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 preparations such as evacuation. Much research is focused on looking for early warning signs of an impending quake, with the hope that we could use early warning systems to initiate evacuations and reduce the damage to human lives that occurs with such events.
Researchers have had little luck successfully predicting most destructive earthquakes. Occasionally a large earthquake will be preceded by smaller earthquakes, slight tremors, volcanic activity, or changes in land level relative to sea level. Such was the case in Haicheng, China in 1975, when warnings a day before a large earthquake occurred saved many lives. However, very few earthquakes send advance warning signals.
Current research around the Pacific Ocean focuses on trying to measure the amount of strain being put on two crustal plates as they press against one another. (When the pressure builds to a certain point and is released, an earthquake occurs; see Chapter 10 for details.) Unfortunately, many complex factors lead to an earthquake event, which makes the effort to predict them very challenging. Fortunately, scientists love a good challenge!
Climate change
In looking into Earth’s past, one area of intense study is paleoclimatology, the study of past climates. Scientists called paleoclimatologists take long, cylinder-shaped samples called cores from ice sheets. In these ice cores, they find trapped gases and dust from the ancient atmosphere that provide clues to the earth’s temperatures long ago. Similar cores of sediments in the bottom of lakes or the ocean may have fossil remains of microscopic organisms. These remains of plant and animal life help scientists called paleoecologists build a picture of the ancient environment and past climates.
These are just two of the types of records scientists use to understand Earth’s past climate conditions. By combining multiple records and including different types of data, paleoclimatologists and paleoecologists build a picture of climate change throughout Earth’s history.
Through these studies, scientists have learned that the earth’s climate has gone through dramatic shifts of warming and cooling in the past. Factors including Earth’s orbital characteristics and the position of the continents are thought to have affected the climate.
By building a more complete understanding of what changes occurred in the past, scientists hope to be able to predict what changes may occur in the future, particularly in light of industrial civilization’s measurable impacts.
Out of this world: Planetary geology and the search for life
Geology is no longer confined to Earth. Advances in scientific understanding of the earth’s planetary systems have helped scientists apply that understanding to other planets. Fields of current research include the search for extraterrestrial, or alien, life.
When astrogeologists — planetary geologists — look at the surface of Mars, they see features that remind them of features on Earth created by running water. While there is no water on Mars’s surface now, these features suggest that large amounts of water once flowed across the surface, presumably originating from underground sources on the planet.
The exploratory Mars rover project is currently collecting sediment samples and other
