A volcano erupts on the island of Réunion in the Indian Ocean. © www.istockphoto.com/Julien Grondin
A volcano erupts on the island of Réunion in the Indian Ocean. © www.istockphoto.com/Julien Grondin
Humans live on unsteady ground. This fact was made abruptly apparent on January 12, 2010, when a magnitude 7.0 earthquake devastated the Caribbean island nation of Haiti. Because many buildings in this impoverished country were poorly constructed, they could not withstand the shaking and collapsed en masse, particularly in the capital city of Port-au-Prince, 15 miles from the quake’s epicenter. Rescue workers desperately combed through rubble, hunting for survivors. By early February, Haiti’s prime minister estimated that more than 200,000 people had been killed by the earthquake’s effects. This type of earthquake, caused by two separate sections, or plates, of the Earth’s crust sliding against each other is known as a strike-slip earthquake. It is just one example of the restlessness of Earth’s surface.
This book will show how the movement of rock within the Earth is explained by the theory of plate tectonics, the idea that Earth’s outer layer is broken into moving pieces. It will show how Earth adds and subtracts land over time. In addition, it will explain how and why volcanoes erupt and earthquakes shake the ground we live on.
As early as 1620, scholars noticed that the outlines of continents could fit together like puzzle pieces. In 1912, German scientist Alfred Wegener first explained the concept of “continental drift.” He postulated that a single supercontinent, which he called Pangea, once existed, but broke apart into several pieces over geologic time. Wegener cited the existence of similar types of rocks and fossils found from separate continents as proof. He also used continental drift to explain the evidence of major climate and biological changes.
By the late 1960s, scientists had pieced together the “how” of plate tectonics. Earth’s crust—the top layer of the lithosphere—is broken into pieces called plates. The plates rest on and slide over a layer of plastic and molten rock. As the plates move, they shape the features of Earth’s surface above, including continents and oceans. Movement takes time—a mere 5 to 10 cm (2 to 4 inches) per year in some places, less in others. Hundreds of millions of years have passed since Pangea began to separate into the land masses we know today.
The surface of each tectonic plate is made up of continental crust and oceanic crust. The continental crust is less dense and thus floats higher than oceanic crust. It is also about 40 km (25 miles) thick. While oceanic crust averages about 6.4 km (4 miles) thick. Tectonic plates interact at convergent, divergent, and transform boundaries.
When the boundary of oceanic crust meets, or converges with, continental crust, the more buoyant continental crust stays afloat while the oceanic crust is forced beneath it—a process called subduction. The subducting crust melts under the pressure and heat of Earth’s mantle, and the process offers an explanation of why continental crust is generally older than oceanic crust. When two oceanic plates meet, the older plate subducts under the younger one. In contrast, when two continental crusts meet, neither gives way. Instead, the earth is forced up, creating mountains. The thickness of the crust aids this process. The Himalayas were formed when the plate under India and the plate under Asia met. They are still applying enough pressure to cause the Himalayas to grow taller each year.
Two plates slide past each other at transform boundries. These plate interactions neither create nor destroy crust. Most transform boundries lie beneath the ocean: however, some, such as the San Andreas Fault in California, occur on land. Faults associated with convergent and transform boundries appear as two huge masses of rock tensely locked in place. When the plates build up enough pressure to overcome the friction holding them, the releasing energy causes the ground to shake—an earthquake.
Earth’s volume is a constant. As crust moves, subducts, and is consumed by the mantle, new crust is formed. When some plates move together, other plates move apart, or diverge. At divergent bundries, hot, liquid magma from Earth’s mantle fills in the gaps and creates new crust. This process contributes to the spreading and expansion of the ocean floor. On land, between two diverging continental plates, the ascending magma creates a rift. The best example of continental rifting is the East African Rift Valley, and evidence suggests that the land east of the rift will separate from Africa one day.
Some scientists doubted Wegener because his ideas about the driving force behind plate movement were unsubstantiated. One of the leading modern theories contends that heat circulating within Earth’s mantle, essentially convection, causes plate movement.
Paleoclimatologists, for example, provided evidence for plate tectonics. The splitting of Pangea into Gondwana and Laurasia kept Earth’s polar areas much warmer than they are today. Between about 100 million and 70 million years ago, the poles were warm enough to support forests. Once the plates shifted, wind patterns and ocean currents changed to such a degree that polar ice sheets began to grow. Scientists have also speculated about the major effects of plate tectonics on evolution and diversity of life.
Theories of diverging and subducting plates have informed the study of volcanoes to a great degree. In the mid-1700s, Encyclopædia Britannica described volcanoes as “burning mountains.” Today, it is commonly known that volcanoes function as vents that release magma and gases from beneath Earth’s surface. The release is called an eruption.
About 80 percent of volcanoes are located in subduction zones. The line along which subduction occurs is called a trench, and these features, are the deepest areas on Earth. The pressure and water within oceanic crust cause the rock to melt from 70 to 200 km (40 to 120 miles) below the surface. Since the magma is less dense than the rock around it, it rises and melts more rock on its way to the surface. Under the ocean, volcanoes form along the trench, building crust and landforms called island arcs.
Some volcanoes occur where plates are diverging. Although rifting is a constant process, volcanoes only occur intermittently along the rift, when enough pressure has built up. In some places, “hot spots” form volcanoes in the middle of plates. The volcanoes of Hawaii are examples of hot spots.
Sometimes the magma flows out of pre-existing fissures in subducting and diverging plates. Sometimes it collects below a vent until it creates enough pressure to produce an explosive eruption. Where does this force come from? Besides rock, magma contains many gases. Rapid expansion of these gases can cause violent volcanic eruptions.
Naturally, the most violent, destructive volcanoes have the most gaseous magma. When magma reaches the surface, it is called lava. The more gas within the lava, the faster lava flows and spreads. Alone, volcanic gases—including carbon monoxide, carbon dioxide, and hydrogen sulfide—can quickly kill plants, animals, and people. In 1986, a gas cloud released through a volcanic crater lake took the lives of more than 1,700 people in Cameroon, West Africa.
Ash from volcanic eruptions ranges from fine dust to heavy blocks. The force of the volcanic blast can be so powerful that large pieces of rock have been thrown over 20 km (12 miles)! Ash falls can have negative repercussions on crop production near volcanoes. However, in the long run, ash makes soil very fertile. The ash from major volcanic explosions can reach such heights that they can reflect incoming sunlight and influence global climate. Krakatoa (1883), Mount Agung (1963), and Pinatubo (1991) were three volcanic eruptions that lowered the world’s temperature by 0.5 °C (0.9 °F)! Pyroclastic flows are another feared element of volcanoes. These mixes of volcanic materials and gases travel down the slopes of volcanoes at speeds of 160 km (100 miles) an hour Since the material within pyroclastic flows may be as much as 1,300 °C (700 °F). they burn everything in their path.
The six degrees of eruptions in order from least to most explosive are: Icelandic, Hawaiian, Strombolian, Vulcanian, Pelean, and Plinian. The least explosive still have slow-moving-lava, which allows the gradual release of gases. At the other end of the spectrum, Plinian volcanoes produce the most damage, because the gases contained within them are released rapidly.
Most volcanoes are located on islands and mountains bordering the Pacific Ocean plate. This boundry is known as the “Pacific Ring of Fire,” or the Circum-Pacific Belt. It is also home to most of the world’s earthquakes, another by-product of plate tectonics. About 50,000 earthquakes happen each year, although usually only one or two cause major damage.
Plate tectonics is the process largely responsible for earthquakes. It does not cause all of them, however. Some are initiated by human activity, such as mining, whereas others can be triggered by large meteors crashing to Earth’s surface. The most common source is the shearing of two plates of Earth’s crust. These plates meet at faults, where they are held in place by friction until enough pressure forces movement.
Harry Fielding Reid’s “elastic rebound” theory first explained how an earthquake released energy similarly to an elastic band breaking after being stretched. The rocks fracture first at the earthquake’s focus, usually several miles below the surface. Fracturing can continue along the fault for many miles. Reid formulated his hypothesis after the San Andreas Fault ruptured in 1906, wreaking destruction on San Francisco. The fractures occurred a distance of about 430 km (270 miles), moving one side of the fault about 6 m (20 feet).
Faults can move in four different ways: strike-slip, normal, thrust, and reverse. In each of these faults, one mass of rock moves one way, while the other moves another way. When the rock fractures and slipage occurs, energy travels as seismic waves. There are four types of seismic waves. Primary (P) and Secondary (S) waves travel through Earth, while the third and fourth types, Love and Rayleigh waves, travel at the surface. They all move in different ways, at different speeds, and through different materials. Love and Rayleigh waves are slower than P and S waves; however, these surface waves are the ones that cause the most damage.
Seismologists use seismographs and other equipment, along with their knowledge of seismic waves to pinpoint the focus of an earthquake. Once they know the focus of the quake, they can better predict where other fault slips, called aftershocks, will occur following the initial event. There may be more than 1,000 aftershocks per day after an earthquake, and some may cause more damage than the initial event.
Earthquakes can cause massive loss of life. In underwater subduction zones, an earthquake is capable of displacing water, which may result in a tsunami. In 2004, more than 200,000 people were killed as a result of a tsunami occuring in the Indian Ocean near southeast Asia. Earthquakes can also cause other natural disasters, including landslides and avalanches. Many people have been hurt or have lost their lives when buildings and other structures collaps during earthquakes as well. In recent years, engineers have worked successfully with seismologists to build with materials “flexible” enough to withstand strong earthquakes.
This photo shows a tsunami hitting Marina beach in Madras, India, on December 26, 2004. More than 1000 people were killed in the Madras area alone. AFP/Getty Images
Plate tectonics is a relatively recent theory. In less than 100 years since Wegener proposed continental drift, scientists have designed underwater seismographs, harnessed geothermal volcanic energy for power, and explored the deepest trench in the world. Much more will be discovered over the next hundred years. Many more questions will likely be answered: How can we better prepare for earthquakes and volcanoes? How can we predict them? This book looks back at our planet’s past, describing the processes that shape the Earth and its features. It attempts to explain the natural disasters that affect us today, and offers a glimpse into our planet’s ever-changing future.
