1. Part 1: The Creation of New Oceanic Crust
Seafloor Spreading
Part 1: The Creation of New Oceanic Crust
AUTHORS: Ashlee Henig, Scripps Institution of Oceanography, and Stephen Halpern, San Diego High School
WHY: This presentation combines all of the information students have learned about the seafloor (continental drift fossil, rock, mountain evidence, etc., seafloor age distribution, and the pattern in seafloor magnetic stripes) to explain the creation of new oceanic crust in a process called seafloor spreading. Did you know that the movement of the seafloor causes some of the biggest and most destructive earthquakes in the world?
SUMMARY: Seafloor spreading is the basis for the modern theory of Plate Tectonics. It is the process by which new seafloor is created as the plates are pulled apart and new material emerges in the center gap. Main points of this presentation are: 1) New seafloor forms as seafloor spreading centers and spreads out to both sides of the middle causing the symmetric/mirror image pattern of seafloor ages and magnetic stripes; 2) Seafloor forms as different rates in different oceans, but always has the same mirrored pattern; 3) Because seafloor spreading occurs on a sphere, the spreading center is broken up into many “ridge segments” that are connected by other features called transform faults and fracture zones.
NOTE: This presentation is broken into two parts (separated by a part 1 conclusion slide). Students will complete some activities in class before moving on to the second part of the lecture.
PICTURE/GRAPHICS CREDITS: Credits given on each individual slide
WEBSITES USED IN THIS PRESENTATION: N/A
ADDITIONAL READING: Wikipedia article on seafloor spreading and plate tectonics; any earth science textbook.
CONTEXT FOR USE: Activity is good for middle or high school classes. It is the apex of the Seafloor Spreading Centers Unit and explains the process of seafloor spreading and how all of the evidence fits into the theory.
MISCONCEPTIONS: The Earth is expanding because seafloor is created but not destroyed. The continents are “drifting” through the oceans.
EVALUATION TIPS: The pop quiz, worksheet, and paper spreading center model can be used for evaluation of understanding (PopQuiz_SeafloorSpreadingUnit.doc, Worksheet_SF_Spreading.doc, Activity_SeafloorSpreadingModel.pdf).
TEACHING NOTES. Slide notes are contained within each slide. Please view the lesson plan for additional information on this activity, including required files and materials.
SUPPORTING DOCUMENTS:
Activity_SeafloorSpreadingModel.pdf
Lecture_SeafloorSpreading.ppt
Lesson_Plan_SeafloorSpreading.doc
PopQuiz_SeafloorSpreadingUnit.doc
Worksheet_SeafloorSpreadingModel.doc
Worksheet_SeafloorSpreadingModel_key.doc
Worksheet_SF_Spreading.doc
Worksheet_SF_Spreading_key.doc
Part Two:
Worksheet_RingOfFire_Video.doc
Worksheet_RingOfFire_Video_key.doc
Scripps Classroom Connection

2. “Continental Drift” Evidence
This slide should be review of material from the Continental Drift lesson. Simply remind students of the evidence/arguments for Continental Drift. 1) The “fit” of the continents. 2) Animal and plant fossils. 3) Mountain Belts. 4) Evidence of glaciation. Ask students if these could be evidence of something else besides continental drift? (all of this is also evidence for seafloor spreading). It is important to note that the continents were once together, but now they are separated. Something had to be put in between the continents in order for them to move apart.
For additional information on each of these lines of evidence, see the notes in the Continental Drift lecture: Lecture_ContinentalDrift.ppt
Figure Credits:
Scripps Classroom Connection

3. Seafloor Ages & Magnetic Anomalies
This slide should be review of the seafloor ages and seafloor magnetic stripes lectures. Remind students of the symmetric patterns in seafloor ages and magnetic anomalies. Ask what must be happening at the part of the seafloor with 0 Myr age (they should be able to tell you that new crust is forming there). Ask students what all of this evidence together could mean? (Someone should know seafloor spreading).
For additional information on seafloor ages and magnetic stripes, see the lectures on Seafloor Age and Magnetic Stripes: Lecture_SF_MagneticStripes.ppt and Lecture_SeafloorAges.ppt
Figure Credits:
Age
Magnetics
Scripps Classroom Connection

4. Seafloor Spreading Scripps Classroom Connection
Figure: Cartoon showing two oceanic plates (brown) overlying mantle (orange). The plates are separated by a seafloor spreading center. A magma chamber (red) lies beneath the seafloor spreading center and feeds new material to form new oceanic crust.
Seafloor Spreading occurs when the seafloor is pulled open in opposite directions and new crust forms in the gap. As this process repeats, the seafloor grows. This is how the symmetric age and magnetic stripes pattern is formed. The new material inserted between the plates magnetizes under the influence of the Earth’s magnetic field at the time it formed. That same seafloor also ages progressively as it moves away from the oceanic ridge. There are various factors that may cause seafloor spreading, but the two most commonly accepted reasons are a force called slab pull (discussed later) where the old, dense parts of the plates pull the younger parts apart, and mantle convection in which currents and upwelling in the mantle may pull the plates apart.
This is the process that explains all of the evidence we have talked about up to this point: the evidence of continental drift (fossils, glaciers, mountain belts, shape and fit of the continents), symmetric seafloor magnetic stripes, symmetric seafloor ages. Seafloor spreading is the mechanism by which all of these lines of evidence came to exist, and they all point toward the creation of new seafloor in such a way that some new material moves to one side of the spreading center and some material moves to the other side.
Figure credit:
Scripps Classroom Connection

5. Each side of the ridge is the same!!
Age and Magnetic Field recorded in the seafloor and spread to each side of the ridge
Each side of the ridge is the same!!
Figure: Magnetic stripes created in the seafloor are spread away from the spreading center in both directions over time.
NOTE: This slide is similar to the following slide (which has a website to demonstrate this concept). You may choose to use one slide or the other depending on internet access, or use both.
As new crust is created some of it is pulled to each side of the ridge where is ages as it gets farther away. The magnetic field of Earth is recorded in this newly formed crust as it cools below a certain temperature (The Curie point). Some new rock is pulled in each direction and the magnetic stripes make a mirror image. Each new block (in the figure) shows the seafloor after a new eruption that has been magnetized and spread to the sides.
Figure Credits:
Scripps Classroom Connection

6. Creation of Magnetic Stripes on the Seafloor
If you have internet in the classroom, this website is useful for demonstrating how new seafloor is formed, magnetized, and spread off axis. If there is no internet, you can simply describe the process using the previous slide.
Go to this website:
You can adjust the speeding of spreading and switch the polarity of the magnetic field. Stripes will appear on the seafloor for each reversal of the magnetic field.
Scripps Classroom Connection

7. How Fast is the Seafloor Spreading?
Slow: km/Myr
Intermediate: Km/Myr
Fast: 90+ km/Myr
Map: Map showing the ages of the seafloor, which can be used to understand seafloor spreading rate/speed. Atlantic is a slow spreading ocean, while Pacific is a fast spreading ocean.
The seafloor spreads at different rates in different areas. Different forces and amount of magma affects spreading rate. Full spreading rate is the total amount of material that forms on BOTH sides of the ridge in a certain amount of time. A fairly simple way to see the difference in spreading rates of the various seafloor spreading centers is to look at the ages of the seafloor and note how much seafloor was formed in a certain amount of time in various locations. For example, if you look at the youngest group of seafloor ages (the “reds”) in both the Pacific and Atlantic Oceans, you can see that there is much more “red” in the Pacific. Another way to say this is that the 40 Myr isochron is farther from the spreading center in the Pacific than it is in the Atlantic. This means that much more seafloor was made in the Pacific in the last 40 Myr (the “reds) than what was made in the Atlantic in that amount of time. Based on the amount of new seafloor created in each ocean in the same amount of time, one can see that the Pacific Ocean is spreading much faster than the Atlantic (Atlantic Ocean – about the speed that your toenails grow while Pacific Ocean – about the rate your fingernails grow). Discuss the division of spreading rates into slow, intermediate, and fast (as noted on the slide).
Figure adapted from:
Full Spreading Rate (km/Myr)
Scripps Classroom Connection

8. Ridge Segment Scripps Classroom Connection
Figure (top left): Illustrates the the seafloor spreading center is not a straight line. Instead it is jagged.
Figure (Right): Zoomed in image of parts of a seafloor spreading center with labels and arrows showing direction of motion.
This slide explains a few features of the seafloor created by seafloor spreading. If we zoom in on a section of the mid-oceanic ridge, we can easily see the zig-zag geometry formed by seafloor spreading centers. Since spreading is happening on a globe/sphere instead of a flat Earth, the spreading centers cannot develop in straight lines. Instead, they are made up of lots of spreading “segments” that each spread individually to create new seafloor. The segments are connected by Transform Faults (where two plates are sliding past each other between segments). This geometry forms features called Fracture Zones, which are fossils of transform faults where seafloor made at two adjacent spreading segments move in the same direction. The two sides of a fracture zone DO NOT move relative to each other (but instead move in the same direction); once the seafloor moves to the outside bounds of two adjacent ridge segments, a fracture zone is formed and is an inactive feature. Fracture zones create deep grooves in the seafloor that can stretch across entire ocean basins (bottom left). Fracture zones can tell us about the positions of old ridge segments.
Figure Credits:
Scripps Classroom Connection
Scripps Classroom Connection

9. Conclusions Part 1
There is lots of evidence pointing to seafloor spreading!
New seafloor forms at seafloor spreading centers.
Seafloor forms about as fast as your nails grow!
Seafloor spreading does not happen in a straight line.
This conclusion slide is optional for the students. If you prefer, simply reiterate these main concepts without showing this text slide to the students.
1) The evidence for Continental Drift is also evidence for seafloor spreading: fit of continents, matching fossil types and mountain belts, and glacial markings on the continents. The symmetric pattern of magnetic stripes is evidence for seafloor spreading, indicating that the new seafloor is volcanic in origin (so it can record the magnetic field), and that the newly formed seafloor spreads out to both sides of the oceanic ridge. The pattern of seafloor ages are also evidence for seafloor spreading. Youngest seafloor is at the middle of the pattern indicating that new seafloor is being formed there. The oldest seafloor on the edges indicates that new material is being created in the middle and pushing older seafloor to each side.
2) New seafloor forms at seafloor spreading centers and spreads out to both sides causing the symmetric/mirror image pattern of seafloor ages and magnetic stripes. New seafloor forms when the oceanic plates are pulled apart and new material fills the gap between them.
3) Seafloor forms as different rates in different oceans. On average it spreads at about the rate that your nails grow! The Pacific Ocean is a fast spreader, while the Atlantic is a slow spreader. This can be seen by analyzing the amount of seafloor that forms in each ocean during the same amount of time – much more forms in the Pacific than in the Atlantic!
4) Because seafloor spreading occurs on a sphere, the spreading center is broken up into many “ridge segments” that are connected by other features called “transform faults” and “fracture zones”. New seafloor is created at individual ridge segments, which together create the entire oceanic ridge. At transform faults, two plates move in opposite directions. Fracture zones are fossil transform faults where the seafloor on each side moves in the same direction.

10. Part 2: The Life Cycle of the Seafloor
Seafloor Spreading
Part 2: The Life Cycle of the Seafloor
AUTHORS: Ashlee Henig, Scripps Institution of Oceanography (Originally taught in the classroom of Stephen Halpern, San Diego High School)
WHY: This presentation combines all of the information students have learned about the seafloor (continental drift fossil, rock, mountain evidence, etc., seafloor age distribution, and the pattern in seafloor magnetic stripes) to explain the creation of new oceanic crust in a process called seafloor spreading. Did you know that despite seafloor spreading, the Earth is NOT growing?
SUMMARY: Seafloor spreading is the basis for the modern theory of Plate Tectonics. As seafloor ages and moves outward, it often eventually collides with continents and is destroyed causing catastrophic earthquakes and volcanoes. This is the life cycle of the seafloor. Main points of this presentation are: 1) Seafloor is destroyed at areas called subduction zones, where the seafloor dives underneath another plate and into the mantle; 2) The deepest places on earth (called Trenches) are associated with subduction zones and are located near the coasts of the continents (in most cases).; 3) Subduction causes devastating earthquakes and explosive volcanoes, many of which are located around the coasts of the Pacific ocean and termed the Ring of Fire; 4) The Pacific ocean is shrinking over time due to subduction, while the Atlantic ocean is growing because it has no subduction.
NOTE: This is the second part of the seafloor spreading lecture. Please consult the lesson plan for activities that should be completed before moving on to this section of the lecture.
PICTURE/GRAPHICS CREDITS: Credits given on each individual slide
WEBSITES USED IN THIS PRESENTATION: N/A
ADDITIONAL READING: Wikipedia article on subduction, Ring of Fire, and plate tectonics; any earth science textbook.
CONTEXT FOR USE: Activity is good for middle or high school classes. It is the an important part of the Seafloor Spreading Centers Unit and explains the process of seafloor destruction.
MISCONCEPTIONS: The Earth is expanding because seafloor is created but not destroyed.
EVALUATION TIPS: The paper spreading center worksheet and Ring of Fire Video worksheet can be used for evaluation of understanding (Worksheet_SeafloorSpreadingModel.pdf; Worksheet_RingOfFire_Video.doc).
TEACHING NOTES. Slide notes are contained within each slide. Please view the lesson plan for additional information on this activity, including required files and materials.
SUPPORTING DOCUMENTS: Worksheet_RingOfFire_Video.doc
Worksheet_RingOfFire_Video_key.doc
Figure Credits:
Scripps Classroom Connection

11. Is the Earth Expanding? Scripps Classroom Connection
Figure: cartoon showing the Earth as two different sizes.
Up to this point in the seafloor spreading centers unit, we have learned a lot of evidence that points to the creation of new seafloor and we have learned about the mechanism of seafloor creation. If the seafloor continues to spread and new material is created, one might expect that the earth would swell up and expand over time as new seafloor is added to the oceans. THIS IS NOT THE CASE! As this lecture will show, seafloor is also being destroyed and Earth remains the same size. The students don’t know this yet though. Try to get them to reason out what must be happening to the seafloor if it is being created but Earth isn’t growing:
Ask students if the seafloor is spreading, is the Earth growing? (No, of course not). Ask students why isn’t the Earth growing? What is happening that keeps it from growing? (seafloor is destroyed at subduction zones)
Figure Credit:
Scripps Classroom Connection

12. “Pacific Ring of Fire” Scripps Classroom Connection
This figure shows the Pacific Ring of Fire, which is the boundary around the pacific ocean that is created by a large number of earthquakes and volcanoes.
Scientists noticed some patterns in the global distribution of earthquake and volcano locations. In this figure, the black triangles represent volcanoes, while the colored circles represent earthquakes occurring in and around the Pacific Ocean. As shown above, the earthquakes and volcanoes occur mostly around the boundary of the Pacific Ocean (and what we now know to be boundaries of other small oceanic plates). Scientists realized that something must be happening at the Pacific boundary (lots of energy is being released there in the form of earthquakes and volcanoes). By studying the earthquakes closer, you will notice that there is a pattern with respect to the depth at which they occur (purple is shallow, red is deep). Earthquakes become progressively deeper moving away from the oceanic plate – something is angled downward into the earth and is causing the quakes. You may ask students what causes the earthquakes (two plates break and slip past each other). This process that causes earthquakes goes deeper and deeper into the earth with increasing distance from the trench. This is the evidence that scientists needed to determine that the outer edges of the seafloor in the Pacific Ocean are diving back down into the earth, causing earthquakes on their way down through the mantle! The seafloor is being destroyed here!! The process in which (usually old and dense) seafloor dives back into the earth and is destroyed is called SUBDUCTION (shown in greater detail in the next slide).
Figure Credit:
Scripps Classroom Connection

13. Seafloor Destruction: “Subduction Zones”
Figure: subducction zone showing earthquakes along the downgoing plate and volcanoes created by melting caused by water introduced to the mantle by the downgoing plate.
Here is a larger image of a subduction zone, showing the sinking oceanic plate diving at an angle beneath the continental plate. It is usually old seafloor that has become dense during the aging process (from cooling and addition of dense mantle to the base of the plate) that “sinks” beneath an adjacent plate and returns back into the earth. Earthquakes (red circles, here) occur where the oceanic plate scrapes the continental plate, and also within the subducting plate. The area within the subducting plate where earthquakes occur is called the Benioff Zone, in which earthquakes become progressively deeper away from the subduction zone due to the diving angle of the downgoing plate.
Also point out how volcanoes form on the continental plate over the subducting plate. This happens because water trapped in the subducting oceanic plate causes the mantle to melt into magma. The process of adding water to the mantle to cause melting is similar to adding salt to ice to lower the freezing point of ice. By adding salt to ice, the ice doesn’t freeze until a lower temperature is reached – this is used for making ice cream or de-icing sidewalks and driveways in cold areas. Similarly, adding water to the mantle decreases the melting temperature of the mantle meaning that it will melt at a lower temperature (and turn into magma) when it is mixed with water.
Figure Credit:
Earthquakes
Scripps Classroom Connection

14. Seafloor Destruction: “Subduction”
Figure: cartoon showing how seafloor spreading and subduction are connected. This figure shows how seafloor can be created and destroyed at the same time.
It is still not completely clear to scientists what forces drive plate tectonics. Some believe that convection currents in the mantle (symbolized by red arrows) pull the plates along. Just like a conveyor belt, a plate would subduct where the mantle convection cell moves downward. This process is driven by buoyancy forces. Others believe that the main force is due to the sinking of the dense downgoing slab. This is called “slab pull.” As the old end of a plate sinks back into the mantle, it pulls the rest of the plate behind it. In either case, the downgoing plate sinking back into the earth causes the deep oceanic trenches to occur at subduction zones. These are the deepest places on the seafloor, and now we can see why they are not in the middle of the seafloor. Rather, they occur closer to the edges of the continents than to the center of the seafloor.
Figure Credit:
Scripps Classroom Connection

15. Scripps Classroom Connection
This figure shows the location of global earthquakes. Point out the Pacific Ring of Fire and ask students what is different about the Atlantic ocean. Why are there no/few earthquakes along the continents bordering the Atlantic ocean? (there is no subduction occurring in the Atlantic). Ask students what they think will happen to the Atlantic if no seafloor is being destroyed there? (It will continue to grow). What will happen to the Pacific? (It will shrink and disappear).
Figure Credit:
Scripps Classroom Connection

16. Conclusions Part 2
The Ring of Fire is defined by earthquakes and volcanoes.
The Ring of Fire is caused by subduction.
Seafloor is destroyed by subduction.
Deep trenches happen at subduction zones.
The Pacific ocean is shrinking; the Atlantic ocean is growing.
This conclusion slide is optional for the students. If you prefer, simply reiterate these main concepts without showing this text slide to the students.
1) Scientists noticed a pattern of earthquakes and volcanoes occurring around the edges of the Pacific Ocean that they termed the Ring of Fire.
2) Subduction causes devastating earthquakes and explosive volcanoes, many of which are located around the coasts of the Pacific ocean on the Ring of Fire.
3) Seafloor is destroyed at areas called Subduction Zones, where the seafloor dives underneath another plate and into the mantle.
4) The deepest places on earth (called Trenches) are associated with subduction zones and are located near the coasts of the continents (in most cases). These form as the dense, old seafloor sinks into the earth.
5) The Pacific ocean is shrinking over time due to subduction around its edges, while the Atlantic ocean is growing because it has no subduction.