The concept of plate tectonics describes the outer rigid shell of the Earth (the lithosphere) as consisting of six major plates and a number of smaller ones. These plates converge, move apart and slide past each other, movements which explain the dynamics of earthquakes and volcanoes. The framework of plate tectonics, now generally accepted by scientists, encompasses the controversial theories of continental drift and sea-floor spreading, and has been revolutionary for the history of science. Over the past 20 years, through plate tectonics scientists in a wide range of Earth science disciplines have found their work to be interdependent.
How could continents actually move? For most geologists, this was the great stumbling block of Alfred Wegener’s proposal in 1912 that the geological characteristics of opposing continents such as Africa and South America could only be adequately explained if they were once joined. Years later, suggestions that volcanic eruptions on the sea floor and intrusions in the Earth’s crust below it could push the continents apart (so-called sea floor spreading) were substantiated by studies of the magnetic properties of oceanic crust. The Earth’s magnetic field periodically reverses polarity, and the magnetic minerals of erupting basalts record the ambient orientation of the field as they cool. They define a linear pattern of magnetic reversal stripes about the axis from which they were produced. Each ocean has such an axis, which takes the form of a linear (usually central) submarine mountain chain, or mid-ocean ridge system.