Questions: Seafloor Spreading and Mid-Ocean Ridges

Question 1

Why are symmetric magnetic anomaly stripes on the ocean floor considered compelling evidence for seafloor spreading?

• A. The stripes are parallel to coastlines, confirming that oceanic crust forms at continental margins and spreads toward ocean centers
• B. The stripes are symmetric about ridge axes and their widths match the independently dated timescale of Earth's magnetic reversals, showing continuous symmetric creation of new crust
• C. The presence of alternating normal and reversed magnetic bands confirms that Earth's magnetic field reverses on a predictable schedule
• D. The stripes are stronger in older crust, confirming that oceanic crust gains magnetic intensity as it ages and moves away from the ridge

Answer: (B)

The symmetry is the key. If seafloor spreading were occurring, both plates moving away from the ridge would receive identical magnetic imprints as the same lava cooled at the same ridge axis during the same interval. The result would be a mirror-image stripe pattern on both sides of the ridge — exactly what oceanographers found in the 1960s. Correlating stripe widths to the independently established magnetic reversal timescale (from dated continental rocks) allows calculation of spreading rates and confirms the mechanism. Option C is true on its own but is not why the stripes are evidence for *spreading* specifically — it's the symmetry about the ridge axis that is diagnostic.

Question 2

The Mid-Atlantic Ridge spreads at ~2 cm/year while the East Pacific Rise spreads at >10 cm/year. Which ridge is most likely to have a prominent central rift valley, and why?

• A. The East Pacific Rise, because faster spreading creates stronger tensional forces that pull the ridge center downward
• B. The Mid-Atlantic Ridge, because slow spreading means intermittent magma supply, allowing the axial region to subside into a rift valley between eruptions
• C. Both ridges have similar rift valleys because the same divergent process generates the same morphology at any spreading rate
• D. Neither ridge has a rift valley; rift valleys only form at continental divergent boundaries, not at oceanic spreading centers

Answer: (B)

Spreading rate controls magma supply. Fast-spreading ridges like the East Pacific Rise have an abundant, near-continuous magma supply that keeps the ridge axis thermally supported and inflated, producing a broad, gently domed rise. Slow-spreading ridges like the Mid-Atlantic Ridge have an intermittent magma supply; between eruptions, the axial region cools, contracts, and subsides, creating a deep rift valley (a graben) flanked by fault-bounded mountains. The Mid-Atlantic rift valley can be 1–2 km deep and 10–30 km wide — structurally similar to continental rift valleys.

Question 3

The oldest oceanic crust is dramatically younger than the oldest continental crust because oceanic crust is continuously created at ridges and destroyed at subduction zones.

• T. True
• F. False

Answer: True

The oldest oceanic crust is approximately 200 million years old, found in the western Pacific near the Mariana Trench. The oldest continental rocks exceed 4 billion years. This disparity exists because oceanic crust, being denser and thinner than continental crust, is subducted back into the mantle at convergent boundaries. Continental crust is too buoyant to subduct and accumulates over billions of years. The continuous recycling of oceanic crust means the ocean floor is geologically young despite the oceans themselves being ancient features of Earth's surface.

Question 4

Seafloor spreading causes the total surface area of Earth's ocean basins to increase continuously over geological time as new crust is created at mid-ocean ridges.

• T. True
• F. False

Answer: False

New crust created at mid-ocean ridges is balanced by destruction of old crust at subduction zones. Earth's total surface area remains constant; the creation of new seafloor at ridges is compensated by the consumption of old seafloor at trenches. If creation outpaced destruction, Earth would need to expand — a hypothesis that was proposed (the 'expanding Earth' hypothesis) but is not supported by evidence. The approximately constant total area of ocean basins over time is a constraint that requires subduction to consume crust at the same average rate that ridges create it.

Question 5

Explain how magnetic anomaly stripes on the ocean floor can be used to determine the age of the seafloor at a specific location and calculate the spreading rate of the ridge.

Model answer: As basaltic lava erupts at the ridge and cools, magnetic minerals freeze in the orientation of Earth's magnetic field at that moment. Because the field periodically reverses polarity, successive eruptions record alternating normal and reversed orientations, creating a striped pattern as the crust spreads away from the ridge. By correlating these stripes to the independently dated magnetic reversal timescale (established from dated continental rocks), scientists can assign an age to each stripe. The age of any point on the seafloor is the age of the reversal whose stripe it lies in. Spreading rate is then distance from the ridge axis divided by the age of that stripe.

This is a beautiful example of cross-calibration between independent datasets. The magnetic reversal timescale was established from continental igneous rocks, independently dated by radiometric methods. The stripe pattern on the seafloor then acts as a tape recording of that timescale, stretched or compressed by the spreading rate. Wide stripes mean fast spreading; narrow stripes mean slow spreading during that interval. The match between the continental timescale and the seafloor stripe pattern was one of the most powerful validations of both plate tectonics and geomagnetic reversal history.