Questions: Mid-Ocean Ridge Spreading Rates and Seafloor Aging

Question 1

Seafloor at 1,000 km from the Mid-Atlantic Ridge axis is 40 million years old. What age would you expect for seafloor at 2,000 km from the same ridge axis, assuming constant spreading rate?

• A. 40 million years — seafloor age plateaus once crust has fully cooled
• B. 80 million years — because age increases linearly with distance from the ridge axis
• C. 160 million years — because thermal contraction accelerates aging at greater depths
• D. Cannot be determined without knowing the current half-spreading rate

Answer: (B)

Seafloor age increases linearly with distance from the ridge because new crust forms at the axis and is carried away at constant velocity. Age = distance / half-spreading rate. If 1,000 km corresponds to 40 Ma, the half-spreading rate is 25 km/Ma = 2.5 cm/yr. At 2,000 km, the age is 2 × 40 = 80 Ma. This linear relationship is fundamental to paleogeographic reconstruction and to reading the magnetic barcode. Departures from linearity indicate changes in spreading rate over time.

Question 2

The Mid-Atlantic Ridge has a prominent axial rift valley along its crest, while the East Pacific Rise is a broad, smooth swell without a rift valley. What is the primary cause of this morphological difference?

• A. The Mid-Atlantic Ridge is younger and has not yet developed the smooth profile that emerges with age
• B. The East Pacific Rise spreads faster, maintaining higher magma supply and keeping the crust hot enough to resist the extensional faulting that creates the rift valley at slow ridges
• C. The Mid-Atlantic Ridge sits in shallower water where erosion and mass wasting carve the valley
• D. The East Pacific Rise has thicker oceanic crust that is mechanically too strong to fault into a rift

Answer: (B)

Spreading rate directly controls ridge morphology through the thermal state of the crust. Fast-spreading ridges (>8 cm/yr, like the East Pacific Rise) have continuous, high magma supply keeping the crust thin, hot, and ductile — extensional stresses are accommodated by magmatism rather than faulting, producing a smooth swell. Slow-spreading ridges (2–5 cm/yr, like the Mid-Atlantic Ridge) have episodic, lower magma supply; the crust cools between magmatic pulses, becomes rigid and brittle, and normal faults develop to accommodate the plate divergence, forming a deep axial rift valley.

Question 3

The symmetric pattern of magnetic anomaly stripes on either side of a mid-ocean ridge encodes Earth's geomagnetic polarity reversal history, allowing geophysicists to reconstruct past spreading rates and plate motion history.

• T. True
• F. False

Answer: True

As new basalt cools at the ridge axis, it records the orientation of Earth's magnetic field at that moment. Because the field periodically reverses polarity, successive strips of crust record alternating normal and reversed magnetization. Since the seafloor spreads symmetrically from the axis, these stripes appear as mirror images on either side. Matching the stripe pattern to the independently dated geomagnetic polarity timescale gives the age of each stripe, and the stripe widths divided by their ages give the spreading rate at each period. This is the 'tape recorder' that captures plate tectonic history.

Question 4

Oceanic crust can be found at ages up to several billion years on Earth's seafloor, preserving a nearly complete record of plate tectonic activity since Earth's formation.

• T. True
• F. False

Answer: False

The oldest oceanic crust is only about 200 million years old — a tiny fraction of Earth's 4.5 billion year history. Oceanic crust is continuously recycled back into the mantle at subduction zones. Because it is denser than continental crust, it inevitably sinks when it collides with a continental margin or another oceanic plate. The oldest surviving oceanic crust (in the western Pacific) is Jurassic in age; anything older has been subducted. Continental crust, being less dense, survives subduction and preserves rocks billions of years old.

Question 5

Why is there no oceanic crust older than about 200 million years on Earth's seafloor, and what does this tell us about the fate of oceanic plates compared to continental plates?

Model answer: Oceanic crust is basaltic — denser than continental crust — and therefore sinks into the mantle at subduction zones when it meets resistance. As oceanic plates age, they cool, thicken, and become even denser (negative buoyancy increases), making subduction more efficient. The spreading of new seafloor at mid-ocean ridges continuously pushes old seafloor toward subduction zones, where it is recycled. No oceanic crust survives long enough to accumulate beyond ~200 Ma. Continental crust is granitic — less dense — and resists subduction, so continental rocks can survive for billions of years. The ocean floor is therefore a recycling conveyor belt, while continents are long-term archives.

This asymmetry between oceanic and continental crust is fundamental to plate tectonics. It explains why all Wilson cycle reconstructions older than ~200 Ma rely on continental geology and paleomagnetic data from continents rather than preserved ocean floor. It also explains why the seafloor spreading tape recorder only extends 200 Ma back — beyond that, the tape has been erased by subduction.