Plate kinematics

History of ideas on continental drift and plate tectonics

Isostatic compensation was discovered in the 19th century, when surveyors mapping the Indian subcontinent realized that the mass of the Himmalayan mountains should be causing deflection of plumb lines used to measure vertical. When they calculated the deflection it was found to be substantially less than predicted, indicating that the mass of the mountains above sea level was isostatically compensated by lighther than average rocks below.

Continental drift was proposed by Wegener in the early 20th century to explain certain features of the continents. In Wegener's concept, continents drifted somewhat like large icebergs through surrounding material that formed the mantle and the ocean floors. Wegener's continental drift was not supported by most European and North American geologists, because geophysical information indicated that the ocean floors were too rigid to flow.

Paleomagnetism was used by geophysicists who found evidence in the 1950s that supported relative movements of continents, and more geologists began to support a mobile Earth.

Also in the 1950s, major jumps in seismic properties were used to define the base of the crust (the Moho), the mantle, and the core.

Magnetic reversals: Paleomagnetic studies in the 1960s indicated that the Earth's field had reversed itself spontaneously over time.

Linear magnetic anomalies on the ocean floor, also discovered in the early 1960s provided a record of those reversals, and indicated that the ocean floors, though rigid, were continually growing at the mid-ocean ridges by the process of ocean floor spreading.

A world-wide standardized seismograph network, initially set up to monitor nuclear testing, provided new types of information about the fault movements that produced earthquakes, allowing slip vectors to be calculated.

In 1967, slip vectors at deep ocean trenches, at mid-ocean ridges, and at the San Andreas fault were shown to be consistent with movements of rigid plates on a sphere.

Plate tectonics combined these ideas in the late 1960s and 1970s to explain many (not all) major tectonic features of the Earth's surface.

Direct measurement of lithosphere movement was initially attempted with laser ranging devices, but became much easier from about 2000 through the application of GPS (global positioning system) technology.

Numerical modelling of motions in the mantle and core have become increasingly possible due to increases in computer speed and capacity, to the point where it's now possible to simulate circulation of material within the Earth, subject to uncertainties about the properties of those materials, which are not directly observable.