Comment: Can we measure both above and below sea level? What patterns are revealed in elevation data? Why might these patterns exist?
Comment: In the activities we did with you on this unit, you were learning just as your students were learning by thinking about a question and then devising a method and collecting data to try to answer that question. One of the most important parts of a teacher’s job, when teaching with the inquiry method, is to help students sum up what they have learned.
Comment: At this point, I want to give you a little background on the theory of plate tectonics.
Comment: Historically, people only knew about what they could see above sea level (termed topography). They could see only the topography of the continents and an endless, flat sea. This led to lots of speculation, mythology, and other hypotheses, such as: The earth was riding on the back of a giant turtle Or on Atlas’s shoulders Or was flat
Comment: There were lots of problems with understanding anything as big as the whole earth back then. Travel was difficult and early maps generally included only known areas. Anything outside the known area was a guess. Measurements were different in different parts of the world. So there was little consistency, and no comparisons, from one part of the world to another.
Comment: Detection of patterns in the shape of the continents was difficult, at best. As we found out more, some questions were answered Was the earth flat? - no Was the land on earth bigger than just what one person could see? - yes Was the sea floor like a bathtub? - no But some new questions were generated, too. Just looking at this topographic map might make you wonder: Is there a pattern to how the continents are arranged on the globe?
Comment: Even more recently we began to get a picture of what is under that endless, flat sea. Inquiry began with questions like these: (Read the slide).
Click on the slide to reveal the test and comment: Finding the shape of the unseen ocean floor is remarkably easy in one sense: You did it this morning in your “black box”. But remember the discussion: How many samples are needed? How many are needed when looking at the whole globe? How many are needed to understand big, broad patterns? Does that change if you look for small, more detailed patterns? And how good a picture do we have now? Is it as good as what we will have in the future? What should happen to our understanding of the sea floor as we accumulate more measurements?
Comment: And once we have information on the actual shape of the ocean floor, deeper questions come up (see slide and have each question come up sequentially).
Comment: The development of the Theory of Plate Tectonics, from a point where all we knew about elevations of the earth was that some parts on land had mountains and some were flat, and the oceans were just one flat expanse, is one of the great examples of how science is supposed to work! Construction of the theory of plate tectonics began with hypotheses explaining patterns of elevation on earth. In the activities we did today, we’ve learned about elevation patterns.
Comment: We’ve also tried to reinforce to you, and hope you will reinforce to your students, that… (Read the slide). We did the same thing today with our measuring sticks and black boxes as ships do - measured the depth to the bottom. Ships use more sophisticated technology, in the form of echo sounding. But it’s still the same measurements.
Comment: There are even more ways to measure ocean depth, but they are still doing basically the same thing as we were doing. The thing you want to keep reminding your students of is that these data are real measurements made by real people.
Comment: Again, we want students to realize that data are real measurements made by real people all over the world.
Comment: The development of the theory of plate tectonics, from a point where all we knew about elevations of the earth was that some parts on land had mountains and some were flat, and the oceans were just one flat expanse, is one of the great examples of how science is supposed to work! Construction of the hypothesis of plate tectonics began with bathymetry.
Comment: Later, with the hypothesis of continents east and west of the Atlantic possibly having been joined in mind, French scientist Antonio Snider-Pellegrini looked at fossil plants in the coal beds of North America and Europe and found them to be identical. He then added to the hypothesis of von Humboldt and suggested that N. America and Europe had once been joined.
Comment: By the late 1800s, scientists had further studied the flora and fauna of the continents - a discipline called biogeography - and found many similarities. So, by the late 1800s, it was no surprise that the Austrian geologist Edward Seuss suggested all of the continents had once been part of one huge southern continent - Gondwanaland.
Comment: One thing that scientists must deal with is uncertainty. And one has to say that the fit of Africa and Europe with North and South America is not perfect. But one also has to admit that it’s pretty good, especially on the scale of continents. This example is a perfect one to pull out for your students to discuss the uncertainly of patterns and how scientists continue to gather data to either support or not support (refute) the hypothesis. And the difference between a theory and a hypothesis lies squarely here: A hypothesis is one suggested explanation to an observed pattern or phenomenon - collection of more data may support the hypothesis, in which case it will be kept as a possible solution, or collection of more data may not support the hypothesis, and it would be discarded as a viable option. A theory has been “tested” - that is, more data have been collected and have supported it. And so it is our current “best” hypothesis, and has stood the stand of rigorous scientific questioning. Are theories ever discarded? Of course. That’s the nature of science - we keep collecting data, and evaluating that data, and when new data show that the theory is no longer a valid option, it is discarded.
Comment: Another pattern that was noticed in the early 1900s was in the rocks and minerals of the world’s mountains. In 1908, a U.S. geologist looked at the rock formations and minerals of the Caledonian Mts. in Europe and the Appalachians in the U.S. So, this theory broke away from the traditional thought that continents were stationary and took the bold step that continents could move or “drift” … the beginning of the continental drift theory.
Comment: The idea that continents might actually move - as normal as it seems to us today - seemed ridiculous when it was first proposed. It was first proposed by Frank Taylor of the U.S. but his hypothesis was pretty much ignored. It wasn’t until Alfred Wegener put all the evidence together in a convincing manner that anyone even considered that this hypothesis was possibly credible.
Comment: Wegener’s pursuit of the theory of continental drift was the purest of inquiry - the scientific method in action. He started with an observation: he noticed the “fit” of South America and Africa, and wrote to his wife, &quot;This is an idea I&apos;ll have to pursue.&quot; Then he found papers that supported this hypothesis of a continental fit. He became fascinated and continued to search out everything he could find about the topic, and finally formulated his hypothesis using such strong data that even scientists who wouldn’t accept the theory were fascinated by the possibility of continents moving.
Comment: Most of the attacks on the theory were personal - aimed at Wegener himself - this upstart (32 years old) meteorologist (not even a geologist) was telling geologists what the earth was doing! It’s hard to give up long-held beliefs. The main problem with Wegener’s theory was that no force was known that could cause continents to move. He suggested two possibilities: (read the slide).
Comment: 1922 - the first transect across the Atlantic using echo-sounding 1925-27 - 5 more transects by German oceanographic ship, but kept quiet because of political climate And then WW II --&gt; no further scientific exploration until it ended 1940’s - full steam ahead! What lies ahead for our next unit? Earthquakes.
Earth Science Teacher Institute
Topography and Bathymetry
Early History of Plate Tectonics
Goals and Objectives
• Define topography and bathymetry
• Consider questions about the elevations of
continents and the ocean floor
• Understand what methods were used
historically and are used today to determine
the topography of the earth
• Understand the connection between
elevation/bathymetry and the theory of plate
• Think about a question
• Think about devising a method and collecting
data to try to answer that question
• Help students sum up what they have learned
Theory of Plate Tectonics
The theory of plate tectonics was created and
subsequently supported by a variety of data,
starting with elevation.
Topography of Earth above sea level:
It took centuries to develop
a map of “the earth”
• Eratosthenes created the
first known map of the world
(ca. 220 B.C.)
• Crates of Mallos constructed
the first globe
(ca. 150 B.C.)
Patterns: Random? Clumped? Linear? ? ? ?
Why? - one cause? many causes?
tackled by early
How deep is the ocean?
Where is it deepest?
How deep is the center of the ocean?
Is the ocean floor flat or bumpy? Where?
Is there a relationship between topography on land and in
the ocean? If so, what is that relationship?
•Elevations of Earth below sea level is
Today we know that the ocean floor has
mountains, valleys, plains, and other
features similar to what we see on land.
Why does the ocean have the depths it does?
Why do ocean depths have the patterns we see?
Are land and ocean elevations related?
The Theory of Plate Tectonics
• Elevation patterns
– Not random - that is, there ARE patterns
– Continents are clumped
– Mountains are linear high areas surrounded by
vast expanses of lower, flatter areas
– The ocean floor shows similar patterns of high
ridges, abyssal plains, and deep trenches
– Most elevations on earth fall either around
200 m above sea level or 4,000 m below sea
–Scientific data are real measurements
–They are measured by real people
–They do not just appear by magic
–They are not much different from what
we did - mostly differ in sophistication of
equipment - like echo-sounding above
Today, scientists use
remote sensing of
multibeam echo sounding
and satellite altimetry,
predicting topography of
the bottom from the surface
of the ocean.
Satellite altimeter data of world’s oceans, measuring the surface of the
ocean as it bulges over underwater features. From:
The same goes for measuring
topography on land.
We used a carpenter’s level
and a pole and did exactly the
same thing that surveyors do
with their telescopic levels
Workings of a Global
Positioning System. From:
But back to patterns …
One of the first patterns
recognized, historically, had
to do with the shapes of the
1800 - German naturalist Alexander von Humboldt
hypothesized that the continents on either side of
Atlantic Ocean were once joined based on how the
bulge of South America fit into the curve of Africa.
•A historical note - this was first proposed by Abraham Ortelius three
1858 - Snider-Pellegrini’s maps from http://pubs.usgs.gov/publications//text/historical.html
Then around 1850 - French scientist Antonio Snider-Pellegrini
went a step further:
• Suggested N. Am. and Europe were once connected based
on identical fossil plants in coal deposits
• Pennsylvanian period - 325 to 286 mya (million years ago)
Late 1800s - Austrian geologist Edward Seuss
• Continents had all been part of one ancient continent
• Coined term Gondwanaland
• Based on similarities of plant fossils in South America, India,
Australia, Africa and Antarctica
One thing that scientists must
deal with is uncertainty.
• A hypothesis is one suggested explanation to
an observed pattern or phenomenon
•A theory has been “tested.”
•Are theories ever discarded? Of course.
And back to patterns again…
1908 - U.S. geologist Frank Taylor
• Continents had collided at some time
• Those collisions had created some of the
world’s mountain ranges
• Based on rock formations and minerals in
Caledonian Mts. of Europe and the
Appalachian Mts. of North America
• Also thought the mid-Atlantic Ridge (a high
ridge dividing the Atlantic Ocean from north
to south) was the former boundary between
Development of the theory of
• First proposed by Frank Taylor, but was
developed using all the available evidence
by Alfred Wegener, a German
meteorologist, in 1912
– Geology (mountain range, rock and mineral patterns)
– Climatology (tropical fossils in Antarctica)
– Paleontology (fossils)
– Continental shapes, including continental shelf contours
– Differing densities of continental and oceanic rock
• Wegener’s hypothesis of “continental drift”
– "Entstehung der Kontinente und Ozeane”
(The Origin of Continents and Oceans)
"It placed an easily comprehensible,
tremendously exciting structure of ideas upon a
solid foundation. It released the continents from
the Earth's core and transformed them into
icebergs of gneiss [granite] on a sea of basalt. It
let them float and drift, break apart and
converge. Where they broke away, cracks, rifts,
trenches remain; where they collided, ranges of
folded mountains appear.”
• Wegener’s hypothesis was not popular
• Major problem - what forces could cause
continents to move?
• Two possibilities:
– Centrifugal force
– Westward tidal drag generated by the
gravitational pull of the sun and moon
– But these were just hypotheses with no data …
Although down, the theory of continental
drift was not completely out. Some
scientists were making their own
hypotheses about what patterns could be
explained by continents moving around.