
our genes and therefore can be passed down
from parents to children.
• For example, two tall parents are likely to have
tall children.
• But mutations in genes caused by damage to the
genes (by cosmic rays, chemicals etc.) or
copying errors during reproduction can cause
children to have different physical traits from the
parents.
• For example, two very tall parents could have a
short child.
• Even if all adults in the human race started out
with the same height (i.e., they are clones of each
other), random genetic mutations would result in
a normal distribution.
4

• This same process would also occur for every
human genetic trait (height, skin color, hair
curliness etc.).
This is the random element of evolution: the range
of traits that species exhibit is the result of random
changes in their genetic cells caused by cosmic rays,
chemicals, copying errors etc.).
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Natural selection can cause systematic changes in
the traits of a population.
• Random genetic changes provide a variety of
characteristics within a species.
• The environment influences the characteristics of
species in a non-random way.
• Some characteristics help individuals of the
species survive longer and, therefore, reproduce
more: that shifts the gene pool over the course of
generations.

• As time goes by, these changes can be so large
that after many generations the animals or plants
that develop are so different from their ancestors
that they have become different species (i.e.,
they no longer look alike and can no longer
breed together).
• In other words, random genetic mutations
interacting with environmental pressures result
in the evolution of species.
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The histogram below illustrates how a gene pool
shifts in response to some aspect of the environment.
Examples of processes that affect biologic
evolution
I. Continents drifting to different climate zones
• As continents drift across the globe, some
move from warmer to colder climates and
vice versa.
• Below is a sequence showing the movement
of continents over the last 139 million years.

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Note the movement of India (dark green) and
Antarctica (in orange).
8
• The changes in climate cause by continents
drifting to different climates zones can cause
1) Evolutionary changes in animals and
plants. Remember this from above?:
2) Extinction, for example, for Antarctica:
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II. Continents colliding and separating
• Evolutionary changes can result from the

collisions and separations of continents even
without any change in climate.
• To see how that happens it is useful to
understand the relationship between the size of
an area and the number of species that can exist
in that area.
• The species-area relationship (SAR) states that
the number of species that live in an area
increases in a systematic way as the area
increases.
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A. Continental collisions: decrease worldwide
species diversity. Here is a simplified example to
explain why that happens.
1. Before the collision:

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2. Immediately after the two continents collide:
12
3. The stable situation after the collision:
• The effect of continental collisions on diversity
can be illustrated by the example of the North
and South America connection that occurred
about 2.7 million years ago.
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B. Continental separations: worldwide diversity

increases. Here is a simplified example to explain
why that happens.
1. Before the separation:
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2. Immediately after separation:
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3. The stable situation after separation:

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III. The mystery at Gubbio, Italy
A discovery made in Gubbio, Italy, in 1977
eventually revealed how a geologic process that
occurred 66 million years ago had a major impact
on the course of biologic evolution.
• Geologic time is divided into named periods
characterized by different fossil assemblages
• The boundary between two of these periods,
the Cretaceous and the Tertiary, divides
periods with very different life forms.
• This so-called K-T boundary marks a mass
extinction event at 66 million years ago.
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• In 1980, Luis and Walter Alvarez (a father and son

team) were investigation rocks that spanned that
boundary in Gubbio, Italy.
A close-up:
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• The transition in life forms was abrupt and sharp.
19
• The Alvarez team measured the chemical iridium
(in the hope of finding out how long the clay layer
took to accumulate).
• Instead of accomplishing that, to their surprise,
they discovered an iridium anomaly.
• The Alvarez team looked at K-T sites elsewhere in
the world and found the same iridium anomaly.

• They proposed a theory for the worldwide
extinctions and the iridium spike: a meteorite
impact.
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• The scientific community was skeptical.
• Gradualism was a prevailing theory, there was not
crater large enough, etc.
• But over a decade evidence accumulated:
1) Osmium isotopes (1983)
2) Tsunami deposits the right age
3) Tektites the right age
4) Shocked quartz deposits
5) And in 1991, the smoking gun: a crater large
enough and the right age near Chicxulub,
Mexico. (93 miles wide, 12 miles deep).

21
• This case reflects how resistant the scientific
community can be to new theories (as it was with
Wegener).
• It also provides these lessons.
o Science can test hypotheses about events that
happened long ago.
o Scientific ideas are tested with multiple lines of
evidence.
o Science relies on communication within a
diverse scientific community.
o The process of science is non-linear,
unpredictable, and ongoing.
o Science often investigates problems that require
collaboration from those in many different

disciplines.
22
IV. Magnetic reversals
• As you learned in previous lessons, the Earth’s
magnetic field occasionally reverses itself.
• During reversals, the Earth is no longer
shielded from cosmic and solar radiation
(which include particles such as electrons and
protons).
• Some evidence suggests that this can affect the
course of biologic evolution
23
A. The extinction of some radiolaria species

• Radiolaria are a group of microscopic one-
celled species that float in the surface waters
of the oceans. They have hard shells (usually
made of silica).
• Evidence from deep-sea cores suggests that
the evolution of radiolaria may be affected
by magnetic reversals.
24
B. The extinction of large mammals
• Evidence suggests that this may have
affected the evolution of large mammal.
• A possible mechanism exists: UV exposure

25
IV. Ice Ages
• The Earth experiences periods of colder
climate that allow for ice sheets (i.e., glaciers)
to expand.
• North America and Europe, experienced the
last one about 115,000 to 12,000 years ago.
• That Ice Age may have affected hominid
evolution.
• Neanderthals were a human-like species living
that existed from 400,000 years ago until
40,000 years ago in Europe.
26
• Neanderthals:
o made tools, clothing, and jewelry
o hunted and used fire created cave art

o buried their dead, sometimes with tools
• Modern humans:
o first evolved in Africa around 200,000
years ago;
o migrated to Europe around 45,000 years
ago;
o lived side by side with Neanderthals from
about 45,000 years ago until 40,000 years
ago, the date when Neanderthals went
extinct.
Why did the human species survive the glaciation
but Neanderthals did not?
That question is not settled, but the evidence suggests
a number of plausible explanations:
1) Cultural differences in a harsh environment
2) The harsh environment as the main cause

Rock Deformation
Introduction
• When rocks are subject to forces, they may
experience deformation (i.e., change their size or
shape).
• Under some circumstances the rock folds or
stretches. This is plastic deformation.
• Under other circumstances, the rock breaks. This is
brittle deformation.
Faults: an example of brittle deformation
• A fault is a fracture in rock along which the rocks
on either slide in opposite directions.
Types of faults
• Classified based on how the blocks of rock on
either side of the fault plane move relative to each
other.

• Different fault types are created when the forces
applied to the rock are different.
• Two common fault types are classified on the basis
of the relative movement of footwalls and hanging
walls
I. Normal Faults
• The hanging wall moves down relative to
the footwall.
• They are caused by tension (i.e., forces that
pull the rock in opposite directions).
Normal faults as evidence of processes:
• The existence of normal faults in a location
reveals that the rocks there have been pulled
apart.
• The 3700-mile long East African Rift Zone
(The Great Rift Valley) is an example.

• It is a series of connected valleys located in
the eastern part of Africa. They extend for
about 3,700.
• Its location is shown on this map:
• It’s so large feature that it can be seen from
space:
• The valley is formed by a series of normal
faults in which blocks of rock have moved
downward to form the valley, as illustrated
here:
• Here’s a photo of the valley taken in
Ethiopia:

• Because normal faults require tensional
forces this major feature must have been
created by extremely large tensional forces
pulling apart Africa:
• Scientists did not know what those forces
were for a long time.
• Luckily, you’ll just have to wait for a few
more lectures before I explain them to you.
II. Reverse Faults
• The hanging wall moves up relative to the
footwall.
• They are caused by compression (i.e., forces
that squeeze rock.
Here’s a photo of a reverse fault:

Reverse faults as evidence of processes:
• The existence of reverse faults in a location
reveals that the rocks there have been
squeezed.
• The Appalachian Mountains on North
America’s east coast are just one of many
examples.
• Reverse faults throughout the Appalachian
Mountains provide one line of evidence to
suggest that the mountain range formed by
enormous compressional forces.
III. Strike-Slips Faults
• Unlike normal and reverse faults, the fault
plane in a strike-slip fault is vertical (i.e.,
perpendicular to the surface).
• That means that there is no footwall and
hanging wall.

• Blocks of rock on the opposite sides of a
strike-slip fault do not move up or down.
Rather, they slide sideways along the
vertical fault plane:
• A strike-slip fault provides evidence of the
nature of the forces that created it, but a
discussion of that is beyond the scope of this
course.
• I have described this type of fault for one
reason. The only specific fault that a person
is likely to be able to name is the San
Andreas Fault in California and it is a 750-
mile long strike slip-fault.
• And here’s an aerial photo of the San
Andreas Fault. As you can see, its location is
clearly visible as a long straight valley at the
Earth’s surface.

Folds: an example of ductile deformation
• Under some conditions, rocks will bend rather than
break when compressional (squeezing) forces are
applied to them.
• This can result in originally horizontal layers of
rock (e.g., sedimentary rock) forming a wave like
pattern
• The arches (upward folds) are called anticlines
and the troughs (downward folds) are called
synclines.
• Here’s a photo of folded rock. These rocks were
originally horizontal, but were folded as the
result of being squeezed.
What factors determine whether a rock undergoes
faulting or folding?
1) Temperature

2) Pressure
3) The rate at which the force is applied
• Rocks near the surface tend towards faulting,
unless the force is applied relatively slowly
• Rocks at depth tend to bend (or stretch, or flow)
1
Volcanoes
Introduction
A volcano is a mountain formed by the accumulation
of solidified lava and rock fragments around an
opening (vent) in the ground.
• Mount Fuji in Japan is an example:
• Volcanoes typically have craters at their
summits, for example, Mount Fuji’s:

2
Why are parts of the Earth’s interior melted?
• The Earth gets hotter with depth.
• There are two sources of the Earth’s heat:
1) radioactive elements
2) heat remaining from the Earth’s formation
• The Earth’s pressure also increases with depth.
• The pressure increase tends to keep materials
solid
• The rock at some locations inside the Earth melt
because they are at the right combination of
pressure and temperature.
• Volcanoes are created when melted rock inside
the Earth rises towards the Earth’s surface and
escapes through vents.
Materials associated with Volcanoes
• Lava: melted rock forms at certain depths in the
Earth and rises to escape through vents as lava.

3
• Gases: melted rock at depth (called magma)
contains gases dissolved in it that can escape
when the lava surfaces.
• Pyroclastic material: rock fragments of various
sizes that sometimes form during eruptions.
Volcanic ash, tiny rock particles, is an example
of a pyroclastic material.
Here’s an example of an ash cloud (in gray)
rising above an erupting volcano in Bali,
Indonesia:
4
Geographic distribution of volcanoes
• Volcanoes are found concentrated in certain
geographic areas (with some exceptions) as shown
below.
• The volcanic zones are largely the same as the
earthquake zones

Why? Later lectures will reveal that.
5
Types of volcanoes
Two main types of volcanoes exist:
I. Shield Volcanoes
• Broad, dome-shaped: The Hawaiian volcanoes
are examples.
Mauna Loa:
Notice that there is snow at its peak, although it
is in a tropical climate. This shows that its peak
reaches a great height.
6
• Produce lava and gases, but not very much
pyroclastic material:
• Do not tend to erupt explosively: In other
words, NOT THIS:

Santorini:
7
Note that people live on Santorini
Erupt frequently: One Hawaiian volcano has
been erupting continuously every few months
since 1983.
Have very low viscosity lava: It is so fluid that
it is common for shield lava to flow at 20 miles
per hour and can even get up to 60 miles per
hour (although rarely)
8
II. Composite Volcanoes
• Cone-shaped: Mount Vesuvius and Aetna in
Italy are examples. So are Mount Fuji in Japan,
and Mount Rainier near Seattle, Washington.

Mount Rainier:
• Produce lava, gases, and pyroclastic materials
(like volcanic ash): for example, of an erupting
composite volcano near Mexico City.
9
• Tend to erupt explosively: remember Santorini,
which erupted explosively in 1470 BCE:
Do not erupt frequently: there may be 100s or
1000s of years between eruptions.
Have very high viscosity lava: it may move as
little as inches or feet per hour
10
The reason why shield and composite volcanoes

have different characteristics
• The difference in lava viscosity between
shield and composite volcanoes accounts for
the differences in their:
1) Shape
2) Explosiveness
3) Production of pyroclastic material
4) Frequency of eruption
The reason why shield volcanoes have less
viscous lava than composite volcanoes
• They originate in different places and have
different chemical compositions.
• Shield volcano lava originates in the mantle.
• Composite volcano lava is created when
some of the crust is melted.
11
Volcanic hazards

• Volcanoes can harm people and property in a
variety of ways.
• But these dangers (or hazards) can often be
minimized by a variety of methods.
I. Lava flows
A. Damage caused
• Destroys structures.
For example, setting fire to wooden houses:
12
Or knocking down houses (as in Italy in 1944):
13
Or burying houses (as in Iceland, 1973):
• Less direct lava damage, for example:

1) To harbors, as in Iceland 1973
14
2) To farmland:
B. Ways to mitigate damage from lava:
• Avoid valleys – despite all the advantages of
valleys
• Divert the lava
Such as this case in Italy:
15
• Freeze the lava
Here’s an example of that method in the
1973 Iceland eruption. (That wall of
advancing lava is 90-feet high.)

16
II. Volcanic ash: damage caused and ways to
mitigate it
Ash falls: ash in rising plumes that eventually
settles to the ground gently (like
falling snow).
• A health hazard
17
• Structural damage
• The Jakarta Incident – would a plane
avoid crashing into volcanic mountains
The story of British Airways Flight 9 on
route from Malaysia to Australia in June
1982 is instructive.

18
Imagine being aboard a commercial jet
and hearing this: “Ladies and gentlemen,
this is your captain speaking. We have a
small problem. All four engines have
stopped. We are doing our damnedest to
get them going again. I trust you are not in
too much distress.”
Car engines can be also be subject to
mechanical failure, as with jet engines.
• A global problem
In 1815, Tambora, a volcano in
Indonesia, erupted killed 10,000 people
near the volcano, and 80,000 people in
Europe and North America (10,000
miles away).
How?
19
Ash explosions

20
For example, the ash explosion in
Martinique in 1902.
The pyroclastic flow travelled down the
mountain at about 100 miles per hour and at
about 800-degree Fahrenheit.
It killed all but a few of the 30,000 people in
the nearby town by scalding them to death.
Here are before and after views of a street in
that town:
Other than evacuating the area when
eruptions are expected, there is little that can
be done to mitigate this hazard. There are no
good defenses against a red-hot cloud of ash
and gas travelling at you at 100 miles an
hour or more.
21
III. Damage from volcanic gases
• Mostly water vapor and carbon dioxide
• But some gases are toxic, and some also
caustic.

• Even a non-toxic, non-caustic gas can be
deadly.
• A 1986 incident near Lake Nyos, in
Cameroon, Africa illustrates the danger.
22
• Mitigation includes
1) Warning systems
2) protective equipment
3) pressure release
IV. Volcanic mudflows (lahars)
• Rapid downslope movement of muddy water
made up of pyroclastic material and water.
• Composite volcanoes have all the right
conditions to create mudflows.
23

• In 1985, a small volcanic eruption of a volcano
in Colombia melted part of its snowcap. That
created a mudflow that that killed about
20,000 people.
• Mitigation includes
1) Zoning laws
2) Warning systems
3) Sediment traps
24
Predicting volcanic eruptions
• Unlike with earthquakes, volcanic eruptions can
be predicted very well.
• The prediction timescale allows for mitigation.
Advantages of living near a volcano
• Agriculture
• Geothermal energy

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