3. Fracture zones and transform faults

‘Fracture zones and transform faults’ introduces fracture zones, huge, long linear scars in the seafloor first mapped in the 1950s, and their interpretation in terms of a new concept, transform faulting. Fracture zones are made at mid-ocean ridges, where the seafloor spreads apart. Segments of zones of spreading intersect fracture zones at right angles, along which transform faulting transfers the spreading on one spreading zone to another. As the seafloor spreads, and plates move apart at mid-ocean ridges, fracture zones grow longer. Testing this idea relied on the study of earthquakes that occurred on the transform faults, using seismographs on distant continents. This chapter introduces readers to the pertinent seismological methods by which this was achieved.

5. Rigid plates of lithosphere

‘Rigid plates of lithosphere’ explains that because of the lithosphere’s strength, essentially rigid plates of lithosphere move with respect to one another across the surface of the Earth. Their rigidity allows their rates of relative motion and of their total displacements to be described by rotations about axes passing through the centre of the Earth, or poles of rotation. Global Positioning System (GPS) measurements corroborate the inferences drawn both from rates of seafloor spreading determined using magnetic anomalies and from directions of relative plate motion determined using orientations of transform faults and fault plane solutions of earthquakes along plate boundaries.

4. Subduction of oceanic lithosphere

‘Subduction of oceanic lithosphere’ begins with the notion that for the Earth not to expand, the sum total of new lithosphere made at a spreading centre (or mid-ocean ridge) must be matched by the removal, by subduction, of an equal amount of lithosphere elsewhere. The subduction process is asymmetric: one plate will slide beneath the other at island arcs and continental margins like the Andes of South America. Before it plunges beneath the island arc, the subducting plate of lithosphere bends down gently to cause a deep-sea trench. The subducting plate slides beneath the region between the trench and volcanoes, commonly in large earthquakes, and plunges to great depth, pulled down by gravity acting on the dense slab of subducted lithosphere. Water carried to depth by the subducting plate lowers the melting temperature of the adjacent rock and enables volcanoes to form.

2. Seafloor spreading and magnetic anomalies

‘Seafloor spreading and magnetic anomalies’ begins with the Vine–Matthews Hypothesis, which proposed that strips of seafloor parallel to the mid-ocean ridges, where two plates diverge from one another, were magnetized in opposite directions because the Earth’s field had reversed itself many times. A test of the Vine–Matthews Hypothesis, which required determining the age of the seafloor, became a test of seafloor spreading. Dating the ocean floor using magnetic anomalies detected by magnetometers towed behind ships and core samples extracted during the Deep-Sea Drilling Project confirmed the hypothesis. With magnetic anomalies to date the seafloor and a curve relating seafloor depth and age, the difference between the Atlantic, with its ‘ridge’, and the Pacific and its ‘rise’ became comprehensible. With a theory for predicting the depths of oceans, it was also possible to understand the history of sea-level changes.

7. From whence to whither?

‘From whence to whither?’ discusses two examples that illustrate the role of plate tectonics in topics related to society and science: the recurrence of great earthquakes and the link between plate tectonics and glaciation. It also considers how plate tectonics has affected the way questions are approached in Earth Science. Has plate tectonics facilitated the discovery and acquisition of petroleum resources and ore deposits? Can plate tectonics be observed on other planets? Plate tectonics accelerated a shift from geology being a largely descriptive science aimed mostly at the history of our planet to a quantitative physical science focused on the processes that have made the present-day Earth what it is.

6. Tectonics of continents

‘Tectonics of continents’ shows that the much greater thickness of continental than oceanic crust makes continental and oceanic lithosphere behave differently. First, because crust is less dense and therefore buoyant, compared with the mantle, thick continental crust resists subduction into the asthenosphere. Slices of the upper part of the crust detach from underlying parts and become stacked atop one another to form a mountain range, like the Alps or Himalaya. Second, continental lithosphere is weaker than oceanic lithosphere and when put under stress it deforms. When the horizontal dimension of a region of continental crust is shortened, the crust thickens. Because of isostasy, thick buoyant crust stands higher than thin crust, creating mountain ranges. Various mountain ranges around the world are used to illustrate these principles.

1. The basic idea

‘The basic idea’ presents the principles of plate tectonics and describes how this revolutionary theory took hold. It begins with Alfred Wegener in 1912, who proposed the concept of continental drift and a former huge continent, Gondwanaland. In the face of strong opposition, this theory was supported by the development of palaeomagnetism in the 1950s and, in the 1960s, became subsumed within the broader framework of plate tectonics. Three major events precipitated this change: a switch in emphasis from continents to ocean basins and their exploration; rapid growth in seismology; and a shift in perspective from the chemical stratification of the Earth, in terms of crust and mantle, to another that emphasized strength—a strong lithosphere, some 100–200 km thick, overlying a weak asthenosphere.