Chapter 5: The Earth-Moon System

Continental drift fossil evidence. As noted by Snider-Pellegrini and Alfred Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined.

For centuries, geologists studied observable characteristics of earthquakes, mountain building, and volcanic eruptions without understanding the underlying geophysics creating these features. It wasn't until the last century that progress was made thanks to one simple observation on a map. If you stare at a globe long enough you may notice that the continents seem to appear to be puzzle pieces that could be fit together. Sir Francis Bacon first noted this in the 17th century, but it wasn't until the early 1900s that the implications of this observation were realized by German geophysicist Alfred Wegener. He pointed out that the coastlines of some continents, such as South America and Africa, could fit together, and in fact have matching geology and biology on either side of the sea. Wegener boldly proposed that those two continents once were joined, and had split and drifted apart in the ancient past. He called this phenomenon continental drift.

Portrait of Alfred Wegener

Portrait of Francis Bacon

Continental plates of the Earth

The theory that Earth's surface features are molded primarily by moving plates, dragged along by currents in the underlying plastic asthenosphere, is called the theory of plate tectonics. This theory is now universally accepted among geologists. Its acceptance was the major revolution in geological science in the 20th century.

Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North American and Eurasian Plates. The map also shows Reykjavik, the capital of Iceland, the Thingvellir area, and the locations of some of Iceland's active volcanoes (red triangles), including Krafla.

The blocks of the lithosphere that move are called plates, and they include both continental masses and large regions of the seafloor. Where two plates move apart, fresh basalt wells up from the mantle and forms new crustal rock to fill in the gaps. This is called a spreading center, one example of which is the Mid-Atlantic Ridge, a mostly underwater seam where young lava extrudes. Iceland, a highly volcanic island, is an exposed section of the Mid-Atlantic Ridge where volcanos rise above the sea. 

Oceanic plates are subducted creating oceanic trenches.

The collision of two continental plates can compress the lithosphere laterally and crumple it to form mountain ranges. For example, the Himalayas are young mountains formed where the Indian plate has pushed into the southern Asian plate. In places where a continental plate collides with an oceanic plate, the denser, basaltic ocean floor is forced below the less dense, granitic continent. This process is called subduction, and the surface expression is a deep trench where the oceanic plate is driven downward. Eventually, the oceanic plate is forced deep enough to melt. This cycle of new rock being created at mid-ocean ridges and destroyed at subduction zones ensures that the ocean floors are much younger than the continents, which are never subducted.

The location of the Ring of Fire.

Plates can also slip along next to each other at transform boundaries. The San Andreas Fault in California is a transform fault and the source of many earthquakes. In fact, a global map of seismic activity reveals an outline of all of the Earth's tectonic plates. This is because earthquakes are concentrated along plate boundaries, where stresses build up from plates moving against each other. An earthquake is the sudden release of that stress. Likewise, volcanic activity is clustered along mid-ocean ridges and subduction zones, where molten mantle material has access to the surface. Unfortunately, over half a billion people live on the "ring of fire" that traces the plate boundaries at the rim of the Pacific Ocean.

Simplified animation of the break-up of Pangaea and the formation of modern continents.

The motions of the plates might be imperceptible, but they are real. The techniques of radio astronomy have been used to measure directly the motion of continents from space. Europe and North America are moving apart at a rate of about 5 centimeters per year, as fast as a fingernail grows. The Nazca plate in Peru is racing along at 16 centimeters per year, as fast as hair grows. About 150-200 million years ago, Europe and North America were locked in a geological embrace. In fact, all the continents were joined in a primeval landmass called Pangaea.