Following World War II, oceanographers equipped with new marine tools and ample funding from the U.S. Office of Naval Research embarked on an unprecedented period of oceanographic exploration. Over the next 2 decades, a much better picture of large expanses of the seafloor slowly and painstakingly began to emerge. From 1957 to 1977, pioneering cartographer, geologist, and oceanographer Marie Tharp painstakingly drew bathymetry maps with pencil and paper using data from these studies ▼. These maps revealed not a flat seafloor but one comprised of deep basins, even deeper trenches, and expansive mountain ranges. From this work came the discovery of a global oceanic ridge system that winds through all the major oceans.

In other parts of the ocean, more discoveries were being made. Studies conducted in the western Pacific demonstrated that earthquakes were occurring at great depths beneath deep-ocean trenches, contradicting the belief that earthquakes only occur at shallow depths. Of equal importance was the fact that dredging and drilling of the seafloor did not bring up any oceanic crust that was older than 180 million years. Further, sediment accumulations in the deep-ocean basins were found to be thin, not the thousands of meters that had been predicted. By 1968 these developments, among others, had led to the unfolding of a far more encompassing theory than continental drift, known as the theory of plate tectonics.

Rigid Lithosphere Overlies Weak Asthenosphere

According to the plate tectonics model, the crust and the uppermost, and therefore coolest, part of the mantle constitute Earth’s strong outer layer, the lithosphere (lithos = stone). The lithosphere varies in both thickness and density, depending on whether it is oceanic or continental ▼. Oceanic lithosphere is about 100 kilometers (60 miles) thick in the deep-ocean basins but is considerably thinner along the crest of the oceanic ridge system—a topic we will consider later. In contrast, continental lithosphere averages about 150 kilometers (90 miles) thick but may extend to depths of 200 kilometers (125 miles) or more beneath the stable interiors of the continents. Further, oceanic and continental crust differ in density. Oceanic crust is composed of basalt, a rock rich in dense minerals of iron and magnesium, whereas continental crust is composed largely of less dense granitic rocks. Because of these differences, the overall density of oceanic lithosphere (crust and upper mantle) is greater than the overall density of continental lithosphere. This important difference will be considered in greater detail later in this chapter.

The asthenosphere (asthenos = weak) is a hotter, weaker region in the mantle that lies below the lithosphere ▲. In the upper asthenosphere (located between 100 and 200 kilometers [60 to 125 miles] depth), the pressure and temperature bring rock very near to melting. Consequently, although the rock remains largely solid, it responds to forces by flowing, similarly to the way clay may deform if you compress it slowly. By contrast, the relatively cool and rigid lithosphere tends to respond to forces acting on it by bending or breaking but not flowing. Because of these differences, Earth’s rigid outer shell is effectively detached from the asthenosphere, which allows these layers to move independently.

Earth’s Major Plates

The lithosphere is broken into numerous segments of irregular size and shape called lithospheric plates, tectonic plates, or simply plates, that are in constant motion with respect to one another ▼. Seven major lithospheric plates are recognized and account for 94 percent of Earth’s surface area: the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. The largest is the Pacific plate, which encompasses a significant portion of the Pacific basin. Each of the six other large plates consists of an entire continent, as well as a significant amount of oceanic crust. Notice in the figure below that the South American plate encompasses almost all of South America and about one-half of the floor of the South Atlantic. Note also that none of the plates are defined entirely by the margins of a single continent. This is a major departure from Wegener’s continental drift hypothesis, which proposed that the continents move through the ocean floor, not with it.

Intermediate-sized plates include the Caribbean, Nazca, Philippine, Arabian, Cocos, Scotia, and Juan de Fuca plates. These plates, with the exception of the Arabian plate, are composed mostly of oceanic lithosphere. In addition, many smaller plates, called microplates, have been identified but are not shown in the figure above. For example, the relatively small Anatolian plate, situated between the African, Arabian, and Eurasian plates, is part of a complex tectonic system responsible for the devastating 2023 Turkey-Syria earthquake.

Plate Movement

One of the main tenets of the plate tectonics theory is that plates move as somewhat rigid units relative to all other plates. As plates move, the distance between two locations on different plates, such as New York and London, gradually changes, whereas the distance between sites on the same plate—New York and Denver, for example—remains relatively constant. However, parts of some plates are comparatively “weak.” For example, southern China is literally being squeezed as the Indian subcontinent (on the Australian-Indian plate) rams into Asia proper (on the Eurasian plate).

Because plates are in constant motion relative to each other, most major interactions among them occur along their boundaries, and this is, therefore, where most deformation occurs. In fact, plate boundaries were first established by plotting the locations of earthquakes and volcanoes. Plates are delimited by three distinct types of boundaries, which are differentiated by the type of movement they exhibit. These boundaries are briefly described here:

• Divergent plate boundaries—where two plates move apart, resulting in upwelling and partial melting of hot material from the mantle to create new seafloor ▲ 

• Convergent plate boundaries—where two plates move toward each other, resulting either in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed into the mantle, or possibly in the collision of two continental blocks to create a mountain belt ▲ 

• Transform plate boundaries—where two plates grind past each other without the production or destruction of lithosphere ▲ 

Divergent and convergent plate boundaries each account for about 40 percent of all plate boundaries. Transform boundaries account for the remaining 20 percent. In the following sections we will discuss the three types of plate boundaries.

• Research conducted after World War II led to new insights that helped revive Wegener’s hypothesis of continental drift. Exploration of the seafloor uncovered previously unknown features, including an extremely long mid-ocean ridge system. Sampling of the oceanic crust revealed that it was quite young relative to the continents.

• The lithosphere, Earth’s outermost rocky layer, is relatively stiff and deforms by bending or breaking. The lithosphere consists both of crust (either oceanic or continental) and underlying upper mantle. Beneath the lithosphere is the asthenosphere, a relatively weak layer that deforms by flowing.

• The lithosphere consists of numerous segments of irregular size and shape. There are seven large lithospheric plates, another seven intermediate-size plates, and many relatively small microplates. Plates meet along boundaries that may be divergent (moving apart from each other), convergent (moving toward each other), or transform (moving laterally past each other).

• lithospheric plates (a.k.a., tectonic plates, or plates): Coherent units of Earth's rigid out layer that include the crust and uppermost mantle.

• theory of plate tectonics (plate tectonics): A well-tested theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.