Science Matters (27 page)

The acceptance of plate tectonics by the scientific community in the 1960s is one of the great events in the history of science. Despite the fact that almost all geologists considered moving continents to be the rankest sort of heresy, when the data began to accumulate they willingly abandoned the teachings of a lifetime in deference to the new information. Every scientist must be willing to change his or her mind when the data require it.

The most striking evidence for plate tectonics came from an unlikely source: measurements of rock magnetism on the ocean floor. When molten rock comes to the surface, as it does wherever plate boundaries diverge, it usually contains small grains of iron minerals. These grains act like tiny compasses, lining up to point to the north pole. When the rocks solidify, the grains are
locked in, and the rock “remembers” where the north pole was when the cooling occurred.

Earth’s magnetic field has reversed its polarity frequently over geological time. Your compass needle now points north, but a million years ago it might have pointed south. Iron minerals formed from molten rock a million years ago thus have the opposite orientation of more recently formed minerals.

In the 1960s ocean scientists discovered distinctive patterns of magnetic stripes in rocks on both sides of what we would now call diverging boundaries. As new rock rises from the mantle and fills the space left as plates diverge, the iron grains point north and become locked in that position. As time goes by and the older rock moves aside to make way for new material, Earth’s magnetic field eventually undergoes one of its reversals and iron grains in those rocks point in an opposite direction from those of their older neighbors. Repeated reversals of the field produce the striped pattern, which can occur
only
in an environment where new crust is being created continuously
and
the magnetic field is undergoing sporadic reversals.

Plate tectonics suggests that plates will move a few inches per year. Until fairly recently, the idea that continents move was buttressed only by indirect evidence such as that provided by rock magnetism. No one had actually measured continental motion. In 1985 new confirmation for plate tectonics arrived from an unexpected source: extragalactic astronomy. In that year astronomers announced the result of measurements of the radiation from distant quasars. They measured the difference in arrival times for radio waves from the quasars at three observatories: one in Massachusetts, one in Germany, and one in Sweden. From these measurements, they obtained a very precise number for the distances between those observatories. In just over two decades, this distance has grown by more than three feet as the
separation between Europe and North America slowly increases, confirming that the continents do indeed move.

Before the 1960s, scientists who studied the Earth tended to work in isolation from one another. The oceanographers measured currents and temperatures, but they never talked to the paleontologists, who studied fossils, and neither group spoke to the geophysicists, who probed Earth’s deep interior. The various groups seemed to have little in common.

The advent of plate tectonics has changed all that. The model has supplied a common language, a common paradigm, and common ground of concern to all scientists who study planet Earth. Oceanographers now know that what goes on under Earth’s crust affects ocean basins, paleontologists routinely use the evidence available in fossils to track the wandering of the continents across the globe, and geophysicists understand Earth’s interior as a dynamic convecting system that drives the restless crust. Scientists now see planet Earth as a single integrated whole, rather than a series of isolated systems that have nothing to do with one another.

With the discovery of plate tectonics, a lot of seemingly random geological data began to make sense. Seismologists had known that most earthquakes strike in broad circular belts around the world, but they didn’t know why. We now know that those belts coincide with scraping and colliding plates. Volcanoes are most common in long chains of young mountains; we now see that those mountain chains correspond to plate boundaries. Mining geologists found that the largest ore deposits, precipitated in the hot mineralized waters of volcanic districts, often occur above subducting plates; new deposits have been discovered
as a result. And for the first time geologists and paleontologists can explain why distinctive ancient rock formations and fossil deposits match up across vast oceans. The simple idea of plate tectonics illuminates and unifies much of today’s earth science research.

Efforts to predict earthquakes reveal the strengths, as well as the limitations, of plate tectonics. We now know why major earthquakes shake the Los Angeles and San Francisco areas from time to time. Two massive plates are inexorably grinding past each other, and in the process California is being ripped apart. But knowing why doesn’t necessarily tell us when. At the present rate of movement—a few inches per year—a major quake should hit every fifty to one hundred years. But specific events are almost impossible to pinpoint. Sometimes “swarms” of little quakes precede major shocks, but it is not practical to evacuate Los Angeles or San Francisco every time a few small earthquakes are registered.

We do know that over time, as the two plates whose boundary is the San Andreas fault move in opposite directions, stress builds up in Earth’s deep interior. The process is like winding up a spring: eventually the rock fails and the energy is released. We can measure the amount of strain in rocks near the surface, and thereby guess where large earthquakes will most likely occur. But at the moment all we can say with certainty is that another “big one” will happen sooner or later. The 1989 San Francisco earthquake is regarded by some geologists as little more than a warm-up for the large release of energy they expect to happen some time in this century—an impression reflected by their naming it
“the pretty big one.” At the moment, that’s the best our science can do.

The media usually describe the strength of an earthquake by the Richter scale, introduced in the 1930s by the California seismologist Charles F. Richter. Richter assigned a value of 0 to the weakest rumble he could measure with his equipment. Each increment of 1 in the scale means a tenfold increase in seismic signal—equivalent to about thirty times more earthquake energy A magnitude 4 earthquake (a noticeable event) is 810,000 times stronger than one of magnitude 0. The Richter scale is completely open-ended—any number is possible. Today’s sensitive seismometers record earth twitches much weaker than 0 (they are given negative numbers). The 1989 Loma Prieta earthquake, which killed 63 people and caused significant damage in the San Francisco Bay area, registered about 7 on the scale, while the catastrophic 2004 Sumatra-Andaman earthquake registered about 9.2 and was the second strongest ever recorded.

Earth is unique among the sun’s rocky planets. Mercury, Venus, Mars, and our moon are unchanging worlds. Why should our globe be different? Why don’t our neighbors also possess continents that ride on mobile plates?

The critical factor is size. The other worlds are small enough that all the heat generated by radioactivity inside leaks out by conduction as fast as it is produced. You experience a similar effect every time you eat hot food: a potful of soup can stay hot for hours, and a bowl stays hot for several minutes, but a spoonful loses its heat in a matter of seconds. Mars, Mercury, and the moon, all mere spoonfuls of earth-like material, have long since
frozen to inert balls. Any new heat generated in their interiors quickly flows to the surface and radiates away. They have no plates to collide, no great earthquake fault zones or chains of mountains. Venus, which is only slightly smaller than Earth, may once have had its own sluggish version of plate tectonics and may even have active volcanism today. But Venus also proved too small, and its interior apparently no longer convects. Earth, because it is slightly larger and traps its internally generated heat, continues to roll and boil. Given enough time, it too must cool and stop changing, but that won’t happen for billions of years.

The deepest mine descends only about two miles; the deepest borehold penetrates less than ten miles. Scientists are usually wary of placing limits on what humans may eventually achieve, but at present we cannot conceive (even in our wildest fantasies) of a way to journey to Earth’s center. Given such physical limitations, how can anyone possibly know what’s in the Earth’s deep interior? One group of scientists, seismologists, use sound waves to unlock Earth’s hidden secrets.

Seismology is a global-scale variation of sonar. Sonar measures the time it takes for a sound wave (the ubiquitous
ping
sound of submarine movies) to travel to an object (the ocean bottom or another submarine), bounce off, and return. Seismology is almost the same thing. Instead of a ping, seismologists use dynamite or earthquakes to generate a loud enough sound wave to travel through Earth. Any softer sound would be lost in the noise of landslides, construction equipment, and interstate traffic. The time it takes for sound to travel through the planet
depends on the kind of rock through which it travels. By measuring many waves traveling along many different paths from an earthquake or explosion, the seismologists gradually build up a picture of Earth’s deep interior.

The result of seismic explorations is an understanding of Earth’s interior as a series of concentric layers. The innermost layer, called the core, is about 2,600 miles in radius and made primarily of heavy metals like nickel and iron. The inner part of the core is solid, but the outer layer is a sea of liquid metal. Temperatures in the core may reach 7,000°C—enough to vaporize any known material at Earth’s surface.

Surrounding the core and reaching to within a few miles of the surface is the mantle. Made of lighter materials, it is the mantle rocks that move slowly in response to the heat in Earth’s interior and whose motion ultimately results in tectonic activities at the surface.

Finally, Earth’s outer surface, or crust, contains the mountains and valleys, oceans and plains, that make up our familiar surroundings. The crust contains Earth’s least dense materials—those that floated to the top when the planet was molten.

Most seismologists work for oil companies or mining concerns and study small-scale geological features, usually only a mile or so across. They wear rugged clothes and sturdy boots and travel from site to site with a drilling rig and a truckful of detectors called seismometers. The exploration team drills a hole, packs it with explosives, sets off the charge, and records the seismic echos in the hope of finding telltale rock structures that might indicate nearby deposits of valuable minerals or oil.

Government and academic seismologists often study Earth on a much larger scale. They have established hundreds of permanent listening stations around the world. These stations play a vital role by monitoring the location and severity of earthquakes.
By comparing the arrival time, duration, and strength of seismic waves at many different stations, scientists can deduce the exact location and force of each quake. Civic planners depend on that information to predict zones of future earthquakes and thus guide development.

It may seem that earthquake seismologists spend most of their lives waiting for something bad to happen, but these earth scientists also play a key role in preserving peace, since they provide the technical basis for verifying nuclear test ban treaties. An underground explosion, which pushes rock out in all directions, has a different seismic “signature” than natural earth movements, during which rocks slide against each other. No large-scale nuclear test can escape the notice of the global array of seismometers. By computer analysis of the signals, scientists can determine the place and size of any underground blast, even at a distance of thousands of miles.

Plate tectonics directly affects how much you pay at the gas pump, for a knowledge of the positions of ancient plates and continents can lead us to untapped natural resources. One of the major challenges facing today’s oil and mining geologists is to unravel Earth’s tectonic history. Geologists and geophysicists can discover the locations of ancient plates and continents, oceans and mountain ranges by integrating many types of studies—fossil distributions, rock magnetism, field mapping, and seismology.

Oil fields formed eons ago from thick accumulations of organic materials in tropical or temperate zones. Early in the twentieth century, before anyone conceived of the notion of wandering
continents, no one would have predicted the discovery of major oil reserves in the Arctic regions of Alaska, but with our new picture of moving plates and continents it is obvious that once-tropical lands could end up literally at the ends of the Earth. The search for fossil fuels has expanded accordingly.

Plate tectonics has also changed the way we look for metal mines. Many metal deposits lie near ancient plate boundaries, where hot volcanic mineral waters concentrated ore, so modern prospectors study the history of Earth’s wandering plates. Rich mines of gold in China, copper in Chile, nickel in Australia, and molybdenum (used for making hard steels) in the American West have been revealed by the new science of metallogeny.

Other scientists devote their research lives to understanding more about Earth’s deep interior. The mantle forms most of the solid earth, but we don’t know its composition or temperature profile. We know the mantle convects, causing plates and continents to move, but not the details of how this process works.

Today’s earth scientists approach these questions in two ways. One group, called mineral physicists, studies the properties of rocks and minerals that they subject to the very high pressures and temperatures that exist in the mantle. By learning in the laboratory how minerals respond to extreme conditions, they can identify which combination of minerals most closely matches Earth’s deep interior. Mineral physics complements the work of the second group, seismologists, who increasingly focus on determining Earth’s three-dimensional structure. Seismologists once had to analyze signals from one earthquake at a time by hand. Today, however, supercomputers collate data on thousands of earthquakes, from hundreds of seismic stations around
the world. Each piece of data places additional constraints on Earth models. Eventually we hope to obtain a detailed three-dimensional picture of the convecting Earth, a picture that will tell us as never before where our planet has been and where it is going.