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time, scientific understanding of the earth’s crust as a continuous, solid, rigid layer did not allow for moving continents, and without a clear explanation for how they moved, Wegener’s hypothesis was strongly rejected by other geologists.

A dramatic breakthrough occurred in the decades after World War I when the use of submarines in warfare led to mapping of the seafloor with sonar. The 1960s in particular was a time of new discovery and understanding. Geologist Marie Tharp discovered a long, rocky ridge, a rift, in the middle of the Atlantic Ocean while drawing an ocean floor map from sonar data. (Her map, created with Bruce Heezen, is still the standard map of the ocean floor used by all scientists.)

      The idea of seafloor spreading — the moving apart of oceanic crust along ridges on the ocean floor — was being explored by scientists, led by the ideas of Harry Hess. But there were still many who “knew” the earth’s crust and the mantle below were solid rock and could not be moving. One skeptic, Canadian geologist J. Tuzo Wilson, eventually published his ideas about plates moving across hotspots (see Chapter 10 for details) and his ideas provided a foundation on which the unifying theory of geology was built. The theory of plate tectonics combines ideas about plate movement with evidence for seafloor spreading, as well as incorporating explanations for volcanoes, earthquakes, and other geologic features and phenomena. Because this theory is so crucial, I devote Part 3 of this book to it.

      Scientists never stop exploring, of course, so even with a well-accepted, well-tested explanation of how the surface of the earth constantly transforms, they don’t stop asking questions.

Forging Ahead into New Frontiers

      After geologists had the theory of plate tectonics laid out, they had a framework within which they could propose and test specific hypotheses to fill in the details. This work continues today, right now, as you read these words! The frontiers of earth science are being expanded in many directions. In this section, I describe just a few areas of current, exciting research and discovery.

      Asking how, where, and why: Mountain building and plate boundaries

      Plate tectonics theory explains that the movement of plates creates mountains by pushing crustal rocks together and up (see Chapters 9 and 10). But scientists have not gathered enough evidence to agree on what forces drive the uplift of mountains. Some suggest that a pushing force, exerted by the neighboring plate, forces the rocks upward. Others suggest that the removal of rocks by erosion (explained in Part 4) leads the continental rocks to “float” upward, like an iceberg melting in the ocean.

In Chapter 8, I present a line drawing of plate boundaries. It may seem very straightforward, with lines neatly separating continental plates from one another. But some areas of that map are almost unknown, and the lines have been drawn based on best-guess estimates. In regions such as the northeast Pacific plate, near Kamchatka (a peninsula in eastern Russia), researchers today map earthquake and volcano events in an attempt to pinpoint plate boundaries.

      Mysteries of the past: Snowball earth, first life, and mass extinctions

      Later in this book (in Chapter 16), I explain exactly how long and complex Earth’s history is. Many of the events in Earth’s history can be interpreted from patterns in rocks that scientists observe today. The downside to a history told in rocks is that many chapters are missing. These gaps in the record of Earth’s past provide fascinating topics for further scientific exploration.

      Snowball earth

      A hypothesis currently being debated proposes that at some point (between approximately 600 million and 1 billion years ago) the entire planet was covered with ice. This idea is called the snowball earth hypothesis.

      Some of the evidence to support this hypothesis includes rock formations that are the result of massive layers of ice (glaciers, which I describe in Chapter 13) covering the continents near the equator (at that time). Some scientists argue that an earth covered in ice could not sustain life, and there is evidence of life in rocks from both before and after the suggested time of the snowball. Others want to know what caused the snow and ice to eventually melt. Another hypothesis based on the same evidence suggests that rather than a snowball, the earth was merely a “slushball” that could have, in some areas, still supported life during an extended, very cold period.

      Will the snowball earth hypothesis fade into history as a fanciful idea? Or will it be revived, perhaps proven partially true by future studies and incorporated into an accepted geologic theory? Only time (and more research) will tell.

      Earliest life

Fossils found in rocks provide a long history of life on Earth, going back nearly 3.6 billion years. These early life forms were tiny, single-celled, simple organisms, such as bacteria. But even at that level, life is a very complicated thing. How did nonliving matter become living matter? Scientists suspect that energy of some kind acted on chemical elements, creating the proper combination to spark life. They have even re-created such a scenario in a laboratory. But until fossil evidence in the rocks is found that can provide clues about the nature of earliest life, the question is still up for debate.

      Mass extinctions

      Long after the first life forms existed, Earth experienced periods when many different species thrived, filling the oceans and eventually the land. At least five times in Earth’s long history, thousands of species were wiped out in a very short time. (Geologically speaking, a “short time” can span a few million years; I explain geologic time in Chapter 16.) Such events are called mass extinctions.

      Even if you haven’t heard about the other extinctions, you likely know about the extinction of the dinosaurs. But the extinction of the dinosaurs was not the largest mass extinction event in Earth’s history. Hundreds of millions of years before the dinosaurs, an extinction took place that killed 80 percent of all the plant and animal groups existing at the time.

      In Chapter 22, I describe what is currently known about the mass extinctions in Earth’s past. However, geologists and paleontologists (people who study fossils) have many unanswered questions about how and why these periods of major extinction occurred. Some propose changes in climate as the culprit, and others point to meteor impacts or extreme volcanic activity. Still others claim only a combination of all these factors could have led to such dramatic mass extinctions.

      Predicting the future: Earthquakes and climate change

      Scientists in many different fields hope someday to understand enough about Earth’s systems to be able to predict what changes may occur in the near future. Two examples I describe here are efforts to predict earthquakes before they occur and the science of future climate change.

      Earthquake warnings

      You may have firsthand experience with the literally earth-shaking event of an earthquake. If not, you certainly have seen news reports of the terrible devastation that occurs in some regions of the world when strong earthquakes occur. The ability to predict an earthquake event could lead to lifesaving

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