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Seafloor spreading is the process by which new oceanic crust is created at mid-ocean ridges as tectonic plates diverge. Magma upwells and solidifies, recording Earth's magnetic field orientation at the time of formation. Symmetric magnetic anomaly stripes on either side of ridge axes provided key evidence confirming plate tectonics. The ocean floor is geologically young (< 200 Ma) compared to continents because it is continuously created at ridges and destroyed at subduction zones. Spreading rates range from slow (< 2 cm/yr at the Mid-Atlantic Ridge) to fast (> 10 cm/yr at the East Pacific Rise), controlling ridge morphology.
Interpret ocean floor magnetic anomaly maps: correlate stripe widths to spreading rate and age, and identify ridge axis locations. Connect magnetic reversal timescale from paleoclimatology to the seafloor tape recorder.
You already understand that Earth's lithosphere is divided into rigid plates that move relative to one another, and that the ocean floor sits on oceanic plates that are created and destroyed over time. Seafloor spreading is the specific mechanism of creation: at a mid-ocean ridge, two plates pull apart (diverge), and hot mantle rock rises to fill the gap. As this upwelling material reaches the surface, it melts partially, producing basaltic magma that erupts on the seafloor and solidifies into new oceanic crust. The process is continuous — new crust pushes older crust aside symmetrically on both sides of the ridge, like a conveyor belt running in two directions.
The most elegant evidence for seafloor spreading comes from magnetic anomaly stripes. As basaltic lava cools at the ridge, iron-bearing minerals in the rock align with Earth's magnetic field and freeze in that orientation. Because Earth's magnetic field periodically reverses polarity (north and south switch), the newly formed crust records a series of alternating normal and reversed magnetic bands. These stripes are symmetric about the ridge axis — a mirror image on each side — because both plates receive the same magnetic imprint as they move apart. When oceanographers towed magnetometers across the seafloor in the 1960s and discovered this zebra-stripe pattern, it provided some of the most compelling confirmation of plate tectonics. By matching the stripe widths to the independently dated magnetic reversal timescale, scientists can calculate spreading rates and determine the age of the ocean floor at any point.
Spreading rate profoundly controls the character of the ridge itself. Fast-spreading ridges like the East Pacific Rise (full rates exceeding 10 cm/year) have robust magma supplies that keep the ridge axis inflated, producing a broad, gently sloping rise with a shallow axial summit trough. The crust formed here tends to be relatively uniform in thickness and layering. Slow-spreading ridges like the Mid-Atlantic Ridge (full rates around 2 cm/year) have intermittent magma supply, so the ridge axis is dominated by a deep rift valley — a graben-like depression 1–2 km deep and 10–30 km wide — flanked by rugged, fault-bounded mountains. The crust at slow ridges is more heterogeneous, with stretches where tectonic extension exposes mantle rock directly on the seafloor rather than building basaltic crust.
Because all oceanic crust is eventually consumed at subduction zones, the ocean floor is remarkably young by geological standards — the oldest seafloor is only about 200 million years old, compared to continental rocks exceeding 4 billion years. This continuous cycle of creation at ridges and destruction at trenches means the ocean basins are geologically ephemeral features. Mid-ocean ridges are also sites of intense hydrothermal activity: seawater circulates through the fractured young crust, heats up near the magma source, leaches metals and minerals, and vents back into the ocean as superheated fluid at hydrothermal vents — ecosystems sustained entirely by chemosynthesis rather than sunlight. Seafloor spreading thus connects plate tectonics to ocean basin geometry, magnetic field history, deep-sea biology, and the chemical cycling of elements between Earth's interior and its oceans.