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📘 How does the seafloor spread?

A mid-ocean ridge is a nearly 65,000-kilometre volcanic seam where tectonic plates pull apart and new oceanic crust forms. Follow the evidence outward: hot mantle rises at the axis, basalt cools into fresh lithosphere, and that crust gradua

6
lessons
~30 min
to learn
Adults
level
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What you’ll learn

  1. A crack becomes a ridgeExplain how divergent plate motion creates the global mid-ocean ridge system.Ridges are active boundaries where plates separate and mantle melt fills the opening.
  2. Magma becomes new crustDescribe how basaltic magma cools into oceanic lithosphere and interacts with seawater.Magma becomes basaltic crust, while cooling and hydrothermal circulation leave physical and chemical evidence.
  3. The seafloor keeps timeExplain how magnetic reversals become symmetrical stripes in oceanic crust.Cooling basalt records field polarity, and spreading carries matching bands away from the axis.
  4. Age increases away from the ridgeUse crustal ages, trenches, and transforms to trace the life of oceanic lithosphere.Ocean crust gets older away from ridges and is eventually recycled at subduction zones.
  5. How we know spreading happensCombine bathymetry, heat flow, earthquakes, and modern motion measurements as evidence.Independent observations reveal active spreading boundaries beneath the oceans.
  6. Read the whole spreading storySynthesize the formation, magnetic recording, movement, and recycling of seafloor.A persistent centimetre-scale process can build ocean basins over geological time.

Questions this course answers

What happens at a mid-ocean ridge?

Divergence lets mantle melt rise and cool into new oceanic lithosphere.

Match each ridge feature to its role.

Different features record different parts of ridge construction and movement.

Why can mantle rock melt beneath a spreading ridge?

Decompression melting occurs when rising mantle experiences lower pressure.

Put the basic crust-forming sequence in order.

Divergence opens the setting, melting supplies magma, cooling makes crust, and plate motion transports it.

Why do magnetic stripes form on oceanic crust?

Cooling magnetic minerals preserve successive normal and reversed field states.

Match the observation to its meaning.

Magnetic surveys turn rock magnetization into a time and motion record.

Grounded in trusted sources

  • U.S. Geological Survey, This Dynamic Earth: Developing the theory, https://pubs.usgs.gov/gip/dynamic/developing.html
  • U.S. Geological Survey, This Dynamic Earth: Magnetic stripes and isotopic clocks, https://pubs.usgs.gov/gip/dynamic/stripes.html
  • U.S. Geological Survey, Sea-Floor Spreading and Subduction Model, https://pubs.usgs.gov/of/1999/ofr-99-0132/
  • Woods Hole Oceanographic Institution, Mid-ocean ridges, https://www.whoi.edu/ocean-learning-hub/ocean-topics/how-the-ocean-works/seafloor-below/mid-ocean-ridges/
  • Woods Hole Oceanographic Institution, Mid-Ocean Ridges: Magnetics and Polarity, https://www.whoi.edu/ocean-learning-hub/multimedia/mid-ocean-ridges-features/
  • NOAA National Centers for Environmental Information, Earth Magnetic Anomaly Grid 2, https://www.ncei.noaa.gov/products/earth-magnetic-model-anomaly-grid-2
  • Wikimedia Commons MediaWiki API, image metadata and thumbnails, https://commons.wikimedia.org/w/api.php

Every Wunder lesson is built from real, reputable sources — never invented.

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