Make a record as rock forms
The ridge model adds rock at the center and moves it outward symmetrically. A parcel keeps the magnetic-polarity label assigned when it formed. The current field does not repaint all the older rock.
How can moving rock leave a clock on the ocean floor?
Pull a ridge apart over millions of years. Read the age and magnetic pattern left behind, then compare spreading, sliding and subduction.
Enable JavaScript to change the conditions and run the interactive experiment.
Make a discovery
At a spreading ridge, new crust forms near the axis while older rock moves away. Rock age and recorded magnetic polarity leave patterns that help scientists reconstruct motion.
Make a prediction
At 3 cm/year per side, how far apart are two matching parcels after one million years?
- 3 km
- 30 km
- 60 km
Read the explanation
Each parcel travels 30 km from the ridge in opposite directions. Their separation is 60 km.
Understand it
Read width as distance
A longer interval of one polarity makes a wider band at a fixed spreading rate. A faster rate also makes wider bands for the same time interval. Real reversal intervals are unequal; our two schedules are hypothetical examples.
Compare boundary behavior
Divergent boundaries separate. Transform boundaries slide past one another. At selected convergent boundaries an oceanic plate can descend beneath another. These describe relative motion; they are not a calculated cause of it.
Look closer at the science
Rate × time
A half-spreading rate of 3 cm/year moves one side 30 km in one million years. The matching pair is 60 km apart. The slider is a half-rate, not the total separation rate.
Magnetic evidence is interpreted
Rock can preserve a magnetic record, while measured magnetic anomalies depend on geometry and magnetization. The colored strips are an ideal record, not a measured anomaly profile or a map of colored seafloor.
The mantle is not a global lava ocean
Earth’s plates include oceanic and continental lithosphere. Much of the mantle is solid yet deforms over geological timescales. This kinematic model prescribes plate motion; it does not solve mantle flow, rock fracture, magma generation or earthquake prediction.
Try it yourself: Make a paper ocean floor
Supplies
- Two paper strips
- Card base with an adult-prepared slot
- Two colored pencils
- Ruler and tape
- Bring new paper up
Feed paired strips through the central slot and fold them outward in opposite directions. Hands supply the motion underneath.
- Mark unequal intervals
Pull equal distances for each pretend time step. Change marker color after unequal numbers of steps. Mark both sides at the ridge.
- Read the mirror
Cover one side and predict its pattern. Repeat with a faster pull but the same time schedule. Wider bands can reflect a faster rate.
Can you infer the opposite side’s pattern before revealing it?
A kinematic analogy, not a model of mantle forces, heat or earthquakes. An adult prepares slots; no learner blade work is needed.
Sources and model limits
- Generic symmetric ridge with prescribed constant half-rate, not a reconstruction of a specific ocean.
- Hypothetical irregular reversal schedules. Colors mean polarity in the record view and material in the boundary views.
- The narrow warm-colored ridge identifies a formation region. The underlying layers remain solid-model context; subducting rock is not shown instantly melting.
Plate boundaries can diverge, converge or slide past one another.
This Dynamic Earth is a historical agency synthesis; used here for boundary classification and kinematics.
USGS · Plate motionsPaired magnetic patterns and independent dates helped support seafloor spreading.
Agency evidence account. Our stripes are hypothetical and are not copied measured profiles.
USGS · Magnetic stripes and clocksPaper strips can model spreading and subduction routing.
John C. Lahr’s original activity. Its simplified force/melting descriptions are not imported.
USGS · Paper spreading modelIndependent subject review is pending.
Read the sources and model assumptions
