Earthquakes And Volcanoes Draw Some Lines

One of the most important things that geologists discovered when they conducted seismic studies of the crust was that it was not made up of one single piece. Instead, the crust is cracked and broken into many different pieces. Scientists discovered this fact as they began to learn more about the causes of earthquakes. It had long been known that earthquakes do not happen with the same frequency all over the planet. Some places, like California and Mexico, have many earthquakes. Places like Michigan and Maine have relatively few. The big question that geologists had to answer was why this should be. To understand the problem, it helps to understand what causes an earthquake in the first place.

During an earthquake, forces inside the planet cause two pieces of the crust to move past each other. The breaks between the pieces of crust are called faults, and they come in many different sizes. As might be expected, large earthquakes usually happen at big faults and small quakes usually happen at small ones. In the early 1960s, several geologists began taking the really simple approach of plotting the location of earthquakes on a map of the world. In a short time, an interesting picture began to develop.

Drawing Earthquake And Volcanoes
Figure 5.4 Volcanoes (red triangles) and major earthquakes (yellow dots) are concentrated along the boundaries of tectonic plates (red lines).

It seems that most earthquakes occur in the ocean along narrow bands. The single biggest concentration of them can be found in a broad ring that roughly circles the Pacific Ocean. Another area of earthquake concentration is found along the mid-ocean ridges. Earthquakes that occur on land are usually found in mountainous regions.

After geologists plotted the earthquake locations, they did the same for active volcanoes. Volcanoes are openings in the Earth where hot liquid magma rises to the surface from deep within.

the ring of Fire

So many active volcanoes are located around the rim of the Pacific Ocean that geologists have nicknamed it the "Ring of Fire." While all volcanoes are dangerous, the size and explosiveness of most of the volcanoes found in this ring make them particularly hazardous. In recent years, eruptions at Mount St. Helens in the United States and Mount Pinatubo in the Philippines have been responsible for the destruction of a tremendous amount of property and loss of life.

These eruptions are small compared to past eruptions, however. In 1883, Krakatau, located between the islands of Java and Sumatra, blew up with such force that it was heard in Australia, over 2,000 miles (3,219 km) away. An even bigger eruption had occurred earlier, in Indonesia in 1815, when the entire top of Mount Tambora blew off, killing over 50,000 people. More, possibly bigger, problems with the volcanoes in the Ring of Fire lie in the future. Many of the Ring of Fire volcanoes are among the world's largest, including Mount Fuji in Japan and Mount Rainier in the United States, both of which are just waiting to blow. Because the populations surrounding these volcanoes are so large, their eruptions could impact millions of people.

Table 5-1: Deadly Volcanoes in the Ring of Fire

Year

Volcano Name and Location

Estimated Number Killed

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