How do modern scientists measure the rate of seafloor spreading?

Renewed interest in oceanographic research in the 1950's led to extensive mapping of the world's ocean basins and to the discovery of the world wide oceanic ridge system.

In 1962, Harry Hess postulated that the ocean floors are spreading apart due to convection currents in the mantle. According to this theory, magma from the interior of the earth is injected at mid-ocean ridges resulting in new oceanic crust spreading symmetrically away from mid-ocean ridges.

Magnetometers towed behind research ships can detect small variations (about 1%) in the geomagnetic field called magnetic anomalies.

The anomalies (magnetic stripes) are due to the magnetized rocks on the sea floor. Ocean crust magnetized in the normal direction shows a positive (+) anomaly; reversely magnetized crust shows a negative (-) anomaly.

Repeated intrusions of magnetized crust at the ridge and sea-floor spreading produces a symmetrical series of magnetic anomalies (highs and lows) on both sides of the ridge.

Hess's hypothesis of sea-floor spreading was confirmed by Vine, Matthews, and Morleys' interpretation of magnetic anomalies in terms of parallel stripes of normal and reversely magnetized oceanic crust produced at mid-ocean ridges.

When lava is extruded and solidifies at any mid-ocean ridge, the rock becomes magnetized and acquires the magnetic polarity that exists at the time the lava cools through its Curie Temperature.

As sea-floor spreading continues, new intrusions split the previously formed magnetized crust in half and the two blocks are transported laterally away from the ridge.

The sequence of magnetic polarities preserved within the oceanic crust on both sides of the ridge is identical to the K-Ar dated magnetic polarity time scale preserved in continental lava flows. The highly magnetized rocks of the ocean crust are pillow basalts.

The ocean crust behaves like a giant magnetic tape recorder imprinted with the geomagnetic polarity time scale!

Age of the ocean crust and determination of spreading rates

Marine magnetic anomalies can be used to determine the absolute age of the ocean crust beneath a particular anomaly with reference to the geomagnetic polarity time scale.

Knowing the age of the ocean crust beneath an anomaly and by measuring its distance from a mid-ocean ridge, the RATE OF SEA-FLOOR SPREADING (plate motion) can be estimated:

velocity of plate = distance of anomaly from ridge/age of anomaly

Source