Continental Drift and Plate Tectonics

CONTINENTAL DRIFT AND PLATE TECTONICS

1. Map fit (jig saw puzzle effect)--mentioned earlier

I. Continental drift concept

2. Mountain chains formed with longitudinal axes perpendicular to the movement of continents

A. Introduction and early concepts · 1. Introduction

an example is the formation of the Himalayan Mountains--the trend of the axis is east-west and formed from the Indian subcontinent moving northward buckling up material while colliding with the Asian continent--some believe another example is the Rocky and Andes mountain chains (axes trending mostly N-S) formed in part from the western movement of the North and South American continents

·

the big question is "did the continents originate at their present locations or did they drift to where they are today?"--even with much data indicating the continents drifted there are those who refute the continental drift idea--we will treat some data supporting the drift idea in I.B. in the outline below

·

a solid lithosphere floating on top of the asthenosphere gives an idea how plates can move

3. Same fossils, rocks, mountain ranges, or glacial features located in areas on different continents representing prejoined positions prior to continental drift

·

Accretion of continental material is also an important during continental drift to help shape the continents today

4. The presence and shape of the global ocean ridge

·

formulation of the drift concept supplied much information to the rock plate and plate tectonic concept

·

our earlier studies on the Earth's interior structure should give us a better understanding of how the lithosphere is able to drift (solid upper Earth floating on the asthenosphere)

·

the global ocean ridge is a raised region on the ocean basin which is believe to represent the prejoined or splitting area of separated continents-note the shape of the ridge contours the shape of the coast lines of the separated continents--this is most evident in the mid Atlantic ocean

·

convection cells in the Earth's interior are the force which split the lithosphere and are the driving force in continental separation

2. Initial ideas of continental drift ·

·

·

the most obvious feature which would lead someone to believe the continents drifted apart is the jig saw puzzle or map fit of continents. Looking at a global map it would appear that the continents could be brought together to fit like a jig saw puzzle--this would seem to imply that the continents must have been together at one time and drifted apart

5. Seafloor spreading

who was the first to see the map fit phenomenon?---Alfred Wegener was the first to publish a summary of the ideas of the continental drift concept in 1912 and for this was given the title of "The Father of Continental Drift" some facts noted in the Wegener's publication were: o

the (protocontinent) was named Pangaea

o

the southern portion was named Gondwana (Gondwanaland) and the northern portion, Laurasia

·

the age of rocks located the same distance away from the center of the ridge on both sides of the ocean ridge are the same--rocks are youngest nearest the ridge center and progressively older away from the center-this indicates new rocks form at the ridge centers and older rocks are pushed away to make room for younger rocks resulting in spreading of the ocean floor and continental drifting

·

also the magnetic intensities in the rocks are found to be the same on equivalent sides of the ocean ridge--on both sides of the ridge the magnetic intensity alternates between normal (high intensity) and abnormal (low intensity)

II. Lithospheric plates and plate tectonics

o

the protocontinent broke in pieces about 150-200 million years ago

B. Some important data confirming continental drift



if the ocean floor is spreading from ocean ridges and continents are drifting, where are the spreading sections going?-- the answer to this question was important in initiating the concept of rock plates--the question was answered with the discovery of subduction zones in the lithosphere-rock sections moving away from spreading zones are moving towards subduction zones

A. Rock plates 

Ocean.

ocean basin and continental sections float in the asthenosphere, some plates move towards each other, some away from each other, and some parallel to each other

the evidence is that there are similar rocks in areas very far apart, 2 is that there are coal mines in cold regions when all scientists know that coal is made in tropical(warm) temperatures and lastly the finding the fossils of animals to small to swim the long distances between land masses

1858

Antonio Snider-Pellegrini suggests that continents were linked during the Pennsylvanian period (325 million to 286 million years ago), because Pennsylvanian plant fossils from Europe and North America were similar. (See "carboniferous" on this time line.

1885

Australian geologist Edward Seuss sees similarities between plant fossils from South America, India, Australia, Africa and Antarctica, and coins "Gondwanaland" for a proposed ancient super-continent with these land masses.

1910

American physicist F.B. Taylor proposes concept of continental drift to explain formation of mountain belts.

1912-1915

German meteorologist Alfred Wegener proposes theory of continental drift, based on evidence from geology, climatology and paleontology. Wegener names one of the ancient super-continents "Pangea," and draws maps showing how the continents moved to today's positions.

1920-1960

Assorted arguments are used to debunk continental drift, most importantly the lack of a mechanism strong enough to move continents across or through ocean basins.

1937

South African geologist Alexander du Toit maps out a northern supercontinent, "Laurasia," to explain coal deposits, which presumably indicate the remains of equatorial plants, in the Northern Hemisphere.

1960-present

A mass of evidence for continental drift, or plate tectonics, starts accumulating, including:

Paleomagnetism: British scientists find that magnetic fields recorded in rocks from Europe and North America indicate the rocks were formed in far different locations than paleomagnetism their present positions. The pattern of continental drift recorded by rocks show Europe and North America have drifted away from each other for more than 100 million years. This movement opened the Atlantic

fossils

Fossils of the plant genus Glossopteris occur on all five Gondwana continents. The seeds were too heavy to be carried by wind, and would have died quickly in salt water, indicating that the continents were once joined. Similarly, fossils of several reptiles occur on several continents.

mountains

The Appalachian Mountain chain can be linked with mountains in Greenland, the United Kingdom and Norway, indicating that these land masses were joined at the time the mountain chain formed.

Glacial scars

Marks left by glaciers on rocks in Africa, India, South America and Australia make no sense -- unless these continents were joined and arrayed around the South Pole. Then, the glacial scars would have all pointed away from the Pole when they were made. Today, glaciers form near the poles and move away as they travel and eventually melt.

Seafloor spreading

The ocean floors are spreading away from mid-oceanic ridges, indicating that the ocean floor is moving. Mid-oceanic ridges, more than 65,000 kilometers long, are the largest mountain range on Earth. In 1962, Harry Hess of Princeton University proposed that seafloor spreading would explain this movement.

Age of rocks

The oldest rocks on the ocean floor are younger than 220 million years, while the oldest terrestrial rocks are about 4 billion years old, indicating that the ocean floor is recycled back into the Earth.

Magnetic data

Earth's magnetic field periodically changes polarity (your compass would point to the South Pole). When magma solidifies at the midoceanic ridges, it records the polarity of the Earth's magnetic field. Bands of seafloor basaltic rocks paralleling the mid-oceanic ridges carry a record of this alternating polarity.

Geologists think convection cells in the mantle (the hot, plastic rock Thermal under the crust) power continental convection cells movements, overcoming an early objection to continental drift.

There is much evidence that Pangaea existed and that the seven continents used to be as one. Here are just some: 1. The most obvious piece of evidence is that the eastern coast of South America fits almost perfectly with the western coast of Africa. 2. In the fossil record, remains of the therapsid Lystrosaurus as well as the plant Glossopteris have been found in South Africa, India, and Australia, three regions currently separated by thousands of miles of ocean.

Step 1: Pick a spot 3. Similarly, fossils of the freshwater reptile Mesosaurus have been found on the coasts of Brazil and West Africa. 4. A polar ice cap covered the southern end of Pangaea during the Carboniferous period, and glacial deposits of the same age and structure are found throughout the southern continents. 5. The continuity of mountain chains also provides evidence. For example, the Appalachian Mountains extend from the northeastern US to the Caledonides of Ireland, Britain, and Scandinavia. 6. Paleomagnetic study of rocks can help scientists reconstruct the exact positions of continents at specific points in Earth's history. 7. There is direct evidence from very accurate measuring techniques that the movement of the continents is continuing today. It has been proven that the Earth's present continents were once together as a Pangaea as seen from:

Pick a spot to measure your object (I measured a telephone pole). You should be far enough away from your object that you can see the top of it, and you need to be on level ground with the base of the object. I like to set something down by my feet once I've picked my spot, so that I can easily come back to it. Step 2: Measure angle

Continental Coastlines Appearing To Fit Together One prominent example of continental coastline fitting together is to fit the coastline of the West Coast of Africa with the coastline East Coast of South America. It can be seen that they fit well, like pieces of a jigsaw puzzle. This helps to prove that these continents were once joined together as one whole Pangaea and broke away to form these two land masses now. Fossil Distribution Matching fossil of reptiles have been found in Africa and South America, further proving that these two continents were actually so close to each other or even joined, that reptiles could travel to and fro between them easily. Identical fossil ferns have also been found in all southern continents, and also embedded in the same layer sequence, suggesting the proximity the southern continents were in millions of years ago that allowed the growing of these ferns in the same climate and soil. Distinctive Rock Strata Geologists have discovered that the geological structures of the rocks in South West Africa and South East Brazil were distinctively identical, and the age of the rocks at these two areas was the same. This distinctive rock strata shared by the two land masses suggests that these two areas were once joined together. Coal Distribution Coal can be found underneath the cold and dry Antarctic ice cap, though coal can only form in warm and wet conditions. This could mean that Antarctica was once together with the other continents as part of the Pangaea, and was once in a warm and humid region. Coal was formed before Antarctica drifted away to its present cold and dry climate. That is why the coal can be found buried under the thick layer of ice and snow.

Here's where we bust out our handy clinometer. Look through the straw of your clinometer at the top of the light pole (or whatever object you're measuring). The weighted string should hang down freely, crossing the protractor portion of the clinometer. Read the angle shown, and subtract from 90° to find your angle of vision from your eye to the top of the pole (it can be helpful here to have an assistant to read the measurement while you look through the straw). Record your results on your paper. From my spot, my clinometer (read by my assistant) showed 55°. Subtracting from 90°, that indicated that I looked at an angle of 35° to the top of the telephone pole. Step 3: Measure distance

Once you have your angle of vision, use your tape measure to find the distance from the spot you're standing to the base of the object you're measuring (an assistant comes in handy here, too). We must know how far away you are to accurately calculate the height.

Time to move inside. In calculating the height of the object you just measured, I find it helpful to begin by drawing a picture and labeling it with all of the information I have. Step 6: Model as a triangle

My spot was 15.6 meters from the base of the telephone pole I measured. Step 4: Find your eye-height

The next step is to simplify your drawing to model your system as a right triangle. Label your triangle with the angle you read on your clinometer as well as the distance you were standing from the object (we don't need the eye-height just yet). Step 7: Solve for x The last piece of data you need to calculate the height of your object is the height from the ground to your eye (your eye-height). Have your assistant help you measure this using your tape measure. My eye height was recorded for this example as 1.64 meters. Step 5: Draw a picture

We can find x in this triangle (which represents the portion of the height from eye-level up) by using some basic trigonometry, specifically the tangent ratio of the triangle: tan(angle) = x / distance Multiply by the distance on both sides and you get: x = tan(angle) * distance

Use a calculator to multiply these together and get a decimal value (be sure your calculator is in 'degrees' mode, rather than 'radians'!). In my example: tan(35°) = x / 15.6 x = tan(35°) * 15.6 x = 10.92 meters Step 8: Combine with eye height

To find the height of your object, bring this x value back to the original drawing. By labeling it, we can see that the height of the object, h, is equal to the x value we just found plus the eye-height we measured earlier: h = x + (eye-height) In my example: h = 10.92m + 1.64m h = 12.56m There you have it! A few basic classroom materials and a little bit of trigonometry and you can measure the height of anything around you!

Expected results: Depending on the paperclips students used and the amount of liquid poured in each cup, students’ results may vary a bit. However, it should be clear that the vegetable oil weighs less than the water and that corn syrup weighs more than the water. Table 1. Expected results for weighing equal volumes of vegetable oil, corn syrup, and water using paper clips. Liquid Weight in Paperclips Vegetable oil 24 Water 29 Corn Syrup 41