Transcript
Transcript: What is Geology? - Part 1
Slide 1: What is Geology? Part 1
Welcome to Geology 11040 – How the Earth Works – an introductory Physical Geology
course offered at Kent State University.
Slide 2: What is Geology?
So what is Geology? Well ‘logy’ means the ‘the study of’ and Geo is the Earth, so
Geology is the study of the Earth. Perhaps you took a Geology course in high school –
but if so, it was more likely called Earth Science – which is the same thing, and I will use
both terms interchangeably throughout this course. Now Geology is a natural science,
and a fairly young science compared to most other sciences including Biology,
Chemistry, or Physics. So what do Geologists or Earth Scientists study? First, they are
interested in what Earth is made of – its materials. Mostly solid materials like minerals,
rocks, and fossils, but also liquid materials like magma, lava, surface water, subsurface
fluids (oil and gas), and even vapors or gases such as those released during volcanic
eruptions or released by the burning of fossil fuels. Second, Geologists are interested in
Earth processes, such as the processes by which rocks, mountains, oceans, and even
continents form and the processes by which rocks, mountains, oceans, and continents
are destroyed. Thirdly, Earth Scientists are very interested in the history of Earth
including the history of life on Earth. In fact many of the time periods and eras are
defined in what dominated life at the time. We won’t be going into too much about the
history of Earth in this course since our historical geology course Earth and Life through
Time is devoted to that topic. In this physical geology course you’ll be learning primarily
about earth materials and processes.
Slide 3: Why is Geology Important?
Never has Earth Sciences been as important or as practical as it is in today’s world. And
that’s largely because of global population. Earth processes have a huge impact on our
environment, but also with so many people our modern societies today are using Earth’s
natural resources to a degree never seen before and in doing so humans are having a
huge impact on the Earth’s environment as well. Essentially humans are now agents of
geologic change.
Let’s look at one dramatic impact humans have had on earth’s most important resource,
water. The Aral Sea, in western Asia, was once the 4th largest lake in the world. In the
1960s, the Soviet Union undertook major water diversion projects on nearby rivers,
capturing water that once fed into the Aral Sea for irrigation purposes. Now this worked
great for crop production in the surrounding area but it was a disaster for the natural
freshwater lake. These images show the shrinking of the Aral Sea from 1977 on the left
to 1989 in the middle and to 2006 on the right. Today the Aral Sea is virtually gone.
Wow…and it only took humans about 4 to 5 decades to do this.
Slide 4: It’s Relevance Today…Soil Degradation
Soil is another critical natural geologic resource that humans are using like never before.
We’ll go over how soils form later in the course but it’s important to realize that a welldeveloped
soil can take a few hundred years to form, and if humans degrade the
existing soil faster than it can be replenished naturally, well then we have a problem.
This map uses colors to visualize the state of soil on earth with the warm or hot color
red showing the location of very degraded soil, orange showing somewhat degraded
soils, and the cream colored regions showing stable soils. The United Nations has
determined that 10s of millions of acres of cropland are lost annually to soil degradation
especially in Africa, Latin America, and Asia. Even in the U.S., our soil is being eroded at
a rate 10 times greater than it can be replenished! Wow.
Slide 5: Course Themes
As we proceed through this course, there will be a number of recurring themes that will
crop up within the major topics we cover. One is that Earth is constantly changing and
its components (its interior, its land surface, oceans, atmosphere, and life forms)
interact with one another and evolve. We call this interconnected web of interlinking
components the Earth System. Another theme is that internal and external processes
drive geologic change – internal processes (represented by earthquakes, volcanoes, and
mountains) are driven by the earth’s internal heat. External processes (represented by
weathering, erosion, sediment transport by rivers and humans) are driven by heat from
the sun and by gravity. A third recurring theme is that Plate Tectonics explains many
Earth processes – which is why the first topic in this course is entirely devoted to Plate
Tectonics. Another emphasis in this course is the Geologic processes and phenomena
affect our environment and society. Natural hazards are dramatically increasing and
natural resources are vitally important to society. People really need to know about
these earth-based hazards and resources. Lastly, science stems from observations and
our measurements of these observations. So we will use lots of visuals and talk a lot
about societal impacts in this course.
Slide 6: Earth System Science
I want to emphasize the point of viewing the Earth as a System. When I was learning
about Geology as a student some time ago, I learned primarily about the solid
components of Earth –Geosphere – primarily Earth Materials, Earth’s crust and
underlying mantle, Earth’s tectonic plates, etc. Now students of Geology understand our
changing Earth as consisting of four realms or components all influencing and
interacting with one another. In addition to the Geosphere there’s the atmosphere, the
mixture of gases that surround Earth; the hydrosphere, all of the water on Earth
including the oceans, rivers, lakes, but also including water in the subsurface and in the
atmosphere; and lastly the biosphere, the realm of all living organisms, including
mankind importantly. Within the Earth System certain materials cycle among rock, sea,
air, and living organisms and this cycling and interaction of the different spheres or
worlds is energized by the Sun, by Earth’s internal heat, and by the force of gravity.
Slide 7: Plate Tectonics
We will begin the course on the topic of Plate Tectonics, the theory that the outer layer
of Earth consists of rigid moving plates. Plate Tectonics is one of the top 10 scientific
discoveries of the 20th century and it serves as the foundation for understanding most
geologic phenomena. This image outlines in black the boundaries of the large plates that
constitute the rigid outer shell of earth. And it shows the location of active volcanoes on
earth with red dots. We see that the majority (about 80%) of earth’s 1900 active
volcanoes occur around the outer rim of the Pacific. This Pacific “Ring of Fire” is a direct
result of active Plate Tectonics.
Slide 8: Earth Materials and Riches in Rocks
Another topic we will cover is the formation and characteristics of rocks and the mineral
and rock resources that come from them. Most of the resources that humans use come
from geologic materials and people living in developed societies use these resources
extensively – on average every American born will use almost 3 million pounds of
minerals, metals, and fuels in their lifetime. Earth’s resources are not infinite and some
countries have already run out of some of these economically important resources. We
will learn about the processes by which rocks are formed and the conditions which
concentrate valuable resources, including minerals and natural ores.
Slide 9: Natural Hazards: When Nature Strikes!
Many geologic phenomena affect society, often in catastrophic and devastating ways. In
this topic on Geologic Hazards, we will learn about the hazards of volcanoes,
earthquakes, and landslides, why these hazards are increasing, and about what can be
done to mitigate their impacts. This is an aerial photo of the Oso landslide (seen in the
upper center) in Washington state. In March 2014 the landslide catastrophically
engulfed 49 homes, killing 43 people, and dammed the river, causing extensive flooding
upstream. Excluding landslides caused by volcanic eruptions, earthquakes or dam
collapses, the Oso slide is the deadliest single landslide event in the United States
history.
Slide 10: Geology of Water
Freshwater is earth’s most important natural resource. This topic covers where
freshwater occurs, how freshwater sculpts and shapes landscapes and transports earth
materials from the continental interiors and mountains to the margins of the oceans.
Especially important is how humans extract and use freshwater and how people have
created a global water crisis through the overuse and misuse of water resources. For
example, Lake Erie has been experiencing harmful algal blooms that are toxic and
contaminating an important freshwater resource. The increase in harmful algal blooms
is being studied by scientists who have determined that the heavy use of pesticides and
fertilizers in Ohio and other places is at least part of the reason why there are more
blooms.
Slide 11: Geology of Energy and Environmental Impacts
There has been a dramatic increase in use of and demand for energy resources with
industrialization and global population growth. This topic covers the nature and origin of
the most widely used energy sources (oil, gas and coal, the fossil fuels), the discovery
and search for fossil fuels, and the methods and environmental impacts of extracting
and burning these energy sources.
Slide 12: Climate Change
No doubt you have heard and read a lot about climate change in the present. But what
was climate like in the geologic past and how do Earth Scientists recognize what climate
was like just one-two million years ago during the ice ages but hundreds of millions of
years ago when Earth may have been completely immersed in ice? And what earth
processes might have been responsible for past climates and how and why is climate
changing today?
Transcript: What is Geology? - Part 2
Slide 1: What is Geology? - Part 2
Welcome back to the Introduction to Geology 11040 How the Earth Works. Questions are one
really good way to increase our understanding of a subject and to promote interest. So for fun
think about a few no-stakes (non-graded) questions on your own.
Slide 2: Answer these Questions
Here are the questions. The age of the Earth is a) 4600 years, b) 46000 years, c) 46 million
years, d) 4.6 billion years, and e) 46 billion years. Rocks can be folded: True or False? Continents
are older than oceans: True or False? The Rocky Mountains are older than the Appalachian
Mountains: True or False? Ice is a mineral: True or False? And lastly, the largest historical
earthquake in the lower 48 states occurred in a) California, b) Oregon, c) Missouri, d) Ohio.
Now the answers: The age of the Earth is d) 4.6 billion years. The reason I start with this
question in this course is to emphasize that the Earth is really old, so old in fact that it’s hard for
humans to fathom the vast amount of geologic time that encompasses Earth history. In fact
scientists give a name for the past 4.6 billion years –Deep Time, a topic we will come back to
soon. Another reason I start with this question is that it allows me to follow up with another
question – namely how do we know the Earth is 4.6 billion years old? Interestingly, there are no
known rocks or minerals on earth that are 4.6 billion years old; the oldest radiometrically dated
rocks and minerals are about 4 billion years old and there are very few of those. Most rocks are
much, much younger. We don’t have the time or the background to go into how we know the
Earth’s age right now, but my point is this: “it’s not so much what you know, but how you know
it.”
Slide 3: Yes, Rocks can be Folded
Here’s a beautiful photo that proves that yes, rocks can be folded. Rock folds are some of the
most spectacular features of Earth structures. They are common features in many belts and
they occur at all scales – I have hand size samples of rocks in my office that have beautiful folds.
There are larger size folds that are about the size of road cuts, and there are very large folds
that are the size of mountains. I ask this question to show you how answers to questions in
Geology often come from simple observations of the rock record. I also ask it to get you
thinking about how folds form and the tremendous forces that must be involved to cause rocks
to fold.
Slide 4: Yes, Continents are older than the Oceans
Ok, which do you think is older; continents or oceans? Continents are indeed older than ocean
basins – in fact they are much older. For instance, the North American continent started
forming about 2 billion years ago and was largely complete 1 billion years ago. In comparison,
the Atlantic Ocean that borders North America to the east started forming less than 200 million
years ago. I ask this question to get you thinking about why there are oceans and continents,
and to stress that they are fundamentally different geologic features. Oceans are born and die
all the time, but when continents form, they tend to stick around.
This map of the U.S. is also helpful in answering which mountain chain is older – the
Appalachians on the eastern side of the U.S. or the Rocky Mountains to the west on the other
side of the Great Plains? The answer lies in how tall the mountains are – note how the Rocky
Mountains stand tall compared to the Appalachian Mountains to the east. Older mountains
have had lots of time to be eroded, so they tend to be worn down and have a more subdued
topography. In contrast, younger mountains tend to be higher and can often have dramatic
topography.
Slide 5: Grand Tetons, Wyoming
Let’s take a look at some mountains out west. Here’s a favorite photo of mine of Geology
majors attending our summer field course in the Rocky Mountains – great looking bunch of
people I might add – and behind them are the spectacular Grand Tetons, which are majestic
mountains in Wyoming that are only about 2 million years old. If you get a chance to go west, I
encourage you to visit the Grand Teton National Park – you won’t be disappointed. It’s one of
my favorite places!
Slide 6: Ice is by Definition a Mineral
Many students are surprised to learn that ice is indeed a mineral– so these beautiful glaciers
could be considered flowing rivers of rock, all made of a single mineral – ice. Now everyone
knows that ice melts and becomes water. So is water a mineral? Actually no, because by
definition minerals are solid. So what about mineral water? Heehee. Mineral water is water
that is obtained from mineral springs and that contains dissolved solids and gases.
OK, so what about ice cubes? Are they minerals? Actually, no! Because ice cubes are not
naturally occurring, and a mineral by definition must also be naturally occurring. Anyway, we’ll
talk more about ice and water in this course as both are important components of the Earth’s
System.
Slide 7: U.S. Seismic Hazard Map
Here’s a map of seismic hazard in the United States. The hotter red and orange colors represent
regions where there is very high potential of earthquakes and hence high seismic hazard risk,
and the cooler colors, the blues and greys, are areas of few or no earthquakes historically and
therefore low seismic hazard. Very likely you are well aware of the high seismic hazard along
the entire west coast (shown in the red), but you may be surprised to see that there is also high
seismic hazard potential in the great plains area along what is known as the New Madrid
seismic zone (the bulls-eye pattern in the interior). It turns out the historically largest
earthquake (2 earthquakes actually) in the lower 48 states actually occurred in Missouri in 1811
and 1812, in the interior of the North American plate. So although most earthquakes do occur
along the edges of geologic plates, some significant earthquakes have occurred in the middle of
plates and we will talk about why that is when we cover geologic hazards. Another interesting
point about earthquakes is that humans are now causing earthquakes by the process of
removing gas from the subsurface via fracking.
Slide 8: Geology is the study of Earth Materials, Processes and History
Geology, the study of Earth, has changed a lot since I was a student learning about Earth
Science. The older view of Geology focused on identifying Earth materials (rocks, minerals,
fossils), boots on the ground mapping of the earth’s surface, and development of the Plate
Tectonic theory as the over-riding paradigm for understanding our planet.
Modern views of Earth Sciences focuses much more on human environmental impacts;
recognizing that people are a major force on earth, causing significant environmental changes
and dramatically changing Earth’s landscape. Energy and mineral resource extraction and usage
are exceedingly important to societies. And Earth is studied today by remote sensing
techniques (satellites, radar, infrared cameras, etc.) and is viewed as an interconnected system
(the Earth System).
Slide 9: Mapping the Invisible
Here’s a great example of using remote sensing to document an environmental problem that
occurred near Los Angeles. In 2015 a leak was discovered in a natural gas storage facility near
Los Angeles. The gas leak was invisible and no one new how extensive it was, although
residents in a nearby neighborhood were experiencing significant health problems and many
residents had to be evacuated.
Slide 10: Methane
The center photo of these 3 photos shows the Aliso Canyon area during the gas leak. Obviously
it looks very normal. However, the images on the left and right were taken by infrared cameras
which reveal a huge cloud of methane gas. The purple stuff on the left and the orange cloud
and vent on the right. The image on the right clearly shows the gas pouring out of the ground
like an erupting volcano – you could view this essentially as a human made methane volcano.
These types of remote sensing images enabled scientists to measure the amount of methane
being released and to understand the magnitude of the problem. The Governor of California
declared a state of emergency for this area and sometime later the leak was capped, but not
before a tremendous amount of methane was released into the atmosphere. Methane is a very
potent Greenhouse Gas, nearly 100 times, or two orders of magnitude, more potent than
carbon dioxide. The methane released from this one storage facility was thought to be about
equivalent to greenhouse gases released by 7 million cars driven for an entire year.
Slide 11: Geologic Processes take place across Vast Scales of Space and Time
Geologic materials exist at many different scales of space from the size of sand grains (and
smaller) to the size of mountains and tectonic plates. Likewise geologic processes occur across
vast scales of time – on the order of seconds and minutes for earthquakes to millions of years
for the construction of mountains. One way to think about different scales of space or size is to
think about size variations by multiples or factors of 10, which scientists also refer to as an
order of magnitude. Now humans are about 1-2 meters in length (a meter is 3 feet). Something
that’s about 10 times our length might be a 3 story apartment building – that would be a one
order of magnitude different or 101
. A very tall structure, say the Eiffel tower, would be closer
to 100 times our length or two orders of magnitude or 102
. What about Mount Rushmore, the
giant sculpture of Presidents carved into granite in the Black Hills of South Dakota? We would
say that’s roughly 1000 times larger than us, or 3 orders of magnitude larger, or 103
. This image
nicely depicts scales of space using multiples of 10 from a carbon atom (10-10 meters across) to
the size of the universe (1026 meters across). In this physical geology course we will refer to
scales about as small as coffee beans (10-2 meters or 100 times smaller than us) up to the size of
earth which is on the order of 107 meters across.
Slide 12: Grasping 4.6 billion years
Earth is not only large, but it also extends almost unimaginably far back in time. So how can we
fathom or comprehend 4. 6 billion years of time? Again, let’s go backwards in time using factors
of 10 years. Most of us have no problem thinking in timescales of 1 year or 101
years ago, but
what was happening say about 100 years ago, or 102
years ago? The first cars were being mass
produced. 1000 years ago? That was the Middle Ages and people thought that the sun and
planets orbited earth. About 10,000 or 104
years ago was the dawn of civilizations. The rise of
Homo Sapiens occurred on the order of 100,000 or 105
years ago, and early hominids first
appeared about one million or 106
years before that. 10 million years ago was the Age of
Mammals, 100 million years ago was the Age of Dinosaurs, and 1,000 million years ago (1 billion
years) only single celled organisms existed. And Earth’s history started 4.6 billion years ago
which is Earth’s birthday – woohoo!
Slide 13: Another view of Earth’s History
Another way to view Deep Time (the last 4.6 billion years) is to compress it into a single
calendar year – which is a manageable amount of time for us. So Earth’s birthday would be
January 1, the oldest dated rocks would be about mid-February, single cell organisms would
come on the scene in early March, but the first fossils of animals with hard parts wouldn’t
arrive until middle October. And the first dinosaurs, which lived on the order of 1-200 millions
of years ago, would not arrive until December 11 and they would disappear December 26. The
first modern humans would come on the scene only 23 minutes before midnight, the last day of
the year.
Slide 14: Geologic History
Geologic history is often depicted with a vertical geologic timescale with the top of the scale
being the present and time going further and further backwards going down the scale. This is a
simple geologic timescale that highlights the most recent 542 million years. These 542 million
years are divided into three parts, the Paleozoic Era where ‘Paleo’ means ancient and ‘zoic’
means life. This 200 million year era was when hard-shelled organisms evolved, and also fishes
and plants. The Mesozoic or middle life period was another approximately 200 million years
from 250 to 65 million years ago when dinosaurs wandered earth. And this is the Age of
Dinosaurs. The extinction of the dinosaurs 65 million years ago was the start of the Cenozoic
Era, the Age of Mammals. I know that’s a lot of information to take in about geologic time, but
hopefully it didn’t just go into one Era and out the other Era.
Note how the lowest segment is the Precambrian – which is the first 4 billion or 7/8 of geologic
time, lumped into one word called the “Precambrian.” That doesn’t seem quite right but
geologists tend to know much more about the more recent 500 million years of Earth history
than we do about the Precambrian, so maybe that’s OK.
Slide 15: Oceans and Continents
Geology is an outside science – it’s the study of our physical world and therefore it’s helpful to
know very broadly about the primary geographical features of Earth – namely the oceans and
the continents and where some of the major geologic features are located – like mountains.
Now I know that everyone is familiar with the names and locations of the four oceans, the
Pacific, Atlantic, Indian and Arctic Ocean at the top of the world. And the continents Australia,
Asia, Africa, Europe, North and South America and Antarctica at the bottom of the world. Now
can you locate these geologic features, the Rocky Mountains, Denali, the Alps, the Himalaya
Mountains, the Great Rift Valley, and the Andes Mountains? We’ve already talked about the
Rocky Mountains, but while you are thinking about the others, you should know that Denali is
the new name for what was formerly called Mt. McKinley, originally named after President
McKinley from Ohio.
Slide 16: Mountain Locations
Okay, here are the locations of those geologic features. Denali is in Alaska and is the highest
point in North America. The Rockies (RM) are in the western US (we knew that already). The
Andes Mountains (AM) are the long mountain chain on the western side of South America. The
Alps are young spectacular mountains in southern Europe. The Himalaya Mountains (HM) are
the huge mountains in Asia formed by the collision of India into Asia about 45 million years ago
(and still growing today). And lastly, the Great Rift Valley (GRV) is in the northeastern part of
Africa and it represents a place where the African continent is breaking apart. Okay, very good.
Slide 17: Why does the Ocean Roar?
I’d like to end this Introduction to How the Earth Works with one of my favorite Geology jokes.
Why does the Ocean roar? Well, you would roar too if you had crabs all over your bottom!
Hope you enjoy the course.
Transcript: Intro to Plate Tectonics – Part 2
Slide 1: Continental Drift Hypothesis
In the early 1900’s what evidence did Wegener present to the community of scientists and in
his 1915 book for the existence of Pangea and its breakup by continental drift, the slow
movement of continents over time?
Slide 2: Wegener’s used Fit of Continents too
Wegener noted the jigsaw puzzle fit of the continents as others had before him, but with a
twist. Instead of using the coastlines of the continents, Wegener realized that the true edge of
continents actually extend out a ways from the coastline, to where the underwater continental
shelves start to steepen or slope down into the ocean. By including the continental shelves as
the drowned true edge of continents, and as shown in the cyan color bordering the continents
of this map, Wegener was able to show that the continents fit together even better, with fewer
gaps between the puzzle pieces.
Slide 3: Glacial Till
As an Arctic climate scientist who worked extensively on ice sheets in Greenland, Wegener
knew that glaciers are rivers of ice that flow across the land surface. He recognized that glaciers
carry rock debris that gets laid down or deposited on the Earth’s surface when the glaciers melt.
The rock debris left behind is typically very chaotic with a mixture of different rock fragments
and of different sizes (a jumbled mess essentially) called glacial till. The jumbled, mixed nature
of the different fragments, such as you can see in this photo, are evidence that this rock from
South Africa was deposited by a glacier.
Slide 4: Wegener used Glacial Evidence
Wegener recognized and mapped 260 million year old Paleozoic glacial till deposits in parts of
South America, Africa, southern Australia, and even in tropical India, places today that are
much too warm to harbor glaciers! He also recognized glacial grooves and scratches on rocks
(called striations) that were carved by rocks embedded in the base of the glacier as it was
moving. And he noted that on a map these striations were oriented so that they appeared to be
radiating outward from a location in southern Africa.
Slide 5: Glacial Deposits and Striations fit together near the South Pole in
Wegener’s Pangea
Wegener hypothesized that these ancient glacial features, which are now widely separated
from one another, and even extend to north of the equator today, could have been a single ice
sheet located near the South Pole at the end of the Paleozoic (around 250 to 260 million years
ago).
Slide 6: Wegener Used Climatic Evidence
As a meteorologist, Wegener recognized that Earth has climate zones in which different land
and marine deposits and life forms exist. Coal deposits and marine limestone reefs form in
warm tropical deposits, for instance, whereas salt deposits and sand dunes form in hot and dry
climates. Since his Pangea supercontinent extended from the South Pole to the equatorial
regions, Wegener expected to find different rock types within different climate belts across
Pangea – and that’s what he found. For instance, he found coal and limestone deposits that
would be predicted to form in the tropics of Pangea shown here in the green.
Slide 7: Wegener used Fossil Evidence
Wegener also used fossil evidence to connect continents that are now widely separated. Since
land animals and plants cannot cross oceans, they evolve independently on different continents.
But when continents are together, land animals and plants can migrate across them. By
examining the late Paleozoic and early Mesozoic fossils, Wegener was able to show that certain
species had indeed lived on several continents, including species of ferns and reptiles as shown
in this reconstruction.
Slide 8: Wegener matched Geology Across Oceans
Finally, Wegener examined geologic maps and recognized that rock types, structures, and ages
matched up across continents and fit together very well in the absence of the Atlantic Ocean.
The left image shows how the geologic core of South America and Africa match up when they
are adjacent. And the right image shows how a single mountain belt exists when Africa and
North America are in contact with one another.
Slide 9: Pangea (~250 my ago) = “All Land”
On the left is a simple cartoon image of Wegener’s proposed Pangea Supercontinent
reconstruction annotated with the names of the continents. And on the right is an amazing
more realistic image of what Wegener’s supercontinent-superocean world may have looked
like around 250 million years ago. So I have question – do you think Wegener makes a strong
argument for moving continents and the existence Pangea?
Slide 10: Criticism of Continental Drift Hypothesis
After publishing his book and when presenting his idea that continents move slowly or drift to
the scientific community, Wegener’s proposal was ridiculed and rejected by nearly everyone.
He convinced essentially no one at the time and although he kept looking for further evidence,
he ultimately died in 1930 at the age of 40 on an ice sheet in Greenland when he was on a
supply expedition. And since he was the sole advocate for continental drift, the drift hypothesis
‘drifted away’ and died with him.
Now why couldn’t Wegener convince anyone of continental drift? Some people point out that
being a German at the time probably didn’t help him (during and after World War I); others
argue that being a meteorologist also probably didn’t help him either since he was not trained
and educated as a Geologist. But certainly the most important reason however, was that
Wegener couldn’t explain how continental crust could drift or plow through oceanic crust. He
lacked a mechanism for continental drift.
Slide 11: How do continents plow through oceans?
In his writings and at conferences Wegener wasn’t able to provide a mechanism or process by
which the continents could move and he couldn’t explain the great forces that would be
needed to move huge masses of continents.
Slide 12: New Evidence after WWII
The argument for continental drift disappeared and was largely forgotten about until after
World War II when new evidence came to light largely driven by renewed political and scientific
interest in the oceans. Remember, Wegener used only land based evidence. The oceans and
the Earth’s magnetic field were two other key elements in the Plate Tectonic story.
Transcript: Paleomagnetism – Part 1
Slide 1: Paleomagnetism
The idea of continental drift, the notion that large masses of continents are somehow drifting
through oceans of basalt, died with Wegener in 1930, but was inadvertently rediscovered after
World War II in the 1950s with renewed interest in studying rock magnetism and especially the
record of ancient magnetism preserved in rocks. In order to understand Paleo-magnetism, we
first have to understand the Earth’s magnetic field.
Slide 2: Aurora Borealis from Space
How do we know the Earth has a magnetic field? One way we know is the existence of the
amazing northern and southern lights – the Aurora Borealis and Aurora Australis. This is a
beautiful photo of the Aurora Borealis taken from space. These are the lights seen above the
magnetic poles of the northern hemisphere.
Slide 3: Aurora from Earth
The lights are even more amazing to see from the ground, they are these amazing green lights
that dance around the sky. They exist because a magnetic field permeates the space around
Earth. The dancing lights are actually collisions between electrically charged particles from the
sun that enter the Earth's atmosphere. These charged particles flow toward Earth’s magnetic
poles and cause gases in the atmosphere to glow. Of course the other way we know Earth has a
magnetic field is because we can use compasses, which point toward the magnetic North Pole.
Slide 4: Iron Filings surround a Bar Magnet
So what does the magnetic field look like? Now you may have done this interesting experiment
in a secondary school science class. If you take a bar magnet and place it on a paper thinly
coated with randomly oriented iron filings, the filings will move and orient themselves into the
direction shown here, and what they are doing is aligning themselves parallel to the magnetic
force field that surrounds the bar magnet. So in essence this allows us to see the shape of the
normally invisible magnetic field.
Now many rocks have tiny magnetic minerals in them, magnetite being perhaps the best known
magnetic mineral. And these magnetic minerals in rocks align to Earth’s magnetic field just like
those iron filings aligned to the magnetic field of the bar magnet.
Slide 5: The Earth’s Magnetic Field
Earth is essentially a huge dipole magnet – meaning it has force field lines that go around the
Earth and enter and exit Earth at the magnetic poles. You can see how the shape of the Earth’s
magnetic field is thought to be pretty much the same as the magnetic field surrounding a bar
magnet – which is also dipolar, consisting of two poles.
Slide 6: Why Does Earth have a magnetic field?
So why does Earth have a magnetic field? It turns out that internally Earth is layered, kinda like
a layer cake, and that it has a very dense iron rich core, part of which is liquid iron, that’s the
outer part of the core. The motion of earth spinning through space causes circulation of the
outer liquid iron core, which generates a magnet field that surrounds and shields Earth from
dangerous solar winds.
Just as a sidebar, this graphic nicely shows how Earth is layered, with its outermost layering
being atmosphere, the oceans and ice caps forming the hydrosphere on the surface beneath
the atmosphere, the thin outer shell of granitic continental and basaltic oceanic crust below
that, which is underlain by a very thick and dense layer of mantle, which in turn surrounds the
densest iron core. Each layer I just described is more dense or heavier than the previous layers,
so we know that Earth is layered by density.
Slide 7: Geographic and Magnetic Poles do not Coincide
Now the geographic pole, the location where the Earth’s rotational axis intersects Earth’s
surface, does not coincide exactly with the magnetic pole. Today, the magnetic pole is several
hundred miles away from the geographic pole and measurements over time show that the
magnetic pole moves around a bit, but it’s never too far from the geographic pole. Since
compasses point toward magnetic north, people need to correct their compasses to point
toward true north. The angle between the magnetic pole and the geographic pole is known as
the magnetic declination, which varies depending on where you are on Earth. If you go to
magnetic-declination.com you will see that its 8 and 1/2 degrees west in Kent, whereas in
Duluth Minnesota, where I grew up, its only about 1 degree west. So when Minnesotans go
hiking in the northwoods, they don’t really need to worry about magnetic declination since the
angular difference is so small, but here in northeast Ohio the magnetic declination is quite
significant!
Slide 8: Curved Magnetic Field Lines
If we look again at Earth’s magnetic field we see that the field lines are curved – that is they are
about parallel to the Earth’s surface near the equator and then start to tilt downward toward
the surface moving away from the equator. Since your compass needle wants to align itself
with the magnetic field lines, this curving of the magnetic field causes your compass needle to
tilt – and this tilt is the magnetic inclination. Now practically compass needles, like the Brunton
Compass, are weighted so that they can’t tilt, because the needle needs to be kept horizontal
so that it can spin freely.
Slide 9: Magnetic Inclination
This image shows how the tilt of a compass needle would change moving from the equator to
the North Pole if it weren’t artificially weighted, that is held horizontal. The inset circle in the
lower right shows that inclination is the angle of tilt down from the horizontal. Looking at the
Earth’s surface near the equator, the tilt or inclination of the needle is zero as the field lines
there are parallel to Earth’s surface near the equator. In contrast, at the North Pole, the
inclination is 90 degrees or vertical, as the needle would be perpendicular to the Earth’s
surface, just as the magnetic field lines are oriented. And in-between the equator and the pole,
the inclination steepens gradually going from the equator to the pole.
The take-home message about magnetic inclination is that it varies from zero (or no tilt) at the
equator and increases with increasing latitude toward the polar regions, until it’s very steep or
even vertical (90 degrees). When certain rocks form, they freeze in the magnetic inclination at
the location where the rock formed – so rocks with low magnetic inclination must have formed
near the equator, and rocks with high or steep inclination must have formed at high latitudes
near the poles.
Transcript: Paleomagnetism – Part 2
Slide 1: Paleomagnetism: Ancient Magnetism Preserved in Rocks
Welcome back! There are two aspects about Paleomagnetism, the study of ancient
magnetism preserved in certain rock types. The first is that rocks have a magnetic
inclination, or tilt, that is related to the latitude where the rock formed. And the second,
which we will cover today, is that rocks record regular reversals of the magnetic field,
that is the flipping of the N and S magnetic poles in the past. So what type of rocks
preserves a record of the Earth’s magnetic field when it forms?
Slide 2: Recent Lava Flows at Hawaii Volcanoes National Park
Lava is liquid or molten melt, from the subsurface, that flows onto the land surface or
the ocean floor, cools quickly, and crystallizes into volcanic rock. As lava cools like this
one in Hawaii, magnetic crystals that form within the melt align themselves with the
magnetic field much like iron filings do when placed next to a bar magnet. Active lava
flows today are in a sense ‘freezing-in’ magnetic inclinations that will be permanently
recorded in the volcanic rock. So here’s a question, what do you expect would be the
ballpark magnetic inclination angle for Hawaiian basalt flow like these? Would you
expect a low, medium, or steep magnetic inclination to be preserved? The right answer
would be a low inclination since Hawaii is located in the tropics at low latitudes.
Slide 3: The Magnetic Memory of Rocks
This photo shows a stack or sequence of multiple basalt flows that erupted millions of
years ago in Washington State. In sequences like this, where each layer flowed onto the
surface sequentially, we know that the oldest visible flow would be at the base of the
sequence, at about lake level, and that younger flows formed on top of older flows in
sequence with the youngest visible flow seen at the top of the cliff and along the flat
plateau extending away from the cliff in the background. Now Washington State is at an
intermediate latitude today, but the magnetic minerals in these ancient basalt flows do
not align with the Earth’s current magnetic field – they aligned to the field that existed
when the rock originally cooled from lava millions of years ago. In a sense, rocks have a
magnetic memory from when they initially formed, from their birthdays.
Slide 4: Basalt Flow, Grand Coulee, WA
Let’s take a look at another sequence of basalt flows in a cliff near Grand Coulee,
Washington. And let’s say, just for funsies, that paleomagnetic scientists, I sometimes
call them paleo-magicians, measured the magnetic inclination in three flows, the highest
one, one in the middle of the sequence and one near the bottom of the cliff. And let’s
say the highest flow has a very gentle magnetic inclination – a tilt of only 5 degrees. The
middle flow, however, has somewhat steeper inclination of about 30 degrees and the
lowest flow has the steepest inclination, say 60 degrees. Since these flows are all from
the same geographic location today, why are their magnetic inclinations different? The
answer is that the latitude of these rocks changed as they were erupted over time. The
lowest oldest flow formed when North America was at higher latitudes, and over time,
the continent moved southward toward the equator, as suggested by the decreasing
magnetic inclination. And since, eruption of the highest, youngest flow, this part of the
continent has migrated northward to its present location at an intermediate latitude.
Slide 5: Which moves – the Continents or the Magnetic Pole?
Studies of magnetic inclinations in ancient volcanic rocks in the 1950’s revealed that
many flows preserved inclinations that were very different from the expected
inclination based on their current distance from the equator (or the latitude). Geologists
realized that these unexpected inclinations could be explained if the continents had
moved since the volcanic flow initially formed millions of years ago – and hence these
studies inadvertently resurrected Wegener’s hypothesis of Continental Drift. Now it had
been 25 years since Alfred Wegener died and about 40 years since his book about
Pangea and Continental Drift was published. Many of these young scientists had never
even heard of Wegener, so you can imagine their surprise when they rediscovered his
idea of Continental Drift.
As a short aside, the paleomagnetic results generated a debate about what actually
moves – the continents or the magnetic poles? These diagrams show how the new
results can be explained by a) assuming a fixed pole and a drifting continent or by b)
assuming a fixed continent and a wandering pole. Suffice it to say that with further
study, the scientists learned that the magnetic poles move only slightly and that it’s the
continents that have somehow drifted substantially.
Slide 6: Studies of Magnetic Inclination
So it was studies of magnetic inclination in volcanic rocks in the 1950’s that resurrected
Wegener’s hypothesis of Continental Drift, although scientists still did not know how
they could drift. More information or data was needed and that would come, in part,
from learning about the second key aspect of Earth’s magnetic field, that it changes
polarity or reverses itself fairly regularly.
Slide 7: Normal and Reversed Polarity
Normal polarity of Earth today is represented by a dipole magnet with an arrow pointing
toward the South Pole with force field lines exiting the South Pole and entering at the
North Pole (as shown on the left). During reverse polarity the arrows point in the
opposite directions (exiting the North Pole and entering the South Pole shown on the
right). All volcanic rocks formed in the last 700,000 years or so record normal polarity
like we have today. However, volcanic rocks formed between 700,000 years and about
1.2 million years or so show reverse polarity.
Slide 8: Volcanic Rock Layers Permanently Record Magnetic Reversals
This sketch of a volcano shows the different volcanic layers that erupted onto the
surface as the volcano grew. The purple lines represent the magnetic field lines at the
volcanoes’ location on Earth. The angle of tilt of these lines represents the magnetic
inclination angle (shown here as moderately steep) and the arrowheads on these lines
reflect periods of normal polarity and reverse polarity (with arrows pointing in the
opposite direction). We see that the youngest layer on top, which is 0.4 million years old
or m.y. (or 400,000 years old) has normal polarity, but the 800,000 year old volcanic
rock below it has a reverse polarity. We therefore know that there must have been a
polarity flip or reversal between the eruption of these two rock layers. And we know
from the third and oldest sample that there must have been another reversal again
between 1.2 million years and 800,000 years.
Slide 9: Magnetic Reversal Timescale
By dating easily accessible piles of layered volcanic rocks on continents like shown in this
drawing, and measuring their polarity, the paleomagicians were able to reconstruct a
timescale of all of the magnetic reversals that have occurred in the past several hundred
million years of Earth history. This drawing shows the intervals of normal (in grey) and
reverse (in white) polarity extending back to about 2.3 million years ago.
Slide 10: Normal, Reverse
Looking again at the pile of volcanic flows from Grand Coulee, Washington, we can now
add the additional information of normal and reverse polarity in this volcanic sequence,
with N being the abbreviation for normal polarity and R being the abbreviation for
Reverse polarity. The discovery that the magnetic poles have flipped positions fairly
regularly in the geologic past was a key element in recognizing that it was not just the
continents that moved but that oceanic crust also moved as I’ll explain later.
Slide 11: Marine Geology
Prior to World War II, we knew very little about the ocean floor and about the oceanic
crust other than it was made of primarily basaltic volcanic rocks. During World War II,
navy battles in the South Pacific and the presence of German U-boats off the east coast
of the United States, resulted in renewed political and scientific interest in the oceans.
Use of submarines in the future would require knowledge of the variation of ocean floor
depths (or ocean topography) and this and other information from the oceans was key
for discovering plate tectonics.
Transcript: The Ocean Floor – Part 1
Slide 1: Observations of the Ocean Floor
What would the ocean floor look like without water? Prior to World War II people knew
very little about the ocean floor. Military needs during World War II gave a boost to sea
floor exploration. With submarines and warfare, suddenly countries became very
interested in mapping the ocean floor. Prior to World War II, researchers used plumb
lines (that is a weight at the end of cable) to determine the ocean depth at one location
(a pinpoint), an incredibly tedious task, but multibeam sonar technology greatly
enhanced ocean floor observations.
Slide 2: Sonar Mapping (Echo Sounding) of the Ocean Floor
Sound Navigation and Ranging or Sonar mapping is also known as echo sounding, which
is the same method that bats use to navigate. Sound waves from a ship bounce off the
ocean floor and return as an echo. The time it takes for a sound wave to echo back is
related to depth, the distance to the ocean floor. By cruising back and forth with
multibeam sonar as shown here, physical features of the ocean floor can be mapped in
detail. Today, these types of maps are made even more quickly using satellite data.
Slide 3: Harry Hess
Dr. Harry Hess is another rock star that I want to introduce to you. He was a Princeton
University Geology Professor and during World War II he served as Captain of the Cape
Johnson in the south Pacific. He was keen on using the new Sonar technology to study
depth variation of the ocean floor so he would do sonar mapping as he moved from
battle to battle. Amazing. He also made some key observations of the ocean floor
sediment and of volcanic island chains that I will come back to later in this presentation.
Slide 4: Key Discoveries from the Oceans
This is one of the early sonar images of a segment of a ridge in the Pacific Ocean. Warm
red colors represent shallow water depth or ocean ridges and cooler blues depict
deeper depths or lower ocean floor elevations. These types of images were the first to
show that the ocean floor was not merely flat like a shallow bowl or saucer – sonar
revealed a rich underwater world of dramatic physical features. In addition to
underwater mountains like the Pacific ridge, other key discoveries included great
fracture zones (which are some of the largest faults on Earth), deep sea trenches (which
I already mentioned earlier with regard to the Pacific Ring of Fire), and seamounts.
Slide 5: The Mid-Atlantic Ridge
Sonar studies in the Atlantic Ocean revealed a 60,000 kilometer (or 40,000 mile) long
mountain range which splits nearly the entire Atlantic Ocean from north to south as
shown in this map illustration. These ocean floor highs are not tremendously tall
mountains; they rise about 2 kilometers above the deeper flat plains of the ocean floor,
which is why we call them ridges, and they are roughly symmetrical. Clearly these ridges
are not straight features but curve, maintaining its presence in the middle of the Atlantic
Ocean.
Slide 6: Mid-Atlantic Ridge in Cross-Section
Another way to visualize the ocean floor topography is with a cross-section, or a vertical
slice through the ocean. The bottom image is a cross-section slice that’s oriented about
east-west and extends from the U.S. east coast (near Florida) on the left (or the west)
through the Atlantic ocean and over to Africa on the right. This cross-section shows the
peak of the ridge in the middle of the Atlantic Ocean with both sides of the ridge
gradually dropping off in elevation to the deepest and flat parts of the ocean. These vast
flat regions of oceans are called abyssal plains, or I like to say abysmal plains, and
represent some of the flattest regions found anywhere on Earth. Near the continental
margins of the Atlantic, the edges of the oceans start to rise up and form a slope that
extends to the continental shelf, or the shallow drowned margin of continents.
Slide 7: Ocean Ridges form the Longest Single Mountain Chain on Earth
The Pacific Ocean also has a ridge, although it’s not in the middle of the Pacific but is
located closer to the Americas in the east Pacific. This spectacular world ocean floor
map shows how the ridges wind their way between the continents much like the seam
on a baseball forming the longest single mountain chain on Earth. Note also how the
ridges appear segmented or chopped up by closely spaced fracture zones.
Slide 8: Deep Sea Trenches Bordering Volcanic Islands
The other physical feature about the Pacific Ocean that’s different from the Atlantic
Ocean is that it’s bordered by deep ocean trenches. These Google images show a deep
trench adjacent to the Aleutian volcanic islands in the north Pacific and the Japanese
volcanic islands in the west Pacific. These narrow trenches at the edge of the Pacific are
8 to 12 kilometers deep troughs that occur adjacent to arcs of volcanic islands or directly
adjacent to continents like South America.
Slide 9: Oceanic Fracture Zones
Let’s look a little more closely at those fracture zones that segment the ocean ridges.
The map on the left is an image of the south Pacific ridge that is cut by two very long
and closely spaced fractures that are about perpendicular to the ridge segments. Note
how the ridge seems to be offset or displaced by each fracture. The right ridge shows an
oblique view created by multibeam sonar showing the offset segments of ocean ridges
(in orange) between two fracture zones, which consist of broken up fractured oceanic
crust. The blue regions on both those images represent the flatter deeper parts of the
ocean floor, located away from the ridges.
Slide 10: Ocean Ridge, Great Fracture Zones, and Seamounts
In addition to ocean ridges and great fracture zones (labeled here as transform faults),
Sonar mapping revealed isolated lines or chains of underwater extinct volcanoes such as
these shown in the upper right of this beautiful seafloor image. Lines of active volcanic
islands in the middle of oceans have been known for about for a long time – such as the
Hawaiian Islands in the middle of the Pacific. But sonar revealed lines of extinct
underwater volcanoes called seamounts.
Slide 11: Atolls of the Maldives
Atolls are circular reefs like the one shown here in the upper left that sometimes occur
next to each other in a straight line. In the 1800’s Charles Darwin, on his famous Voyage
of the Beagle, explained that atolls start out as reefs that surround or fringe an active
volcano. When the volcano becomes extinct, subsides, and is eventually submerged, the
marine organisms continue to grow and build upward, first forming a barrier reef
around the sinking volcano and then forming an atoll when the volcano is completely
submerged.
Slide 12: Harry Hess Recognized that Volcanic Islands Moved
As Captain Harry Hess was moving around the south Pacific during World War II, he
recognized that volcanic islands moved horizontally as they progressively sank and
evolved into atolls. He noted that submerged extinct volcanoes often had flat tops from
being eroded when they were islands. Flat topped submerged volcanos are now known
as guyots (g-u-y-o-t-s). Perhaps most importantly, Captain Hess noticed that seamount
chains always moved away from ocean ridges. As a prestigious Geology professor, Hess
was well aware of the new magnetic inclination data that resurrected Wegener’s
continental drift. But now he correctly deduced, in an Oh-Ah moment, that oceanic
crust was also slowly moving along the earth’s surface and that everything was moving
away from the oceanic ridges.
Slide 13: Thin Ocean Sediments and High Heat Flow
I want to return to this close-up image of mid-ocean ridge to mention two other
observations of the ocean floor: 1) that it’s covered by a thin layer of ocean sediments
and 2) that lots of heat is rising from below the ridges.
Ocean sediment is formed by clay and shell-like material from organisms constantly
raining down and settling on the ocean floor. Since the ocean sediment was quite thin,
Hess realized that the oceans must be fairly young. If the oceans were very old, then
there would be more time for sediment to settle and accumulate on the ocean floor and
the sediment should be very thick. He also observed that the thin sedimentary layer on
the ocean floor was a little thicker moving away from the ridges, which suggested to him
that the ridges were younger than the abyssal plain regions which had been around long
enough to collect more sediment. In fact the abyssal plain regions are flat (as seen in the
right side of this image) because the sediment there blankets the oceanic floor there
and smooths out the rough topography that makes up the ridges.
The high heat flowing out of the ridge area suggested to Hess that the ridge area was
underlain by liquid magma which was erupting volcanic basalt onto the ocean floor at
the ridges.
Slide 14: New Maps of Earthquakes
Finally, new maps of earthquake locations produced in the 1950s revealed that oceanic
earthquakes were localized in distinct narrow belts along the ocean ridges and at deep
sea trenches. This 1954 map nicely shows how the Atlantic Ocean earthquakes occur
north-south, right along the curved Mid-Atlantic Ridge on the left side of this map.
Slide 15: Sea-Floor Spreading and Subduction
In 1960, Professor Harry Hess wrote an elegant explanation for the many new
observations about the ocean floor that he dubbed an “Essay in Geopoetry.” He
proposed that molten magma formed beneath ocean ridges erupted and solidified to
form new oceanic crust. Earthquakes along the ridges suggested that the new crust was
cracking and splitting at the ridges and was moving away from the ridge on both sides.
The process Hess described is now known as Sea-floor Spreading, the gradual widening
of an ocean basin as new oceanic crust forms at and moves away from an ocean ridge.
Since new ocean floor was being created at the ridges, Hess realized that it had to be
consumed somewhere else, and he proposed that it was being consumed or sinking
back into the earth at the trenches, a process now known as subduction.
Slide 16: Hess’s Hypothesis: Continents “Drift”
Hess’s elegant Essay in Geopoetry is famous for instantly providing the long-sought
explanation for how continental drift occurs. The idea is that continents are attached to
the oceans and passively pushed across the Earth’s surface by the formation and
destruction of oceanic crust. They don’t plow through the ocean floor but instead ride
along with the ocean floor as it does all the work of spreading.
Transcript: The Ocean Floor – Part 2
Slide 1: Observations of the Ocean Floor
Welcome back. Ocean studies were critical to the discovery of plate tectonics.
Recognition of major physical features of the ocean floor, including ocean ridges, great
fracture zones, deep sea trenches, and seamounts led to Dr. Hess’s idea of sea-floor
spreading as the mechanism for continental drift. Another key discovery from the
oceans related to magnetic reversals provided a key test for the concept of sea-floor
spreading and cemented the realization that the continents and the ocean floor
together make up the mobile plates.
Slide 2: Passive Continental Margin
This block diagram nicely summarizes the physical features making up the ocean floor.
The ridges, great fracture zones, abysmal plains, and seamounts constitute the ocean
floor interior. Note however that there are two different types of submerged
continental margins where the oceans and continents meet.
The continental margin on the left side consists of a broad shallow shelf that steepens to
form a slope that extends down to where the ocean floor starts to rise up from the
abysmal plain region. Continental shelf-slope-and-rise regions are sometimes down cut
by narrow and deep valleys called submarine canyons. Broad continental shelves form
along passive continental margins that are not plate boundaries and thus lack
earthquakes and volcanoes. An example is the east coast of North America. In contrast,
active continental margins have a narrow drowned shelf that slopes steeply into a
trench which typically borders active volcanic islands or continents with active
volcanoes. Active continental margins do constitute a plate boundary with lots of
earthquakes and examples include the continental margins which surround the Pacific
Ocean and make up the Pacific Ring of Fire.
Slide 3: 2
nd Key Discovery from Oceans
The second key discovery from the ocean studies was the documentation of magnetic
stripping along the ocean floors like that shown here off the Pacific Northwest coast.
The bold dashed lines represent oceanic ridges and the colored zones or stripes
represent patterns or stripes of alternating magnetic strength preserved on the ocean
floor. These stripes are always oriented parallel to ocean ridge segments and the
pattern of stripes are the same on both sides of the ridges. This symmetrical magnetic
stripping pattern exists in all oceanic crust and was documented when magnetometers
towed behind research vessels recorded changes in magnetic strength across the ocean
floor.
Slide 4: Strength of Magnetic Field Varies away from Mid-Ocean Ridges
The top diagram shows how magnetic strength swings up and down from high to low
away from ocean ridges. Note how the broad high or positive magnetism in the middle
of the diagram coincides with mid-ocean ridge and how the positive and negative (or up
and down) swings are the same on either side going away from the ridge. If we color the
positive high strength stripes dark and the negative (or low) strengths light, the
magnetic pattern appears like stripes on a zebra oriented parallel to ridges. These
magnetic zebra stripe patterns, which are recorded on all ocean floor, were interesting
but very mysterious patterns until the paleomagicians recognized that the magnetic
field reverses itself or flips.
Slide 5: Volcanic Sequence
Remember that volcanic sequences preserved on continents record normal and reverse
magnetic polarity and indicate reversals or flips of the Earth’s magnetic field in the past.
The volcanic sequence on the left represents a vertical stack of volcanic rocks with the
oldest 2.3 million year flow on the bottom and the youngest 300,000 year old flow on
the top. N stands for normal polarity like we have today and R stands for reversed
polarity. On continents, volcanic rocks get younger vertically from bottom to top (as
younger flows erupt atop older flows on the land surface). But according to the sea-floor
spreading hypothesis, oceanic flows get younger horizontally towards the ridges where
new oceanic flows are erupting today. To show this on the right I have tipped the
volcanic continental sequence sideways with the oldest flow on the left side and
youngest on the right side. Looking down on this sideways sequence hopefully you can
now see how the ocean floor would appear with zebra stripes recording past magnetic
reversals.
Slide 6: Formation of Magnetic Stripping on Ocean Floor
This diagram shows the creation of oceanic crust by sea floor spreading over time. New
oceanic basalt forming continuously at the ridges records or freezes in the polarity of
the Earth’s field at the time of crystallization of the melt. At is moves away from the
ridge on both sides, the oceanic crust and sea floor becomes increasingly older and the
magnetic stripes preserve the time of prior periods of normal and reverse polarity in the
geologic past. This explanation for the magnetic stripping recorded in the ocean regions
was an extraordinary test of and perfectly consistent with Hess’s sea-floor spreading
idea.
Slide 7: World Ocean Floor Topography Map
Let’s return to this amazing image of the world’s ocean floor topography, which
represents one of the most important datasets collected about Earth. We now know
that the underwater mountains or ridges are where new oceanic real estate is forming
and you can perhaps imagine the pattern of magnetic zebra stripes which would be
parallel to these ridges. The presence of past magnetic reversals permanently recorded
in the sea floor meant that they could be used to date the entire sea floor without the
need to sample any oceanic rocks. The magnetic reversal timescale obtained from the
easily accessible continental volcanic rocks could be correlated with the record of ocean
floor magnetic reversals.
Slide 8: Age of the Sea Floor
This beautiful color image of sea floor age was created with using magnetic ocean floor
reversals. It reveals how the youngest sea floor in red occurs at the ridges with oceanic
crust getting progressively older away from the ridges in both directions. The coolest
color blue represents the oldest oceanic crust preserved on Earth today which is only
180-200 million years old, the middle of the Mesozoic Era when dinosaurs roamed the
Earth. You can see that the oldest (deep blue colored) oceanic crust occurs in the
western Pacific Ocean region and on the continental margins of the Atlantic Ocean.
Note how this image of the age of the sea floor mimics the world ocean floor
topography map. Their remarkable similarity tells us that the ocean mountains stand
high because they are the warmest parts of the ocean floor (recall that lots of heat
emanates from the ridge regions) – and therefore they are less dense than the off-ridge
portions of the ocean floor. Essentially they are thermal mountains elevated by their
lower density compared to the rest of the older, cooler parts of oceanic crust. Density is