Date ______Period ______
Objectives:
1. The student should be able to construct a model of seafloor spreading and oceanic crust subduction.
2. Be able to relate magnetic reversal patterns, age of rocks and thickness of sediments to the creation of new crustal material at mid-ocean ridges.
3. Relate the process of seafloor spreading to divergent plate boundaries, convergent plate boundaries, Subduction, and ocean trenches.
Background:
It is not easy to see how some processes work, especially if you cannot observe them directly. Thus, it is often helpful to create a model. In this activity you will make a model that helps to show how ocean-floor spreading works.
Figure 1. Seafloor spreading / The Ocean Floor – A map of the ocean floor shows a variety of topographic features: flat plains, long mountain chains, and deep trenches. Mid-ocean ridges are part of a chain of mountains some 84,000 km long. The Mid-Atlantic Ridge is the longest mountain chain on Earth. These ridges are spreading centers or divergent boundaries where the upwelling of magma from the mantle creates new ocean floor. The rift zone is the valley between the two ridges where the plates are pulling apart and new material is rising. Deep-sea trenches are long, narrow basins that extend 8-11 km below sea level. Trenches develop adjacent to subduction zones, where oceanic lithosphere slides back into the mantle. (Figure 1.)Earth’s Magnetic Field – The Earth’s magnetic field is thought to arise from the movement of liquid iron in the outer core as the planet rotates. The field behaves as if a permanent magnet were located near the center of Earth, tilted about 11o from the geographic axis of rotation (Figure 2.). Note that magnetic north (as measured by a compass) differs from the geographic north that corresponds to the planet’s axis of rotation.
At present, the lines of force of the Earth’s magnetic field are arranged as shown in Figure 2; the present orientation of Earth’s magnetic field is referred to as normal polarity. In the early 1960s, geophysicists discovered that Earth’s magnetic field periodically reverses; i.e. the north magnetic pole becomes the south pole and vice verse. Hence, Earth has experienced periods of reversed polarity alternating with times (like now) or normal polarity. Although the magnetic field reverses at these times, the physical Earth does not move or change its direction of rotation. / Figure 2. Earth’s magnetic field.
Basaltic lavas contain iron-bearing minerals such as magnetite, which act like compasses. That is, as these iron-rich minerals cool below their melting point, they become magnetized in the direction of the surrounding magnetic field. Studies of ancient magnetism (paleomagnetism) recorded in rocks of different ages provide a record of when Earth’s magnetic field reversed its polarity.
Figure 3. Diagram of magnetic anomalies on the seafloor. Black represents normal polarity; white represents reversed polarity. / In 1963, F. Vine and D.H. Matthews reasoned that, as basaltic magma rises to form new ocean floor at a mid-ocean spreading center, it records the polarity of the magnetic field existing at the time magma crystallized. As spreading pulls the new oceanic crust apart, stripes of approximately the same size should be carried away from the ridge on each side (Figure 3.). Basaltic magma forming at mid-ocean ridges serves as a kind of “tape recorder,” recording Earth’s magnetic field as it reverses through time. If this idea is correct, alternating stripes of normal and reversed polarity should be arranged symmetrically about mid-ocean spreading centers. The discovery of such magnetic stripes provided powerful evidence that seafloor spreading occurs.
The age of the seafloor also supports seafloor spreading. As the seafloor spreads, the youngest oceanic crust is found at the ridges and progressively older crust is found moving away from the ridges towards the continents. The oldest known ocean floor is dated at about 200 million years, indicating that older ocean floor has been destroyed through subduction at deep-sea floor. The Atlantic Ocean has not been destroyed by subduction and did not even exist before that time, instead it was created as the continents on either side of it broke and moved apart.
Procedure:
1. Obtain 2 sheets of unlined paper, a scissor, a colored pencil or marker, and a metric ruler.
2. Using the colored pencil or marker, draw stripes across one sheet of paper, parallel to the short sides of the paper. The stripes should vary in spacing and thickness. Fold the paper in half lengthwise. Write the word “start” at the top of both halves of the paper. Then, using the scissors, cut the paper in half along the fold line to form 2 strips. (Figure 4.) / Figure 4.3. Take the second sheet of paper and lightly fold it into eight sections.
4. Draw lines 5.5 cm long, starting at the fold, on the middle crease and the 2 creases closest to the ends of the paper. Then use the scissors to cut along the lines you drew. Unfold the paper. You should have 3 slits in the center of the paper. (Figure 5.) / Figure 5.
5. Put the 2 striped strips of paper together so that the “Start” labels touch one another. Insert the strips up through the center slit, then pull them toward the side slits.
6. Insert the stripes into the side slits. Pull the ends of the strips and watch what happens at the center slit.
7. Practice pulling the strips through the slits until you can make the stripes come up and go sown at the same time. / Figure 6.
Analysis Questions:
1. What structure does the center slit represent on your seafloor model? ______
2. What structure do the 2 side slits represent? ______
3. What type of plate boundaries are demonstrated in the model? ______
4. What do colored strips of paper represent? ______
5. What do the stripes on the strips of paper represent? ______
6. Describe the process of seafloor spreading in your own words. ______
7. How is this model useful to studying paleomagnetism? ______
Objective: To work a puzzle using plate tectonics vocabulary terms.
Procedure: Identify the terms described below, then locate them in the puzzle. They may be frontward, backward, horizontal, vertical, or diagonal.
Descriptions: Write the word being described in the blank after the statement, then find it in the puzzle below.
1. The sideways horizontal movement that occurs along some plate boundaries. ______
2. The name of the plate upon which we are traveling. ______
3. In a zone of subduction, these creations are produced from the rising of hot magma. ______
4. In this zone, a heavy oceanic plate meets a lighter continental plate and dives under. ______
5. This describes a plate boundary in which the plates come together. ______
6. The name of the supercontinent in which the plates come into one big hunk. ______
7. The name of the once single continent in the Southern Hemisphere. ______
8. The deformation of Earth materials on a large scale. ______
9. A large chunk of the lithosphere. ______
10. A rather narrow rise that has steep sides. ______
11. A deep valley caused by a break in the lithosphere, following along the crest of the mid-oceanic ridge. ______
12. A long, narrow depression formed in the zones of subduction. ______
13. A type of plate boundary in which 2 plates move away from each other. ______
14. A large mountain range in Asia formed when 2 plates crunched into each other. ______
15. The crust plus some upper mantle. ______
16. The mass movement of mantle material caused by heat flow. ______
L / O / L / I / T / H / O / S / P / H / E / R / EI / T / N / E / G / R / E / V / I / D / O / S / D
N / N / H / R / B / O / M / O / D / E / G / N / S
C / E / T / H / I / M / A / L / A / Y / A / S / U
O / G / I / J / K / O / N / C / I / L / M / Q / U
N / R / I / F / T / P / R / S / S / E / E / P / A
V / E / R / I / D / G / E / N / B / T / R / A / L
E / V / C / A / L / U / A / O / P / A / I / N / A
C / N / U / V / Y / W / S / A / X / L / C / G / T
T / O / C / I / D / A / P / I / L / P / A / A / E
I / C / O / N / T / R / E / N / C / H / N / E / R
O / G / O / C / I / N / O / T / C / E / T / A / A
N / G / S / U / B / D / U / C / T / I / O / N / L
Harry Hammond Hess: Spreading the seafloor
URL: http://pubs.usgs.gov/publications/text/HHH.html
Last updated: 05.05.99
Harry Hess (1906-1969) in his Navy uniform as Captain of the assault transport Cape Johnson during World War II. After the war, he remained active in the Naval Reserve, reaching the rank of Rear Admiral. (Photograph courtesy of Department of Geological and Geophysical Sciences, Princeton University.)
Harry Hammond Hess, a professor of geology at Princeton University, was very influential in setting the stage for the emerging plate-tectonics theory in the early 1960s. He believed in many of the observations Wegener used in defending his theory of continental drift, but he had very different views about large-scale movements of the Earth.
Even while serving in the U.S. Navy during World War II, Hess was keenly interested in the geology of the ocean basins. In between taking part in the fighting in the Marianas, Leyte, Linguayan, and Iwo Jima, Hess -- with the cooperation of his crew -- was able to conduct echo-sounding surveys in the Pacific while cruising from one battle to the next. Building on the work of English geologist Arthur Holmes in the 1930s, Hess' research ultimately resulted in a ground-breaking hypothesis that later would be called seafloor spreading. In 1959, he informally presented this hypothesis in a manuscript that was widely circulated. Hess, like Wegener, ran into resistance because little ocean-floor data existed for testing his ideas. In 1962, these ideas were published in a paper titled "History of Ocean Basins," which was one of the most important contributions in the development of plate tectonics. In this classic paper, Hess outlined the basics of how seafloor spreading works: molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge crest and, eventually, sinks into the deep oceanic trenches.
Hess' concept of a mobile seafloor explained several very puzzling geologic questions. If the oceans have existed for at least 4 billion years, as most geologists believed, why is there so little sediment deposited on the ocean floor? Hess reasoned that the sediment has been accumulating for about 300 million years at most. This interval is approximately the time needed for the ocean floor to move from the ridge crest to the trenches, where oceanic crust descends into the trench and is destroyed. Meanwhile, magma is continually rising along the mid-oceanic ridges, where the "recycling" process is completed by the creation of new oceanic crust. This recycling of the seafloor also explained why the oldest fossils found on the seafloor are no more than about 180 million years old. In contrast, marine fossils in rock strata on land -- some of which are found high in the Himalayas, over 8,500 m above sea level -- can be considerably older. Most important, however, Hess' ideas also resolved a question that plagued Wegener's theory of continental drift: how do the continents move? Wegener had a vague notion that the continents must simply "plow" through the ocean floor, which his critics rightly argued was physically impossible. With seafloor spreading, the continents did not have to push through the ocean floor but were carried along as the ocean floor spread from the ridges.
In 1962, Hess was well aware that solid evidence was still lacking to test his hypothesis and to convince a more receptive but still skeptical scientific community. But the Vine-Matthews explanation of magnetic striping of the seafloor a year later and additional oceanic exploration during subsequent years ultimately provided the arguments to confirm Hess' model of seafloor spreading. The theory was strengthened further when dating studies showed that the seafloor becomes older with distance away from the ridge crests. Finally, improved seismic data confirmed that oceanic crust was indeed sinking into the trenches, fully proving Hess' hypothesis, which was based largely on intuitive geologic reasoning. His basic idea of seafloor spreading along mid-oceanic ridges has well withstood the test of time.
Hess, who served for years as the head of Princeton's Geology Department, died in 1969. Unlike Wegener, he was able to see his seafloor-spreading hypothesis largely accepted and confirmed as knowledge of the ocean floor increased dramatically during his lifetime. Like Wegener, he was keenly interested in other sciences in addition to geology. In recognition of his enormous stature worldwide, in 1962 Hess -- best known for his geologic research -- was appointed by President John F. Kennedy to the prestigious position of Chairman of the Space Science Board of the National Academy of Sciences. Thus, in addition to being a major force in the development of plate tectonics, Hess also played a prominent role in designing the nation's space program.
Questions:
1. What did Hess and his crew do while cruising from one battle to the next during World War II? ______
2. Name the book which was one of the most important contributions in the development of plate tectonics? ______
3. How did Hess outline the process of seafloor spreading? ______
4. What was one of the questions that a mobile seafloor explained? ______