In the volcanic regions of the Earth, volcanoes are gathering strength and stoking up for eruptions. Volcanic eruptions can cause temporary changes in Earth’s climate. When they occur near a populated area, they are likely to cause death and social and economic catastrophe. Fortunately, large volcanic eruptions occur much less frequently than smaller ones do.

Scientists who study volcanoes, known as volcanologists, try to predict the likelihood of an eruption as well as the severity of an eruption. They use seismographic devices and other instruments to monitor geophysical signs that may indicate when a volcano is getting ready to erupt. These events or signals, known as precursors, include earthquake activity and swelling of the ground beneath the volcano. Volcanologists can usually determine if a volcano will produce a minor or major eruption on the basis of the volcano’s precursor activity. A larger eruption, for example, would be preceded by a long period of vigorous earthquakes and a change in the shape of the volcano due to underground swelling of magma (molten rock). In addition, before a volcano erupts it usually has active steam vents, fumaroles (vapor-emitting vents), and hot springs in its crater or on its flanks. The temperature of these vents and the chemical composition of the gases and fluids emerging from them may show changes that can help volcanologists predict an eruption. An increase in the amount of sulfur or chlorine gases, for example, would indicate that magma may be rising toward the surface.

Many of the world’s potentially most deadly volcanoes, or those located near populated areas, are closely monitored by scientists. The number of permanent volcano observatories is growing rapidly. Of the hundreds of active volcanoes on Earth, more than 20 are closely monitored today. However, some of the most dangerous volcanoes are those that are not known to be active and suddenly spring to life without warning. El Chichün in Mexico was one of these volcanoes, erupting in 1982 with deadly results. Even more dangerous are mountains that are not even known to be volcanoes. This was the case with Mount Lamington, which exploded suddenly in 1951 in Papua New Guinea.

Although volcanologists cannot always forecast exactly when or where the next eruption will occur, they can predict which volcanoes are likely to erupt in the future. The following volcanoes have perhaps the greatest potential to wreak havoc.

Campi Flegrei Caldera, Italy

This dangerous volcano is located west of Naples, Italy. It shook with gigantic explosive eruptions about 35,000 and 12,000 years ago. These eruptions created a large caldera, or volcanic crater, that is partially filled in by the Gulf of Pozzuoli. Since a small eruption in 1538, the volcano has remained quiet at the surface. However, the caldera floor has moved up and down by several meters at various times during the last two centuries, indicating that the volcano is in a state of unrest. A major eruption of Campi Flegrei would have a direct impact on more than 1 million people who live in Naples and nearby cities. However, much of the eruption would be directed into the Tyrrhenian Sea.

Fuji (Volcano), Japan

Fuji is the tallest mountain in Japan, rising to 3,776 m (12,387 ft). The volcano is located less than 100 km (62 mi) west of Tokyo. With a population of about 30 million people, the Tokyo metropolitan area is the most populated metropolitan region in the world. The beauty of Fuji has been a source of inspiration for artists for centuries, but it has also been a source of terror in Japan. Its last eruption occurred in 1707, when huge quantities of volcanic ash fell over a large area to the east of the volcano, burying villages under several feet of volcanic ash. The ash fallout was also intense in the Tokyo region, where it blocked out sunlight and caused days of darkness. A repeat of this eruption would have devastating effects for Japan’s economy and the way of life of millions of people living in the greater Tokyo region and elsewhere in central Japan. The length of Fuji’s dormant period, since 1707, is a cause of concern. Typically, the longer the dormant period, the greater the likelihood that a violent eruption will follow.

After a dormancy of nearly 300 years, Fuji may be coming back to life, judging from a large number of earthquakes that started in October 2000. Hundreds of tremors have been detected, all of them originating below the volcano. Fuji has been placed on official eruption watch by the Japanese authorities, and monitoring efforts have been stepped up in order to provide advance warning. An eruption like the one in 1707 would spread ash over a wide area, bring Tokyo’s economy to a halt, force the closure of airports, damage water supplies, and cause serious health risks from the volcanic dust.

Mount Rainier, United States

Located in the western United States in Washington state, Mount Rainier is the largest volcano in the Cascade Range, reaching a height of 4,392 m (14,410 ft). The majestic volcano is situated near the large urban areas of Tacoma and Seattle. It is one of the most dangerous volcanoes to be located near a huge population and economic center. The main hazard from an eruption of Mount Rainier would be the creation of lahars (landslides or mudflows of volcanic debris that resemble wet concrete). An eruption would probably melt the glaciers capping the mountain, causing flash floods that would mix with loose volcanic ash and other rock debris. This concretelike mixture would rapidly flow down the flanks of the volcano and into the surrounding lowlands, destroying everything in its path. In past eruptions, lahars from Mount Rainier have reached Puget Sound, a large and sheltered arm of the Pacific Ocean that forms the waterfront of several cities, including Seattle. A volcanic eruption of Mount Rainier about 5,700 years ago caused one of the world’s largest-known lahars, and 600 years ago another large lahar was generated on the mountain’s western flank. Even a small eruption of Mount Rainier is almost certain to generate lahars. Today the effects would be devastating to the millions of people who live in the area.

Mount Vesuvius, Italy

Anyone interested in volcanoes has heard about the deadly eruption of Mount Vesuvius in ad 79 that buried the Roman cities of Pompeii and Herculaneum. The volcano had many more eruptions afterward, some of them violent, such as the eruption of 1631. The last eruption of Mount Vesuvius was in 1944, and this notorious volcano has remained quiet ever since. Quiet at the surface, that is. From August to November in 1999 the volcano alarmed volcanologists in nearby Naples when they detected earthquake activity at depths of up to 4 km (2 mi) below the crater. The activity then subsided. About 3 million people live near the volcano, so even the smallest eruption is almost certain to have a large impact. Probably no other active volcano on the planet is surrounded by such a dense population. Mount Vesuvius is therefore closely monitored for any new signs of unrest.

Santoríni Caldera, Greece

In about 1640 bc a gigantic eruption occurred on the eastern Mediterranean island of Thíra (Thera), also known as Santoríni. The eruption was so destructive that it was probably a factor in the decline of the Minoan culture. Many scholars attribute the legend of the lost continent of Atlantis to this eruption. Today the volcano is a huge caldera called Santoríni that is submerged in the sea and nearly encircled by what remained of Thíra after the eruption. The island is now one of the Aegean Islands of Greece. Several minor lava eruptions have built up two small islands in the caldera’s center. The last of these eruptions was in 1950. Although that eruption was minor, the history of this volcano shows that it is capable of truly enormous explosions that would have devastating effects.

Reykjanes Peninsula, Iceland

The submarine mid-ocean ridges form Earth’s longest and most active volcanic zones. The only place where a mid-ocean ridge appears above sea level is in Iceland on the Reykjanes Peninsula. This peninsula has been volcanically quiet since the Middle Ages, but one day it will burst to life. The peninsula contains Iceland’s capital city of Reykjavík and nearby towns, with about half the country’s population and most of its industries. An eruption on the Reykjanes Peninsula would create large and fast-moving basaltic lava flows, or flood basalts, that would threaten this vulnerable community.

Long Valley Caldera, United States

This enormous volcano in east central California has been in a state of unrest since 1979, with earthquakes along the boundary of the caldera, periodic uplift of the caldera floor, and increased emission of gases. The suffocating gases have already killed trees and other plant life. The volcano is actually a large oval-shaped depression located 20 km (12 mi) south of Mono Lake on the eastern side of the Sierra Nevada mountain range. About 700,000 years ago the Long Valley Caldera exploded in one of the largest eruptions on the North American continent. Ash fallout occurred over much of the area of the United States, and pyroclastic flows (a mixture of hot ash, rock fragments, and gas) spread widely over the region surrounding the caldera. A major eruption of this volcano would have severe effects throughout northern and central California and Nevada.

Rabaul Caldera, Papua New Guinea

The Rabaul Caldera on the island of New Britain in Papua New Guinea erupted in 1937, 1944, and 1994. In the 1994 eruption, the two craters of Tavurvur and Vulcan erupted simultaneously, covering the nearby town of Rabaul in ash and causing the evacuation of about 90,000 people. The volcanic activity has continued at a low level to the present day. The recent eruptions were small events compared to the great eruption that took place in the 6th century, when the caldera was formed by the collapse of the volcano. The low-level activity of Rabaul Caldera could be a prelude to a giant eruption.

Taupo (Volcano), New Zealand

Most volcanoes are high mountains, but some of the most violent volcanoes are almost invisible, such as Taupo in New Zealand. This volcano had such a violent eruption in ad 186 that it blew apart and created a huge hole in the ground to form Lake Taupo. It was one of the world’s most violent eruptions. Numerous earthquakes have shaken the area in recent times, although this activity appears to have ceased. Taupo volcano bears watching closely because more than 200,000 local residents would be affected by volcanic activity.

Krakatau (Volcano), Indonesia

The volcanic island of Krakatau in Indonesia is probably second only to Mount Vesuvius in its notoriety as one of Earth’s killer volcanoes. In 1883 more than half of Krakatau Island disintegrated during an explosive eruption. The entrance of pyroclastic flows into the sea during the eruption caused huge tsunamis (tidal waves) to strike the neighboring islands of Java and Sumatra, killing about 34,000 people. The eruption was so violent that hot pyroclastic flows, riding on an air cushion, swept over the surface of the sea and reached Sumatra 40 km (25 mi) away, fatally burning 2,000 people. Subsequent eruptions formed a new island inside the submarine caldera. This new island is called Anak Krakatau, meaning “child of Krakatau.” The volcano’s current eruptions are small and frequent, but another large one is possible.

Paektu-san (Baitou Shan), East Asia

In about ad 1050 a colossal volcanic eruption occurred on the present-day border of China and North Korea. The explosion formed a caldera now filled by lake Chŏn-ji (Tian Chi). It was one of the largest explosive eruptions of the Holocene Epoch, the period spanning the last 10,000 years. It ejected about 150 cu km (36 cu mi) of material and spread ash as far away as northern Japan. This eruption and the volcano itself have only recently come to the attention of volcanologists. Unconfirmed reports indicate that this little-known volcano erupted again in the early 18th century. It is clearly not an extinct volcano, and we are likely to see much more from it in the future.