Heating of the inner portions of Earth result from fusion of hydrogen atoms. Whereas the population of our planet reached the first billion mark near the year 1830, today's population is approximately 6 billion with an additional billion added in increasingly shorter periods of time. Both the Red Sea and the Gulf of California are the recent result of rifting and sea-floor spreading. Convergence of two ocean plates leads to the construction of island arcs such as the Aleutians, the Tonga Islands, and Japan. The epicenters of subduction zone earthquakes range in depth from shallow at the trench to several hundred kilometers beneath the overriding plate.
Transform faults connect ends of offset ocean ridges.
Sedimentary rocks at the bottom of a geocline are as old as the initial rifting that formed the ocean basin. As early as 1911, Alfred Wegener presented ideas about sea floor spreading. California is destined to sink into the ocean. The fluid motion of the asthenosphere is the cause of the Earth's magnetic field. Basalt magma is a more fluid liquid than that of andesite because it contains less silica and is at a higher temperature.