Science Matters

by Robert M. Hazen

  Why Plate Tectonics Is Important to Science

  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.

  Earthquakes

  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.

  Why There Are No Mountain Chains on Mars

  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.

  A WINDOW INTO THE SOLID EARTH

  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 th
e 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.

  FRONTIERS

  Searching for Buried Treasure

  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.

  The Earth’s Deep Interior

  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.

  Unstable Magnetic Poles

  Earth’s magnetic field owes its origin to the rotation of the liquid outer core, but beyond that we know little about why our planet behaves like a giant magnet. Since the core is electrically neutral, its rotation does not produce an electric current and cannot, in and of itself, produce a magnetic field. There are, however, somewhat more complex ways in which a rotating neutral conductor can create a field, so that’s not the real quandary scientists face.

  The real problem is that Earth’s north and south poles have not always been where they are now. The magnetic pole wanders, usually remaining near the north pole, but changing position by about twenty-five miles every year. Today the pole lies near Ellesmere Island in northern Canada at a latitude of about 83 degrees north, and it is moving steadily northward.

  In addition, at various times in the past, Earth’s magnetic field has reversed, a process that may take a few hundred or thousand years while the “north” pole shifts to Antarctica. We can see over 300 of these reversals in the geological record, and we really don’t have a very good idea of why they happen. Geologists are faced with the unenviable task of producing a theory that predicts a steady magnetic field that at seemingly random times flips directions.

  CHAPTER FOURTEEN

  Earth Cycles

  NEXT TIME YOU’RE AT the beach, pick up a handful of sand and look at it—really examine it. You’ll notice that each grain differs from its neighbors. Some may be black, others shiny; still others may be green or white or various shades of brown. If you look at the grains under a microscope, more differences appear. Some look smooth and rounded, some sharp and angular. All of these differences arise because the grains of sand you are holding, despite their differences, share one important property: all are part of one of the great cycles that operate on our planet.

  The grains of sand are different colors because each comes from a different rock inland from the beach. The grains have different shapes because they may have been washed to the beach, buried, incorporated into new rocks, uplifted, and washed to new beaches many times. The great cycles of weathering and erosion of rocks, sedimentation, and creation of new rocks has gone on since Earth’s beginning, and will continue until the sun burns out and the planet dies. Because of this cycle, it is possible that you hold in your hand the very first grain of sand that formed on the very first beach when Earth was young.

  As scientists examine nature in operation, they recognize many ongoing processes—natural actions that constantly change the surface of the globe. Rain falls, gradually washing away rocks and soils and creating sand and silt. Rivers flow, carrying those sediments from hills and mountains to the valleys below. Ocean and lake waters evaporate, creating new rain clouds. Rocks, water, and atmosphere—the matter that forms the outer layers of our planet—are forever being shifted from place to place.

  Water evaporates from the oceans and flows back, sometimes on the surface, sometimes underground, and sometimes stopping for a while in an inland lake. When the climate turns cold, water is taken up in huge ice sheets that spread out from the poles, and sea levels fall around the world. With warmer weather the ice sheets retreat and the water flows back into the sea. Like rocks, water moves in cycles.

  Even the air moves in stately cycles, from the prevailing winds that bring us our daily weather to the long-term effects that constantly change the climate.

  In fact,

  Earth operates in cycles.

  Today scientists recognize that all of Earth’s cycles are connected, each influencing the others. We are beginning to see our planet as a kind of marvelous machine, full of turning gears and moving parts. And most wonderful of all, we are beginning to understand how that machine works and how all the parts fit together.

  CYCLES OF CHANGE

  No feature on Earth is permanent. Mountains weather away, continents break apart, oceans disappear, glaciers form and melt. Change is the hallmark of our planet. Yet amidst all this change, there is constancy. For all practical purposes Earth has a fixed budget of atoms. For an atom to be used in one structure, it must be taken away from another. Like a child in a room filled with wonderful building blocks, Earth has a large but finite number of pieces to play with.

  THE ROCK CYCLE

  Earth’s surface displays a remarkable variety of rocks. But despite this variety, geologists classify rocks into only three basic
types: igneous, sedimentary, or metamorphic. Cataloging rocks is not just an academic exercise. Each type of rock records a different complex past—a past revealed by mineral textures and form. Each type of rock can be changed from one form to another and then back again. Geologists call these transformations the rock cycle.

  Igneous Rocks

  The crust of our planet began as molten rock; from space that early Earth must have appeared as a spectacular incandescent ball. The rock cycle could not begin until that glowing outer layer began to solidify. In the beginning, all rocks on Earth were igneous—fire-formed.

  Volcanism is the most spectacular process that produces new igneous rocks today. Volcanic rocks arrive at the surface, either in the air or underwater, as magma—the molten form of rock. The most obvious and destructive volcanoes occur on land, where huge fountains of incandescent molten rock light the night sky and rivers of lava destroy life and property while reshaping the landscape. Most of those volcanoes produce dark basalt lavas, which possess a sticky fluid consistency before they harden. Occasionally, as in the Mount St. Helens eruption in 1980, lavas are thick and viscous like tar so that little flow can occur. An epic explosion may be required to relieve the pent-up pressure.