The study of earthquakes and their causes has been an active area of research since the late nineteenth century. John Milne's development of the seismograph
in the 1890s and subsequent establishment of a network of seismographs enabled seismologists to detect and locate earthquakes. It also allowed scientists to use seismic waves to probe the interior of the planet and infer the planetary structure (see From Earthquakes to Continental Drift). Against this backdrop of knowledge, Alfred Wegener suggested in 1915 that the seven continents were once one large land mass that broke apart creating the continents, which then drifted to their current locations. Strong support of this idea did not come until the mid 1950's, when paleomagnetic data supported Wegener's claim (see Magnetic Clues). Scientists used sonar measurements to map the ocean floor leading to the discovery of a large undersea mountain ranges with midocean ridges. To explain the ridges, Henry Hess theorized that the seafloor was slowly spreading away from the ridges, driven by convection currents in the mantle (see Insights from the Ocean). Confirmation of Hess's theory of seafloor spreading came from a magnetic survey of the material on either side of a midocean ridge (see Of Magnetism and Time). Examining the ridges and faults radiating from the ridges helped show that the earth's crust is made up of several plates (see Plates in Motion).
The understanding of plate tectonics has led to improved building methods in earthquake-prone areas. Maps of the earth's surface have also helped identify areas for mining and petroleum exploration (see Putting Geophysical Insights to Work). An unexpected bonus has been the discovery of entire new ecosystems that live near the hydrothermal vents, created by the spreading of the sea floor (see Window on a New World).
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