Why did it take until the 1500s for people to realize that the world's land masses had moved around?  In a word, technology.  We couldn't figure out that the continents had moved until we could figure out where they were, and what their shape was.  In this case, the technology necessary is the ability to navigate precisely, which led to good quality maps.  What technology was necessary?  Two devices come to mind - the Sextant, which enabled exact latitudes to be determined, and the Chronometer, which enable longitude to be determined.

A sextant is used to measure elevation of astronomical objects above the horizon, which gives you latitude.

This is the first chronometer made by John Harrison in 1741.  He also made the first successful chronometer, in 1761. Note that this is AFTER the 1500s, which is one reason why maps from that time sometimes look significantly different from modern maps.

Technology will continue to be important in the development of Plate Tectonics as well.  Keep this in mind!

Other ways and people that showed the continents had moved

• Alexander von Humbolt (1769-1859) - noted similarities of continental outlines - and geologic features - on either side of the Atlantic.
• Antonio Snider - 1858 - noted similarities btw. America and European fossil plants of Carboniferous (300 m.y. BP) age; therefore all continents once together.

In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Left: The formerly joined continents before (avant) their separation. Right: The continents after (aprés) the separation. (Reproductions of the original maps courtesy of University of California, Berkeley.) (from USGS website - click on images to go to it)

Of all the people showing this, the most important was Alfred Wegener.  He (and at the same time, Frank Taylor) came up with a concept that Wegener called "Continental Drift".  Wegener is often called "the Father of Continental Drift".  (Note: "Continental Drift" is NOT the same as "Plate Tectonics".)


• Frank B. Taylor and Alfred L. Wegener - 1908-1910 to 1930 - common historical record for both parts of Atlantic and many other continents 200 m.y. BP, suggested once formed a supercontinent called PANGEA.  Wegener named process "Continental Drift".
  A must see: https://www.youtube.com/watch?v=T1-cES1Ekto
  
• Wegener Websites that also include information about both Continental Drift and Plate Tectonics:
• http://pubs.usgs.gov/publications/text/wegener.html
• http://www.ucmp.berkeley.edu/history/wegener.html
  
• 
  

• Gondwana (or) Gondwanaland
• India
• Africa
• S. America
• Australia
• Antarctica
• Laurentia
• Eurasia
• North America
  
• Pangaea
• All the continents together
• You might occasionally hear the word "Rodina" used as well.
• Rodina existed long before the others - ~1 billion to ~700 million years ago.
• Composed of North America and parts of:
• Europe
• South America
• Africa
• Australia
• Antarctica
• India
  
• So the continents have been moving around for some time.
  

• Fit of Continents
• Today - used 2000 m contour interval or edge of continental shelf
• Fossil Evidence

(Figure is from USGS website) Glossopteris Flora Mesosaurus Cynognathus Lystrosaurus

• Glossopteris is a plant that lived during the late Paleozoic.
  

• 
• Glossopteris (and its associated flora) fossils are found in Australia, Antarctica, India, Madagascar, Africa, and South America.
• It or its seeds could not have crossed oceans.

• Mesosaurus was a reptile that lived during the early Permian.
  
• Long snout filled with teeth.  Short legs.  Long, powerful tail.  About 1 m long.
  
• Mesosaurus fossils are found in coastal marine (not deep water) sediments.
• Not likely to have been able to swim across Atlantic, but fossils are present nonetheless in southern South America and south Africa.
  
• Lystrosaurus and Cynognathus were "mammal like reptiles" - not the ancestors to mammals, but cousins of our ancestors.
  
• Cynognathus - Middle Triassic (overlapped with Lystrosaurus)
• About 4 ft long
• short legs and tail.
• Not good swimmers
• in what is now South America and Africa
• Lystrosaurus - around 250 million years ago, survived "the great dying" (Permian extinction)
  
• They ranged in size from that of a small dog to 2.5 meters long.
• short legs and tail.
• Not good swimmers
• in what is now Antarctica, India, and Africa.
• This is a nice video (dating from 1969, so quite a bit later than Wegener) detailing the discovey of Lystrosaurus in Antarctica.  Please watch it.
• And this is a photo series showing a number of the organisms on display at a special presentation at the Florida Museum of Natura History in 2018
  

• None of these organisms should have been able to cross oceans.
  

• Geological Features
• Mountain Belts
• Rock Types
• Mineral Deposits
• All of these features match up across oceans.
  
• Climatic evidence
• Alfred Wegener was actually a meteorologist, so it's not surprising that he used climatic data as well.
  
• Glacial evidence
• Striations (grooves left behind by glacial movement)

• Till (unsorted sediment left behind by melting glaciers)
• Both striations and till are found around the world, often in places where there are now no glaciers.
• The distribution of striations is especially interesting.
  
• Distribution of Glacial Features:
• 
• Glaciers (ice sheets) approaching land from the ocean in South America, Australia, and India.
• This is odd because glaciers ONLY form on land, when snows last through the summer.
• If snow falls on the ocean, it just melts.
• Glaciers NEVER form in the ocean.
• Why then do they seem to be approaching land from the ocean?
• Glaciers moving into India from the tropics, and spreading out from the subtropics in Africa.
• Glaciers form in polar areas (or at high altitudes).
• Why did they seem to be forming in warm areas?
• The answer, of course, is that if you reconstruct Gondwana, the distribution makes sense.
  

Glacial striations and deposits of till show direction of glacial motion only make sense if the continents were together as shown at right.

• Coal deposits
• Coal forms when vegetation does not completely decay, gets buried, and subjected to heat and pressure long enough to be turned to coal.
• The kind of environment where this happens tends to be warm and wet.
• Environments like that?
• Amazon Rain Forest
• Everglades
• Paine's Prairie
• Generally speaking, tropical and subtropical environments.
• Where is coal found today?
• West Virginia
• Kentucky
• Pennsylvania
• Colorado
• Wales (Great Britain)
• In other words, lots of places that are not NOW tropical and subtropical environments.
  
• Coral Reefs

• 
• Present day distribution of coral reefs
• Coral reefs need warm, shallow water.  They are almost all confined to the tropics
  
• Fossil coral reefs, on the other hand, are found all over the world.
• Why?
  

• Evaporites
• Form in enclosed basins in dry environments,
• typically (because of global atmospheric wind patterns) near 30^o north and south of the Equator.
• Yet they are today found in many other areas.
• Why?
• Wegener's answer to "Why?" of course is that the continents have moved over time,
  
• a concept he called "Continental Drift"
• He gathered a HUGE data set showing this,
  
• yet his ideas were not accepted.

There may be a number of reasons for this.

• This was shortly after the Great War (WWI) and Germans were not popular.
• He was a Meteorologist by training and geologists may have resisted his ideas because of this.
• He was, by all accounts, a difficult person to get along with.
• But the big problem? Mechanism
  

• Tidal Forces
• The moon's gravity pulls on the earth, which SHOULD make the continents get pulled towards the equator.
  
• Flight from Poles
• The spin of the earth should fling landmasses away from the poles and towards the equator.
• The problem with that?
  
• both forces are too weak to cause this to happen
  
• Continents plowed through oceanic Crust (but oceanic crust too strong and forces too weak)
• No evidence of "plowed oceanic crust" existed
  
  

What alternatives to Wegener's mechanism were suggested?
The data still had to be explained, even if his mechanism was rejected.  There were a number of ideas proposed:
Other ideas explaining Wegener's data:

• Expanding Earth Hypothesis
• The idea here is that if the earth expanded, continents would be spread apart.

• 
• The problem with this, of course, is that there is no way to physically make the Earth expand.  The Earth is not a balloon.
  

• Oceanization
• Oceanic crust is more dense than continental crust.  If a continent was intruded with oceanic magma, it would make the crust more dense, and it would subside.
• The problem is that this would not explain the parallel coasts or the presence  of similar rock structures on opposite sides'
• In addition, oceanic drilling has never found any evidence of continental crust like this in the ocean floors.
  
  
• A number of ideas were proposed to explain why fossil organisms were spread out on distant continents.
  
• Rafting - organisms float either themselves or on floating vegetation across oceans.

• There is an element of truth to this - we can observe animals and especially plant seeds floating across oceans (think coconuts) to form new populations on distant shores.
• However it always results in "depauperate" faunas and floras, where only a few members of a fauna and flora make it across.
  
• Land Bridges - This has definitely happened.
  
• 
• During Ice Ages, sea level has been low enough that the Bering Strait (between Asia and Alaska) was dry land, and many animals walked one way or another,
• Horses and Camels walked from east to west
• Elephants and Humans walked from west to east
• Roughly 5 million years ago, the Panama Isthmus came into being and the "Great American Interchange" took place.
• Cats and llamas went south.
• Armadillos went north.
• HOWEVER, there is no evidence that there were ever land bridges between, say, Africa and South America, Australia and India, or Australia and Antarctica, so that canNOT be the explanation for organisms being present on these diverse continents.
  
• Island Stepping Stones - We also know this happens.  The idea is that organisms can swim a short distance to an island, set up a population, and when that population gets large, some animals swim to the next island in the chain and do the same thing.
  
• 
• This is how the Galapagos and the Florida Keys got populated.
• But AGAIN, there is no evidence that there were ever island chains between, say, Africa and South America, Australia and India, or Australia and Antarctica, so that canNOT be the explanation for organisms being present on these diverse continents.
• This will also lead to depauperate faunas and floras.
  

• Sonar (SOund Navigation And Ranging) - sending pulses of sound out to reflect off of submarines AND the bottom of the ocean.  They can be used to measure depth in the ocean.  Harry Hess discovered a ridge running the length of the Atlantic - the Mid-Ocean Ridge.
  

• Magnetometer - detects strength of the "ambient" (local) magnetic field - can be used to detect submarines submerged under the ocean because submarines are made of steel, which is magnetic.  The magnetometers are towed behind a ship.
  
• 
• When military ships (and eventually researchers) looked at what the magnetometers found, there was a surprise:
  

• Instead of being a few hundred feet long (the length of a submarine), they were far longer - kilometers to hundreds of kilometers long.
• There were both positive and negative, where the magnetic field strength was either more or less as strong as the Earth's average field.
• The positive (dark, in the drawing above) and negative (light, above) anomalies changed seemingly randomly.
• They were symmetrical.  One side is a mirror image of the other.
  
• The center of their symmetry was right over the crest of the Mid-Ocean ridge.
• By sailing back and forth over a Mid-Ocean Ridge and measuring the magnetic anomalies, it was clear that a map of these anomalies showed that the same MMA pattern was present over the entire ridge.

• Note how the same pattern is present from east to west, all along the north-south ridge.
  

Top: South Atlantic magnetic anomaly pattern, made by the RV Vema. Middle: North Pacific magnetic anomaly pattern, made by the RV Vema. Bottom: Southern Indian Ocean magnetic anomaly pattern, made by the RV Eltanin. In all, "0 km" (near the right hand side of these patterns) is at the crest of the respective mid-ocean ridges.  Note that the anomaly pattern is symmetrical around the 0 km line. The same anomalies (numbered in the top pattern) are present, slightly compressed or expanded, in all of the patterns.  The black and white striped pattern represents positive (black) and negative (white) marine magnetic anomalies.

• In this diagram, the big arrow represents the direction of the Earth's magnetic field today.  Each of the small layers represents a lava flow (oldest on the bottom of course). Each small red or blue arrow represents the direction of magnetic field recorded in each lava flow. When paleomagnetic geologists analyzed the direction of magnetization in each lava flow, they found: Some (including the most recent) lava flows are magnetized in the same direction as the Earth's magnetic field today (the small red arrows).  We call that direction "Normal". Other lava flows are magnetized 180o opposite to the direction of today's magnetic field (the small blue arrows).  We call that direction "Reverse". The pattern of Normal and Reverse directions switches back and forth seemingly at random.  BUT: We can measure the radiometric age of lavas fairly easily, and: The uppermost transition between Reverse and Normal ALWAYS (no matter where on Earth you measure it) occurs at 700,000 years ago. The transition before that always occurs at 1 million years ago. And other transitions occur consistently as well. (green dashes)

• Eventually paleomagnetists realized that this meant that the Earth's magnetic field flips every now and then, everywhere at once.  This can be recorded and is a good way to tell time around the world.
• Further research has shown that the flips usually (on average) occur about every 1/4 to 1 million years;
• When a reversal occurs, it takes about 10,000 years for the magnetic field to decay to zero then build up again in the opposite direction.
• (Note: the Earth's magnetic field is currently gradually decreasing.  If we extrapolate this 10,000 years into the future, the field would have reversed.  However, the Earth's magnetic field varies over time, so while it has been suggested that our field may be in the process of flipping, the only way to be sure is to wait 10,000 years!)
  
• The timing of these flips is well enough known that we now have what is called the "Geomagnetic Polarity Time Scale".

• In this GPTS, periods of Normal magnetic polarity are represented by black bars, and periods of Reversed magnetic polarity are represented by white bars.  Age in millions of years (Ma = "mega annum") is on the right hand side.  Names of the various geologic periods, ages, etc. are on the left. This part of the GPTS in the diagram goes back about 88 million years, but the entire GPTS goes back today to well over 200 million years before the present.

  • If we take real anomaly patterns such as the South Atlantic below:
  • (This is the same as in the figure of the 3 anomaly patterns)
    
  • and compare it to the GPTS, we find that they correlate almost perfectly.

  • On the left is the South Atlantic Marine Magnetic Anomaly pattern shown just above, but rotated 90o. On the right is the Geomagnetic Polarity Time Scale or GPTS. Part of the Theory of Plate Tectonics suggests that seafloor is being created at the crest of mid ocean ridges (left), so brand new crust (zero age) is present at the 0 km mark.  Draw a line from there to the 0 M.Y. point on the GPTS (top right).  Then see how many other correlations you can draw in.  Once you've tried that, click on the image to see a nearly finished version. The conclusion draw from this is that marine magnetic anomaly patterns nearly perfectly record changes in the earth's magnetic polarity, being formed at mid-ocean ridges, then moving away from the ridge as new crust is sequentially created, very similar to the action of a tape recorder.

• The final interpetation: (Vine and Matthews) -
  
• As magma comes up at a mid-ocean ridge, it solidifies, and as it cools below the Curie Point, becomes magnetized in the direction of the Earth's magnetic field at the time.
• As the plates pull apart, the rocks separate, creating strips of crust that are magnetized in the same direction, and in the opposite direction, as the Earth's magnetic field flips back and forth.
• When a magnetometer is pulled over rocks that are magnetized in the same direction as the field, the magnetic field increases, creating a positive anomaly.
• When a magnetometer is pulled over rocks that are magnetized in the opposite direction as the field, the magnetic field decreases, creating a negative anomaly.
• See images below. 

Fred Vine (l), Drummond Matthews (r)

Result: Marine Magnetic Anomalies (MMA) - used by Fred Vine and Drummond Matthews (King's College, Cambridge) to infer Seafloor Spreading in 1963, based on MMA obtained during the 1950s and 60s.  (click on figure above to see USGS discussion of MMA's)

• take about 10,000 years to occur, gradually decreasing to zero, then building up in the other direction,
  
• and last (in either polarity) for (on average) 1/4 to 1 million years in either the Reversed or Normal direction.  Longer and shorter periods in either direction occur.
• It is not presently understood how or why these reversals occur, and it is not known how to predict when the next one may occur.
  

The MMA's are produced by sequential changes in the the earth's magnetic field, as seafloor spreading occurs:

(from USGS Website - click on image to go to that site)

Result (map view):

(Click on this image to see the entire Earth)

This has also allowed us to map out probable spreading patterns over time.

	From this website: http://www.searchanddiscovery.net/documents/97019/index.htm
	From this website: https://sos.noaa.gov/catalog/datasets/plate-movement-200-million-years-ago-to-today/ Click on the image to the left to see an animation.

The net result is Divergent Boundaries.

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• This creates a problem.  We have already seen that the Earth cannot be expanding.  But Divergent Boundaries CREATE new crust.  This implies that somewhere else, crust must be getting destroyed.
• Where? Hugo Benioff (in 1954, even before Vine and Matthews published their work in 1963) got the evidence from seismology.
  

Hugo Benioff, 1954 - discovered the "Benioff Zone" - American (of Russian heritage) seismologist. Invented seismological instruments Also invented electronic musical instruments! Examined earthquakes near Kamchatka Peninsula (Russia)

• Looked at earthquakes in Sea of Okhotsk and Kamchatka Peninsula
• (We now call this a subduction zone)

• Middle: cross sections of this region along transects from west to east.
• The data shows a downward dipping zone of earthquakes (which is known as the "Benioff Zone").
• Further studies shows that the relative motion indicates something pushing down from the east under the western part.
• At the time (note that this was BEFORE the work of Vine and Matthews), that's all anyone could say.
• But combined with the information from Vine and Matthews showing Seafloor Spreading (Divergent Boundary), it became clear that plates were coming together in a Convergent Boundary.