- Explore_ Magnetic Stripes and Plate Tectonics (2023)

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Harvard University *

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180

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Geology

Date

Oct 30, 2023

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Uploaded by AmbassadorGiraffeMaster852

Magnetic Stripes & Plate Tectonics For millennia, we have been making discoveries that add to our understanding of the planet we call home. About 2000 years ago, scholars such as Aristotle determined that the Earth was in fact a sphere. But not much else was known about the composition of that sphere or the mechanisms operating under the crust. In 1596, Flemish mapmaker Abraham Ortelius was the first to notice that the coastlines appeared to fit together like puzzle pieces. In 1680, Sir Isaac Newton calculated that the density of the interior of the planet is far greater than the surface rock, indicating that the surface of the Earth was not composed of the same material as the interior. Little was known about the inner workings of our planet for quite some time, though. It wasn’t until 1912 that German meteorologist and geophysicist Alfred Wegener proposed that the continents were once joined in a supercontinent he called Pangaea, and that slowly over time they have moved apart, a phenomenon he called “continental displacement” which we now call “continental drift”. His ideas were rejected by his peers until the 1950s and 60s, however, because he could not explain how the plates had drifted apart over time. In today’s activity, we’ll discover a piece of evidence that enabled scientists such as Wegener to demonstrate that the continental plates are in fact moving. And it has everything to do with something we’re now intimately familiar with: Earth’s magnetic field. Age of the Lithosphere The color-coded map linked above shows the age of the lithosphere, which consists of the crust and upper mantle. What color is used to show young ocean floor (less than 20 million years old)? red What color is used to show old ocean floor (more than 240 million years old or more)? blue In the center of the Atlantic Ocean is an underwater mountain range that we call the “Mid-Atlantic Ridge”. Is the age of the ocean along this ridge young or old? How do you know? Young because its red Compare the age of the ocean floor near Africa and near North America. Based on this observation, what happens to the age of the ocean floor as you move away from the mid-atlantic ridge? Explain how you know this, using evidence from the map. Its moving You will sometimes come across the term “seafloor spreading” when reading about the ocean floor. Based on your answer to the question above, what is meant by the term “seafloor spreading”? Continents moving Is the seafloor under the Atlantic Ocean spreading at the same speed as the seafloor under the Pacific Ocean? Explain how you know using the map as evidence. No because the pacific ocean is lot younger

Visualizing Seafloor Spreading Click the link above to watch an animation of seafloor spreading. Use the animation to help with the drawing. Click on the drawing below and click edit. Using arrows and text boxes, add the following information: *Label the Mid-Ocean Ridge *Include arrows to show which way the seafloor is moving *Label crust, mantle, magma *Label old crust and new crust Based on the animation, what seems to be pushing the plates apart? Where does this material come from? The magnetic polarity What Magnetic Stripes Tell Us During World War II, Navy ships used magnetometers (devices that help detect magnetic fields) to help them locate enemy submarines. In doing so, it was discovered that the seafloor was also partially magnetic. ( What?!? ) Further study and analysis showed a regular, symmetrical and repeating pattern. Not only was the seafloor partially magnetic, the magnet material within the basaltic rocks (the small bits of iron) had a regular arrangement such that some rocks had magnetic material that pointed north, much like a compass needle would point today. Other rocks had a regular pattern with magnetic “needles” that pointed in the opposite direction. While we cannot actually see the magnetic material “pointing north” or “pointing south” with our eyes, we can imagine it like magnetic stripes on the seafloor, much like the image to the right (a positive anomaly means it points in the same direction as a compass needle would today). Take a look at THIS ANIMATION . The scale is on a timescale showing “millions of years ago”, along with the orientation of the stripes. (Remember, these stripes are not actually visible features on the ocean floor, so anytime you see a stripe, it is a model showing the magnetic field direction. The magnetic fields are measurable, however, so that’s how we are able to “see” them.) Notice that the stripes are arranged symmetrically around the crest of a mid-ocean ridge. Where is the new ocean floor (rock) formed? Near the ridge What state of matter is magma in when it first arrives to form new rock at the mid-ocean ridges? liquid Considering that the liquid magma has small bits of magnetized iron in it, why does this material all point in the same direction? Cause of a magnetic field Once the magma cools and solidifies, could the magnetic material change direction? Explain why or why not. It wont change

When the pattern of stripes in the Atlantic Ocean is compared to the pattern of stripes in the Pacific Ocean, it can be seen that the reversals happen at the same time in history. Rock that is 780,000 years old - no matter where it is found on the ocean floor - has its “magnetic compass” pointing South, not North. In fact, every stripe reversal seems to happen at the same time in Earth’s history. What does this indicate that probably happened to Earth’s magnetic field at these times in Earth’s history? ( CLICK HERE to learn more.) It changes Take a look back at the map showing the age of the ocean floor. How old is the oldest ocean floor? (240 million years old. ) Knowing that the Earth is much older than this, what do you think happens to the ocean floor to make this true? Mediterranean sea After looking at THIS MAP , would you say the average age of the oceanic crust is about the same as, younger than, or older than the average age of the continental crust? Explain. No because the color is different

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